THE CHANDRA LARGE AREA SYNOPTIC X-RAY SURVEY (CLASXS) OF THE LOCKMAN HOLE-NORTHWEST: THE X-RAY CATALOG

We present the X-ray catalog and basic results from our Chandra Large Area Synoptic X-ray Survey (CLASXS) of the Lockman Hole-Northwest field. Our nine ACIS-I fields cover a contiguous solid angle of ~0.4 deg2 and reach fluxes of 5 ? 10-16 ergs cm-2 s-1 (0.4?2 keV) and 3 ? 10-15 ergs cm-2 s-1 (2?8 keV). Our survey bridges the gap between ultradeep pencil-beam surveys, such as the Chandra Deep Fields (CDFs), and shallower, large-area surveys, allowing a better probe of the X-ray sources that contribute most of the 2?10 keV cosmic X-ray background (CXB). We find a total of 525 X-ray point sources and four extended sources. At ~10-14 ergs cm-2 s-1 (2?8 keV), our number counts are significantly higher than those of several noncontiguous, large-area surveys. Such a large difference is an indication of clustering in the X-ray sources. On the other hand, the integrated flux from the CLASXS field, combined with ASCA and Chandra ultradeep surveys, is consistent with results from other large-area surveys, within the variance of the CXB. We see spectral evolution in the hardening of the sources at fluxes below 10-14 ergs cm-2 s-1, which agrees with previous observations from Chandra and XMM-Newton. About one third of the sources in the CLASXS field have multiple observations, which allow variability tests. Above 4 ? 10-14 ergs cm-2 s-1 (0.4?8 keV), ~60% of the sources are variable. We also investigated the spectral variability of the variable sources. While most show spectral softening with increasing flux, or no significant spectral change, there are a few sources that show a different trend. We observed four extended sources in CLASXS, which is consistent with the previously measured log N?log S relation for galaxy clusters. Using X-ray spectra and optical colors, we argue that three of the four extended sources are galaxy clusters or galaxy groups. We report the discovery of a gravitational lensing arc associated with one of these sources. Using red-sequence and brightest cluster galaxy methods, we find that the redshifts of the extended sources are in the range z ~ 0.5?1. The inferred masses within the Einstein radii are consistent with the mass profiles of local groups scaled to the same virial radii.

[1]  H. Liang.  AMiBA 2001: High-z Clusters, Missing Baryons, and CMB polarization , 2002 .

[2]  S. Borgani,et al.  New Results from the X-Ray and Optical Survey of the Chandra Deep Field-South: The 300 Kilosecond Exposure. II. , 2001, astro-ph/0103014.

[3]  L. Bassani,et al.  Exploring the spectral properties of faint hard X-ray sources with XMM Newton , 2002 .

[4]  G. Di Cocco,et al.  An XMM-Newton study of the hard X-ray sky , 2003, astro-ph/0309606.

[5]  D. M. Alexander,et al.  The Chandra Deep Field North Survey. VI. The Nature of the Optically Faint X-Ray Source Population , 2001 .

[6]  G. Hasinger.  The X-ray background and AGNs , 2003, astro-ph/0310804.

[7]  R. F. Mushotzky,et al.  Resolving the extragalactic hard X-ray background , 2000, Nature.

[8]  P. Capak,et al.  An Optical Catalog of the Chandra Large Area Synoptic X-Ray Survey Sources , 2004, astro-ph/0409088.

[9]  Kirpal Nandra,et al.  ASCA Observations of Seyfert 1 Galaxies. I. Data Analysis, Imaging, and Timing , 1997 .

[10]  H. Matsuhara,et al.  ISO deep far-infrared survey in the “Lockman Hole” - III. Catalogs and source counts at 90 & 170 μm , 2004 .

[11]  M. Gladders,et al.  A New Method For Galaxy Cluster Detection. I. The Algorithm , 2000, astro-ph/0004092.

[12]  N. Gehrels.  Confidence limits for small numbers of events in astrophysical data , 1986 .

[13]  Allan Sandage,et al.  Redshifts and magnitudes of extragalactic nebulae. , 1956 .

