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Strong X­ray Absorption in a BALQSO: PHL5200
Smita Mathur 1 , Martin Elvis 1 and K. P. Singh 2;3
1. Harvard­Smithsonian Center for Astrophysics,
60 Garden St, Cambridge MA 02138
2. Code 668, Lab. for High Energy Astrophysics,
NASA/GSFC, Greenbelt, MD 20771.
3. NRC Senior Research Associate, on leave from
Tata Institute of Fundamental Research, Bombay, India.
Internet: smita@cfa.harvard.edu
October 3, 1995

Abstract
We present ASCA observations of the z=1.98 prototype BALQSO: PHL5200.
The source was detected in both SIS and GIS. A power­law spectrum (ff E =
0:6 +0:9
\Gamma0:6 ) with large intrinsic absorption (NH = 1:3 +2:3
\Gamma1:1 \Theta 10 23 cm \Gamma2 ) best de­
scribes the spectrum. Excess column density over the local Galactic value is
required at the 99% confidence level. This detection suggests that, although
BALQSOs are X­ray quiet, it is strong absorption in the BAL region that
makes them appear faint to low energy X­ray experiments. The required
intrinsic absorbing column density is two to three orders of magnitude larger
than earlier estimates of column densities in BALQSOs. This implies that
the BAL systems are much more highly ionized than previously thought.

1 Introduction
Associated absorption is common in the optical and ultraviolet spectra of
quasars (Ulrich, 1988). A subset of these have very broad absorption line
profiles extending up to \Deltav = 0:1 \Gamma 0:2c redwards with respect to the quasar
rest frame (see e.g. Turnshek 1988). These Broad Absorption Line quasars
(BALQSOs), show absorption features due to high ionization lines of C +3 ,
Si +3 , and other ions. Low ionization BALQSOs have also been observed
which show Mg +1 and/or Al +2 absorption troughs. BALQSOs have been
estimated to have column densities NH ¸ 10 20\Gamma21 cm \Gamma2 (Turnshek 1984,
Hamann et al. 1993). As a class, BALQSOs share some common properties:
they are always radio­quiet (Stocke et al. 1992), may have abundances 10­
100 times solar (in their emission lines; Turnshek 1988, Hamann & Ferland
1993) and are X­ray quiet ( Green et al. 1995). Recent work suggests that
BALQSOs are normal radio­quiet quasars seen from an unusual direction
(Weymann et al. 1991, Hamann et al. 1993). In this case all radio­quiet
quasars have collimated BAL outflows, which however are pointed out of our
line of sight in some 90% of cases. Thus BALQSOs, far from being exotic
objects, give us a special probe into the gas dynamics around the typical
quasar.
However, physical conditions in the absorbing gas in the BALQSOs are
poorly determined from optical/UV absorption line studies (Lanzetta et al. 1991).
This is because only a few, usually saturated, lines are measured yielding
lower limits to column densities for a few ions, but little information on
the ionization state. If, as in the narrow line associated absorbers, there is
X­ray absorption as well as optical and UV, then the combined X­ray and
UV analysis would allow us to derive the physical conditions in BALQSO
absorption systems (Mathur et al. 1994, Mathur 1994, Mathur et al. 1995).
This, however, has been difficult, since BALQSOs are elusive X­ray sources,
and so are otherwise essentially unconstrained in their X­ray properties. In a
soft X­ray study of quasars with EINSTEIN only four out of nine BALQSOs
were detected (Zamorani et al. 1981). Initial results from ROSAT are mainly
upper limits (Kopko et al. 1993, Green et al. 1995) implying that they are
relatively faint in soft X­rays (i.e. have steep ff ox ). Our understanding of
BALQSOs is incomplete without knowing their X­ray properties. In fact,
lack of knowledge of the underlying ionizing continuum is one of the major
uncertainties in the models of BALQSOs: Are they intrinsically X­ray quiet
1

