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The Astrophysical Journal, 618:123 ­ 138, 2005 January 1
# 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A.

A

HARD X-RAY­ EMITTING ACTIVE GALACTIC NUCLEI SELECTED BY THE CHANDRA MULTIWAVELENGTH PROJECT
J. D. Silverman,1, 2 P. J. Green, W. A. Barkhouse, D.-W. Kim, T. L. Aldcrof t, R. A. Cameron, B. J. Wilkes, A. Mossman, H. Ghosh, and H. Tananbaum
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; jsilverman@cfa.harvard.edu, pgreen@cfa.harvard.edu

M. G. Smith and R. C. Smith
Cerro Tololo Inter-American Observatory, National Optical Astronomical Observatory, Casilla 603, La Serena, Chile

P. S. Smith
Steward Observatory, The University of Arizona, Tucson, AZ 85721

C. Foltz
National Science Foundation , 4201 Wilson Boulevard, Arlington, VA 22230

D. Wik
Astronomy Department , University of Virginia, P.O. Box 3818, Charlottesville, VA 22903-0818

and B. T. Jannuzi
National Optical Astronomical Observatory, P.O. Box 26732, Tucson, AZ 85726-6732 Receivv 2004 May 24; accepted 2004 Augg 29 ed ust

ABSTRACT We present X-ray and optical analysis of 188 active galactic nuclei (AGN ) identified from 497 hard X-ray ( f2:0 8:0keV > 2:7 ; 10þ15 ergs cmþ2 sþ1) sources in 20 Chandra fields (1.5 deg2) forming part of the Chandra Multiwavelength Project. These medium depth X-ray observations enable us to detect a representative subset of those sources responsible for the bulk of the 2 ­ 8 keV cosmic X-ray background. Brighter than our optical spectroscopiclimit,weachieveareasonabledegreeofcompleteness (77% of X-ray sources with counterparts r 0 < 22:5 have been classified): broad emission-line AGNs (62%), narrow emission-line galaxies (24%), absorption line galaxies (7%), stars (5%), or clusters (2%). We find that most X-ray unabsorbed AGNs (NH < 1022 cmþ2) have optical properties characterized by broad emission lines and blue colors, similar to optically selected quasars from the Sloan Digital Sky Survey but with a slightly broader color distribution. However, we also find a significant population of redder ( g 0 þ i 0 > 1:0) AGNs with broad optical emission lines. Most of the X-ray ­ absorbed AGNs (1022 cmþ2 < NH < 1024 cmþ2 ) are associated with narrow emission-line galaxies, with red optical colors characteristically dominated by luminous, early-type galaxy hosts rather than from dust reddening of an AGN. We also find a number of atypical AGNs; for instance, several luminous AGNs show both strong X-ray absorption (NH > 1022 cmþ2) and broad emission lines. Overall, we find that 81% of X-ray ­ selected AGNs can be easily interpreted in the context of current AGN unification models. Most of the deviations seem to be due to an optical contribution from the host galaxies of the low-luminosity AGNs. Subject headings: galaxies: active -- quasars: general -- surveys -- X-rays: galaxies g Online material: color figures, machine-readable table 1. INTRODUCTION In the era of Chandra and XMM-Newton, X-ray surveys of the extragalactic universe are for the first time able to probe the demographics and evolution of active galactic nuclei (AGNs) irrespective of any moderate obscuration. Current deep surveys such as the CDF-N ( Barger et al. 2002), CDF-S ( Tozzi et al. 2001), and the Lockman Hole ( Mainieri et al. 2002) are unveiling both bright quasars and lower luminosity Seyfert galaxies with significant absorbing gas columns. This obscuration can be large enough to effectively hide any optical signature
1 Astronomy Department, University of Virginia, P.O. Box 3818, Charlottesville, VA 22903-0818. 2 Visiting Astronomer, Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation.

