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arXiv:astro­ph/0110308
12
Oct
2001
**TITLE**
ASP Conference Series, Vol. **VOLUME**, **PUBLICATION YEAR**
**EDITORS**
An Overview of the Performance and Scientic Results
from the Chandra X-Ray Observatory (CXO)
M. C. Weisskopf
NASA Marshall Space Flight Center, MSFC, AL 35812
B. Brinkman
SRON, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands
C. Canizares
MIT, 77 Massachusetts Avenue, Cambridge MA 02139-4307
G. Garmire
PSU, 525 Davey Lab, University Park, PA 16802
S. Murray
SAO, 60 Garden Street, Cambridge MA 02138
L. P. Van Speybroeck
SAO, 60 Garden Street, Cambridge MA 02138
Abstract.
The Chandra X-Ray Observatory (CXO), the x-ray component of
NASA's Great Observatories, was launched on 1999, July 23 by the Space
Shuttle Columbia. After satellite systems activation, the rst x-rays fo-
cussed by the telescope were observed on 1999, August 12. Beginning
with the initial observation it was clear that the telescope had survived
the launch environment and was operating as expected. Despite an ini-
tial surprise due to the discovery that the telescope was far more eô-
cient for concentrating CCD-damaging low-energy protons than had been
anticipated, the observatory is performing well and is returning superb
scientic data. Together with other space observatories, most notably
XMM-Newton, it is clear that we have entered a new era of discovery in
high-energy astrophysics.
1. Introduction
The Chandra X-Ray Observatory (CXO), formerly known as the Advanced
X-Ray Astrophysics Facility (AXAF), has joined the Hubble Space Telescope
(HST) and the now defunct Compton Gamma-Ray Observatory (CGRO) as one
1

2 Author & Co-author
of NASA's "Great Observatories". Chandra provides unprecedented capabilities
for sub-arcsecond imaging, spectrometric imaging, and for high-resolution dis-
persive spectroscopy over the band 0.08-10 keV (15-0.12 nm). Therefore, a wide
variety of high-energy phenomena in an all-encompassing range of astronomical
objects is being observed.
Chandra is a NASA facility that provides scientic data to the international
astronomical community in response to scientic proposals for its use. The Ob-
servatory is the product of the eorts of many organizations in the United States
and Europe. NASA Marshall Space Flight Center (MSFC, Huntsville, Alabama)
manages the Project and provides Project Science; TRW Space and Electronics
Group (Redondo Beach, California) served as prime contractor; the Smithsonian
Astrophysical Observatory (SAO, Cambridge, Massachusetts) provides technical
support and is responsible for ground operations including the Chandra X-ray
Center (CXC) which distributes and archives the data. There are ve scientic
instruments (x2.6.) aboard the Observatory.
In 1977, NASA/MSFC, in collaboration with SAO, performed a study which
led to the denition of the mission. This study was a result of an unsolicited
proposal submitted to NASA in 1976 by Prof. R. Giacconi and Dr. H. Tanan-
baum. Since then, much has transpired, including the highest recommendation
by the National Academy of Sciences Astronomy Survey Committee, selection of
the instruments, selection of the prime contractor, demonstration of the optics,
restructuring of the mission, and ultimately the launch.
We begin by brie y describing the Chandra systems (x2.). We then de-
scribe Chandra's on-orbit performance (x3.) and highlight certain scientic re-
sults (x4.).
2. Chandra Systems
2.1. Mission and Orbit
The Space Shuttle Columbia launched and deployed the Observatory into a low
earth orbit at an altitude of about 240 km. Subsequently, an Inertial Upper
Stage, a two-stage solid-fuel rocket booster developed by the Boeing Company
Defense and Space Group (Seattle, Washington), propelled the Chandra ight
system into a highly elliptical transfer orbit. Subsequently, over a period of
days, Chandra's Internal Propulsion System placed the observatory into its ini-
tial operational orbit - 140,000-km apogee and 10,000-km perigee, with a 28.5 o
inclination. The highly elliptical orbit, with a period of 63.5 hours, yields a
high observing eôciency. The fraction of the sky occulted by the earth is small,
as is the fraction of the time when the detector backgrounds are high as the
Observatory dips into Earth's radiation belts. Consequently, more than 70% of
the time is useful and uninterrupted observations lasting more than 2 days are
possible.
The specied design life of the mission is 5 years; however, the only expend-
able (gas for maneuvering) is sized to allow operation for much more than 10
years and the orbit will remain useful for decades.

APS Conf. Ser. Style 3
Figure 1. Expanded view of the Chandra ight system, showing
several subsystems. TRW drawing.
2.2. Spacecraft
The spacecraft is made up of:
1. The Pointing Control and Aspect Determination subsystem which per-
forms on-board attitude determination, solar-array control, slewing, point-
ing and dithering control, and momentum management.
2. The Communication, Command, and Data Management subsystem which
performs communications, command storage and processing, data acquisi-
tion and storage, and computation support, timing reference, and switch-
ing of primary electrical power for other systems or subsystems.
3. The Electrical Power Subsystem which generates, regulates, stores, dis-
tributes, conditions, and controls the primary electrical power.
4. The Thermal Control Subsystem which furnishes passive thermal control
(where possible), heaters, and thermostats.
5. The Structures and Mechanical Subsystem which encompasses the space-
craft structures, mechanical interfaces among the spacecraft subsystems
and with the telescope system and external structures.
6. The Propulsion Subsystem which comprises the Integral Propulsion Sub-
system - deliberately disabled once nal orbit was obtained - and the Mo-
mentum Unloading Propulsion Subsystem.
7. The ight software which implements algorithms for attitude determina-
tion and control, command and telemetry processing and storage, and
thermal and electrical power monitoring and control.
2.3. Telescope system
Kodak integrated the Telescope System. Its principal elements are the High-
Resolution Mirror Assembly (HRMA, x2.6.) and the Optical Bench Assembly
(OBA). Composite Optics Incorporated (COI, San Diego, California) developed
the critical light-weight composite materials for the optical bench and for other
Chandra structures. The Telescope System also provides mounts and mecha-
nisms for the Chandra Observatory's 2 objective transmission gratings (x2.6.).
In addition, Ball Aerospace and Technologies Corporation (Boulder, Colorado)
fabricated the Aspect Camera Assembly (Michaels 1998), a visible-light tele-
scope and CCD camera which attaches to, and is coupled with, the HRMA
through a ducial-light transfer system, which maps the x-ray focal plane onto
the sky.

