THE DEEP LOOK AT THE HARD X-RAY SKY: THESWIFT–INTEGRALX-RAY (SIX) SURVEY

Abstract

The super-massive black-holes in the centers of Active Galactic Nuclei (AGN) are surrounded by obscuring matter that can block the nuclear radiation. Depending on the amount of blocked radiation, the flux from the AGN can be too faint to be detected by currently flying hard X-ray (above 15 keV) missions. At these energies only ∼1% of the intensity of the Cosmic X-ray Background (CXB) can be resolved into point-like sources that are AGNs. In this work we address the question of the undetected sources contributing to the CXB with a very sensitive and new hard X-ray survey: the SIX survey that is obtained with the new approach of combining the Swift/BAT and INTEGRAL/IBIS Xray observations. We merge the observations of both missions. This enhances the exposure time and reduces systematic uncertainties. As a result we obtain a new survey over a wide sky area of 6200 deg 2 that is more sensitive than the surveys of Swift/BAT or INTEGRAL/IBIS alone. Our sample comprises 113 sources: 86 AGNs (Seyfert-like and blazars), 5 galaxies, 2 clusters of galaxies, 3 Galactic sources, 3 previously detected unidentified X-ray sources, and 14 unidentified sources. The scientific outcome from the study of the sample has been properly addressed to study the evolution of AGNs at redshift below 0.4. We do not find any evolution using the 1/V max method. Our sample of faint sources are suitable targets for the new generation hard X-ray telescopes with focusing techniques.

Unveiling the nature of INTEGRAL objects through optical spectroscopy

“…Using the same approach as in our previous Papers I-IX, we considered the IBIS surveys of , Krivonos et al (2010Krivonos et al ( , 2012, Coe et al (2010), Grebenev et al (2013), and the Swift/INTEGRAL ("SIX") joint catalogue of Bottacini et al (2012). Among them, we selected unidentified or unclassified hard X-ray sources containing a single bright soft X-ray object within the IBIS 90% confidence level error box.…”

“…For the source naming in Table 1, we directly adopted the names as they appear in the relevant works Bottacini et al 2012;Ricci et al 2012;Coe et al 2010;Grebenev et al 2013;Krivonos et al 2010Krivonos et al , 2012, and the IGR alias when available. However, we remark that we chose not to use the ROSAT name as reported in the fourth IBIS Survey ) for one of the selected objects (1RXS J090320.0+533022), because the soft X-ray emission detected with Swift/XRT within the corresponding 90% IBIS error circle is not positionally consistent with the one reported in the ROSAT Faint Catalogue (actually, no detectable emission appears to be associated with this ROSAT object from Table 1).…”

“…In these tables, X-ray luminosities were computed from the fluxes reported in McDowell (1994), Voges et al (1999Voges et al ( , 2000, ROSAT Team (2000), Saxton et al (2008), Rodriguez et al (2009, Cusumano et al (2010), Landi et al (2010Landi et al ( , 2012b, Maiorano et al (2010), Malizia et al ( , 2012a, Bottacini et al (2012), Krivonos et al (2012), Molina et al (2012a,b), Ricci et al (2012), Tomsick et al (2012), Baumgartner et al (2013), and Grebenev et al (2013).…”

“…The optical spectra of the firm or likely counterparts of 30 unidentified, unclassified, or poorly studied sources belonging to one or more IBIS surveys Coe et al 2010;Krivonos et al 2010Krivonos et al , 2012Bottacini et al 2012;Grebenev et al 2013) are shown in this paper. We moreover added to our sample the spectra of two INTEGRAL sources reported in Ricci et al (2012) and one in Townsend et al (2011), thus reaching a total of 33 objects hereby explored.…”

The NuSTAR Serendipitous Survey: The 40-month Catalog and the Properties of the Distant High-energy X-Ray Source Population

“…These photon energies are also low compared to the peak of the CXB (at ≈ 20-30 keV), meaning that spectral extrapolations are required to characterize the AGN population responsible for the CXB peak. High energy (> 10 keV) X-ray surveys with non-focusing X-ray observatories (e.g., Swift BAT and INTEGRAL) have directly resolved ≈ 1-2% of the CXB peak into individual AGNs (Krivonos et al 2007;Ajello et al 2008;Bottacini et al 2012). These surveys have been successful in characterizing the local highenergy emitting AGN population (e.g., Tueller et al 2008;Burlon et al 2011;Vasudevan et al 2013;Ricci et al 2015) but, being largely confined to z 0.1, there is limited scope for evolutionary studies.…”