of an active nucleus. With the unprecedented sensitivity and resolving power of these current observatories, we are able to probe large volumes to determine the prevalence of X-ray ­ emitting AGNs and their evolution. The study of AGNs enshrouded by dust and gas is not new. Obscured AGNs (e.g., narrow-line radio galaxies, Seyfert 2 galaxies, IRAS sources) have been under investigation for many years, though a complete census of the population has been out of reach. The spectrum of the cosmic X-ray background (CXRB) has provided evidence of the preponderance of the hidden AGN population. While ROSAT has shown that unabsorbed AGNs dominate the soft (0.1 ­ 2 keV ) CXRB ( Hasinger et al. 1998), its high-energy spectrum (2 ­ 30 keV ) is harder than that of known AGNs. Models based on the CXRB spectrum and the X-ray luminosity function have predicted the existence of large numbers of heavily obscured AGNs that have been missed in past surveys (Comastri et al. 1995; Gilli et al. 2001). 123


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How do these sources fit into the AGN unification scheme (e.g., Antonucci 1993; Antonucci & Miller 1985)? Many of the absorbed X-ray sources lack optical AGN signatures (e.g., Barger et al. 2003). Is this a result of host dilution ( Moran et al. 2002) or some other geometry/structure that prevents us from viewing the emission-line gas? While optical extinction and X-ray absorption are statistically correlated (Smith & Done 1996; Turner et al. 1997), there are a number of counterexamples. X-ray observations of Seyfert 2 galaxies do not always provide evidence for a large intrinsic obscuring column ( Panessa & Bassani 2002; Georgantopoulos & Zezas 2003). Equally compelling, a number of X-ray ­ selected type 1 AGNs ( Mainieri et al. 2002; Akylas et al. 2003) have significant intrinsic absorption in the X-ray band (1020 cmþ2 < NH < 1023 cmþ2 ). Wilkes et al. (2002) find that a large fraction of the IR-selected AGNs found in the Two-Micron All Sky Survey (2MASS) have broad optical emission lines and a wide range of X-ray absorption. Given the complex environment of some of these AGNs, one line of sight might not always exemplify the overall geometry. While the Chandra and XMM-Newton deep fields do cover a large volume, wide-field surveys are needed to compile a significant sample of sources with 2 ­ 8 keV flux levels around 10þ15 ­10þ14 ergs cmþ2 sþ1. Such sources comprise most of the flux of the 2 ­ 8 keV CXRB ( Moretti et al. 2003; Cowie et al. 2002). The deep fields provide relatively few sources at these flux levels to characterize the absorbed AGN population. Many ongoing surveys at intermediate flux levels are currently contributing to our understanding of the X-ray ­ emitting AGNs. For example, the HELLAS2XMM ( Fiore et al. 2003) and XMM-Newton SSC ( Barcons et al. 2003) take advantage of the large field of view and high collecting area of XMM-Newton. The SEXSI ( Harrison et al. 2003) survey joins the Chandra Multiwavelength Project in utilizing Chandra's small PSF and low background to detect the faint AGNs and unambiguously find optical counterparts. With large samples of all AGN types, we can characterize the dominant population contributing to the CXRB and determine the relative importance and nature of interesting AGNs that defy a simple unification model. 2. THE CHANDRA MULTIWAVELENGTH PROJECT (ChaMP) The Chandra Multiwavelength Project (ChaMP; Kim et al. 2004a, 2004b; Green et al. 2004) is providing a medium-depth, wide-area survey of serendipitous X-ray sources from archival Chandra fields covering $14 deg2. The broadband sensitivity between 0.3 ­ 8.0 keV enables the selection to be far less affected by absorption than previous optical, UV, or soft X-ray surveys. Chandra's small point spread function ($100 resolution on-axis) and low background allow sources to be detected to fainter flux levels, while the $100 X-ray astrometry greatly facilitates unambiguous optical identification of X-ray counterparts. The project effectively bridges the gap between flux limits achieved with the Chandra deep field observations and those of past ROSAT and ASCA surveys. A total of about 8000 serendipitous extragalactic X-ray sources are expected when the project is complete. A primary aim of the ChaMP is to measure the luminosity function of quasars and lower luminosity AGNs out to z $ 4 with the inclusion of the obscured population (J. D. Silverman et al. 2004, in preparation). We present results from the ChaMP using a subsample ( f2:0 8:0keV > 2:7 ; 10þ15 ergs cmþ2 sþ1 and r 0 < 22:5) of 497 X-ray sources detected in the hard band (2.5 ­ 8.0 keV ) in 20

