Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://hea.iki.rssi.ru/SXG/PROJECT/Yam_SRG_f.doc Äàòà èçìåíåíèÿ: Mon Jan 19 16:00:40 2004 Äàòà èíäåêñèðîâàíèÿ: Mon Oct 1 22:43:47 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: http www.astronomy.ru forum index.php topic 4644.0.html

Russian Academy of Sciences

Space Research Institute

















Spectrum-X-Gamma Project


based on Yamal platform






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2003

contents

CONTENTS 1

INtroduction 1

1. high apogee orbit 1

1.1 Active Galactic Nuclei (AGNs) 1
1.2 A supermassive black hole in the center of the Galaxy 1
2. the near-earth orbit 1

2.1 Sensitivity to resolve objects with a low surface brightness 1
2.2 Scientific objectives for the JET-X telescope 1
2.2.1 Cosmic X-ray background 1
2.2.2 Observations of the outer regions of clusters of galaxies 1
2.2.3 Shock waves in the interstellar medium 1
2.2.4 Diffuse emission in the Galactic center region 1
3. Launch facilities and the ballistic scenario for s/C delivery to the
operationAL orbit 1

3.1 Results of the ballistic analysis for the Spectrum-X-Gamma S/C
operation orbits and launch vehicles 1
4. instruments 1

References 1



INtroduction

Recent limitation of funding in Russia has forced the Spectrum-X-Gamma
collaboration to change from the very expensive heavy PROTON to a less
expensive middle class launcher with a booster. This decision makes
possible to decrease the launch cost several times thus bringing the
Spectrum-XG (SXG) launch feasible within the next few years. Otherwise the
launch date is shelved for a considerably period.
However this decision strongly governs the SXG design. The middle class
launcher with a booster (e.g. SOYUZ LV + FREGAT booster) is able to put one
ton at most in the SXG orbit. The present spacecraft dimension doesn't fit
the installation requirements for the SOYUZ's fairing. As a result IKI is
considering the option of SXG without the SODART telescope and the YAMAL
bus provided by the ENERGY Rocket & Space Corporation (Fig. 1). This bus is
noted for its several successful launches with the communications payload
and its performance and adapters well fit the SXG modified design. In
addition this decision also allows to save money for the modern Spectrum
satellite development.

Fig. 1. Overview of the SXG spacecraft

According to the requirements to get an approval of the SXG modified
design IKI with RSC Energy should present a report on scientific and
technical advantages of the new SXG concept to Space Council of the Russian
Academy of Sciences and Rosaviacosmos at the end of this year.
IKI and the Energy Corporation are studying two possible launch options
(high and low apogee orbits) for modified SXG.

high apogee orbit

In the proposed configuration the observatory will be focused on the two
scientific objectives:
- Extended (up to several months) quasi-continuous observation of a
thoroughly selected set of 5-10 sources - AGNs and the supermassive
black hole in the center of our Galaxy within a wide energy range
from the UV to the hard X-rays. Optionally: agreed multi-wavelength
observations in radio and optical ranges from the Earth.
- Extensive (up to 30-40% of the observation period) TOO program
(gamma-ray bursts afterglow, X-ray transients, supernovae in nearby
galaxies, etc.) in a broad energy range from UV to hard X-rays.
The operating observatories (like Chandra or XMM-Newton) possess
significantly higher sensitivity in the X-ray band. However a very
versatile research program (this is unavoidable and typical for «large»
observatories) hinders from long-term observation of a particular object
due to frequent changes in the instrument orientation due to the
observation schedule agreed. Typical exposure time does not exceed 30-100
ks for these observatories, while TOO share does not exceed 5% of the
observation time. Moreover these observatories offer limited multi-
wavelength capabilities.