[14]  P. Barr.  EXOSAT observations of the Seyfert galaxy NGC 7469; rapid hard X-ray variability and a soft X-ray excess , 1986 .

[15]  D. B. Sanders,et al.  Imaging Large-Scale Structure in the X-Ray Sky , 2003 .

[16]  S. Ettori,et al.  Constraining the cosmological parameters with the gas mass fraction in local and $\mathsf{{\vec z}>0.7}$ galaxy clusters , 2003 .

[17]  Toru Yamada,et al.  Optical Identification of the ASCA Medium Sensitivity Survey in the Northern Sky: Nature of Hard X-Ray-Selected Luminous Active Galactic Nuclei , 2003, astro-ph/0307164.

[18]  S.Campana,et al.  The Resolved Fraction of the Cosmic X-Ray Background , 2003, astro-ph/0301555.

[19]  Fiona A. Harrison,et al.  The Serendipitous Extragalactic X-Ray Source Identification Program. I. Characteristics of the Hard X-Ray Sample , 2003 .

[20]  T. Lauer,et al.  Brightest cluster galaxies as standard candles , 1995 .

[21]  The Chandra Deep Field North Survey. IX. Extended X-Ray Sources* , 2001, astro-ph/0112002.

[22]  F. R. Harnden,et al.  Chandra Multiwavelength Project. I. First X-Ray Source Catalog , 2003, astro-ph/0308492.

[23]  W. Forman,et al.  ROSAT Extended Medium-Deep Sensitivity Survey: Source Counts for 130 Fields , 1995 .

[24]  Roberto Gilmozzi,et al.  Chandra Deep Field South: The 1 Ms Catalog , 2002 .

[25]  The nature of dark matter in elliptical galaxies: Chandra observations of NGC 4636 , 2002, astro-ph/0205359.

[26]  W. N. Brandt,et al.  The 2-8 keV X-Ray Number Counts Determined from Chandra Blank Field Observations , 2002 .

[27]  J. Dickey,et al.  H I in the Galaxy , 1990 .

[28]  A. Finoguenov,et al.  Details of the mass-temperature relation for clusters of galaxies , 2001 .

[29]  W. Cash,et al.  Parameter estimation in astronomy through application of the likelihood ratio. [satellite data analysis techniques , 1979 .

[30]  D. M. Alexander,et al.  Optical and Infrared Properties of the 2 Ms Chandra Deep Field North X-Ray Sources , 2003, astro-ph/0306212.

[31]  An Upper Limit on Omega_matter Using Lensed Arcs , 1999, astro-ph/9904245.

[32]  R. Wijers,et al.  BeppoSAX : proceedings of the BeppoSAX Conference "the Restless High-Energy Universe" Amsterdam, The Netherlands, 5-8 May , 2004 .

[33]  N. Yamasaki,et al.  Study of the X-Ray Background Spectrum and its Large-Scale Fluctuation with ASCA , 2002, astro-ph/0204224.

[34]  D. M. Alexander,et al.  The Chandra Deep Field North Survey. XIII. 2 Ms Point-Source Catalogs , 2003, astro-ph/0304392.

[35]  L. Kewley,et al.  The Chandra Deep Field-South: The 1 Million Second Exposure , 2001, astro-ph/0110452.

[36]  D. M. Alexander,et al.  The Chandra Deep Field North Survey. V. 1 Ms Source Catalogs , 2001, astro-ph/0108404.

[37]  R. Mushotzky,et al.  THE NATURE OF THE HARD X-RAY BACKGROUND SOURCES: OPTICAL, NEAR-INFRARED, SUBMILLIMETER, AND RADIO PROPERTIES , 2000, astro-ph/0007175.

[38]  Astrophysics,et al.  The HELLAS2XMM Survey. I. The X-Ray Data and the log N-log S Relation , 2001, astro-ph/0108514.

[39]  Robert Rosner,et al.  A Wavelet-Based Algorithm for the Spatial Analysis of Poisson Data , 2001 .

[40]  The spectrum of the cosmic X-ray background observed by RTXE/PCA , 2003, astro-ph/0306569.