fields. This work is an extension of the six fields analyzed by Green et al. (2004), here limited to the hard X-ray band. From this subsample, we classify 188 as AGNs based on their X-ray luminosity (L2 8keV > 1042 ergs sþ1). Our motivation is to determine the demographics of the hard X-ray ­ emitting AGNs, measure the range of intrinsic obscuration, and determine the extent to which obscuration of X-rays translates to extinction in the optical. After briefly discussing the X-ray and optical data acquisition, reduction and analysis (xx 3 and 4), we describe the characteristics of the hard X-ray sources (x 5) and the AGN properties (x 6) including selection and completeness. In x 7, we present the results. Throughout this paper, we assume H0 ¼ 70 km sþ1 Mpcþ1, ö ¼ 0:7, and M ¼ 0:3. 3. X-RAY OBSERVATIONS We have chosen 20 Chandra fields ( Table 1) for which we have acquired extensive follow-up optical imaging and spectroscopy. These fields have been selected from the first 2 years of Chandra archival data. Only ACIS observations at high Galactic latitude (jbj > 20 ) with no special observing modes (e.g., gratings) are used. The deepest observations have exposure times that are sensitive to sources with f2 8 keV > 2 ; 10þ15 ergs cmþ2 sþ1. At this flux limit, we resolve $70% of the 2 ­ 8 keV CXRB ( Moretti et al. 2003; Fig. 5). The target of each observation has been excluded to avoid any bias toward specific objects such as AGNs associated with clusters. A full description of the ChaMP image reduction and analysis pipeline XPIPE can be found in Kim et al. (2004a). In short, we have an automated reduction routine that filters out high background intervals, bad events such as cosmic rays and hot pixels to produce a clean and robust X-ray source catalog. Source extraction is performed using a wavelet detection algorithm (CIAO wavdetect; Freeman et al. 2002) in three energy bands [ broad ( B), 0.3 ­ 8.0 keV; soft (S), 0.3 ­ 2.5 keV; hard ( H ), 2.5­8.0 keV ]. For the following analysis, we require a S= N > 2 in the 2.5 ­ 8.0 keV band to generate a hard X-ray ­ selected sample that minimizes any inherent bias against the absorption of soft X-rays. We restrict the off-axis angle of the detections to less than 120 since the sensitivity beyond this is significantly reduced. We do not use chip S4 (ccd_id = 8) since this CCD is severely affected by a flaw in the serial readout, causing a significant amount of charge to be randomly deposited along pixel rows as they are read out. Each detection has a unique effective exposure time that includes vignetting. The conversion from X-ray count rate to flux units (ergs cmþ2 sþ1) is determined from simulated detections on each CCD of a source with a powerlaw spectrum ( fE / Eþ( þþ1) ;þ ¼ 1:73) and Galactic absorption ( Dickey & Lockman 1990). The effect of varying the photon index ( þ ) from 1.7 to 1.9 results in a $2% difference in flux for both ACIS-I and ACIS-S. We calculate the flux in the conventional 2.0 ­ 8.0 keV band for comparison with other surveys. With Chandra's broadband sensitivity (0.3 ­ 8.0 keV ), we are able to investigate the spectral properties of the sample, though we are limited by the small number of source counts in most cases (90% of sources have 9 < counts < 70 in the 2.0 ­ 8.0 keV band). The hardness ratio ( HR ¼ H þ S=H × S) can be used as a crude assessment of the spectral characteristics. Since the response of Chandra varies as a function of energy and off-axis angle with the additional complication of mixing frontside- and
The ChaMP XPIPE ( Kim et al. 2004a) provides energy conversion factors ( ECF ) for two models with þ ¼ 1:7 and þ ¼ 1:4. We chose the former since the photon index more closely resembles the majority of the hard source detections.
3