(i.e. large ff ox )? Or, is it strong absorption that makes them look faint?
PHL5200, a prototype BALQSO at z=1.98 (Burbidge, 1968), was de­
tected in hard X­rays by the EXOSAT medium energy (ME) experiment
but not by the low energy (LE) experiment (Singh et al.1987). To obtain
consistency between the EXOSAT ME and LE requires a column density of
– 10 22 atoms cm \Gamma2 making it an excellent candidate for examining the BAL
region. We observed PHL5200 with ASCA with this aim in mind. EIN­
STEIN did not detect (Zamorani et al. 1981) and ROSAT has not observed
PHL5200.
2 ASCA Observations and Data Analysis
ASCA (Tanaka et al. 1994) observed PHL5200 on 1994 June 21 for a net ex­
posure time of 17.7 ksec (Table 1). ASCA has two Solid­state Imaging Spec­
trometers (SIS0 and SIS1, Loewenstein & Isobe, 1992) and two Gas Imaging
Spectrometers (GIS3 and GIS4, Ohashi et al. 1991). The SIS were operated
in 2­CCD mode (see figure 1). The source was faint but was clearly detected
in SIS0 and GIS3 (figure 1). The X­ray position from SIS0 is (J2000) 22:28:26,
\Gamma5:18:54; 1.1 arcminute from the optical position (Schneider et al. 1992), con­
sistent with the current satellite pointing uncertainties (Tanaka et al. 1994).
The source was off axis in SIS1 (where it lay close to the gap between two
chips) and GIS2 and was not detected in either. This is consistent with the
fact that the optical axes of telescopes containing SIS0 and GIS3 are much
closer to each other than the others. No other sources were seen in any of
the instruments to a level similar to the count rates of PHL5200.
Data were extracted in standard way using the FTOOLS and XSELECT
software. 1 Standard screening criteria were used as recommended in ASCA
ABC guide: ?10 degree bright earth angle; and a cut­off rigidity of ?6
GeV/c. Hot and flickering pixels were removed from the SIS data using XS­
ELECT. All SIS events of grade 0, 2, 3 and 4 were accepted. Data of both
faint and bright modes with high, medium and low telemetry rates were com­
bined. These data can be combined without any calibration compromises.
ASCA X­ray telescope has a broad point spread function and jittering of
1 FTOOLS is a collection of utility programs to create, examine or modify data files
in FITS format. XSELECT is a command line interface to the FTOOLS, for X­ray
astrophysical analysis. The software is distributed by the ASCA Guest Observer Facility.
2

the spacecraft can appear on arcminute scales. To take this into account,
source counts were extracted from a circular region of 6 arcminute radius
for GIS3 and from a 4 arcminute radius for SIS0. The source was pointed
at the center of chip # 1 of SIS0, putting the bulk of its photons into just
one chip. Detectors SIS0 and GIS3 yielded ¸ 500 total counts each. Data
from these detectors cannot be combined since they have different properties.
The background was estimated using the same spatial filter on the deep field
background images (ASCA ABC guide). A background subtracted count
rate of (1:04 \Sigma 0:15)\Theta10 \Gamma2 was observed by SIS0, and (6:26 \Sigma 1:47)\Theta10 \Gamma3
by GIS3. The data were grouped to contain at least 10 counts (background
subtracted) per pulse height analysis (PHA) channel to allow the use of the
Gaussian statistic. The data have modest signal to noise ratio; however it
can be clearly seen that that there are essentially no counts below ¸ 1 keV
(¸ 3 keV in the rest frame)(see Figure 2). The highest rest energy detected
for PHL5200 is 12 keV at 3oe for 0.5 keV wide bins. Figure 2 shows the SIS
and GIS spectra of PHL5200.
The SIS and GIS spectra extracted in this way were then analyzed us­
ing XSPEC. The March 1995 release of the response matrices was used for
the GIS data, and the November 1994 release for the SIS data. A power
law spectrum with fixed Galactic absorption (4.8\Theta10 20 atoms cm \Gamma2 ; Stark
et al. 1992) provides an acceptable fit to the SIS0 data (Table 2). However,
if absorption is allowed to be a free parameter, then the fit is improved with
?98% confidence (F­test, Table 2). The fitted value (NH (z = 0) = 9 \Theta 10 21
atoms cm \Gamma2 , solar abundance) is much larger than the Galactic column den­
sity towards PHL5200, indicating excess absorption along the line of sight.
This is also much larger than the uncertainties in the SIS low energy re­
sponse, which may overestimate the column density by up to 2 \Theta 10 20 cm \Gamma2
(Day, C. S. R. 1995. ``Calibration Uncertainties'', ASCA GOF WWW page.
URL: http://heasarc.gsfc.nasa.gov/docs/asca/cal probs.html). We then fit­
ted a power­law spectrum with Galactic column and an additional column
of absorber allowing its redshift to be free. We found no preferred redshift
for the additional absorber. Fixing the absorber at the source gives a col­
umn density of 1.4 +2:0
\Gamma1:2 \Theta 10 23 cm \Gamma2 (90% confidence for one parameter, solar
abundance). The power­law energy index is ff E = 0:8 +1:1
\Gamma0:9 .
For the GIS data, a similar fit of a power­law spectrum with fixed Galactic
and additional z=1.98 absorption is acceptable, but does not constrain the
parameters well because the data have large errors (Table 2).
3