1 Active Galactic Nuclei (AGNs)

During the recent 5-7 years a considerable progress has been achieved in
understanding the physics of accretion onto a compact object in the X-ray
binaries through the studies of the X-ray flux fast variability, in
particular using the RXTE data. The RXTE instrument is distinct for a very
good sensitivity and an excellent time resolution of about 1 microsecond.
In this case the time resolution plays the principal role since the
characteristic time scales are of an order of 1-10 milliseconds in the
vicinity of black holes with a mass (10 mass of our Sun. The spectra of
many X-ray binaries are similar to the spectra of many AGNs. This makes it
possible to assume that similar physical mechanisms operate for the both
classes of sources. However the characteristic time scales strongly differ
near a black hole with a mass comparable to the stellar mass and a
supermassive black hole. E.g. typical rotation period for a particle in the
close vicinity of the 108 solar masses black hole is of an order of 105
seconds - i.e. about one day. The data accumulated for the X-ray binaries
makes it possible to assume that the strongest variability is associated
with the time scales being 10-100 longer than the rotation period for the
innermost stable orbits. For AGNs this means the variability on scales of
10-100 days possesses the most interest. Unfortunately duration of a
typical AGN observation is limited to less than a day or this order.
The offered space observatory would be able to change the situation and
provide unique information on the variability of AGNs on the time scales
from minutes to months. Broadband coverage would also help to unravel the
relations between the variability in different energy bands. Long-term
observations would further facilitate multi-wavelength campaigns involving
ground based optical and radio telescopes.

2 A supermassive black hole in the center of the Galaxy

Modern concept of the galaxy formation and evolution postulates that
each galaxy harbors a supermassive black hole at its center. This hole mass
is comparable with the mass of the galaxy itself. The presence of a
supermassive black hole in our Galaxy is of particular interest. In the
seventies a compact radio source (Sgr A*) with a unique spectrum was
discovered in the direction towards the Galaxy's Center. Dynamics of stars
in the vicinity of this object makes it possible to assume that the Sgr A*
mass is of an order of 2-3 million solar masses. Thus these are rather
convincing arguments for the presence of a supermassive black hole in our
Galaxy. Obscuration by gas and dust makes the optical or UV observations of
this region impossible, and one has to rely on the radio, infrared or X-ray
data. The Sgr A* is quasi-weak in the X-ray band. The latest Chandra data
shows that its luminosity is at the level of 1033 erg/s, which is more than
10 orders of magnitude below the maximal possible luminosity (Eddington
limit) for the black hole of such a mass. On the other hand, the same
observations show that the Sgr A* is surrounded with a large cloud of hot
gas, which could be accreted by the hole thus providing much higher
luminosity. Several theoretical models have been proposed to explain this
paradox. But it is obvious that much more data is needed to advance in
understanding the physics of the accretion onto Sgr A*. Recently Chandra
has discovered strong variability of the source on the time scale of hours
(Fig.2). Coordinated observations in the radio and X-ray bands will advance
the studies settling the matter for the different theoretical explanations.
The SXG observatory would be able to quasi-continuously monitor the Sgr A*
for several months per year. This is impossible for the operating
observatories like Chandra or XMM-Newton.
[pic]

Fig. 2. Strong variability of the Sgr A* on the time scale of hours (by
the Chandra data)