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X-RAY SURVEY OF AGNs WITH CHANDRA
TABLE 1 Chand ra Fields Exposurea ( ks) 61.0 34.6 47.0 40.9 41.5 48.7 43.1 26.9 30.0 75.6 114.6 40.9 29.8 50.3 23.6 42.3 29.1 21.9 106.1 65.0

125

Obs. ID 520............. 913............. 796............. 624............. 902............. 914............. 1602........... 377............. 2130........... 512............. 536............. 809............. 541............. 548............. 830............. 551............. 928............. 431............. 918............. 861.............
a b

PI Target MS 0015.9+1609 CL J0152.7þ1357 SBS 0335þ052 LP 944þ20 MS 0451.6þ0305 CL J0542þ4100 Q0615+820 B2 0738+313 3C 207 EMSS 1054.5þ0321 MS 1137.5+6625 Mrk 273X V1416+4446 RX J1716.9+6708 Jet of 3C 390.3 MS 2053.7þ0449 MS 2137þ2340 Einstein Cross CL J2302.8+0844 Q2345+007

ACIS CCDs 0123 012367 0123 2367 2367 0123 2367 367 2367 2367 0123 2367 0123 0123 2367 01236 2367 2367 0123 267

b

R.A. (J2000) 00 01 03 03 04 05 06 07 08 10 11 13 14 17 18 20 21 22 23 23 18 52 37 39 54 42 26 41 40 57 40 44 16 16 41 56 40 40 02 48 33.4 43.0 44.0 34.7 10.9 50.2 02.9 10.7 48.0 00 23.3 47.5 28.8 52.3 48.1 22.2 12.7 30.4 48.1 19.6

c

Decl. (J2000) +16 þ13 þ05 þ35 þ03 þ41 +82 +31 +13 þ03 +66 +55 +44 +67 +79 þ04 þ23 +03 +08 +00 26 57 02 25 01 00 02 12 12 37 08 54 46 08 47 37 39 21 44 57 34.8 30.0 39.0 50.0 07.2 06.9 25.5 00.4 23.0 00.0 42.0 10.0 40.8 31.2 43.0 44.4 27.0 31.0 00.0 21.1

c

UT Date 2000 Aug 18 2000 Sep 08 2000 Sep 07 1999 Dec 15 2000 Oct 08 2000 Jul 26 2001 Oct 18 2000 Oct 10 2000 Nov 04 2000 Apr 21 1999 Sep 30 2000 Apr 19 1999 Dec 02 2000 Feb 27 2000 Apr 17 2000 May 13 1999 Nov 18 2000 Sep 06 2000 Aug 05 2000 Jun 27

Galactic NHd (10 20 cmþ2) 4.06 1.61 4.98 1.44 5.18 3.59 5.27 4.18 4.14 3.67 1.18 1.09 1.24 3.71 4.16 4.96 3.57 5.34 5.50 3.81

Effective screened exposure time for the on-axis chip. The ACIS CCD chips used in the observation with the aim point chip underlined. CCD 8 has been excluded (see text). c Nominal target position, not including any Chandra pointing offsets. Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. d Galactic column density taken from Dickey & Lockman (1990).

