A&A 446, 439-446 (2006)
DOI: 10.1051/0004-6361:20041963
J. Clavel1 - N. Schartel2 - L. Tomas2
1 - Research & Scientific Support Department,
ESTEC, SCI-SA, Postbus 299
2200 AG - Noordwijk, The Netherlands
2 -
XMM-Newton Science Operations Centre,
ESAC, Apartado 50727, 28080 Madrid, Spain
Received 6 September 2004 / Accepted 9 September 2005
Abstract
Very long (172 ks effective exposure time) observations of the
BALQSO LBQS 2212-1759 with XMM-Newton yield a
stringent upper-limit on its 0.2-10 keV (rest- frame 0.64-32.2 keV) flux,
,
while
simultaneous UV and optical observations reveal a rather blue spectrum
extending to 650 Å in the source rest frame. These results are
used to set a tight upper-limit on its optical to X-ray spectral index
.
Given the HI-BAL nature of
LBQS 212-1759, its X-ray weakness is most likely due to
intrinsic absorption. If this is the case, and assuming that the intrinsic
of LBQS 2212-1759 is -1.63 - a value
appropriate for a radio-quiet quasar of this luminosity - one can set
a lower limit on the X-ray absorbing column
.
Such a large column has a Thomson
optical depth to electron scattering
,
sufficient
to extinguish the optical and UV emission. The contradiction becomes even
more acute if
the gas is neutral since the opacity in the Lyman continuum becomes
extremely large,
,
conflicting with
the source detection below 912 Å. This apparent contradiction
probably means that our lines-of-sight to the X-ray and to the UV emitting
regions are different, such that the gas completely covers the
compact X-ray source but only partially the more extended source of
ultraviolet photons. An extended (
)
X-ray source is
detected
to the south-east of the QSO. Given its thermal
spectrum and temperature (
), it is
probably a foreground (
)
cluster of galaxies.
Key words: quasars: absorption lines - quasars: individual: LBQS 2212-1759 - X-rays: individuals: LBQS 2212-1759 - galaxies: active - X-rays: galaxies - ultraviolet: galaxies - galaxies: active
Broad Absorption Line (BAL) quasars are characterized by broad and
blueshifted absorption troughs in their spectrum, from resonance
transitions such as CIV1550, Ly
1216,
NV
1240, indicating the presence of a high velocity (up to
50 000
)
outflow along the line-of-sight (LOS) to the
nucleus. Taking into account selection biases, BALQSOs represent
% of the radio-quiet quasar population (Hewett & Foltz
2003). The fraction of BALQSOs that are radio-loud is
approximately the same as that of non-BAL quasars, but there appears
to be a deficit of broad absorption line objects at large radio
luminosities (Menou et al. 2001; Becker et al. 2000).
Because of the overall similarity of their continuum and emission line
properties with those of non-absorbed quasars, it is sometime thought that
BALQSOs are "normal'' quasars seen at a specific viewing angle such that
our LOS intercepts a nuclear wind (Weyman et al. 1991). The wind
possibly originates from the accretion disk and is driven out radially
by radiation pressure (Murray et al. 1995).
A hydrodynamic model for the BAL wind was developed by Proga
et al. (2000). In the empirical scenario proposed
by Elvis (2000), the wind arises vertically from a
narrow range of disk radii and bends outward to a cone angle
of 60
with a divergence angle of 6
.
In this type of models, it
is the solid angle covered by the outflow that determines the fraction
of BAL quasars.
An alternative class of models speculates that the BAL phenomenon
represents an early "cocoon'' phase in the evolution of a QSO (e.g. Briggs
et al. 1984). Although his results are based on
a small sample that contains only 4 BALQSOs, Boroson (2002)
lent some credibility to this idea by showing that BALQSOs occupy a
specific location in the quasar parameter space, characterized by large
accretion rates and luminosities, close to the Eddington limit.
The evolution scenario is also supported by the large fraction
of BALQSOs found in a spectroscopic follow-up to the VLA FIRST
survey - 29 radio-selected BALQSOs (Becker et al. 2000) -
since the properties of the sample appear inconsistent with simple
unified models.
