The XMM-Newton HBS28 sample: Studying the obscuration in hard X-ray selected AGNs ,

This paper presents the analysis of a statistically complete sample of 28 serendipitous X-ray sources selected in 82 pointed XMM-Newton fields down to a count-rate of 0.002 counts s(-1) (4.5-7.5 keV energy band). This is the first sample selected in this energy range to have complete spectroscopic identifications and redshift determinations for all the objects. Apart from one Galactic source (an interacting binary), all the objects are AGNs. Their optical and X-ray properties (derived from the spectral analysis of the XMM-EPIC data) are compared together. The good correlation between the optical spectral type and the X-ray absorption properties supports the AGN unified model. Only one object that does not fit the relation between optical and X-ray absorption is found, namely a Seyfert 1.9 with no evidence of obscuration in the X-ray band (N-H 10(22) cm(-2)), corresponding to a surface density of 0.7 deg(-2) at the flux limit the sample (4-7 x 10(-14) erg s(-1) cm(-2) in the 4.5-7.5 keV energy band). Among these obscured objects, two sources show a large (intrinsic) luminosity (L[2-10 keV] > 10(44) erg s(-1)) and are thus classified as type 2 QSO. Finally, we have compared the fraction of X-ray absorbed AGNs (26%) with that predicted by the current XRB synthesis models at the flux limit of the survey. We find that the models significantly (similar to90% confidence level) over predict the fraction of absorbed AGNs thus confirming also in this hard energy band (4.5-7.5 keV) similar results recently obtained in the 2-10 keV band.

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

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

[3]  R. Della Ceca,et al.  Coevolution of Black Holes and Galaxies , 2004 .

[4]  Donald E. Osterbrock,et al.  Spectral Classification of Emission-Line Galaxies , 1987 .

[5]  G. Hasinger,et al.  Testing current synthesis models of the X-ray background , 2000, astro-ph/0011341.

[6]  Tommaso Maccacaro,et al.  X-ray observations with the Einstein observatory of emission-line galaxies , 1982 .

[7]  Roberto Gilmozzi,et al.  The Chandra Deep Field-South: The 1 Million Second Exposure* , 2002 .

[8]  P. Serlemitsos,et al.  The Nature of the Low-Luminosity Active Galactic Nuclei in NGC 3147 , 1996 .

[9]  M. Mignoli,et al.  The HELLAS2XMM Survey. II. Multiwavelength Observations of P3: An X-Ray-bright, Optically Inactive Galaxy* , 2002, astro-ph/0202080.

[10]  Robert Antonucci,et al.  Unified models for active galactic nuclei and quasars , 1993 .

[11]  X. Barcons,et al.  H1320+551: a type 1.8/1.9 Seyfert galaxy with an unabsorbed X-ray spectrum , 2003 .

[12]  T. J. Turner,et al.  ASCA Observations of Type 2 Seyfert Galaxies. II. The Importance of X-Ray Scattering and Reflection , 1997, astro-ph/9705087.

[13]  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.

[14]  I. Georgantopoulos,et al.  ASCA observations of deep ROSAT fields – V. The X‐ray spectrum of hard X‐ray selected QSOs , 2001 .

[15]  E. Greisen,et al.  The NRAO VLA Sky Survey , 1996 .

[16]  European Southern Observatory,et al.  XMM-Newton observation of the Lockman Hole - II. Spectral analysis , 2002, astro-ph/0207166.

[17]  A. Marconi,et al.  Dust in active nuclei. I. evidence for anomalous properties , 2000, astro-ph/0010009.

[18]  The BeppoSAX High-Energy Large-Area Survey. V. The Nature of the Hard X-Ray Source Population and Its Evolution , 2001, astro-ph/0112455.

[19]  T. Steiman-Cameron,et al.  X-Ray and Optical Observations of BL Hydri , 1999 .

[20]  W. N. Brandt,et al.  X-Ray, Optical, and Infrared Imaging and Spectral Properties of the 1 Ms Chandra Deep Field North Sources , 2002 .

[21]  R. F. Mushotzky,et al.  An X-Ray Spectral Survey of Radio-loud Active Galactic Nuclei with ASCA , 1999, astro-ph/9905365.