backside-illuminated CCDs, we have converted the raw HR to an effective, on-axis, ACIS-I value by multiplying the count rate in each band by the ratio of the ECF (off-axis CCD) to the onaxis ECF, which ranges between 0.6 and 2.0. 3.1. X-Ray Spectral Fits X-ray spectral modeling provides a robust way of characterizing the spectral properties of our sample, independent of observation and instrument details. With a measured redshift, we can more accurately determine the intrinsic absorbing column than that based solely on hardness ratios. Some objects that look soft in HR may have significant absorption especially at higher redshifts. For each X-ray source in our hard-selected sample, we use an automated procedure to extract the spectrum and fit a model to the data. Owing to a lack of counts, we cannot usefully fit a spectral power-law model, leaving both spectral index and intrinsic absorbing column free for all objects. All processing is done using CIAO 3.0.24 and CALDB 2.26.5 The detailed steps to prepare the PHA ( pulse-height analysis) spectrum follow. First, we define a circular region centered on the X-ray source sized to contain 95% of 1.5 keV photons at the given off-axis angle. The background region is annular with a width of 2000 centered on the source. We exclude any nearby sources from both the source and background regions. We then use CIAO tool ``psextract'' to create a PHA spectrum covering the energy range 0.4 ­ 8 keV. We generate both an ungrouped spectrum and one that is grouped to a minimum of 10 counts per channel. The time-dependent quantum efficiency degradation of ACIS is accounted for when the ARF is generated by the ``mkarf '' tool.
4 5

Spectral fitting is done using the CIAO Sherpa6 tool. For all sources, we fit an absorbed power law containing an intrinsic absorber with neutral column NH at the source redshift. Our choice of photon index ( Frozen at þ ¼ 1:9) is based on previous studies of unabsorbed AGNs. Reeves & Turner (2000) have measured the spectral index for radio-quiet AGNs using ASCA observations to be þ $ 1:9. Piconcelli et al. (2003) have measured a mean photon index 1.8 ­ 1.9 with XMM-Newton that shows no variation over the redshift range 0 < z < 2. This NH fit provides a robust one-parameter characterization of the intrinsic spectral shape for as few as 10 counts. We verified by an extensive Monte Carlo simulation that the parameter uncertainties calculated with projection of confidence contours in Sherpa are reliable. Note that the spectral model contains a fixed Galactic neutral absorber appropriate for each object. Spectra with at least 60 counts are fitted using the grouped spectrum with the hybrid Monte Carlo Levenberg-Marquardt minimization method, while the low count spectra are fitted using the ungrouped data with Cash statistics and the Powell method. Spectra with over 200 counts are also fitted with a twoparameter absorbed power law leaving both þ and the intrinsic NH at thesourceredshiftfreetovary. Theresults from thetwoparameter fitting are included in this paper for a couple of sources discussed in x 7.2. The full analysis will be presented in an upcoming ChaMP paper ( T. L. Aldcroft et al. 2004, in preparation). 4. OPTICAL FOLLOW-UP 4.1. Imagg gg in We have acquired optical imaging for each Chandra field to identify counterparts to X-ray sources. We use the NOAO
6

See http://cxc.harvard.edu /ciao. See http://cxc.harvard.edu /caldb.

See http://cxc.harvard.edu /sherpa.


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TABLE 2 KPNO 4 m Opti cal Imaging Exposure ( Total s) 1800 1500 1500 1000 500 360 4500 2400 2000 1950 1200 900 1800 1200 1200 2100 1200 600 2700 2400 1200 2100 1500 1500 1800 1000 600 1800 1800 1200 3500 3000 2000 4500 3000 1500 520 1800 1800 3000 2700 2500 Air Mass ( Mean) 1.00 1.01 1.04 1.36 1.28 1.25 1.29 1.23 1.29 1.24 1.34 1.46 1.28 1.24 1.22 1.28 1.25 1.24 1.33 1.52 1.28 1.19 1.14 1.10 1.49 1.49 1.49 1.24 1.22 1.24 1.46 1.44 1.52 1.20 1.09 1.12 1.56 1.56 1.56 1.16 1.08 1.06 FWHM a (arcsec) 1.1 1.0 1.1 1.6 1.6 1.2 1.3 1.1 1.3 1.2 1.2 1.4 1.2 1.3 1.7 1.7 1.8 1.4 1.1 1.2 1.1 1.8 1.7 1.8 1.6 1.6 1.6 1.0 1.1 1.0 1.1 1.0 1.1 1.2 1.3 1.2 2.4 2.1 2.5 1.4 1.3 0.9 mTOb Limit 25.4 24.9 24.1 24.1 23.4 22.9 24.9 24.4 23.6 24.1 23.9 22.6 24.8 24.1 23.4 24.4 23.9 22.9 25.1 24.6 23.6 23.1 23.4 23.1 24.1 23.4 22.9 24.9 24.4 23.6 25.1 24.6 23.6 24.9 24.1 23.1 23.3 23.1 22.6 25.1 24.9 24.6