4 Author & Co-author
Figure 2. Photograph of the High-Resolution Mirror Assembly dur-
ing alignment and assembly at Kodak. In the photo, 7 of the 8 mirrors
are already attached to the central aperture plate. Photograph is from
Kodak.
2.4. Integrated Science Instrument Module
Ball Aerospace and Technologies Corporation also built the Science Instrument
Module (SIM) - Skinner & Jordan (1997) - which includes mechanisms for fo-
cussing and translating Chandra's focal-plane science instruments (x2.6.). The
translation is necessary as the instruments cannot realistically share the focal
plane and must be translated into position at the telescope focus.
2.5. Electron Proton Helium Instrument (EPHIN)
Mounted on the spacecraft and near the HRMA is a particle detector: the
Electron, Proton, Helium INstrument (EPHIN). The EPHIN instrument was
built by the Institut fur Experimentelle und Angewandte Physik, University of
Kiel, Germany, and a forerunner was own on the SOHO satellite.
EPHIN consists of an array of 6 silicon detectors with anti-coincidence. The
instrument is sensitive to electrons in the energy range 250 keV - 10 MeV, and
hydrogen and helium isotopes in the energy range 5 - 53 MeV/nucleon. Electrons
above 10 MeV and nuclei above 53 MeV/nucleon are registered with reduced
capability to separate species and to resolve energies. The eld of view is 83
degrees with a geometric factor of 5.1 cm 2 sr. A detailed instrument description
is given in Mueller-Mellin et al. (1995).
EPHIN is used to monitor the local charged particle environment as part
of the scheme to protect the focal-plane instruments from particle radiation
damage. Clearly EPHIN is also a scientic experiment in its own right.
2.6. X-Ray Subsystems
Chandra's x-ray subsystems are the High-Resolution Mirror Assembly (HRMA),
the objective transmission gratings, and the focal-plane science instruments.
High-Resolution Mirror Assembly (HRMA) Hughes Danbury Optical Systems
(HDOS, Danbury, Connecticut) precision gured and superpolished the 4-mirror-
pair grazing-incidence x-ray optics out of Zerodur blanks from Schott Glaswerke
(Mainz, Germany). Optical Coating Laboratory Incorporated (OCLI, Santa
Rosa, California) coated the optics with iridium, chosen for high x-ray re ectivity
and chemical stability. The Eastman Kodak Company (Rochester, New York)
aligned and assembled the mirrors into the 10-m focal length High-Resolution
Mirror Assembly (HRMA, Figure 2). The forward contamination cover houses
16 radioactive sources used for verifying transfer of the ux scale from the ground
to orbit (Elsner et al. 1994, 1998, 2000).
Objective transmission gratings Aft of the HRMA are 2 objective transmission
gratings (OTGs) - the Low-Energy Transmission Grating (LETG) and the High-
Energy Transmission Grating (HETG). Positioning mechanisms are used to in-
sert either OTG into the converging beam where they disperse the x-radiation

APS Conf. Ser. Style 5
Figure 3. Photograph of the LETG and HETG mounted to the
spacecraft structure. Photograph is from TRW.
Figure 4. Photograph of the HRC. The HRC-I (imager) is at the
bottom; the HRC-S (the readout for the LETG), at the top.
onto the focal plane. Figure 3 shows the gratings mounted behind the HRMA
in their retracted position.
Low-Energy Transmission Grating (LETG) The Space Research Insti-
tute of the Netherlands (SRON, Utrecht, Netherlands) and the Max-Planck-
Institut fur extraterrestrische Physik (MPE, Garching, Germany) designed and
fabricated the LETG. The 540 grating facets, mounted 3 per module, lie tangent
to the Rowland toroid which includes the focal plane. With free-standing gold
bars of about 991-nm period, the LETG provides high-resolution spectroscopy
from 0.08 to 2 keV (15 to 0.6 nm).
High-Energy Transmission Grating (HETG) The Massachusetts Insti-
tute of Technology (MIT, Cambridge, Massachusetts) designed and fabricated
the HETG. The HETG employs 2 types of grating facets | the Medium-Energy
Gratings (MEG), mounted behind the HRMA's 2 outermost shells, and the High-
Energy Gratings (HEG), mounted behind the HRMA's 2 innermost shells |
oriented at dierent dispersion directions. With polyimide-supported gold bars
of 400-nm and 200-nm periods, respectively, the HETG provides high-resolution
spectroscopy from 0.4 to 4 keV (MEG, 3 to 0.3 nm) and from 0.8 to 8 keV (HEG,
1.5 to 0.15 nm).
Focal-plane science instruments The Integrated SIM (x2.4.) houses Chandra's
2 focal-plane science instruments | the (microchannel-plate) High-Resolution
Camera (HRC) and the (Charged Coupled Device - CCD) Advanced CCD Imag-
ing Spectrometer (ACIS). Each instrument provides both a so called (as all the
detectors are imagers) imaging detector (I) and a spectroscopy detector (S), the
latter designed especially to serve as a readout for the photons dispersed by the
transmission gratings.
High-Resolution Camera (HRC) SAO designed and fabricated the HRC
(Murray et al. 2000) shown in Figure 4. Made of a single 10-cm-square mi-
crochannel plate, the HRC-I provides high-resolution imaging over a 31-arcmin-
square eld of view. Comprising 3 rectangular segments (3-cm  10-cm each)
mounted end-to-end along the OTG dispersion direction, the HRC-S serves as
the primary read-out detector for the LETG. Both detectors are coated with
a cesium{iodide photocathode and covered with aluminized-polyimide UV/ion
shields.
Advanced CCD Imaging Spectrometer (ACIS) The Pennsylvania State
University (PSU, University Park, Pennsylvania) and MIT designed and fabri-

6 Author & Co-author
Figure 5. Photograph of the focal plane of ACIS, prior to installa-
tion of the optical blocking lters. The ACIS-I is at the bottom; the
ACIS-S (the readout for the HETG), at the top.
cated the ACIS (Figure 5) with CCDs produced by MIT Lincoln Laboratory
(Lexington, Massachusetts). Some subsystems and systems integration was pro-
vided by Lockheed{Martin Astronautics (Littleton, Colorado). Made of a 2-
by-2 array of large-format, front-illuminated (FI), 2.5-cm-square, CCDs, ACIS-I
provides high-resolution spectrometric imaging over a 17-arcmin-square eld of
view. ACIS-S, a 6-by-1 array of 4 FI CCDs and two back-illuminated (BI)
CCDs mounted along the OTG dispersion direction, serves both as the primary
read-out detector for the HETG, and, using the one BI CCD which can be
placed at the aimpoint of the telescope, also provides high-resolution spectro-
metric imaging extending to lower energies but over a smaller (8-arcmin-square)
eld than ACIS-I. Both ACIS detectors are covered with aluminized-polyimide
optical blocking lters.
3. On-orbit Performance
Chandra's mission is to provide high-quality x-ray data. Chandra's performance
advantage over other x-ray observatories is analogous to that of the HST over
ground-based telescopes. The eective area of the Chandra mirror is shown in
Figure 6; it is approximately 800 cm 2 at energies below 2 keV, and approximately
400 cm 2 between 2 and 5 keV. Figure 6 also shows the eective area convolved
with ACIS quantum eôciencies.
Figure 6. Chandra on-axis mirror and mirror/ACIS eective areas.
3.1. Imaging
The angular resolution of Chandra is signicantly better than any previous,
current, or even currently-planned x-ray observatory. Figure 7 qualitatively, yet
dramatically, illustrates this point by comparing the early Chandra image of the
supernova remnant Cassiopeia-A, based on about 2700 s of data, with a 200,000
s ROSAT image. (Prior to Chandra, the ROSAT observatory represented the
state of the art in high-resolution x-ray imaging.) The improvement is dramatic,
and the point source at the center | undetected in the ROSAT image | simply
leaps out of the Chandra image.
Quantitatively, Chandra's point spread function (PSF), as measured dur-
ing ground calibration, had a full width at half-maximum less than 0.5 arcsec
and a half-power diameter less than 1 arcsec. The prediction for the on-orbit
encircled-energy fraction was that a 1-arcsec-diameter circle would enclose at
Figure 7. Chandra (left) and ROSAT (right) images of CAS-A.