BALQSOs are invariably X-ray weak or silent (Green et al. 1995;
Green & Mathur 1996;
Gallagher et al. 1999), suggesting the presence of very large
absorbing columns,
,
2-3 orders of
magnitudes larger than those inferred from UV absorption line studies. This
discrepancy led to the conclusion that the bulk of the absorbing gas is
highly ionised and thus mostly transparent in the ultraviolet while still
providing large X-ray opacities. However,
it was subsequently realized that the column densities derived from curve of
growth analysis of absorption lines may be severely underestimated. High
resolution and high signal-to-noise ratio UV spectra show that the lines
are saturated despite the existence of residual flux at their bottom (e.g.
Arav et al. 1999; Wang et al. 1999). The residual flux
may be due to partial covering of the continuum source or to the scattering
of part of its emission back into our LOS, as indicated by the higher
degree of polarisation of BALQSOs as compared to non-BAL quasars
(e.g. Schmidt & Hines 1999; Ogle et al. 1999).
Here we present very sensitive observations of the z = 2.217 BALQSO
LBQS 2212-1759 (Morris et al. 1991) performed with
XMM-Newton. This quasar was selected because of its optical
brightness (
mB = 17.94) and tentative 3-sigma detection in
the soft-X-ray band with ROSAT (Green et al.1995).
LBQS 2212-1759 displays two CIV1548 absorption troughs
blue-shifted respectively by
and
with respect to its systemic velocity (Korista et al. 1993).
Table 1: Details of the XMM-Newton X-ray observations.
Table 2:
Upper limits to the X-ray flux of
LBQS 2212-1759 derived from the EPIC-pn observations.
The X-ray pn and MOS data were reduced and analyzed in a
standard fashion using the SAS v5.3.
The pipe-line products of observation 0106660601 provide 44
positional coincidences between sources of the USNO-A2.0 Catalogue
(Monet et al. 1998) and X-ray sources in the field of
LBQS 2212-1759. Of these 44 coincidences, 35 X-ray sources have only
one, three X-ray sources have two and one X-ray source has three optical
counterparts. Restricting to the 35 X-ray sources with a unique optical
identification, we infer a mean offset of 2.2
between the X-ray
position and the optical source coordinates. Note that out of these 35
X-ray sources with a unique identification, 25 (i.e. 63%) lie within
2
of their optical counterpart. The BAL quasar was not detected
in either of the X-ray instruments. The nearest detected X-ray point
source is
away from the nominal position of
LBQS 2212-1759 (RA = 22:15:31.6; Dec = 17:44:06 - J2000)
From the rms
background count fluctuations in a
cell centered on the
expected source position, we computed
upper limits to the count
rate in various energy bands.
The HEW of the EPIC-pn point-spread function is 14
and one
CCD pixel projects onto an area of
on the sky.
From the XMM-Newton observation of Q 0056-363, we determined the ratios
between the total count rate of a faint point-source and the
count rates measured in a
-pixel cell centered on the source
for each of the energy bands. These ratio were then applied to the
cell upper limits to derive effective upper limits to the source
count rate. The results are listed in Table 2.
These count rates were converted into flux
upper limits using the PIMMS software available on-line at the
HEASARC web-site. The results are given in Table 2 were we only
list the results from the EPIC pn data, since the less sensitive
EPIC MOS detectors yield consistent but significantly higher
and therefore less constraining limits.
In an attempt to understand why LBQS 2212-1759 was
marginally detected by Green et al. (1995), we checked their
original ROSAT image. There are definitely no excess counts at the
centre of their 3
radius extraction circle, clearly ruling out
the presence of a point-source. However, the merged EPIC data reveal the
existence of a weak extended source, centered at RA = 22:15:37
and Dec = -17:45:35 and whose radius is 1.0
.
This source is most
probably a foreground cluster of galaxies since its spectrum is well
described by a Mekal spectrum with temperature in the range 1.5-3 keV
and a redshift between 0.29 and 0.46. Whatever its origin, this source
clearly lies within the ROSAT extraction region and is likely the origin
of the false detection of LBQS 2212-1759 by Green et al.