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Obs. ID 377..................

UT Date 2001 Feb 21

Filter g r0 i0 g0 r0 i0 g0 r0 i0 g0 r0 i0 g0 r0 i0 g0 r0 i0 g0 r0 i0 g0 r0 i0 g0 r0 i0 g0 r0 i0 g0 r0 i0 g0 r0 i0 g0 r0 i0 g0 r0 i0
0

Dithers 3 3 3 2 1 1 5 3 5 3 3 3 2 2 2 3 4 2 3 3 3 3 3 3 3 2 1 3 3 3 5 5 5 5 5 5 1 3 3 3 3 5

m5 c Limit 26.5 26.0 25.2 25.4 24.7 24.1 26.3 25.8 24.8 25.2 24.9 24.0 26.2 25.4 24.6 25.6 25.0 24.5 26.4 25.7 24.8 24.4 24.6 24.4 25.6 24.8 24.2 26.2 25.6 24.9 26.4 25.9 24.8 26.0 25.6 24.8 24.8 24.8 24.7 26.5 25.9 25.8

431..................

2000 Jun 11

512..................

2001 Feb 21 ­ 22

520..................

2001 Oct 25

548..................

2000 Jun 10

551..................

2000 Jun 10, 12

796..................

2001 Oct 24

809..................

2000 Jun 11

830..................

2000 Jun 11

902..................

2001 Oct 23

913..................

2001 Oct 23

918..................

2001 Oct 23

1602................

2001 Dec 14

2130................

2001 Feb 22

Note.--We tabulated 14 additional ChaMP fields following Green et al. (2004). a FWHM of point sources in final stacked images. b Turnover magnitude limit at $90% completeness, using 0.25 mag bins before extinction correction, as described in the text. c Magnitude limit for a $5 detection.

Blanco and Mayall 4 m telescopes and their MOSAIC cameras to image the full Chandra field of view of our survey fields. The exposure times are scaled from the minimum X-ray flux for a detection per Chandra field to identify more than 90% of ROSAT AGNs ( Yuan et al. 1998). Three filters ( g 0 , r 0 , i 0 ) using the Sloan Digital Sky Survey (SDSS) photometric system ( Fukugita et al. 1996) are implemented to measure broadband colors for preliminary source classification. Table 2 provides some details of the imaging for the 14 fields not included in Green et al. (2004). A full description of the optical follow-up program including strategy, image reduction, source detection, and photometric calibration can be found in (Green et al. 2004). Briefly, optical image reduction on the MOSAIC data is performed with the

``mscred'' ( Valdes 2002) package within the IRAF7 environment. We use the SExtractor ( Bertin & Arnouts 1996) algorithm to detect sources, and measure their positions and brightness. Since the X-ray source positions are only accurate to within $100 , the optical astrometric solution is required to achieve the rms < 0B3 accuracy necessary for spectroscopic follow-up. We require an accuracy of the photometric solution to less than a tenth of a magnitude. The majority of the AGN sample (88%) presented in this paper has magnitude errors of less than 0.05 as

IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.