APS Conf. Ser. Style 7
Figure 8. The predicted and observed encircled energy as a func-
tion of radius for an on-axis point source as observed with the HRC-I.
The calculations, performed at two energies (0.277 keV and 6.40 keV)
include a realistic (0.22") estimate of the contribution from the aspect
solution. Flight data are from the calibration observation of AR Lac.
Figure produced by Chandra Telescope Science.
Figure 9. Image of the dispersed spectrum, including zeroth order,
of 3C273. The jet is clearly resolved in the lower right hand portion
of the gure. The six spikes emanating from the central image are due
to dispersion by the facet holders. Image courtesy Jeremey Drake and
LETGS team.
least half the ux from a point source. A relatively mild dependence on en-
ergy resulting from diractive scattering by surface microroughness attested to
the excellent superpolished nish. The ground measurements were taken under
environmental conditions quite dierent than those encountered on-orbit. The
eects of gravity and the nite distance and size of the various x-ray sources
were unique to the ground calibration. On the other hand, on the ground there
was no observatory motion to deal with. On-orbit, the performance folds in the
spatial resolution of the ight detectors and any uncertainties in the aspect so-
lution which determines, post-facto, the direction the observatory was pointing
relative to the instruments and to celestial coordinates.
The HRC has the best spatial resolution ( 20m, 0.4 arcsec) of the
two imaging instruments and thus is best matched to the telescope. Figure 8
illustrates the extrapolation of the ground calibration to on-orbit and compares
predictions at two energies with an observed PSF. More details as to the on-orbit
imaging performance may be found in Jerius et al. (2000). The performance of
the aspect camera and the attitude control system is discussed by Aldcroft et
al. (2000) and Cameron et al. (2000).
Finally, it is interesting that the use of the zeroth order image for the
observations of extremely bright sources, which would otherwise saturate the
detectors and/or the telemetry, has proven quite useful. The utility for such
observations is illustrated in Figure 9 where we show an image with the LETG
inserted. Both the jet and the central source of 3C273 are clearly resolved.
3.2. HRC
The HRC is performing close to what was expected pre-launch, despite a few
anomalies discussed below.
Background The HRC-I on-orbit counting rate is about 250 c/s ( 2 cts/s/cm 2 ).
These are mostly cosmic ray events and are detected in the anti-coincidence
shield (AC). The on board veto function has been activated and reduces the
valid event rate to about 50 c/s.
In the case of the HRC-S, enabling the anti-coincidence shield does not
reduce the background as expected. The problem appears to be a timing error
in the electronics that can not be changed from the ground. The HRC-S trigger

8 Author & Co-author
pulses arrive earlier than the AC signals and are not held long enough to trigger a
coincidence, resulting in telemetry saturation. To cope, a \spectroscopic region"
has been dened which is a strip of the detector, 9.6 mm wide, centered on the
nominal spectroscopy aim point. This region is about 1/2 of the total HRC-S
area and reduces the valid count rate to about 120 c/s, well below the telemetry
limit.
An (on-ground) event screening algorithm has been developed to help in
identifying non-X-ray events and further reduces the background for data from
both HRC-I and -S. For HRC-I, e.g., this results in a background rate of about 0.8
cts/arcsec 2 for a 10 5 second integration. For HRC-S, this approach reduces the
background to  13 counts in a 0.06  A spectral \slice" per 10 5 second integration.
Image Quality Even with a reduction in high voltage and MCP gain, there is
a degradation of image quality in HRC-I for the higher amplitude events due
to added electronic noise during event processing. Fortunately there is enough
information in the telemetry stream to correct for this eect. A revised ground
system event processing algorithm has been developed which identies distorted
events and ags them as such. The added electronic noise, a systematic eect,
can be partially compensated for and removed. Algorithms for making these
corrections have been added to the data processing system. Using the plate
focus data as a test case, the measured encircled energy near the focus was
improved from about 40% within 1 arcsecond diameter to > 60%.
A second image artefact is the appearance of \ghost" images near strong
sources. These ghost images are due to events where the readout ampliers
are saturated leading to fairly large miscalculation of the event position. The
number of such misplaced events is 1 percent or less of the properly placed
events. Screening the data for events where the ampliers are all near their
peak value eectively eliminates these \ghosts".
Because the HRC-S gain is lower than for HRC-I, electronic saturation
eects are less signicant. As a result the imaging performance appears to be
excellent once the corrections described above for HRC-I have been applied.
Eôciency The count rates from the sources observed are about what was
expected. For LMC X-1, Ar Lac and Cas-A the HRC rates are close to the
pre-launch predictions. There have been no measurable changes to the HRC
eôciency since launch.
HRC Timing The HRC was mis-wired so that the time of the event associated
with the j-th trigger is that of the previous (j-th -1) trigger. If the data from all
triggers were routinely telemetered, the mis-wiring would not be problematical
and could be dealt with by simply reassigning the time tag which is nominally
accurate to 16 sec. Since the problem has been discovered, new operating
modes have been dened which allow one to telemeter all data whenever the
total counting rate is moderate to low, albeit at the price of higher background.
For very bright sources the counting rate is so high that information associated
with certain triggers are never telemetered. In this case, the principal reason for
dropping events is that the on-board, rst-in-rst-out (FIFO) buer lls as the
source is introducing events at a rate faster than the telemetry readout. Events
are dropped until readout commences freeing one or more slots in the FIFO.

APS Conf. Ser. Style 9
Figure 10. The spectrum of all x-ray events detected during a 970
ksec exposure to the Chandra Deep Field-North region.
This situation can also be dealt with (Tennant et al. 2001a) and time resolution
of the order of a millisecond can be achieved even under these conditions.
More details of the HRC and its performance may be found in Murray et
al. 2000, Kenter et al. (2000), and Kraft et al. (2000).
3.3. ACIS
As with the HRC, the ACIS instrument is performing well and contributing to
the success of the Chandra Mission.
Background The background experienced by the ACIS CCDs shows occasional
ares depending upon the orientation of the Observatory's orbit with respect to
the Earth's magnetosphere. The frequency and magnitude of the ares are much
more pronounced in the BI CCDs than in the FI CCDs, consistent with the
suggestion that the aring events are caused by low energy protons that enter
the observatory through re ection o the very smooth iridium coated mirrors
that comprise the HRMA. The gate structure of the FI CCDs absorbs most of
the protons for the majority of the ares.
The region of the Hubble Deep Field-North was observed using the ACIS-I
array, for 970 ksec in a series of twelve pointings spanning a period of 15 months
and thus provides, after removing sources, an excellent representation of the
background and its spectrum. The background (0.5-10.0 keV) was observed to be
constant to within about 10% for eight of the pointings. The rst three pointings
were made with the focal plane temperature at -110C and the background was
about 50% higher than for the background one year later taken at -120C. A
reduction in background was expected at the lower temperature. One pointing,
ObsId 2344, experienced a highly variable background with a are increasing the
background by a factor of between two and three for about 20 ksec, depending
upon the energy band.
The spectrum is shown in Figure 10. The prominent lines in the background
spectrum are from cosmic ray induced uorescence of the gold-coated collimator,
the nickel-coated substrate of the collimator, the silicon in the CCDs and from
aluminum used in various places in the housing and the lter coating. In general
the background produced by the uorescent lines is only about 2.6% of the
background not found in the lines in the soft (0.5-2.0 keV) band and 13.5% of
the ux in the hard (2.0-10.0 keV) band. Brandt et al. (2001) have noted that
by selecting certain ACIS event grades it is possible to suppress the background
by another 36% in the soft band (0.5 - 2.0 keV) and by 28% in the hard band (2.0
- 8.0 keV), while only reducing the source counts by 12% and 14% respectively.
Proton Damage to the Front-Illuminated CCDs The ACIS FI CCDs originally
approached the theoretical limit for energy resolution at almost all energies,
while the BI were of somewhat lesser quality in this regard. Subsequent to
launch and orbital activation, the energy resolution of the FI CCDs has become
a function of the row number, being nearer pre-launch values close to the frame