A combined SIS and GIS analysis does constrain the parameters of the
model slightly better (Figure 3, Table 2). The column density at the source is
1.3 +2:3
\Gamma1:1 \Theta 10 23 cm \Gamma2 and ff E = 0:6 +0:9
\Gamma0:6 . This excess absorption, above Galactic
NH , is required at 99% confidence (F­test).
The 2--10 keV (observed frame) flux is 2.9 +13:9
\Gamma1:3 \Theta 10 \Gamma13 ergs s \Gamma1 cm \Gamma2
(corrected for best fit absorption) and a 2--10 keV (rest frame) luminosity is
9.3 \Theta10 45 ergs s \Gamma1 (H 0 = 50, q 0 = 0). The flux in the EXOSAT ME band (2--6
keV observed) is 2 +6
\Gamma1 \Theta 10 \Gamma13 ergs s \Gamma1 cm \Gamma2 . This is smaller than the EXOSAT
flux (¸ 2 \Theta 10 \Gamma12 ergs s \Gamma1 cm \Gamma2 ; Singh et al. 1987) by at least a factor of 2.5.
The optical continuum of PHL5200 does not vary by such a large amount
(Barbieri et al. 1978). It is possible that it is variable absorption rather than
intrinsic source variability that might be responsible for the difference in the
ASCA and EXOSAT ME fluxes. The ASCA flux is consistent with the upper
limits observed by the EINSTEIN IPC (! 4:5 \Theta 10 \Gamma13 ergs s \Gamma1 cm \Gamma2 ) and
the EXOSAT CMA (! 5 \Theta 10 \Gamma13 ergs s \Gamma1 cm \Gamma2 ).
The ASCA derived monochromatic luminosity at 2 keV (rest frame) is
1.3 \Theta10 28 ergs s \Gamma1 Hz \Gamma1 and at 2500 š A (rest frame) it is 1.2 \Theta10 32 ergs s \Gamma1
Hz \Gamma1 (Zamorani et al. 1981), giving ff ox = 1:5.
An Fe­K absorption edge is not detected (Ü ! 0:9, 90% confidence for
one interesting parameter). The opacity of an Fe edge corresponding to
NH = 10 23 cm \Gamma2 is Ü = 0:1f ion where f ion is the ionization fraction of iron
in hydrogen­like state. Our data are not sensitive enough to detect such an
edge.
The Fe­K emission line (Ross & Fabian 1993) is also not detected (see
figure 2). We place a 0.5 keV upper limit (90% confidence for one interesting
parameter) to the rest frame equivalent width of a narrow (oe ! 10 eV) line
between 2.1 and 2.4 keV (6.3--7.1 keV rest frame). This can be used to place
an upper limit on the covering factor of the absorber. If the absorber is a
uniform spherical shell surrounding the X­ray continuum source, then the
Fe Kff line flux through recombination after photoionization of helium­like
iron is given by I line =(NHA(F e)=10
19:8)(\Omega =4ú) I abs j (Basko 1980). j is the
fluorescent yield, the efficiency with which the flux above 7.1 keV (I abs ) is re­
emitted as an Fe­K line. Assuming solar abundance of iron (A(Fe)=3.3\Theta10 \Gamma5 ,
Grevesse & Andres 1989) and j = 0:5 (Krolik & Kallman, 1987) we estimate
the covering factor of the line emitting
region,\Omega =4ú ! 4f \Gamma1
ion ; which is not an
interesting limit. If, however, the heavy element abundance is 10 times solar
(Hamann & Ferland 1993)
then\Omega =4ú ! 0:4 f \Gamma1
ion , consistent with Hamann
4

et al. (1993).
3 Discussion
The ASCA spectrum of PHL5200 shows excess absorption at 99% confidence.
A column density of 0:2 \Gamma 4 \Theta 10 23 Z fi
Z
cm \Gamma2 is obtained if the absorber is at
the source. A power­law was a good fit to the data with the spectral slope
(ff E = 0:6 +0:9
\Gamma0:6 ) in the normal range (Wilkes et al. 1994). The PHL5200 value
of ff ox = 1:5, is also normal for a radio quiet quasar (Wilkes et al. 1994).
The inferred absorbing column density for PHL5200 is two to three or­
ders of magnitude larger than the earlier estimates of column density in
BALQSOs (Hamann et al. 1993, Turnshek 1984). This implies that the BAL
clouds may be more highly ionized (N HI /NH ¸ 10 \Gamma8 ) than previously thought
(N HI /NH ¸ 10 \Gamma5 , Hamann et al. 1993), as was true with narrow associated
absorbers (Mathur et al. 1994, 1995). The estimates from the saturated UV
lines appear to have been misleading. Recent models of BALQSOs (Murray
et al. 1995), however, do consider column densities as large as we find in
PHL5200. If, on the other hand the abundences are 100 -- 1000 times solar
then the Hydrogen column density would be smaller (NH ¸ 10 20 cm \Gamma2 ). How­
ever, the ionization state would still be high, since the comparison is between
metal line absorption in the UV and absorption in the X­ray. The column
density in PHL5200 is also about an order of magnitude larger than other,
narrow, associated absorption systems (Fiore et al. 1993, Turner et al. 1994).
In this respect, as they are in velocity width, the BALQSOs may be extreme
examples of other associated absorbers.
This is consistent with our earlier conjecture that all associated absorbers
may form a continuum of properties with column density, outflow velocity
and the distance from the central continuum (Mathur et al. 1994). Are
BALQSOs also similar to these in being `XUV absorbers'? i.e. are the broad
absorption lines observed in the UV caused by the same matter producing
X­ray absorption? This can be investigated by combined analysis of X­ray
and UV spectra (Mathur et al. 1994, 1995) of PHL5200; but is beyond the
scope of this paper. If they are indeed the same, it would allow us to further
constrain the physical properties of the absorber and so of the outflowing
circumnuclear matter (Mathur et al. 1995).
The present study implies that BALQSOs are not intrinsically X­ray
5