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127

Fig. 1.--X-ray flux (2 ­ 8 keV ) vs. optical magnitude (r 0 ). Optical spectroscopic classifications are indicated (top right box) with the sample size in parenthesis. X-ray sources with no optical counterparts are shown by an arrow placed at the hypothetical magnitude for a 5 detection from our optical imaging ( Table 2). The dashed vertical and horizontal lines mark the X-ray flux limit and optical magnitude limit for the subsequent analysis. The slanted lines mark the fx =fr ratios of 0.1, 1, and 10.

a result of the bright optical magnitude selection. In addition, we find a mean color offset of 0.05 mag to the red between our g 0 þ i 0 color and the SDSS using sources detected in both surveys. 4.2. X-Ray-to-Optical Source Matchingg We implement an automated routine to match each X-ray source with potential optical counterparts (see Green et al. 2004). The search radius is increased for X-ray detections at large off-axis angles. Each match is visually inspected and a confidence level is determined. For X-ray sources with multiple optical counterparts, the optical source closest to the X-ray centroid is usually given a higher confidence. We have found 415 optical counterparts to 497 X-ray sources (84%; Fig. 1). We have not included 36 sources because the X-ray detection fell on a chip edge or there were multiple optical counterparts for which no single optical source could confidently be assigned. We note that 81% of the matches have an X-ray to optical offset of less than 200 . 4.3. Spectroscopy Optical spectroscopy is crucial for determining the source type and redshift. We acquired the majority of our optical spectra with the WIYN 3.5 m and CTIO 4 m with the HYDRA multifiber spectrographs, which have a field of view (>400 ) that fully covers the Chandra field. To extend spectroscopic classifications beyond r 0 $ 21, the limit of the 4 m class telescopes with HYDRA, we have obtained spectra from Magellan and the MMT. The field of view of Magellan with LDSS-2, a multislit

spectrograph, is 50 , so it takes 5 ­ 6 pointings to cover the full Chandra field. We have been using the FLWO 1.5 m to acquire spectra of the optically bright (r 0 < 17) counterparts. In addition, a number of people, mentioned in the acknowledgements, have graciously acquired long-slit spectra of a few ChaMP sources during their own observing time. All redshifts have an accuracy of à z < 0:001. Table 3 gives a summary of the spectroscopic facilities used by the ChaMP project. We implement a classification scheme of optical spectra similar to the Einstein Observatory Extended Medium-Sensitivity Survey (Stocke et al. 1991). Objects with strong emission lines (Wk > 5 8) are classified as either broad-line AGNs ( BLAGNs; FWHM > 1000 km sþ1) or narrow emission-line galaxies (NELGs; FWHM < 1000 km sþ1). Counterparts with weak emission line or pure absorption line spectra are classified as absorption line galaxies (ALGs). We note that there is a combination of redshift range and spectral bandpass for which we may lose important AGN optical diagnostic features. In some cases, the host galaxy contribution can prevent H from being a useful AGN indicator. Given the low signal-to-noise of our spectra, some NELGs may have weak, broad emission lines. Ho et al. (1997) found that broad H can often be found in lowluminosity ``dwarf '' Seyferts with high S/ N spectra and proper subtraction of the stellar continuum. This type of analysis is not possible given the quality of our spectra. Any stellar source is labeled as a STAR. For the ALGs, we measure the Ca ii break ``CONTRAST'' (Stocke et al. 1991) to look for a power-law AGN component to the continuum to note potential BL Lac candidates. If the associated X-ray emission is extended, the object is further labeled as a possible cluster member.


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TABLE 3 Opti cal Spe ct roscopic Fol low-up Spectral Resolution (8) 7.8 4.6 13.5 13.0 8.8 13 5.9

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Telescope WIYNa ................. CTIO 4 m ............ Magellan............... Magellan............... MMT .................... Keck I .................. FLWO 1.5 m ........

Instrument HYDRA HYDRA LDSS-2 B&C Blue Channnel LRIS FAST

Grating /Grism 316 at 7.0 KPGL3 Med. red and blue 300 lines mmþ1 300 lines mmþ1 300/5000 300 lines mmþ1

k Range (8) 4500 4600 3600 3700 3500 4000 3600 ­ ­ ­ ­ ­ ­ ­ 9000 7400 8500b 8700 8300 9000 7500

R (k /àk) 950 1300 520 384 800 484 850

Number of Spectra 91 48 41 7 15 5 2

a The WIYN Observatory is a joint facility of the University of Wisconsin Madison, Indiana University, Yale University, and the National Optical Astronomy Observatory. b Spectral coverage can vary as a function of slit position in the mask.