10 Author & Co-author
Figure 11. The energy resolution of two of the CCDs (S3 a BI CCD
and I3 a FI CCD) as a function of row number.
Figure 12. Spectral resolving power of the Chandra OTGs. On-
orbit results indicate slightly better performance.
store region and progressively degraded towards the farthest row (Figure 11).
The points are for the FI data and the curves for the BI data. These data were
taken at -120 o C. Note that these curves are representative of the variation with
row number, but do not account for an added row-dependent gain variation
which increases the energy resolution by an additional 15-20% for the larger row
numbers.
For a number of reasons, we believe that the damage was caused by low
energy protons, encountered during radiation belt passages and re ecting o the
x-ray telescope onto the focal plane. Subsequent to the discovery of the degra-
dation, operational procedures were changed so that the ACIS instrument is not
left at the focal position during radiation belt passages. Since this procedure
was initiated, no further degradation in performance has been encountered. The
BI CCDs were not impacted, consistent with the proton-damage scenario, as it
is far more diôcult for low energy protons to deposit their energy in the buried
channels (where damage is most detrimental to performance) of the BI devices,
as these channels are near the gates and the gates face in the direction opposite
to the HRMA. The energy resolution for the two BI CCDs remains at their
prelaunch values.
3.4. Grating performance
The Chandra OTGs allow measurements with spectral resolving power (Fig-
ure 12) of (=
 = (E=
E) > 500 for wavelengths  > 0:4 nm (energies <
3 keV). The on-orbit spectral resolution and eôciencies of both the LETG and
the HETG were as expected based on pre-launch calibrations.
4. Scientic Results
X-rays result from highly energetic processes - thermal processes in plasmas with
temperatures of millions of degrees or nonthermal processes, such as synchrotron
emission or scattering from very hot or relativistic electrons. Consequently, x-ray
sources are frequently exotic:
 Supernova explosions and remnants, where the explosion shocks the am-
bient interstellar medium or a pulsar powers the emission.
 Accretion disks or jets around stellar-mass neutron stars or black holes.
 Accretion disks or jets around massive black holes in galactic nuclei.
 Hot gas in galaxies and in clusters of galaxies, which traces the gravita-
tional eld for determining the mass.

APS Conf. Ser. Style 11
Figure 13. The ACIS-I image of the Orion Trapezium. The full
eld is about 16 arcminutes on a side, with the Trapezium stars in the
center.
Figure 14. The ACIS-I image of the Galactic Center.
Here we give several examples of observations with Chandra which demon-
strate the capability for investigating these processes and astronomical objects
through high-resolution imaging (x 4.1.) and high-resolution spectroscopy (x 4.2.).
4.1. Imaging
Chandra's capability for high-resolution imaging (x 3.1.) enables detailed high-
resolution studies of the structure of extended x-ray sources, including supernova
remnants (Figure 7), astrophysical jets (Figure 9), and hot gas in galaxies and
clusters of galaxies. The capability for spectrometric imaging allows studies
of structure, not only in x-ray intensity, but in temperature and in chemical
composition. Through observations with Chandra , one has begun to address
several of the most exciting topics in contemporary astrophysics.
Normal Stars The aspect solution of the spacecraft typically provides absolute
sky coordinates to about 1 arc second. It is usual for observations exceeding
20 ks to detect several stars in X-rays that have accurate positions (typically
0.4 arcseconds for USNO A2 stars and 0.15 arcseconds for Tyco II Catalog
stars). Using the stellar positions as a local reference, the x-ray sources can be
positioned to about 0.2 to 0.4 arcseconds. One example of using such positions
is the determination of the positions of the 1000 x-ray sources in the Orion
Trapezium region (Figure 13) where the stellar and x-ray positions dier by
about 0.3 arcseconds rms (Feigelson et al. 2001).
The Galactic Center Precise positioning with Chandra was critical for the
unique identication with SgrA* (Bagano et al. 2001a) in the extremely
crowded region of the Galactic Center (Figure 14). The mass of the black hole
at SgrA* has been determined from stellar motion to be 2:6  10 6 solar masses
(Genzel et al. 2000). The X-ray source associated with SgrA* is very faint
compared to other galactic nuclei, emitting only about 2  10 33 ergs/s. A bright
are was detected from this source on 27 October 2000, where the ux increased
by over an order of magnitude for about 10 ksec and then rapidly dipped on a
time scale of 600 seconds (Bagano et al. 2001b).
Supernova Remnants Another example of Chandra's ability to provide high-
contrast images of features of low surface brightness is exemplied by the now
classic image (Figure 15) of the Crab Nebula and its Pulsar (Weisskopf et al.
2000, Tennant et al. 2001a) which shows the intricate structure produced by
the pulsar wind and the synchrotron torus.
In cases where there is suôcient signal, the precisely measured point re-
sponse function of the HRMA permits image deconvolution A good example is
given by the observation of the recent supernova remnant 1987A in the Large

12 Author & Co-author
Figure 15. LETGS image of the Crab pulsar and nebula. The nearly
horizontal line in the gure is the cross-dispersed spectrum produced
by the LETG ne support bars. The nearly vertical line is the dispersed
spectrum from the pulsar.
Figure 16. The ACIS-S3 image of the supernova remnant SN1987A
in the Large Magellanic Cloud. The white overlay lines are from a HST
image.
Magellanic Cloud. The image is shown in Figure 16 where a Lucy-Richardson
algorithm has been used to deconvolve the telescope PSF (Burrows et al. 2000).
Globular Clusters One of the most striking examples of the power of high res-
olution x-ray imaging, is in the spectacular Chandra images of globular clusters.
Figure 17 is the moderately deep exposure (70 ksec) ACIS-I image recently pub-
lished by Grindlay et al. (2001a) of 47Tuc. The upper panel of this "true" color
x-ray image (composed of red/green/blue images derived from counts recorded in
soft (0.5-1.2 keV), medium(1.2-2 keV), hard (2-6 keV) bands) shows the central
2.5' x 2' of the cluster, or approximately central 3 core radii. The enlargement
at the bottom of the gure is  30 arcsec square, and thus the central  0:7
core radius portion of the cluster. Some 108 sources are detected in the eld
excluding the central core, with L x > 10 30 erg/s. Another > 100 sources are
likely present in the central core.
The image, associated spectra, and measured time variability reveals more
about the binary content and stellar, as well as dynamical, evolution of a globular
cluster than achieved with all previous x-ray observations of globulars combined
(and, even arguably, many HST observations as well). All 16 of the millisecond
pulsars (MSPs) recently located from precise pulse timing (Freire et al. 2001),
are detected (circles in Figure 17). Their x-ray spectral properties (colors) vs.
radio pulsation spindown measures shows them to obey a signicantly dierent
L x vs. _
E relation than for MSPs in the eld (Grindlay et al. 2001b) and 50-100
MSPs are likely detected in the cluster.
The second most abundant x-ray source population in 47Tuc are the long-
sought accreting white dwarfs, or cataclysmic variables (CVs), marked as squares
in Figure 17 for those already identied in deep HST images. Many others
(primarily blue or whitish color) candidates are present, so that perhaps a third
of the Chandra sources are CVs. A third signicant population of x-ray binaries
were discovered (and unanticipated) in this image: main sequence star binaries
Figure 17. Chandra image using ACIS-I3 of 47Tuc. The upper
image covers the central 2' x 2.5'. The enlarged central region is 35" x
35". Source identications shown are MSPs (circles), quiescent LMXBs
(circles), CV candidates (squares) and possibly aring BY Dra systems,
or M-S binaries (triangles). Figure courtesy of Josh Grindlay.