the near-earth orbit


1 Sensitivity to resolve objects with a low surface brightness

The X-ray detectors in space are bombarded with charged particles, which
are registered as X-ray photon series. Most of these events are usually
discarded by standard techniques. However, several events are considered
and create the so-called particle induced background noise, which seriously
limits the detector's scientific capabilities. Based on the experience
gained from the observation data by the other X-ray observatories we know
that this background intensity shows non-statistical temporal variability
at the 5-10% level caused by the variations in the particle flux. In
addition to these relatively small variations, Chandra and XMM - launched
on high orbits above the Earth radiation belts - also experience periods of
very high background (sometimes 1-2 orders of magnitude above the quiescent
level) during various intensity Solar flares which mostly take out up to
30% of the observation time. At the same time the ASCA X-ray observatory
launched in a low orbit below the radiation belts does not experience
background flares (excluding the passages above the Southern Atlantic
Anomaly). The Earth radiation belts provide a good shielding against the
cosmic charged particles, and even the quiescent of ASCA SIS was 4-5 lower
than that in a very similar detector ACIS-I onboard Chandra (see Table 1).
The effective area and angular resolution of the JET-X telescope do not
stand comparison with the XMM and Chandra telescopes. However, it would be
an uncontested instrument for studying the very important class of
astronomical sources - extended X-ray sources with low surface brightness,
such as the cosmic X-ray background and clusters of galaxies. The primary
quantity that determines the sensitivity of an X-ray telescope to such
objects is the ratio of the source brightness to the background level
within the same angular area:
[pic]
where a is the combined effective area of the mirror and detector, b is the
background intensity per unit detector area, and f is the focal length of
the telescope. This equation accounts for the non-statistical nature of the
background variations: the background level predicted (and subtracted) with
an accuracy that does not depend on the exposure time on the condition that
the latter is long enough. Note that in general, a small effective area can
be compensated by the corresponding increase in the exposure time, but if
the quantity a/(b(f2) is small, this is fundamental and unavoidable limit
to the ability of the given instrument to study low surface brightness
sources.
A comparison of this parameter for ASCA, XMM, and Chandra with the
estimated properties of the JET-X telescope shows (Table 1) that for LEO
the sensitivity of JET-X telescope will be significantly, a factor of 4-8
depending on the energy band, higher than the XMM sensitivity, and (10
higher than that of Chandra. If JET-X telescope is launched to a high orbit
where the particle flux is several times higher, the JET-X and XMM
sensitivity will be comparable, but still significantly higher than that of
the Chandra telescope.
The JET-X telescope sensitivity to resolve extended sources is
comparable to that for ROSAT PSPC and ASCA (both missions are completed).
However, ROSAT PSPC had virtually low energy resolution and its band pass
was limited to E<2 keV. ASCA had a very poor angular resolution, (3(
compared to 20( for JET-X; what made the study of the most extended sources
with the ASCA telescope almost impossible.
Table 1. Comparison of sensitivities to resolve extended X-ray sources

| |Eff. |Background b |Focal length|Sensitivity |
| |area1 a,|per unit |f, m |a/(b(f2) |
| |cm2 |detector area2 | | |
|At 1 keV: |
|ASCA SIS |120 |0.8 |3.5 |12.2 |
|XMM EPIC-PN |1260 |21 |7.5 |1.1 |
|Chandra ACIS-I |420 |3 |10.0 |1.4 |
|JET-X, high |90 |3 |3.5 |2.4 |
|orbit3 | | | | |
|JET-X, low |90 |0.8 |3.5 |9.2 |
|orbit | | | | |
|At 5 keV: |
|ASCA SIS |110 |2 |3.5 |4.5 |
|XMM EPIC-PN |970 |28 |7.5 |0.6 |
|Chandra ACIS-I |400 |10 |10.0 |0.4 |
|JET-X, high |110 |10 |3.5 |0.9 |
|orbit | | | | |
|JET-X, low |110 |2 |3.5 |4.5 |
|orbit | | | | |
|At 8 keV: |
|ASCA GIS4 |50 |2 |3.5 |2.0 |
|XMM EPIC-PN |630 |28 |7.5 |0.4 |
|Chandra ACIS-I |56 |10 |10.0 |0.06 |
|JET-X, high |36 |10 |3.5 |0.3 |
|orbit | | | | |
|JET-X, low |36 |2 |3.5 |1.5 |
|orbit | | | | |


1 Effective area of an instrument including several telescopes is given
per telescope.
2 Quiescent background.
3 The estimates of the JET-X background make it possible to assume that
its detectors are similar to ASCA SIS and Chandra ACIS-I CCDs; so we
used the actually observed background levels (ASCA for low orbit and
Chandra for high orbit).
4 ASCA SIS has a similar background and a 2 times smaller effective area
at this energy level.

2 Scientific objectives for the JET-X telescope

After Chandra and XMM were launched it became apparent that both
observatories experienced serious problems because of an unexpectedly high
and strongly variable particle flux, unshielded by the Earth radiation
belts. This made a number of very important observations of the low surface
brightness objects either very complicated or even impossible. As a result,
many scientists working with the XMM and Chandra data, including the
designers of the both observatories, now believe that a LEO observatory
operating below the radiation belts would be preferable. Therefore, this
area of the X-ray astronomy remains one of the few where relatively small,
but optimally designed satellites still can compete with the operating
large space observatories. Below we provide an incomplete list of the
related scientific problems.

