Документ взят из кэша поисковой машины. Адрес оригинального документа : http://hea-www.harvard.edu/~pgreen/Papers/pg0946_00.pdf Дата изменения: Mon Sep 19 19:16:12 2005 Дата индексирования: Tue Oct 2 01:09:19 2012 Кодировка:

The Astrophysical Journal, 533:L79­L82, 2000 April 20
2000. The American Astronomical Society. All rights reser ved. Printed in U.S.A.

THOMSON THICK X-RAY ABSORPTION IN A BROAD ABSORPTION LINE QUASAR, PG 0946 301 S. Mathur,1,2 P. J. Green,2 N. Arav,3 M. Brotherton,4 M. Crenshaw,5 M. deKool,6 M. Elvis, R. W. Goodrich,7 F. Hamann,8 D. C. Hines,9 V. Kashyap,2 K. Korista,10 B. M. Peterson,1 J. C. Shields,11 I. Shlosman,12 W. van Breugel,13 and M. Voit14
Received 2000 February 1; accepted 2000 March 1; published 2000 March 24
2

ABSTRACT We present a deep ASCA obser vation of a broad absorption line quasar (BALQSO) PG 0946 301. The source was clearly detected in one of the gas imaging spectrometers, but not in any other detector. If BALQSOs have intrinsic X-ray spectra similar to normal radio-quiet quasars, our obser vations imply that there is Thomson thick X-ray absorption (NH 10 24 cm 2) toward PG 0946 301. This is the largest column density estimated so far toward a BALQSO. The absorber must be at least partially ionized and may be responsible for attenuation in the optical and UV. If the Thomson optical depth toward BALQSOs is close to 1, as inferred here, then spectroscopy in hard X-rays with large telescopes like XMM would be feasible. Subject headings: galaxies: active -- quasars: absorption lines -- quasars: individual (PG 0946 301) -- X-rays: galaxies
1. INTRODUCTION

About 10%­15% of optically selected QSOs have optical/ UV spectra showing deep absorption troughs displaced blueward from the corresponding emission lines. These broad absorption lines (BALs) are commonly attributed to material flowing toward the obser ver with velocities of up to 50,000 km s 1. Broad absorption line quasars (BALQSOs) are probably normal QSOs viewed at a fortuitous orientation passing through a BAL outflow, thus implying a BAL "covering factor" at least 10%­15% in all QSOs. BALQSOs thus provide a unique probe of conditions near the nucleus of most QSOs. The absorbing columns typically inferred from the UV spectra for the BAL clouds themselves are NH 10 20­10 21 cm 2 (Korista et al. 1993). It has been noted, however, that UV studies underestimate the BAL column densities because of saturation (Korista et al. 1993; Arav 1997; Hamann 1998). BALQSOs, as a class, show higher optical/UV polarization than other radioquiet QSOs (Schmidt & Hines 1999; Ogle et al. 1999). Polarization studies reveal multiple lines of sight through high column density gas (Goodrich & Miller 1995; Cohen et al. 1995). With the absorbing column densities as estimated from the
1 Ohio State University, Columbus, OH 43220; smita@astronomy.ohiostate.edu, peterson@astronomy.ohio-state.edu. 2 Har vard Smithsonian Center for Astrophysics, Cambridge, MA 02138; pgreen@cfa.har vard.edu, elvis@cfa.har vard.edu, kashyap@cfa.har vard.edu. 3 University of California, Berkeley, Berkeley, CA 94720; arav@mars .berkeley.edu. 4 National Optical Astronomy Obser vatories, Tucson, AZ 85726; mbrother@ ohmah.tuc.noao.edu. 5 Catholic University of America and NASA Goddard Space Flight Center, Greenbelt, MD 20771; hrsmike@hrs.gsfc.nasa.gov. 6 Research School of Astronomy and Astrophysics, Australian National University, Weston Creek, ACT 2611, Australia; dekool@mso.anu.edu.au. 7 W. M. Keck Obser vator y, Kamuela, Hawaii, HI 96743; goodrich@keck .hawaii.edu. 8 University of Florida, Gainesville, FL 32611; hamann@astro.ufl.edu. 9 Steward Obser vator y, University of Arizona, Tucson, AZ 85721; dhines@ as.arizona.edu. 10 Western Michigan University, Kalamazoo, MI 49008; korista@cloud9 .pa.uky.edu. 11 Ohio University, Athens, OH 45701, shields@helios.phy.ohiou.edu. 12 University of Kentucky, Lexington, KY 40506, shlosman@is.pa.uky.edu. 13 University of California, Lawrence Livermore National Laborator y, Livermore, CA 94550; wil@igpp.llnl.gov. 14 Space Telescope Science Institute, Baltimore, MD 21218; voit@stsci.edu.