APS Conf. Ser. Style 13
Figure 18. Central region of M31 observed with ACIS-I. The cir-
cle is 5 arcseconds in radius and illustrates the ROSAT HRI loca-
tion of the nucleus. The ROSAT source is resolved into 5 individual
sources using Chandra, and the source labeled CXO J004244.2+411609
is within 0.15 arcseconds of the 310 7 solar mass black hole at the
nucleus. Just to south of the nucleus is the supersoft source CXO
J004244.2+411608 (Garcia et al. 2001). A long lived transient source,
CXO J004242.0+411609, is also shown.
(detached), or so-called BY Draconis stars (the brightest few of which, detected
as aring sources, are marked with triangles).
The observations of other globular clusters with Chandra are beginning to
be published with a recent example being the nearby core-collapsed cluster NGC
6397 (Grindlay et al. 2001c). This cluster shows a dramatic contrast with 47Tuc:
although almost as abundant in CVs, it is nearly devoid of MSPs. Grindlay et
al. (2001c) note that this suggests fundamental dierences in the relative neu-
tron star vs. white dwarf content, as well as compact binary formation history.
Clearly the high resolution x-ray view made possible with Chandra is opening a
new era in understanding these oldest, and dynamically most interesting, stellar
systems.
Normal Galaxies In addition to mapping the structure of extended sources
and the diuse emission in galaxies, the high angular resolution permits studies
of ensembles of discrete sources, which would otherwise be impossible owing to
source confusion. A beautiful example comes from the observations of the center
of M31 (Figure 18) performed by Garcia et al. (2000, 2001). The image shows
what used to be considered as emission associated with the black hole at the
center of the galaxy now resolved into ve distinct objects. A most interesting
consequence is that the emission from the region surrounding the central black
hole is unexpectedly faint relative to the mass of the central black hole, as with
the Milky Way.
M81 (NGC 3031) is a Sab spiral at a distance of approximately 3.6 Mpc.
The galaxy was observed by Tennant et al. (2001b) with the S3 chip of the ACIS-
S instrument on Chandra for 50 ksec (Figure 19). Prior to this observation,
the galaxy had been observed with both the Einstein and Rosat observatories.
Nine sources were detected with Einstein by Fabbiano (1988) of which 5 were
in the spiral arms. Twenty-six sources were detected with ROSAT by Immler
and Wang (2001). The Chandra observation detected the bright nucleus at a
(0.2-8.0 keV) luminosity of 4  10 40 ergs/s. In addition, 96 other sources were
detected: 81 with S/N  3:5; 16 with 3:0  S=N  3:5. Here S/N = 3 is one
false detection; 3.5 is 0.1. Based on a canonical spectrum, the (0.2-8.0 keV)
luminosity of the sources, including the nucleus, ranges from 3  10 36 ergs/s to
4  10 40 ergs/s.
There were 41 sources in the bulge of the galaxy and, excluding the nucleus,
these had a total luminosity of 1:6 10 39 ergs/s. One of these sources had a soft
spectrum (T' 70 eV) and an observed luminosity in excess of 2  10 38 ergs/s,
about the Eddington limit for a canonical neutron star. In addition, the bulge

14 Author & Co-author
Figure 19. Chandra observations of M81. Left - x-ray image with
contours. Right - x-ray contours on optical image. Courtesy of Doug
Swartz
shows 0:8  10 39 ergs/s of unresolved emission which follows the starlight and
is not completely consistent with the extrapolation of the LogN-LogS curve for
the galaxy, indicating diuse emission.
There were 56 sources in the disk that were within the ACIS-S3 eld of
view. Twenty one were within 400 pc of the spiral arms. These 21 include 4
of 5 of the soft sources and 7 of the 10 brightest sources. Five of the 21 sources
are near SNR. Interestingly, 35 disk sources are not in the spiral arms and are
typically fainter than those that are, leading Tennant et al. (2001b) to speculate
that these are perhaps high-mass x-ray binaries or black-hole candidates. There
were no associations with any of the 3 known globular clusters in the S3 viewing
eld.
The sheer number of X-ray sources detected by Chandra in a typical nearby
galaxy makes studies of the global properties of the X-ray source populations
possible. For instance, it has been suggested by Sarazin, Irwin, & Bregman
(2001), and supported in a theoretical framework by Wu et al. (2001), that
LogN-LogS distributions can be used as a distance indicator for giant elliptical
galaxies. Wu et al. (2001) further show that LogN-LogS distributions can be
used as a probe of recent star-formation and of the dynamical history of spiral
galaxies. This is only a sample of the use of x-ray data to place constraints on
galaxy evolution.
The remarkable Seyfert 2 galaxy, Circinus, has been observed by Sambruna
et al. (2001), Bauer et al. (2001), and Smith & Wilson (2001). The spectrum of
the nuclear region shows a wealth of emission lines including lines of Ne, Mg, Si,
S, Ar, Ca, Fe and a very prominent Fe-K line at 6.4 keV (Sambruna et al. 2001).
The emission appears to be the reprocessed radiation from the obscured central
source and originates within 60 pc of the object. In addition to the very detailed
spectrum of the nuclear region, sixteen point sources were detected (Bauer et al.
2001, Smith & Wilson 2001), several of which exhibit emission lines and are very
luminous supernova candidates. One bright object has a luminosity of 3:410 39
erg/s and a strong iron emission line at 6.9 keV with an equivalent width of 1.6
keV. An eclipsing x-ray binary was discovered with a period of 7.5 hours and a
0.5-10.0 keV luminosity of 3:7  10 39 ergs/s assuming isotropic emission. If not
local or beamed, such a high luminosity implies that the x-ray emitting object
must have a mass substantially greater than that of a neutron star and is thus
likely to be a black hole with a mass of more than 25 M . These \ultraluminous"
sources appear to be quite common in nearby galaxies (see also Blanton, Sarazin
& Irwin 2001, Sarazin, Irwin & Bregman 2000, 2001, Angelini, Lowenstein &
Mushotzky 2001 and Fabbiano, Zezas & Murray 2001). There appear to be too
many of such objects to give much credence to the idea that they are all more
local (or more distant) than one might think. King et al. (2001) argue that the
ultraluminous sources might be beamed and discuss a link with microquasars.
Gravitational lenses Another unique application of the excellent imaging prop-
erties of Chandra is the study of gravitational lenses, where the image separation

APS Conf. Ser. Style 15
is usually only about an arcsecond. By comparing dierent images it is possible
to measure dierential time delays of temporal changes and obtain an estimate
of the Hubble Constant. More than a dozen lensed systems have thus far been
observed, and one, is shown in Figure 20. The four lensed images of the quasar
are clearly resolved, and a rapid are was seen in one of two images (Morgan et
al. 2001). Several lens systems for which the expected delay is only hours are
under study. The large magnication of some of the lenses allows the study of
objects at very high redshifts that would otherwise not be detectable and the
most distant (z=1.4) x-ray jet has recently been detected in one of the images
of Q0957+561 (Chartas et al. 2001).
Figure 20. A composite of the deconvolved X-ray image of the
gravitational lens RX J0911.4+0551 (top panel) and the light-curves
of the lensed images A2 (left panel) and A1 (right panel).
Clusters of Galaxies Chandra observations frequently exhibit structures with
characteristic angular scales of a few arc seconds in clusters of galaxies which
previously were believed to be simple systems. Two of the more important types
of results involve investigations of the interactions between radio sources and the
hot cluster gas in some clusters, and the existence and implications of cold fronts
in others.
The Hydra A radio galaxy (3C 218) at a redshift of z = 0.052 is associated
with a relatively poor cluster of galaxies. This cluster was observed during the
orbital verication and activation phase of the observatory; these very early
images contained large areas of low X-ray surface brightness, indicating low
density regions or cavities in the intracluster gas. These cavities and other
aspects of the X-ray emission now have been studied by a number of authors
McNamara et al. (2000), David et al. (2001), Nulsen et al. (2001). Similar but
more dramatic cavities are found in the Perseus Cluster (Fabian et al. 2000);
this image is shown in Figure 21.
Brie y, the Hydra A cavities are found to coincide with the emission lobes
of the radio source. The overall temperature of the cluster gas increases from 3
keV in the central 10 kpc to 4 keV at a radius of 200 kpc and then gradually
decreases to 3 keV towards the radial limit of the Chandra observation at
about 300 kpc. However, the cavities are surrounded by bright rims of enhanced
x-ray emission which are cooler than the cluster gas away from the cavities at
comparable radii; this shows that the cavities are not created by expanding radio
lobes which shock the surrounding medium, as expected in some earlier models
of these sources (e.g. Heinz, Reynolds & Begelman 1998).
The cavities probably are in local pressure equilibrium with their surround-
ings. The energy input required to maintain the cavities is comparable to the
current radio emission, and also to that required to substantially inhibit a cooling
ow in the inner part of the cluster.
Figure 21. Adaptively smoothed 0.5-7.0 keV Chandra image of the
X-ray core of the Perseus cluster (Fabian et al. 2000).

