The Palermo Swift-BAT hard X-ray catalogue

III. Results after 54 months of sky survey^⋆

¹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo, via U. La Malfa 153, 90146 Palermo, Italy
e-mail: cusumano@ifc.inaf.it
² Institut für Astronomie und Astrophysik Tübingen (IAAT), 72076 Tübingen, Germany
³ ISDC Data Centre for Astrophysics, Chemin d’Écogia 16, 1290 Versoix, Switzerland
⁴ Università degli studi di Milano-Bicocca, Dipartimento di Fisica, Piazza delle Scienze 3, 20126 Milan, Italy
⁵ INAF – Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate, Italy
⁶ ASI Science Data Center, via Galileo Galilei, 00044 Frascati, Italy
⁷ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, 40129 Bologna, Italy
⁸ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy

Received: 21 June 2010
Accepted: 26 August 2010

Abstract

Aims. We present the Second Palermo Swift-BAT hard X-ray catalogue obtained by analysing data acquired in the first 54 months of the Swift mission.

Methods. Using our software dedicated to the analysis of data from coded mask telescopes, we analysed the BAT survey data in three energy bands (15–30 keV, 15–70 keV, 15–150 keV), obtaining a list of 1256 detections above a significance threshold of 4.8 standard deviations. The identification of the source counterparts is pursued using two strategies: the analysis of field observations of soft X-ray instruments and cross-correlation of our catalogue with source databases.

Results. The survey covers 50% of the sky to a 15–150 keV flux limit of 1.0 × 10^-11 erg cm^-2 s^-1 and 9.2 × 10^-12 erg cm^-2 s^-1 for  |b|  < 10° and  |b|  > 10°, respectively. The Second Palermo Swift-BAT hard X-ray catalogue includes 1079 (~86%) hard X-ray sources with an associated counterpart (26 with a double association and 2 with a triple association) and 177 BAT excesses (~14%) that still lack a counterpart. The distribution of the BAT sources among the different object classes consists of  ~19% Galactic sources,  ~57% extragalactic sources, and  ~10% sources with a counterpart at softer energies whose nature has not yet been determined. About half of the BAT associated sources lack a counterpart in the ROSAT catalogues. This suggests that either moderate or strong absorption may be preventing their detection in the ROSAT energy band. The comparison of our BAT catalogue with the Fermi Large Area Telescope First Source Catalogue identifies 59 BAT/Fermi correspondences: 48 blazars, 3 Seyfert galaxies, 1 interacting galaxy, 3 high mass X-ray binaries, and 4 pulsars/supernova remnants. This small number of correspondences indicates that different populations make the sky shine in these two different energy bands.