[22]  Frederic H. Chaffee,et al.  The Large Bright Quasar Survey.VI.Quasar Catalog and Survey Parameters , 1995 .

[23]  G. C. Perola,et al.  Spectroscopic identification of ten faint hard X-ray sources discovered by Chandra , 2000 .

[24]  H. Tananbaum,et al.  The Unusual Galaxy J2310–43: An Active Nucleus Without Optical Emission Lines and Without a Substantial Optical Continuum , 1997 .

[25]  R. Maiolino,et al.  The Distribution of Absorbing Column Densities among Seyfert 2 Galaxies , 1999, astro-ph/9902377.

[26]  M. Birkinshaw,et al.  New Visions of the X-ray Universe in the XMM-Newton and Chandra Era. Noordwijk, Nov 2001 , 2006 .

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

[28]  Kirpal Nandra,et al.  ASCA Observations of Type 2 Seyfert Galaxies. I. Data Analysis Results , 1997 .

[29]  R. Della Ceca,et al.  A Three-dimensional Diagnostic Diagram for Seyfert 2 Galaxies: Probing X-Ray Absorption and Compton Thickness , 1998, astro-ph/9811074.

[30]  P. Giommi,et al.  The contribution of faint active galactic nuclei to the hard X-ray background , 1999 .

[31]  P. Ciliegi,et al.  The HELLAS2XMM survey IV. Optical identifications and the evolution of the accretion luminosity in the Universe , 2003, astro-ph/0306556.

[32]  P. Veron,et al.  A spectrophotometric atlas of narrow-line seyfert 1 galaxies , 2001 .

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

[34]  Cesare Barbieri,et al.  The SA 94 QSO surveys. V - an UVx sample of 97 QSOs with B not greater than 19.9 , 1992 .

[35]  M. J. Page,et al.  The XMM-Newton serendipitous survey: II. First results from the AXIS high galactic latitude medium sensitivity survey , 2002 .

[36]  A. Caccianiga,et al.  New Results from the REX Survey , 2002 .

[37]  A. Franceschini,et al.  Origin of the X-ray background and AGN unification: new perspectives , 2002, astro-ph/0205529.

[38]  A. C. Fabian,et al.  The unified Seyfert scheme and the origin of the cosmic X-ray background , 1994 .

[39]  J. Melnick,et al.  Observational Evidence of Activity in Galaxies , 1987 .

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

[41]  Isabella M. Gioia,et al.  The Einstein Observatory Extended Medium-Sensitivity Survey. II - The optical identifications , 1991 .

[42]  R. Edelson,et al.  X-ray spectral complexity in narrow-line Seyfert 1 galaxies , 1999 .

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

[44]  G. Matt,et al.  X-ray absorption in Seyfert 2 galaxies , 2001 .

[45]  Loredana Bassani,et al.  Unabsorbed Seyfert 2 galaxies , 2002 .

[46]  Takamitsu Miyaji,et al.  Cosmological Evolution of the Hard X-Ray Active Galactic Nucleus Luminosity Function and the Origin of the Hard X-Ray Background , 2003, astro-ph/0308140.

[47]  G. C. Perola,et al.  The BeppoSAX High Energy Large Area Survey (HELLAS) – III. Testing synthesis models for the X‐ray background , 2001 .

[48]  F. Fiore,et al.  The BeppoSAX High Energy Large Area Survey (HELLAS) , 1998 .

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

[50]  Martin Elvis,et al.  Two optically dull galaxies with strong nuclear X-ray sources , 1981 .

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

[52]  G. Palumbo,et al.  Unveiling the AGN powering the "Composite" Seyfert/Star-forming galaxy NGC 7679: BeppoSAX and ASCA results , 2001, astro-ph/0106444.

[53]  R. Giacconi,et al.  The ROSAT Deep Survey - VI. X-ray sources and Optical identifications of the Ultra Deep Survey , 2001 .

[54]  D. Malin,et al.  A narrrow-line radio galaxy behind the Fornax cluster , 1983 .

[55]  P. Véron,et al.  A catalogue of quasars and active nuclei: 10th edition , 2001 .

[56]  Richard E. Griffiths,et al.  A ROSAT deep survey – VI. Active and passive X-ray galaxies , 1995 .