16 Author & Co-author
The Hydra A cluster contains an unresolved x-ray source coincident with the
radio core. The point source x-ray spectra are highly absorbed, which indicates
that the source must be contained within the high column density material found
towards the nucleus of the galaxy in VLBI radio observations by Taylor (1996).
This limits the size of the X-ray emitting region to 24 pc, (McNamara et al.
2000).
The angular resolution also enables more quantitative studies of the cluster
merger process. The early images of the cluster A2142 (Markevitch et al. 2000)
show two sharp, bow-shaped shocklike surface brightness features; the surface
brightness is discontinuous on a scale smaller than 5-10 arcseconds. However, a
detailed investigation shows that the pressure is continuous across the boundary,
and that the temperature transition has the opposite sign to that expected from
a shock. The most likely explanation is that these edges delineate the dense
subcluster cores which have survived a merger and the associated ram pressure
stripping by the surrounding gas.
The image of the cluster A3667 (Vikhlimin, Markevitch, & Murray 2001a,
2001b) contains a well dened subcluster. The pressure jump across the bound-
ary of the subcluster is approximately a factor of two, which indicates that the
subcluster relative velocity is about equal to the sound speed of the surrounding
medium. The transition width in this case is 3.5 arcseconds or less, smaller than
the Coulomb mean free path of electrons and protons on either side of the front.
A model of the magnetic eld (Vikhlimin, Markevitch, & Murray 2001b) needed
to suppress the particle diusion requires a eld strength of order 10 G. These
and similar studies enabled by the good angular resolution of the Observatory
should lead to signicant improvements in our understanding of cluster merger
phenomena.
The X-Ray Background - The Chandra Deep Surveys The rst sounding rocket
ight that detected the brightest X-ray source in the sky, other than the Sun, also
detected a general background of x-radiation (Giacconi et al. 1962). The nature
of the background radiation has been a puzzle for nearly 40 years, although the
lack of distortion of the spectrum of the Cosmic Microwave Background place
a strong upper limit to the possibility of a truly diuse component (< 3% ,
Mather et al. 1990). Observations with ROSAT at energies below 2 keV made
a major step in resolving a signicant fraction (70-80%) into discrete objects
(Hasinger et al. 1998) and found that the sources reside mainly in AGN at
redshifts from 0.1 to 3.5. ASCA satellite observations extended the search for
sources in the 2-10 keV band, resolving about 30% into mainly AGNs (Ueda et
al. 1998). Observations with Beppo-Sax have continued these studies. Currently
two 1-Ms exposures have been accomplished with Chandra - the Chandra Deep
Fields North (Figure 22, Garmire et al. 2001) and South (Giacconi et al. 2001).
These surveys extend the study of the background to ux levels more than
an order of magnitude fainter in the 0.5-2.0 keV band and resolve over 90%
of the background into a variety of discrete sources. The largest uncertainty
in establishing the fraction is now in the knowledge of the total level of the
background itself.
The spectrum of the X-ray background has been called a \spectral paradox"
by Boldt (1987), because the majority of the bright AGN are found to have a
photon index larger than that for the background itself (1.7 vs 1.4). One of the

APS Conf. Ser. Style 17
Figure 22. Chandra "true -color" ACIS image of the Chandra Deep
Field - North. This image has been constructed from the 0.5 -2.0
keV band (red) and 2.0 - 8.0 (blue) images. The location of the HST
deep eld is shown in outline. Two of the red diuse patches may be
associated with galaxy groups.
purposes of the Chandra Deep Surveys has been to explore the spectra of the
sources. Since it must be the faint sources that modify the spectrum from the
average of the bright AGN sample, the faint source spectrum was estimated by
adding the spectra of individual sources detected in the surveys. Garmire et al.
(2001) found the spectral index to decrease from 1.8 to 1.0 as the ux of the
sources decreased from 1  10 14 to 2  10 15 ergs/cm 2 /s. Similar results are
found in the data from the southern eld. The spectrum of all of the sources
has a slope very near to 1.4 (Tozzi et al. 2001, Garmire et al. 2001), consistent
with the wide eld-of-view measurements made with detectors that could not
resolve the fainter sources.
In each of the Chandra Surveys about 350 sources were detected. The
ux levels attained in the soft and hard bands were (3  10 17 ergs/cm 2 /s and
2  10 16 ergs/cm 2 /s respectively. The highest redshift detected (so far) is a
QSO at 5.2. A highly obscured type 2 QSO at z = 3.4 has been reported in the
Chandra Deep Field - South by Norman et al. (2001).
Since these elds were selected not to have any nearby galaxies in them,
the nearest galaxies detected are at redshifts of  0:08. In one of these galaxies,
located in the HDF-N, the x-ray emission appears to be non-nuclear, based on
the oset from the center of the optical light distribution, perhaps implying
on the basis of the high x-ray luminosity that intermediate mass black hole
candidates are present.
The LogN - LogS function has a change in slope at just over 1  10 14
ergs/cm 2 /s for the 2-10 keV band and at  5  10 15 ergs/cm 2 /s for the 0.5-2.0
keV band. A nominal slope of -1.5 would be expected for a Euclidean geometry
populated with a uniform distribution of sources. In the hard band, the slope
attens to -1 at about 1:4  10 15 ergs/cm 2 /s and then becomes even atter at
lower uxes (Figure 23). In the soft band the slope attens from -1.5 to -0.65
down to 1  10 16 ergs/cm 2 /s, then becomes even atter for fainter uxes (see
Figure 24). Eventually, these curves should change to steeper slopes once the
population of galaxies is reached, Normal galaxies will not contribute very much
to the total X-ray background ux, since they are so faint but will ultimately
become the major contributor at the lowest ux levels.
The majority of the x-ray sources beyond a redshift of 0.5 are AGN or QSOs.
Barger et al. (2001) found, based on the luminosities of the hard-band X-ray
sources, that the accretion rate onto black holes grows linearly with redshift to
a redshift of 1 and then attens with only a slight increase out to a redshift of
3. The volume density of black hole accretion is found to increase as (1+z) 3 .
Gamma-Ray Bursts The moderately rapid response for targets of opportunity
has made possible the study of the afterglows of gamma-ray bursts with the
Observatory. The afterglow of GRB991216 showed the rst X-ray iron line prole

