A&A 446, 439-446 (2006)
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
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, NV1240, 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 . For comparison, a fiducial power-law of index and extending from 2500 Å to 2 keV (rest frame) is shown as a dashed line. The LBQS 2212-1759 flux upper limit is 263 times lower than the flux predicted by extrapolating the power-law to 2 keV.|
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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 MgII2798 line ( upper panel), the AlIII1857 and SiII1808 lines ( middle), and the CIV1549 line ( bottom). The wavelengths at which one expects blue-shifted double absorption troughs are indicated with arrows. Note the absence of significant absorption from MgII, SiII and AlIII.|
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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 .
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.