earlier UV studies, we would have expected ver y little soft Xray absorption in the BALQSOs. However, BALQSOs are found to be markedly underluminous in X-rays compared to their non-BALQSO counterparts (Bregman 1984; Singh, Westergaard, & Schnopper 1987; Green et al. 1995). Green & Mathur (1996, hereafter GM96) studied 11 BALQSOs obser ved with ROSAT and found that just one was detected with aox15 about 2. BALQSOs thus have unusually weak soft X-ray emission, as evidenced by large aox ( 1.9 compared to a ox = 1.51 0.01 [from Laor et al. 1997] for radio-quiet quasars). If BALQSOs are indeed normal radio-quiet QSOs, then their weak X-ray flux is most likely due to strong absorption. Unfortunately, due to the low obser ved flux, there are no obser ved X-ray spectra of BALQSOs to confirm the absorption scenario, with one exception: the archetype BALQSO PHL 5200 (Mathur, Elvis, & Singh 1995a). The ASCA spectrum of PHL 5200 is best fit by a power law typical for non-BALQSOs in the 2­10 keV range, with intrinsic absorption 2­3 orders of magnitude higher than inferred from UV spectra alone (Mathur et al. 1995a). However, the PHL 5200 spectrum suffers from a low signal-to-noise ratio, and while the above was a preferred fit, a model with no intrinsic absorption also fits the data. Recently Gallagher et al. (1999, hereafter G99) studied a sample of six new BALQSOs with ASCA, of which two were detected. G99 derived column densities of 5 # 10 23 cm 2 to explain the nondetections, even higher than the ROSAT estimates (assuming a neutral absorber with solar abundances unless stated other wise). How are the X-ray and UV absorbers related to each other? Are they both part of the same outflow? If so, then the kinetic energy carried out is a significant fraction of the bolometric luminosity of the quasar (see Mathur, Elvis, & Wilkes 1995b for a discussion). With all QSOs likely to contain a BAL outflow, it becomes ver y important to measure the absorbing column density accurately to understand the energetics and dynamics of quasars. We attempt this with a deep ASCA obser vation of a typical BALQSO, PG 0946 301.
15 ° The slope of a hypothetical power law connecting 2500 A and 2 keV is defined as aox = 0.384 log Lo /Lx, so that aox is larger for objects with weaker X-ray emission relative to optical.

L79


L80

THOMSON THICK ABSORPTION
TABLE 1 ASCA Count Rates for PG 0946 301 Background Background 1a ...... Background 2b ...... SIS0 0.855 (2 j), !1.2 1.29 (3 j) SIS0 Hard
!0.74

Vol. 533

SIS1
!1.2 !1.2

GIS2
!0.75 !0.75

GIS3 1.84 (7.9 j) 0.52 (2 j), !0.83

GIS3 Hard 1.4 (8.7 j) 0.48 (2 j), !0.63

0.51 (2 j), !0.74

Note.--Values are given in units of 10 3 photons s 1. The significance of detection is given in parentheses. For nondetections, the 3 j upper limit is given. For 2 j detections, the 3 j upper limit is listed as well. For SIS0 and GIS3, hard band count rates are listed as well. a With background from a source-free region on the detector. b With background from blank sky obser vations.