1 Cosmic X-ray background

The nature of the cosmic X-ray background (CXB) is one of the most
important objects to be studied by the X-ray astronomy. We know, that at
E>1 keV, most of the background is resolved into emission of distant AGNs.
However, it is still unclear what fraction of the background is determined
by the diffuse emission of the interstellar medium in the Galaxy, the
intergalactic gas in the Local group, or the hypothetical «warm»
intergalactic medium, which may contain most of the baryons in the modern
Universe. A convincing detection or even strong upper limits on the
intensity of the diffuse X-ray emission at different energies would be
extremely important for the study of the interstellar and intergalactic
media, as well as for the evolution of the large scale structure in the
Universe (Cen & Ostriker 1999).
For successful observations of the cosmic X-ray background we need an
instrument with a good angular resolution and a low instrument background.
ASCA had a low instrumental background and was able to measure the total
intensity of the CXB, but was unable to separate the truly diffuse emission
from the discrete source flux. Chandra and XMM can easily detect the point
sources, but cannot measure the remaining diffuse background intensity
because of the large uncertainties in the detector background. For LEO the
JET-X instrument has the absolute advantage concerning the required
characteristics.

2 Observations of the outer regions of clusters of galaxies

Clusters of galaxies are the largest gravitationally bound systems in
the Universe and one of the most interesting objects in the X-ray sky. X-
ray observations of clusters have direct applications for cosmology (e.g.,
via the measurement of the dark matter mass, see White et al. 1993), for
the physics of the intracluster medium (Sarazin 1986), and for the study of
interaction of relativistic ejects from the supermassive black holes with
the surrounding gas.
Most of the cluster matter (both visible and dark matter) is located in
the outer regions (Fig. 3). Cluster continuously attract the intergalactic
gas, which is being compressed and heated to the X-ray temperatures as it
passes through the accretion shock, also located at a large radius from the
cluster center. It is currently believed that the accretion shock is a site
for completely unexplored processes of acceleration of relativistic
protons, which then co-exist with the thermal plasma and possibly
contribute a significant fraction of its energy density (Loeb & Waxman
2000). In the low-density cluster regions, now inaccessible for the X-ray
spectroscopy, many of the basic assumptions about the physical state of the
intracluster medium can be violated. For example, one expects deviations
from the electron-ion thermal equilibrium; precise X-ray measurements in
these regions can shed light on the efficiency of the plasma wave processes
for the energy transmission from protons to electrons after passage of the
accretion shock. The MHD phenomena on in the intracluster medium are still
an unexplored area.
[pic]

Fig. 3. Schematic view of a galaxy cluster (galaxies are shown by dots,
and gray color corresponds to the hot intracluster gas)

The X-ray surface brightness in these regions of interest is only 20-50%
of the CXB in hot clusters, and is still lower in cold clusters, which are
especially important for the study of energetic of the intracluster medium
(Cavaliere et al. 1997). For Chandra and XMM, the detector background is
significantly higher that the CXB at useful energies (E>2 keV), and
therefore the cluster brightness is comparable to or less than the
uncertainty in the instrumental background. This makes impossible many
important observations, including a measurement of the virial mass in
clusters. Present popular estimates of the cosmological parameters derived
from the measurements of the cluster mass rely, in fact, on the
extrapolations from the inner regions containing less than one half of the
total mass (Allen et al. 2002). A 5-10 times sensitivity gain, achievable
with JET-X in a low orbit would make it possible to directly observe for
the first time and to study most of the matter in the clusters of galaxies.

3 Shock waves in the interstellar medium

The interaction of the supernova remnants with the surrounding cold
interstellar medium results in a formation of strong shock waves. They are
believed to be the sites of acceleration of relativistic particles observed
on Earth as cosmic rays. Detailed observations of the structure of the
shock waves would significantly further our understanding of the mechanism
and efficiency of the particle acceleration. The observations of the most
strong, external shocks are often limited by their low brightness.
Therefore, JET-X would be an ideal instrument for such observations.