(1995). The extended source was too weak to appear in the ROSAT All
Sky Survey catalogue (1RXS) and
could not therefore be taken into account by Green et al. (1995).
Note that there are no EPIC point-sources within the 10-20
annulus
that these authors used to measure the background in the ROSAT image.
Table 3: Optical and UV fluxes measured though the various OM filters.
In parallel to the X-ray observations, a series of optical and ultraviolet
broad-band filter images of the QSO field were obtained with the Optical
Monitor telescope (OM; Mason et al. 2001) on board
XMM-Newton. The OM data were reprocessed
with the SAS version 6.0 using the script omichain. For each
broad-band filter image, the corrected net count rate of the QSO was
read-off directly from the SWSRLI output files, which lists all sources
automatically detected by the SAS software, together with their count
rate, statistical significance, measured coordinates, the associated
errors and various data quality indicators. We used the close-by
mag star S3211320188 from the HST guide-star
catalog to correct for small (
)
residual astrometric distortions
in the OM coordinate system. After correction, the QSO coordinates
as measured with OM agree to better than
with NED
catalog coordinates. The count rates were converted
into fluxes following the recipe provided on the SAS web page at URL
xmm.vilspa.esa.es/ sas/documentation/watchout/uvflux.html. The final fluxes
are listed in Table 3, which provides: the observation identifier
in Col. 1, the OM exposure number in Col. 2, the date and U.T. time
of the start of the exposure expressed as a fractional day of 2000 in
Col. 3, the filter identifier in Col. 4 and the flux with its associated
statistical rms error in Col. 5. Early observations with
the less sensitive UVW2 filter had exposure times that were too short and
did not yield statistically significant detections. In such cases,
upper limits are listed in Table 3.
A
test shows that the flux of LBQS 2212-1759
remained constant within the measurement uncertainties in all 6 filters.
The reduced chi-squares (d.o.f.) corresponding to the hypothesis of a
constant flux are
(8), 0.53(9), 1.19 (9),
0.52 (10) and 1.20 (6) for the V, B, U, UVW1 and UVM2 filters,
respectively.
We therefore averaged the results
from individual exposures and computed the weighted mean flux in each
filter and the error on the mean. The near-IR J, H and K
fluxes of
LBQS 2212-1759 were retrieved from the 2MASS catalog
(Kleinmann 1994; Barkhouse & Hall 2001). All
fluxes were finally corrected for foreground galactic extinction
(
Eb-v = 0.026; Schlegel et al. 1998; Cardelli
et al. 1989). The results are given in Table 4,
where we list the origin of the data in Col. 1, the effective wavelength
and band-pass of the filter in the observer's frame in Cols. 2 and 3, respectively, the effective wavelength and band-pass of the filter
in the quasar rest-frame in Cols. 4 and 5, respectively and the
averaged de-reddened flux in Col. 6.
![]() |
Figure 1: The flux distribution of LBQS 2212-1759 from 6711 Å to 659 Å (rest wavelengths). All data-points are from the present study, except for the 3 longest wavelengths ones which were retrieved from the 2MASS catalogue. The 1992 spectrum from Korista (1993) is also shown for comparison. All flux values are in the observer's frame and have been corrected for foreground galactic extinction ( Eb-v = 0.026). |
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Table 4: The flux of LBQS 2212-1759 as a function of wavelength, from the near-IR to the EUV. The values in the optical and UV have been obtained by averaging fluxes from individual OM exposures. All fluxes have been corrected for foreground galactic reddening.
The rest-frame optical to EUV energy distribution of LBQS 2212-1759, is shown in Fig. 1. A 1992 spectrum of LBQS 2212-1759 (Korista et al.1993) is also displayed for comparison. Note that while the flux increased by 46% in the OM-V band during the 8.5 years interval between the two observations, the spectral shape remained very similar.