18 Author & Co-author
Figure 23. The integral LogN-LogS plot for the Chandra Deep Field
North of the hard-band sources (small solid squares) found in in three
areas of the eld. The portion of the plot above 10 14 ergs/cm 2 s deg 2
was taken from the full image, the portion between 10 15 and 10 14
ergs/cm 2 s deg 2 from a 6 arcmin radius region, and the faintest sources
from a region of 3 arcmin radius. The large open square and the dash-
dot "bow tie" are from Ueda et al. (1998) and Gendreau (1998); the
small open squares are also from the ASCA Large Sky Survey (Ueda et
al. 1998), and the solid circle and dotted "bow tie" region are from the
Ginga survey and uctuation analysis respectively (Hayashida, Inoue,
& Kii 1991).
Figure 24. The integral LogN-LogS plot for the Chandra Deep
Field-North of the soft-band sources found in the same three areas as
for the hard-band data. The open circles and the dashed region are
from ROSAT (Hasinger et al. 1998).
indicating a very high velocity ( 0:1c) in the ejected material (Piro et al. 2000).
These authors also reported a recombination edge from hydrogenic ions of iron at
a redshift of 1. This observation supports a hypernova interpretation, or delayed
gamma-ray burst following a supernova (Vietri and Stella 1998, Meszaros and
Rees 2001). A second gamma-ray burst, GRB000926, revealed an unusual x-ray
light curve that implied that the burst expanded into a dense medium (n 10 4
cm 3 ) and that the reball was only moderately collimated initially which then
slowed down and become non-relativistic after 5 days (Piro et al. 2001).
4.2. High Resolution Spectroscopy
Owing to their unprecedented clarity, Chandra images are visually striking and
provide new insights into the nature of x-ray sources. Equally important are
Chandra's unique contributions to high-resolution dispersive spectroscopy.
High-resolution x-ray spectroscopy is the essential tool for diagnosing condi-
tions in hot plasmas. It provides information for determining the temperature,
density, elemental abundance, and ionization stage of x-ray emitting plasma.
The high spectral resolution of the Chandra gratings isolates individual spectral
lines which would overlap at lower resolution. The high spectral resolution also
enables the determination of ow and turbulent velocities, through measure-
ment of Doppler shifts and widths. Dispersive spectroscopy achieves its highest
resolution for spatially unresolved (point) sources. Thus, Chandra grating ob-
servations have concentrated on, but are not limited to, stellar coronae, x-ray
binaries, and active galactic nuclei.
Stellar Coronae The spectra of stellar coronae obtained with Chandra contain
a large number of interesting emission line features that serve as diagnostic tools
for temperatures, densities, and emission measures. Figure 25 shows the LETGS
spectra (5-175  A) of Capella and Procyon, two dierent coronal sources. There
are a large number of lines from very many dierent elements, and also a strong
temperature dependence. In Capella there are many lines around 15  A from Fe

APS Conf. Ser. Style 19
Figure 25. The LETGS spectra of the two stars Capella (left) and
Procyon (right).
XVII, while these lines are weak in the Procyon spectrum. Conversely, the Fe
IX line at 171  A, is very prominent in the Procyon spectrum indicating a cooler
corona.
Apart from a chemical and temperature analysis, one can derive densities
from many density-sensitive lines. In the wavelength range around 100  A of the
Capella spectrum, density-sensitive lines of highly ionized Fe-ions appear and in
the short-wavelength region (between 6 and 45  A) of both spectra, temperature-
and density-sensitive lines are present. The latter originate from the He-like ions
Si XIII, Mg XI, Ne IX, O VII, N VI, and C V. For these ions the resonance line
(r) 1s 2 1 S 0 { 1s2p 1 P 1 , the intercombination line (i) 1s 2 1 S 0 { 1s2p 3 P 1;2 and the
forbidden line (f) 1s 2 1 S 0 { 1s2s 3 S 1 are resolved.
From the resonance lines one can obtain an estimate of the temperature |
an estimate as the ux may arise from two dierent regions on the stellar surface.
In the He-like ions, the ratio between the intercombination line and the forbidden
line is strongly density dependent, while the ratio between the the sum of the
intercombination line and forbidden line and the resonance line is temperature
dependent. In a low-density plasma the forbidden line is stronger than the
intercombination line, but for increasing density, the 1s2s 3 S 1 { the upper level
of the forbidden transition { will be depopulated by collisions in favour of the
1s2p 3 P 1;2 { the upper level of the intercombination line. The resonance line
intensity is comparable to the sum of the intensities of the two other lines and
increases at higher temperatures. Table 1 shows the temperature for the coronae
of Capella and Procyon, based on the ratio of the sum of the intercombination
and forbidden lines to the resonance line and also on the ratio between lines of
succeeding stages of ionization. From this table we notice that the coronae of
both stars have a multi-temperature structure and that the temperature of the
corona of Capella is higher and extends to higher ionization stages. Table 2
shows the densities derived from the appropriate line ratios of the He-like ions.
X-ray Binaries The x-ray output of the bright Galactic x-ray binaries is gener-
ally dominated by continuum from an optically thick accretion disk, but grating
spectra are revealing a rich variety of absorption and emission features that
carry new information about material in and around the source (e.g., Brandt &
Schulz (2000), Paerels et al. (2001), Cottam et al. (2001), Marshall, Canizares,
& Schulz (2001), and Schulz (2001). Doppler structure is often seen in the emis-
sion and/or absorption lines, with velocities ranging from hundreds of km/s (e.g.
for 4U 1822-37 where the lines are attributed to recombination in an x-ray illu-
minated bulge where the accretion stream hits the disk; (Cottam et al. 2001) up
to 0.26c for emission lines in relativistic jets of the binary SS433 (Marshall et al.
2001). One interesting example is the mysterious binary Circinus X-1, thought
to contain a neutron star, that at times radiates beyond its Eddington limit
(Brandt & Schulz, 2000). The HETG spectra reveal lines from H-like and/or
He-like Ne, Mg, Si, S and Fe. The lines exhibit broad (2000 km/s) P Cygni
proles, with blue-shifted absorption anking red-shifted emission. Examples

20 Author & Co-author
Table 1. Temperature determination from the line intensity ratio (i+
f)=r for dierent He-like ions and from the ratio of the He- and H-like
resonance lines. Temperatures (in MK) derived from SPEX/MEKAL
and Porquet et al. (2001). For Capella. the data are from Mewe,
Kaastra, & Drake (2001), and for Procyon from Raassen et al. (2001).
Ion Capella Procyon
CV 1.40:5 0.4
NV I 0.5 +0:5
0:2 1.2
OV II 1.80:3 1.1
MgXI 4.6 +1:4
1:0 -
SiXIII 5 +3
2 -
CV=CV I 1.100:15 1.12
NV I=V II 2.500:14 1.62
OV II=V III 3.370:06 2.14
MgXI=XII 6.90:2 -
SiXIII=XIV 9.40:5 -
FeX=IX - 1.25
Table 2. Density determination from the i=f line intensity ratio
for dierent He-like ions. Electron density (in cm 3 ) derived from
SPEX/MEKAL and Porquet et al. (2001). The Capella data are from
Mewe, Kaastra, & Drake (2001); the Procyon data are from Raassen
et al. (2001).
Ion Capella Procyon
CV 32 10 9 4 10 9
NV I 53 10 9 1 10 10
OV II < 7 10 9 6 10 9
MgXI < 4 10 12 -
SiXIII 32:5 10 13 -