4 Diffuse emission in the Galactic center region

The Galactic center region contains the diffuse X-ray emission of a very
complex morphology due to a combination of the thermal emission of the hot
plasma, absorption in the cold clouds, scattering of the emission from
bright Galactic X-ray sources, and emission from numerous supernovae
remnants. A detailed spectroscopic and morphological study of this emission
will make a number of very interesting astronomical studies feasible (such
as the «cosmic archaeology» - historical variations of the flux from the
bright X-ray sources including the supermassive black hole in the Galactic
center). Another possible application is the three-dimensional location of
the bright sources with respect to the numerous giant molecular clouds in
this region.

Launch facilities and the ballistic scenario for s/C delivery to
the operationAL orbit

[pic]
Table 2 presents the launch vehicles considered for bringing the S/C to
its initial orbit, launching sites with the orbit inclination as well as
masses of the stage put into nearly-circular reference orbit with a perigee
of 200 km and an apogee of 245 km.

Table 2. Launchers and the final core spacecraft mass

|Launch |Booster |Mass of the S/C in the reference orbit,|
|vehicle | |kg |
| | |Baikonur, i = |Plesetsk, i = 62.8( |
| | |51.8( | |
|Soyuz-FG |Fregat |7350 |6720 |
|Soyuz-2 (1a)|Fregat |7350 |- |
|Soyuz-2 (1b)|Fregat |8060 |7700 |

Note on the LV feasibility:
- The Soyuz-FG modification is involved in the ISS program.
- The Fregat booster has been used for launching the Cluster series
satellites. At present it is being modernized. The final mass will
be 950 kg in total.

We consider for the YAMAL-SRG S/C the following types of the operational
orbits:
- Low circular orbit (altitude 550-600 km; inclination 29œ); and
- High-apogee orbit (apogee 163 000 km or 200 000 km; inclination
51.8œ/62.8œ).

Ballistic scenario of the S/C launch is as follows:
- Soyuz-FG or Soyuz-2 (1a) LV delivers the S/C with the booster to a
reference orbit. To achieve the proper reference orbit from the
Baikonur launch site an additional boost with a characteristic
velocity of ~260 m/s is performed igniting the booster's propulsion
unit.
- Then a correction maneuver is performed in the first orbit to
increase the apogee altitude without changing the inclination.
In case of a circular low orbit two maneuvers are performed to raise the
S/C from the reference orbit to the operational one. Each maneuver raises
altitude and lowers the orbit inclination simultaneously. Characteristic
velocity of the maneuvers from the reference orbit to operational circular
orbit (550-600 km altitude, 29( inclination) is 3080 m/s (launch from the
Baikonur site).
Table 3 gives masses of the S/C with the adapter delivered to the high-
apogee orbits for different launchers and initial orbital parameters.

Table 3. - S/C masses injected and LV's performance

|Launcher and booster|Launch site and|Mass of the S/C with the |
| |the initial S/C|adapter, kg |
| |orbit | |
| |inclination | |
| | |Hp=250 km |Hp=250 km |
| | |Ha=200 000 km |Ha=163 000 km |
|Soyuz-FG + Fregat |Baykonur, i = |1850 |1880 |
| |518( | | |
|Soyuz-FG + Fregat |Plesetsk, i = |1605 |1635 |
| |628( | | |
|Soyuz-2 (1a) + |Baykonur, i = |1850 |1880 |
|Fregat |518( | | |
|Soyuz-2 (1b) + |Baykonur, i = |2140 |2170 |
|Fregat |518( | | |
|Soyuz-2 (1b) + |Plesetsk, i = |1985 |2015 |
|Fregat |628( | | |