quiet; it is the extreme absorption that makes them appear faint to low
energy experiments. Since the absorption is significant only in soft X­rays,
hard X­ray observations, above a few keV, would reveal their presence as
X­ray sources. This can be done with missions like ASCA, XTE, SAX and
AXAF. We have been awarded XTE time to observe BALQSOs with this
aim.
Acknowledgements:
This research has made use of the NASA/IPAC Extragalactic Database
(NED) which is operated by the Jet Propulsion Laboratory, CALTECH,
under contract with the National Aeronautics and Space Administration.
This work was supported by NASA grants NAGW­2201 (LTSA), NAG5­2563
(ASCA), NAGW­4490 (LTSA) and NASA contract NAS8­39073 (ASC).
6

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Table 1: ASCA Observations of PHL5200
Instrument Total Counts Exposure Net Count Rate
(s) s \Gamma1
SIS0 513 16587 0.01\Sigma0.001
GIS3 505 16788 0.006\Sigma0.001
8

Table 2: Spectral fits to ASCA data of PHL5200
Data Model ff E NH (free) a Normalization b ü 2 (dof) c
SIS Power­law:
+NH 0.9 +1:3
\Gamma1:0 0.9 +1:2
\Gamma0:8 1.3 +4:8
\Gamma0:3 5.2 (12)
+NH (Gal.) fixed \Gamma0:1 +0:4
\Gamma0:4 0.3 +0:1
\Gamma0:2 8.4 (13)
+NH (z=1.98) 0.8 +1:1
\Gamma0:9 14.0 +19:7
\Gamma12:4 1.2 +3:4
\Gamma0:4 4.8 (12)
GIS Power­law:
+NH 2.0 +3:1
\Gamma1:8 4.5 +0:0
\Gamma3:8 9.6 +875
\Gamma0:5 5.4 (9)
+NH (Gal.) fixed \Gamma0:1 +0:5
\Gamma0:6 0.3 +0:2
\Gamma0:1 9.8 (10)
+NH (z=1.98) 2.8 +6:2
\Gamma2:6 130 +0
\Gamma118 47 +170
\Gamma0:5 5.7 (9)
SIS+GIS Power­law:
+NH 0.6 +0:0
\Gamma0:7 0.9 +1:4
\Gamma0:7 1.0 +2:9
\Gamma0:4 14.1 (24)
+NH (Gal.) fixed \Gamma0:1 +0:3
\Gamma0:3 0.3 +0:1
\Gamma0:2 18.4 (25)
+NH (z=1.98) 0.6 +0:9
\Gamma0:6 13.1 +23:2
\Gamma11:1 0.9 +3:0
\Gamma0:4 14.2 (24)
a: \Theta10 22 cm \Gamma2
b: in units of 10 \Gamma4 photons keV \Gamma1 cm \Gamma2 s \Gamma1 at 1 keV
c: degrees of freedom.
9

Figure Captions:
Figure 1: ASCA GIS3 (left) and SIS0 (right) grey scale images around
PHL5200. North is 66.7 degrees clockwise from the top. The GIS field of
view is 50 arcmin diameter, and each SIS chip is 11.1 arcmin on a side (ASCA
Technical Description, 1993)
Figure 2: ASCA spectral data (crosses) with best fit power law with fixed
Galactic and intrinsic absorption models: SIS (top), GIS (middle), Both SIS
& GIS (bottom).
Figure 3: Confidence contours for the combined SIS and GIS spectrum.
Contours of 68%, 90% and 99% confidence regions are shown. The Galactic
column density is shown as a dashed line.
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