APS Conf. Ser. Style 21
Figure 26. Several of the strongest X-ray P Cygni proles seen from
Cir X-1 with the HETG (Brandt & Schulz 2000). The two middle
panels show independent spectra of the Si XIV prole with the HEG
(1 st order) and MEG (3 rd order). Typical bins have 200-1200 counts.
The velocity in each panel gives the instrumental resolution; the lines
are clearly broader
are shown in Figure 26. Brandt & Schulz (2000) interpret these features as the
X-ray signatures of a wind being driven o the accretion disk, making Cir X-1
the X-ray analog of a broad absorption line quasar.
Line ratios are being used to constrain physical properties of the emitting
regions, and time variability of intensities and Doppler structures helps to locate
the source of the lines in the binary system. The HETG spectrum of the ultra-
compact binary 4U 1626-67 shows unexpectedly strong photoelectric absorption
edges of Ne and O, most probably from cool, metal rich material local to the
source (Schulz et al. 2001). The anomalous abundances led the authors to
suggest that the mass donor in this binary is the chemically fractionated core of
a C-O-Ne or O-Ne-Mg white dwarf.
Active Galactic Nuclei (AGN) High resolution spectra of AGN, especially
Seyfert Galaxies, are providing extraordinary new details about the physical
and dynamical properties of material surrounding the active nucleus. In the
case of Seyfert 1 galaxies, whose signal is dominated by the bright X-ray con-
tinuum from the central engine, the partially ionized circum-source material
introduces a prominent pattern of absorption lines and edges. These are the
"warm absorbers" originally discovered in low resolution spectra by Reynolds
(1997), and George et al. (1998) but now revealed in much greater detail.
For example, the LETGS spectrum of NGC 5548 shown in (Figure 27) ex-
hibits three dozen absorption lines, plus a few in emission (Kaastra et al. 2000),
and the HETGS spectrum of NGC 3783 (Kaspi et al. 2000, 2001) has over
ve dozen lines. In both cases, there is evidence for bulk motions of several
hundred km s 1 . For NGC 3783, detailed modeling of an absorber with two
ionization components does a remarkably good job of reproducing the observed
line strengths (Kaspi et al. 2001). The HETGS spectrum of MCG-6-30-15 (Fig-
ure 28) also has dozens of absorption lines from a wide range of ionization states
(Lee et al. 2001). In addition, an absorption feature at 0.704 keV (17.61  A) is
well t by the neutral Fe L3 absorption edge (and associated resonant structure)
with a column density equal to the amount of line-of-sight dust deduced from
earlier reddening studies. (Alternatively, Branduardi-Raymond et al. (2001)
attribute this feature to relativisticly broadened OVIII emission, based on their
XMM-Newton RGS spectrum). The HETG also has been used to (marginally)
resolve a narrow component of the Fe K line in NGC 5548 (Yaqoob 2001).
For Seyfert 2's the strong continuum from the central engine is not seen
directly, so the surrounding photo-ionized regions are seen in emission. The
HETGS spectra of Markarian 3 (Sako et al. 2000) and NGC 4151 (classied
as Sy 1.5 but observed when the continuum was exceptionally faint, (Ogle et
al. 2001) bristle with emission lines, whose ratios provide diagnostics of the

22 Author & Co-author
Figure 27. LETGS spectrum of the Seyfert 1 galaxy NGC 5548,
corrected for order contamination, redshift and Galactic absorption
(Kaastra et al. 2000). Several prominent absorption lines from H-like
and He-like ions are marked, as is the forbidden line of He-like oxygen.
Figure 28. A portion of the HETG spectra of the Seyfert 1 galaxy
MCG-6-30-15: top, corrected for instrumental eective area; see Lee et
al. (2001) and the Seyfert 2 galaxy NGC 4151 (bottom, in counts per
bin; see (Ogle et al. 2001)). The top spectrum also shows the best t
power law including galactic absorption but excluding the region most
aected by the warm absorber) and the magnitude of the one-sigma
uncertainty.
conditions in the emitting clouds (Figure 28). There are clear signatures of
photoionization, such as the relatively strong forbidden line from He-like ions
and narrow features from free-bound radiative recombination. Other diagnos-
tics suggested the presence of a smaller amount of collisionally excited plasma
(Ogle et al. 2001), although more recent observations and modeling indicate
that photoexcitation together with the photoionization and recombination can
explain the line ratios very well (Sako et al. 2000).
Many AGN spectra show primarily a strong, power-law continuum with
little or no evidence for any absorption or emission lines. Examples include the
narrow-line Seyfert 1 galaxy Ton S180 (Turner et al. 2001), BL Lac objects and
radio-loud quasars (Fang et al. 2001).
Young Supernova Remnants Since the X-ray output of a young SNR is dom-
inated by a moderate number of strong emission lines, the dispersed spectrum
resembles a spectroheliogram, showing multiple images of the remnant in the
light of individual lines. Such is the case for 1E0102-72, a SNR in the SMC
estimated to be  1000 years old (Figure 29: Canizares et al. 2001; Flanagan
et al.2001). The monochromatic images are dominated by emission from the
shocked stellar ejecta. For a given element, the images for the He-like resonance
are systematically smaller than those for the H-like Ly line, which is graphic
evidence for the progression of the so-called reverse shock backwards into the
expanding ejecta (also seen in the ACIS image by Gaetz et al. 2000). Doppler
shifts of  2000 km s 1 are measured by comparing the strongest monochro-
matic images from the plus and minus orders. The velocities appear asymmetric,
suggesting that the shock heated ejecta ll a torroidal region inclined to the line
of sight.
Figure 29. A portion of the dispersed HETG spectrum of the SNR
E0102-72.

APS Conf. Ser. Style 23
5. Conclusion
The Chandra X-Ray Observatory is performing as well, if not better, than an-
ticipated | and the results are serving to usher in a new age of astronomical
and astrophysical discoveries.
6. Acknowledgements
We recognize the eorts of the various Chandra teams which have contributed
so much to the success of the observatory. In preparing this overview, we have
used gures and material drawn from their work. We would especially like to
thank Dr. Douglas Swartz and Professor J. Grindlay for their contributions and
assistance.
A Chandra web sites
The following lists several Chandra-related sites on the World-Wide Web (WWW).
http://chandra.harvard.edu/ Chandra X-Ray Center (CXC), operated for
NASA by the Smithsonian Astrophysical Observatory.
http://wwwastro.msfc.nasa.gov/xray/axafps.html Chandra Project Sci-
ence, at the NASA Marshall Space Flight Center.
http://hea-www.harvard.edu/HRC/ Chandra High-Resolution Camera (HRC)
team, at the Smithsonian Astrophysical Observatory (SAO).
http://www.astro.psu.edu/xray/axaf/axaf.html Advanced CCD Imaging
Spectrometer (ACIS) team at the Pennsylvania State University (PSU).
http://acis.mit.edu/ Advanced CCD Imaging Spectrometer (ACIS) team at
the Massachusetts Institute of Technology.
http://www.sron.nl/missions/Chandra Chandra Low-Energy Transmission
Grating (LETG) team at the Space Research Institute of the Netherlands.
http://www.ROSAT.mpe-garching.mpg.de/axaf/ Chandra Low-Energy
Transmission Grating (LETG) team at the Max-Planck Institut fur ex-
traterrestrische Physik (MPE).
http://space.mit.edu/HETG/ Chandra High-Energy Transmission Grating
(HETG) team, at the Massachusetts Institute of Technology.
http://hea-www.harvard.edu/MST/ Chandra Mission Support Team (MST),
at the Smithsonian Astrophysical Observatory.
http://ipa.harvard.edu/ Chandra Operations Control Center, operated for
NASA by the Smithsonian Astrophysical Observatory.
http://ifkki.kernphysik.uni-kiel.de/soho EPHIN particle detector.

24 Author & Co-author
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