The ultraviolet spectrum of LBQS 2112-1759 is however difficult
to reconcile with the above stringent upper limits on its X-ray flux.
The optical-to-X-ray spectral index of a quasar (Zamorani et al.
1981),
,
is defined as the spectral index of
an hypothetical power-law connecting its flux density at
2500 Å and 2.0 keV in the QSO rest-frame,
.
In radio-quiet non-BAL quasars, it is observationally confined to a range
with a weak dependence on the
source luminosity (Vignali et al. 2003; Strateva et al.
2005). Using the same cosmological parameters
as these authors, the monochromatic luminosity of LBQS 2112-1759
at 2500 Å (rest wavelength) is
,
which,
according to Eq. (6) of Strateva et al., predicts
.
Assuming a canonical photon spectral index
for the
0.2-10 keV spectrum of LBQS 2212-11759 (e.g. Laor et al.
1997), and using the X-ray flux upper limits
of Table 2, one can infer an upper limit to the monochromatic
flux density at a rest energy of 2 keV (
),
.
One can derive
the flux at
by interpolation
between the de-reddened fluxes in the J and V bands. Combining the two
yields
,
steeper by
dex than the
index predicted for a radio-quiet quasar of the same luminosity as
LBQS 2212-1759. This is illustrated in Fig. 2,
where we plot the overall Spectral Energy Distribution (SED) of
LBQS 2212-1759. The upper limit to its 2 keV flux is 263
times lower than that predicted by extrapolation of its 2500 Å flux
density with a power-law of index
.
Assuming that the difference is entirely due to intrinsic absorption of the
X-ray flux, one can infer a lower limit to the required absorbing column,
.
Note that this result
depends only weakly on the value assumed for the X-ray spectral index.
For instance, using
instead of -1.9 hardly changes the
results to
.
![]() |
Figure 2:
The overall optical-to-X-ray spectral energy distribution of
LBQS 2212-1759. All flux values are in the observer's frame and have been
corrected for foreground galactic extinction (
Eb-v = 0.026).
The upper limit to the 0.2-10 keV (observer's frame) flux of
Table 2 is shown as a spectrum of photon index
![]() ![]() ![]() |
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If the gas was neutral, an absorbing column as large as or larger than
would create an optical depth at the
Hydrogen Lyman limit,
,
more than
sufficient to extinguish all radiation at wavelengths shorter than
912 Å. This is not the case however, since LBQS 2212-1759 is
detected to
.
Hence, the X-ray
absorbing gas cannot be neutral and cover the UV continuum source.
However, even if the gas is fully ionised,
the Thomson optical depth to electron scattering corresponding to the above
column,
,
is sufficient to attenuate the flux
by a factor
and make LBQS 2212-1759 invisible at
all wavelengths except in the
ray regime. Another difficulty is
that, unless the gas is completely free of dust, extinction will wipe-out
any emerging UV and optical photon. Even if the dust to gas ratio is
100 times lower than the average galactic value (e.g. Gorenstein
1975), an absorbing column of
would still generate
150 mag of visual extinction and
approximately ten times more in the far ultraviolet.
We are thus left with an inconsistency: on the one hand, LBQS 2212-1759 is detected with high statistical significance in the UV and EUV range, and on the other it is not detected in the X-rays, with upper limits on the 0.2-10 keV flux which, taken at face value, imply column densities sufficient to extinguish its ultraviolet emission as well.
In what follows, we briefly explore two possible explanations for this apparent contradiction:
![]() |
Figure 3:
The spectrum of LBQS 2212-1759 around the MgII![]() ![]() ![]() ![]() |
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Table 5:
Intensities and full width at half maximum ( FWHM) of the main
emission lines in the spectrum of LBQS 2212-1759; all intensities are in
units of
and have been corrected
for galactic extinction. Their uncertainties are typically 10%. The FWHM
are expressed in
and have an uncertainty of
.
Acknowledgements
The authors are grateful to Kirk Korista for providing his 1992 optical spectrum of LBQS 2212-1759 in electronic form. The anonymous referee is also thanked for constructive comments which significantly improved this article.
This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.