1 Results of the ballistic analysis for the Spectrum-X-Gamma S/C operation
orbits and launch vehicles

The analysis fulfilled makes it possible to draw the following
conclusions:
- Soyuz-FG LV with the Fregat booster (launch from Baikonur) is able
to deliver the S/C with the adapter either to a high-apogee 4-day
period orbit (51.8( inclination, 250 km perigee, 200 000 km apogee;
total mass is 1850 kg) or to a high-apogee 3-day period orbit (163
000 km apogee; total mass is 1880 kg).
- Soyuz-2 (1b) LV with the Fregat booster (launch from Baikonur) is
able to deliver the S/C with the adapter either to a high-apogee 4-
day period orbit (51.8( inclination; total mass is 2140 kg) or to a
high-apogee 3-day period orbit (51.8( inclination; total mass is
2170 kg).
- In the case of a launch from Plesetsk (orbit inclination is 62.8()
the total mass is reduced by ~150-250 kg compared to the launch
from Baikonur.
- On the condition of a proper choice of initial perigee argument
(320œ-322œ) and the ascending node longitude of 180œ, the perigee
altitude increases rather quickly reaching 10 000 km (the boundary
of the protons influence on detectors) in 220 days for the 4-day
period orbit and in 340 days for the 3-day period orbit. In this
case the estimated ballistic lifetime is up to 20 years.
- Duration of the Earth's shadow entry is less than 3.1 hours per
orbit in case of the 4-day period orbit and 2.6 hours for the 3-day
period orbit. Probability of such a long shadowing is estimated as
once per year for no more than two subsequent orbits.
- Spacecraft will be tracked from the Moscow control & tracking
center for almost 75% of the orbit for the 3-day period orbit and
88% - for the 4-day period orbit.

instruments

YAMAL-SXG instrumentation includes four telescopes (JET-X, MART, EUVITA
and TAUVEX) covering a wide energy range, from the near ultraviolet to the
hard X-ray, plus an X-ray and gamma-ray all-sky monitors (MOXE, SPIN).
Joint European X-ray Telescope (JET-X) is the largest instrument of the
project which consists of the two identical co-aligned multi-mirror grazing-
incidence telescopes with a focal length equal to 3.5-m and an angular
resolution 20( within the 0.3(10 keV energy band. JET-X is equipped with
cooled CCD focal detectors, which provide high spectral resolution data.
Table 4 gives the main technical characteristics of the YAMAL-SXG
instruments.

Table 4. YAMAL-SXG Instruments. Technical characteristics

|Instrument |Energy |Angular |Energy |Eff. area, |Field of|
| |range, keV |resolution|resolution |cm2 |view |
|X-ray mirror |0.3(10 |20( |140 eV |360 (1.5 |40((30( |
|grazing-inciden| | |at 6 keV |keV) 140 (8| |
|ce telescope | | | |keV) | |
|JET-X | | | | | |
|X-ray coded |10(150 |8( |5% |800 |6((6( |
|mask telescope | | |at 60 keV | | |
|MART | | | | | |
|Ultraviolet |0.01(0.014 |10( |- |10 |(1.2( |
|telescope | | | | | |
|EUVITA | | | | | |
|Ultraviolet |0.0091(0.003|4( |Variable |Mirror |54((54( |
|monitor TAUVEX |7 | |filters |(20cm | |
| | | | |(3 | |
| | | | |telescopes)| |
|X-ray all-sky |2(12 |3( |20% |6(3 |4( |
|monitor MOXE | | |at 6 keV | | |
|Gamma-ray burst detectors SPIN |
|Gamma-ray burst|20(3000 |several |9% |8(132 |4( |
|detectors | |deg. |at 662 keV | | |
|X-ray |2(30 |9( |18% |2(100 |2(40((40|
|wide-field | | |at 6 keV | |( |
|cameras | | | | | |
|Optical |Visible band|1( |- |Sensitive |4((4( |
|detector | | | |to m<10 | |

The YAMAL-SXG unique feature consists in its simultaneous observation of
cosmic sources within a wide wave band (from UV to hard X-rays) with high
sensitivity.
As for the science, considering the above listed capabilities of the
mission instrumentation we would like to emphasize the attention of the
world astronomical community on the following significant tasks:
- the all-sky X-ray monitoring;
- study of the cosmic X-ray transient phenomena; and
- study of the gamma-ray bursts and their afterglow emission.


References

Allen, S. W., Schmidt, R. W., & Fabian, A. C. 2002, MNRAS, 334, L11
Cavaliere, A., Menci, N., & Tozzi, P. 1997, Astrophys. Journal Letters,
484, L21
Cen, R. & Ostriker, J. P. 1999, Astrophys. Journal, 514, 1
Loeb, A. & Waxman, E. 2000, Nature, 405, 156
Sarazin, C. L. 1986, Rev. Mod. Phys., 58, 1
White, S. D. M., Navarro, J. F., Evrard, A. E., & Frenk, C. S. 1993,
Nature, 366, 429