2. OBSERVATIONS AND DATA ANALYSIS

2.1. Observations We obser ved PG 0946 301 with ASCA (Tanaka, Holt, & Inoue 1994) on 1998 November 12. ASCA contains two sets of two detectors: the solid-state imaging spectrometer (SIS) and the gas imaging spectrometer (GIS). The effective exposure times in SIS0, SIS1, GIS2, and GIS3 were 72,024, 69,668, 80,910, and 80,896 s, respectively. SIS was operated in 1 CCD mode with the target in the standard 1 CCD mode position. GIS was operated in pulse height mode. The data were reduced and analyzed using FTOOLS and XSELECT in a standard manner (see ASCA Data Reduction Guide or Mathur et al. 1995a and G99 for details of data reduction). 2.2. Image Analysis
2.2.1. XSELECT Analysis

(see G99 for exact formulation of the detection and corresponding count rate estimate).
2.2.2. XIMAGE Analysis

We used XSELECT to create full and hard (2­9.5 keV) band images for each of the four detectors. We also created combined SIS and GIS images. We looked for the target in these images displayed with SAOIMAGE. While there were sources seen within the GIS field of view, there was no obvious source seen at the target position in any of the four detectors. We then smoothed the images with a Gaussian function of j = 1­ 2 pixels. A faint source at the position of the target was then evident in the GIS3 hard-band image and a trace of a source was seen in the full GIS image, but not in any other image. Note that for a standard pointing position, the target lies closest to the optical axis in SIS0 and GIS3. GIS3 is more sensitive in hard X-rays than SIS0. The fact that the source is seen by eye in the GIS3 detector only suggests that the source is faint with flux mainly in the hard band. We extracted the total counts in a circular region with a 3 radius centered on the source position. Because our source is obser ved to be so faint, background subtraction is crucial in determining the net source count rate, so we have done careful background subtraction using different background estimates. Background counts were extracted in two different ways: (1) from a source-free region on the detector and (2) from exactly the same region as the source in the blank sky background files provided by the ASCA guest obser ver facility. The significance of the source detection was therefore different for different background estimates. For SIS, the blank sky background is underestimated because it is available in the BRIGHT mode only, while the source counts were extracted in the BRIGHT2 mode. So the SIS detections are less reliable with background 2. We found that the source was detected in GIS3 and the GIS3 hard band and is marginally detected in SIS0 (2 j). It was not detected in any other detector in either bandpass. The significance of detection for the source in different detectors and the resulting net count rate is given in Table 1. For nondetections, we give a 3 j upper limit of the count rate

Determination of whether or not the source is detected is extremely important to our results. As an independent check, we performed image analysis with XIMAGE (Giommi, Angellini, & White 1997), which is designed for detailed image analysis. The DETECT algorithm in XIMAGE locates point sources in an image by means of a sliding-cell method. We used DETECT on all of our images and looked for a source at the position of the target. Again, we found the source to be detected in the GIS3 hard band. To minimize the number of spurious sources detected, the threshold used by DETECT is somewhat conser vative. As a result, sources with intensity just above the image background can be missed. We found that the source was detected in the full band GIS image if we lowered the detection threshold. The source was not detected in other detectors. These results are consistent with those from the XSELECT analysis discussed above.
2.2.3. CIAO Analysis

We applied more sophisticated wavelet-based techniques (Freeman et al. 2000) to provide independent support to the above detections. Software developed for Chandra Interactive Analysis of Obser vations (CIAO) allows us to decompose the image such that structures at different scales are enhanced. We analyzed the central 20 region of GIS3 images in both the full spectral range and in the harder range. Wavelet analysis of the GIS image at scales approximating the size of the point-spread function shows that detection of PG 0946 301 is complicated by the presence of a strong nearby source 5 away. In the GIS hard band image, this source is significantly weaker, and we detect PG 0946 301 at a probability of spurious detection of 10 4, with a net count rate of (1.26 0.25) # 10 3 counts s 1 (90% confidence). This is consistent with the results discussed above. 2.3. Column Density Constraints Consistency among the methods discussed above gives us confidence in our measurements and in our resulting detections in GIS3 and nondetections in other detectors. If the low obser ved X-ray count rate is due to intrinsic absorption, we can estimate the absorbing column density in PG 0946 301. Since the source did not yield enough net counts in any detector to perform spectral analysis, we use the method discussed in GM96 to determine the column density. We first calculate the flux expected from the source if there was no intrinsic absorption. This was done using the obser ved B-magnitude of the source (B = 16.0 mag) and assuming a ox = 1.6. The redshift of the source (z = 1.216) and the Galactic column density


No. 2, 2000
TABLE 2 Column Density Constraints Detector GIS3 ............ GIS3 hard ...... SIS0 ............ SIS0 hard ...... G 1.7 2.0 1.7 2.0 1.7 2.0 1.7 2.0 Detectiona 0.95 0.52 1.2 0.67 1.4 0.9 ) ) 3 j Lower Limitb 2.1 1.2 2.55 1.55 1.42 0.92 2.12 1.42

MATHUR ET AL.

