The HELLAS2XMM survey

Free access article

Issue		A&A Volume 409, Number 1, October I 2003


Page(s)		65 - 78
Section		Galactic structure and dynamics
DOI		http://dx.doi.org/10.1051/0004-6361:20031107

A&A 409, 65-78 (2003)
DOI: 10.1051/0004-6361:20031107

III. Multiwavelength observations of hard X-ray selected sources in the PKS 0312-77 field

M. Brusa^{1, 2}, A. Comastri², M. Mignoli², F. Fiore³, P. Ciliegi², C. Vignali^{2, 4}, P. Severgnini⁵, F. Cocchia³, F. La Franca⁶, G. Matt⁶, G. C. Perola⁶, R. Maiolino⁷, A. Baldi⁸ and S. Molendi⁸

¹ Dipartimento di Astronomia Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
    e-mail: brusa@bo.astro.it
² INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
    e-mail: comastri,mignoli,ciliegi@bo.astro.it
³ INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio, Italy
    e-mail: fiore,cocchia@quasar.mporzio.astro.it
⁴ Department of Astronomy and Astrophysics, The Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA
    e-mail: chris@astro.psu.edu
⁵ INAF - Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
    e-mail: paola@brera.mi.astro.it
⁶ Dipartimento di Fisica Università di Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
    e-mail: lafranca,matt,perola@fis.uniroma3.it
⁷ INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125, Firenze, Italy
    e-mail: maiolino@arcetri.astro.it
⁸ IASF - CNR, Istituto di Fisica Cosmica, via Bassini 15, 20133, Milano, Italy
    e-mail: baldi,molendi@mi.iasf.cnr.it

(Received 6 March 2003 / Accepted 11 July 2003)

Abstract
We present extensive optical, radio and infrared follow-up observations of a sample of 35 hard X-ray (2-10 keV) selected sources discovered serendipitously in the PV XMM-Newton observation of the radio-loud quasar PKS 0312-77 field, for which also an archival Chandra observation is available. The observations have been carried out as part of the HELLAS2XMM survey, a program aimed to understand the nature of the sources responsible for the bulk of the hard X-ray Background (XRB). The identification of the optical counterparts greatly benefits from the positional accuracy obtained from Chandra and radio observations. As a consequence, the spectroscopic completeness of the present sample (80%) is limited only by the faintness of the optical counterparts. The multiwavelength coverage of our survey allows us to unveil a large spread in the overall properties of hard X-ray selected sources. At low redshift ( z<1), the source breakdown includes Broad Line AGN, Narrow Emission-Line Galaxies, and optically "normal" galaxies. All the ten sources at z>1 are spectroscopically classified as Broad Line AGNs. A few of them show significant intrinsic X-ray absorption ( $N_{\rm H}>10^{22}$ cm ^-2), further supporting previous evidence of a decoupling between optical and X-ray properties at high luminosities and redshifts. Finally, a non negligible fraction (~15%) of the hard X-ray sources are not detected down to the limiting magnitude of the optical images. The corresponding high X-ray to optical flux ratio, X-ray and optical-infrared colors strongly suggest that they are high redshift, obscured AGN.

Key words: surveys -- galaxies: active -- X-ray: galaxies -- X-rays: general -- X-rays: diffuse background

Offprint request: M. Brusa, brusa@bo.astro.it

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