As shown in Figure 1, we have classified 44% (220) of all the hard X-ray sources through our spectroscopic campaign. The sources without redshifts are primarily at faint optical magnitudes (r 0 > 22). In Table 5, we list the numbers of each type for various limits imposed on the sample. 5. CHARACTERISTICS OF THE HARD X-RAY SAMPLE 5.1. X-Ray and Optical Flux We show the optical magnitude (r 0 ) as a function of X-ray flux (2.0 ­ 8.0 keV ) for the 497 sources detected in 20 fields (Fig. 1). Lines of constant fx =fr 0 are determined as follows: log ( fx =fr 0 ) ¼ log ( fx ) × 0:4r 0 × 5:41: Ï 1÷

This relation has been derived using an assumed power law ( fE / E þ ) with spectral index o ¼ 0:5 and x ¼ 0:7. The characteristics of the r 0 filter are taken from Fukugita et al. (1996). Most objects have 0:1 < fx =fr 0 < 10. There exists a significant number of X-ray ­ bright, optically faint sources ( fx =fr 0 > 10), many of which are not detected in our optical imaging. Owing to their optical faintness, we have only identified one such source, an NELG. Based on the 210 spectroscopically identified objects with r 0 < 22:5, we find that 62% of the hard X-ray sources are classified as BLAGN. As shown in Green et al. (2004), these AGNs tend to follow the relation fx ¼ fr 0 over a wide range of optical and X-ray flux. The NELGs, which comprise 24% of the identifications, have flux ratios similar to the BLAGNs. We find a number of counterparts (7% ALGs) that have no evidence for an emitting line region. These galaxies are primarily identified at bright optical magnitudes (r 0 < 21) because of the difficulty of classifying sources without strong emission lines at high redshift (z > 0:5). In addition, a few hard X-ray ­ detected sources are associated with optically bright stars (5%) and galaxy clusters (2%) with extended X-ray emission. 5.2. X-Ray Spectral Properties The X-ray hardness ratio provides a crude measure of the spectral properties and classification of the hard X-ray sources. In Figure 2, we plot the corrected hardness ratio (x 3) as a function of X-ray flux. The flux range shown includes all sources with the exception of the extremely bright cataclysmic variable TX Col (Schlegel & Salinas 2004). The horizontal lines mark the hardness ratio, which corresponds to an X-ray source with a power-law continuum ( photon index þ ¼ 1:9)

absorbed by a column of gas at z ¼ 0. Some of the derived NH detections may be higher (see x 3.1) using an absorber intrinsic to the source. According to Moretti et al. (2003), we are resolving $70% of the full 2 ­ 8 keV CXRB at our chosen flux limit. With a flux-weighted mean HR for the ChaMP sources of þ0.39, the X-ray spectrum of the ensemble is similar to the spectral characteristic of the integrated CXRB ( þ ¼ 1:4), which corresponds to a hardness ratio of þ0.42. As described in many studies with Chandra (e.g., Mushotzky et al. 2000) and XMM-Newton (e.g., Hasinger et al. 2001), the X-ray source population becomes relatively harder at fainter flux levels. As evident in Figure 2, the hardest (HR > 0) X-ray sources in these ChaMP fields are predominately at log10 ( fx ) < þ13:6. At fainter flux levels, the sources have a more even distribution over all hardness ratios. This spectral variation has been attributed to intrinsic absorption rather than changes in the intrinsic spectral energy distribution ( Mainieri et al. 2002; Kim et al. 2004b). This will be further investigated in an upcoming ChaMP paper on the X-ray spectral properties of the AGN sample. 6. HARD AGN SAMPLE 6.1. Selectio