L81

2 j Detection 3.3 1.95 3.2 1.95 1.95 1.3 2.73 1.86

strongly polarized (0.85 0.14%; Schmidt & Hines 1999). However, if true, it again implies the existence of X-ray­thick matter along the direct line of sight.
3. DISCUSSION

Note.--Values are given in units of 1024 atoms cm 2. a If 3 j or better detection (Table 1), either background. Note: the column densities derived here are lower than the 3 j lower limits because the two are from two different background measurements; e.g., for GIS3, detections are with background 1, while the 2 j detections and 3 j lower limits are with background 2. b The upper limit on count rate gives the lower limit on the column density. The 2 j detections and 3 j lower limits are with the same background.

(NH = 1.6 # 10 20 atoms cm 2; Murphy et al. 1996) were taken into account to predict the 2­10 keV flux in the obser ved band (=7.2 # 10 13 ergs s 1 cm 2). An X-ray power-law slope with photon index G = 1.7 was used. We then entered this model into the X-ray spectral analysis software XSPEC (Arnaud 1996), with normalization consistent with the expected flux and simulated spectra using SIS and GIS response matrices. The response of the telescope and detectors was taken into account as well. The column density at the redshift of the source was an additional parameter used in the simulation. If there was no intrinsic absorption, then the predicted count rate was found to be typically an order of magnitude larger than the obser ved one. We then varied the value of the intrinsic absorption, keeping the normalization constant, until the predicted and obser ved column densities matched. The values of intrinsic column density estimated in this way are given in Table 2. This estimate of NH depends upon G and aox. Given the obser ved range of aox (§ 1), our adopted value of a ox = 1.6 gives conser vative estimates of column densities. X-ray spectral slopes also var y among quasars. So we have estimated NH for G = 2.0 as well as G = 1.7. Flatter spectra result in even higher derived column densities. As shown in Table 2, even the conser vative estimate results in Thomson thick X-ray absorption in PG 0946 301, i.e., NH 10 24 cm 2. The column density estimates are consistent with the detection in GIS3 and nondetection in SIS0. Alternatively, is it possible that PG 0946 301 (and BALQSOs in general) is intrinsically X-ray weak? Earlier work (GM96; G99) could not rule out this possibility. To test this, we estimated the obser ved SIS0 hard band count rate for flux consistent with detection in GIS3 hard band, but no intrinsic absorption. We find that the source would have been detected in the SIS0 hard band at greater than 8 j significance (with G = 1.7; 17 j with G = 2.0). So we conclude that the obser ved X-ray weakness of BALQSOs is due to absorption and not due to intrinsic weakness. We cannot, however, rule out the possibility that the source is intrinsically X-ray weak with an unusual spectral shape (turning up at around 10 keV, rest frame). It is also possible that the obser ved flux is only the scattered component, from a line of sight different from the absorbing material. This is unlikely in PG 0946 301, which is not

We have clearly detected the quasar PG 0946 301 in our deep ASCA obser vation, and we infer that there is Thomson thick X-ray absorption (NH 10 24 cm 2) toward this BALQSO. The use of a detection, rather than upper limits, to determine the absorption is highly significant. In earlier work, GM96 and G99 had estimated absorbing column densities of a few times 1022 and 1023 cm 2, respectively. However, these were based on nondetections only and hence yielded only lower limits to the column density. A detection provides a much stronger estimate. Assuming that there is indeed Thomson thick matter covering the X-ray source, can we infer its ionization state? The X-ray absorber will cover the optical and UV continuum sources as well, at least partially. If the absorber is completely neutral, it will result in significant H i opacity, which is not obser ved (Arav et al. 1999). If the absorber is completely ionized, then the opacity due to Thomson scattering would be the same in the optical, UV, and X-rays (up to me c 2). Thus, this scenario by itself cannot account for the unusually large values of aox. If, on the other hand, the hydrogen is mostly ionized but there are still some hydrogen- and helium-like heavy elements, then photoelectric absorption would still be the dominant mechanism in X-rays. In the optical/UV, a Thomson opacity of 1 would result in attenuation by a factor of 2.7. Based on a statistical comparison of polarization properties of BALQSOs and unabsorbed, radio-quiet QSOs, Schmidt & Hines (1999; see also Goodrich 1997) inferred an attenuation factor of 2.4 0.3 for BALQSOs. This is consistent with the attenuation inferred here for PG 0946 301. The X-ray absorber thus must be at least partially ionized and may be responsible for attenuation in the optical and UV. While PG 0946 301 suffers significant attenuation along the line of sight, it is not highly polarized. This might be because the scattering medium is not present or well placed in PG 0946 301 to produce high polarization. The scattering region may be attenuated as much or more than the direct view. Alternatively, it might be a geometric effect in which a large part of the UV continuum is not covered by the X-ray absorber, so it is unattenuated. Whether the X-ray absorber has an ionization state overlapping the range of UV BALs and whether the outflow velocities are similar remain outstanding questions. It is possible that the X-ray absorber is stationar y, at the base of winds producing BALs. The X-ray continuum source might be preferentially covered. X-ray spectroscopy is necessar y to better probe the nuclear region in BALQSOs. For PG 0946 301, we predict about 0.015 counts s 1 with the XMM PN. A reasonable spectrum may be obtained in about 70 ks. We thank K. Arnaud and L. Angellini for help with XIMAGE. This work is supported in part by NASA grants NAG58360 (P. J. G., S. M.), NAG5-3249 (S. M.), and NAG5-3841 (I. S.). The work by W. v. B. at IGPP/LLNL was performed under the auspices of the US Department of Energy under contract W-7405-ENG-48.


L82

THOMSON THICK ABSORPTION

Vol. 533

REFERENCES Arav, N. 1997, in ASP Conf. Ser. 128, Mass Ejection from AGN, ed. N. Arav, I. Shlosman, & R. J. Weymann (San Francisco: ASP), 208 Arav, N., Korista, K. T., de Kool, M., Junkkarinen, V. T., & Begelman, M. C. 1999, ApJ, 516, 27 Arnaud, K. A. 1996, in ASP Conf. Ser. 101, Astronomical Data Analysis Software and Systems V, ed. G. Jacoby & J. Barnes (San Francisco: ASP), 17 Bregman, J. M. 1984, ApJ, 276, 423 Cohen, M. H., Ogle, P. M., Tran, H. D., Vermeulen, R. C., Miller, J. S., Goodrich, R. W., & Martel, A. R. 1995, ApJ, 448, L77 Freeman, P. E., Kashyap, V., Rosner, R., & Lamb, D. Q. 2000, ApJ, submitted Gallagher, S., Brandt, W. N., Sambruna, R., Mathur, S., & Yamasaki, N. 1999, ApJ, 519, 549 (G99) Giommi, P., Angelini, L., & White, N. 1997, The XIMAGE Users' Guide (Greenbelt: NASA/GSFC) Goodrich, R. 1997, ApJ, 474, 606 Goodrich, R., & Miller, J. S. 1995, ApJ, 448, L73 Green, P. J., et al. 1995, ApJ, 450, 51 Green, P. J., & Mathur, S. 1996, ApJ, 462, 637 (GM96) Hamann, F. 1998, ApJ, 500, 798 Korista, K. T., Voit, G. M., Morris, S. L., & Weymann, R. J. 1993, ApJS, 88, 357 Laor, A., Fiore, F., Elvis, M., Wilkes, B. J., & McDowell, J. C. 1997, ApJ, 477, 93 Mathur, S., Elvis, M., & Singh, K. P. 1995a, ApJ, 455, L9 Mathur, S., Elvis, M., & Wilkes, B. 1995b, ApJ, 452, 230 Murphy, E. M., Lockman, F. J., Laor, A., & Elvis, M. 1996, ApJS, 105, 369 Ogle, P., et al. 1999, ApJS, 125, 1 Schmidt, G., & Hines, D. C. 1999, ApJ, 512, 125 Singh, K. P., Westergaard, N. J., & Schnopper, H. W. 1987, A&A, 172, L11 Tanaka, Y., Holt, S. S., & Inoue, H. 1994, PASJ, 46, L37