INTEGRAL All-sky Survey Research Articles

Optical Identification of X-ray Sources from the 14-Year INTEGRAL All-Sky Survey

We present the results of our optical identifications of several hard X-ray sources from the INTEGRAL all-sky survey obtained over 14 years of observations. Having improved the positions of these objects in the sky with the X-ray telescope (XRT) of the Swift observatory and the XMM-Newton observatory, we have identified their counterparts using optical and infrared sky survey data. We have obtained optical spectra for more than half of the objects from our sample with the RTT-150 and AZT-33IK telescopes, which have allowed us to establish the nature of the objects and to measure their redshifts. Six sources are shown to be extragalactic in origin and to belong to Seyfert 1 and 2 galaxies (IGR J01017+6519, IGR J08215-1320, IGR J08321-1808, IGR J16494-1740, IGR J17098-2344, IGR J17422-2108); we have failed to draw definitive conclusions about the nature of four more objects (IGR J11299-6557, IGR J14417-5533, IGR J18141-1823, IGR J18544+0839), but, judging by circumstantial evidence, they are most likely also extragalactic objects. For one more object (IGR J18044-1829) no unequivocal identification has been made.

Accurate localization and identification of six hard X-ray sources from Chandra and XMM-Newton data

We present the results of Chandra and XMM-Newton observations for six hard X-ray sources (IGR J12134-6015, IGR J18293-1213, IGR J18219-1347, IGR J17350-2045, IGR J18048-1455, XTE J1901+014) from the INTEGRAL all-sky survey. Based on these observations, we have improved significantly the localization accuracy of the objects and, therefore, have managed to identify their optical counterparts. Using data from the publicly available 2MASS and UKIDSS infrared sky surveys as well as data from the SOFI/NTT telescope (European Southern Observatory), we have determined the magnitudes of the optical counterparts, estimated their types and (in some cases) the distances to the program objects. A triplet of iron lines with energies of 6.4, 6.7, and 6.9 keV has been detected in the X-ray spectrum of IGR J18048-1455; together with the detection of pulsations with a period of ~1440 s from this source, this has allowed it to be classified as a cataclysmic variable, most likely an intermediate polar. In addition, broadband X-ray spectra of IGR J12134-6015 and IGR J17350-2045 in combination with infrared and radio observations suggest an extragalactic nature of these objects. The source IGR J18219-1347 presumably belongs to the class of high-mass X-ray binaries.

The X-Ray AGN Bias Factor at Zero Redshift

AbstractUsing an AGN sample from the INTEGRAL all-sky survey, we show that the density fluctuations of matter in the local universe translate linearly into the density fluctuations of accreting supermassive black holes. Our results imply that hard X-ray emitting AGNs are unbiased tracers of the galaxy population in the nearby universe and SMBH activity is independent of the density of galaxies.

Optical identification of the source IGR J08390-4833 from the INTEGRAL all-sky survey

Results of the optical identification of the hard X-ray source IGR J08390-4833 recently discovered in the INTEGRAL all-sky survey are presented. We show that the source is most likely a cataclysmic variable, i.e., an accreting white dwarf in a binary. Analysis of its optical light curve clearly reveals intrinsic variability on timescales of the order of an hour or longer. However, the short time of the source’s optical observations does not allow a definitive conclusion about the periodicity of the detected variability to be reached. Further optical and X-ray observations are required for a more accurate classification of the source.

Optical identifications of five INTEGRAL hard X-ray sources in the Galactic plane

The results of optical identifications of five hard X-ray sources in the Galactic plane region from the INTEGRAL all-sky survey are presented. The X-ray data on one source (IGRJ20216+4359) are published for the first time. The optical observations were performed with 1.5-m RTT-150 telescope (TUBITAK National Observatory, Antalya, Turkey) and 6-m BTA telescope (Special Astrophysical Observatory, Nizhny Arkhyz, Russia). A blazar, three Seyfert galaxies, and a high-mass X-ray binary are among the identified sources.

Properties of the Galactic population of cataclysmic variables in hard X-rays

We measure the spatial distribution and hard X-ray luminosity function of cataclysmic variables (CVs) using the INTEGRAL all-sky survey in the 17-60 keV energy band. The vast majority of the INTEGRAL detected CVs are intermediate polars with luminosities in the range 10^{32}-10^{34} erg/sec. The scale height of the Galactic disk population of CVs is found to be 130{+90}{-50} pc. The CV luminosity function measured with INTEGRAL in hard X-rays is compatible with that previously determined at lower energies (3--20 keV) using a largely independent sample of sources detected by RXTE (located at |b|>10deg as opposed to the INTEGRAL sample, strongly concentrated to the Galactic plane). The cumulative 17-60 keV luminosity density of CVs per unit stellar mass is found to be (1.3+/-0.3)x10^{27} erg/sec/Msun and is thus comparable to that of low-mass X-ray binaries in this energy band. Therefore, faint but numerous CVs are expected to provide an important contribution to the cumulative hard X-ray emission of galaxies.

Hard X-ray luminosity function and absorption distribution of nearby AGN: INTEGRAL all-sky survey

We use the INTEGRAL all-sky hard X-ray survey to perform a statistical study of a representative sample of nearby AGN. Our entire all-sky sample consists of 127 AGN, of which 91 are confidently detected (>5 sigma) on the time-averaged map obtained with the IBIS/ISGRI instrument and 36 are detected only during single observations. Among the former there are 66 non-blazar AGN located at |b|>5 deg, where the survey's identification completeness is ~93%, which we use for calculating the AGN luminosity function and X-ray absorption distribution. In broad agreement with previous studies, we find that the fraction of obscured (log NH>22) objects is much higher (~70%) among the low-luminosity AGN (Lx<10^43.6 erg/s) than among the high-luminosity ones (Lx>10^43.6 erg/s), \~25%, where Lx is the luminosity in the 17-60 keV energy band. We also find that locally the fraction of Compton-thick AGN is less than 20% unless there is a significant population of AGN that are so strongly obscured that their observed hard X-ray luminosities fall below 10^40-10^41 erg/s, the effective limit of our survey. The constructed hard X-ray luminosity function has a canonical, smoothly broken power-law shape in the range 40<log Lx<45.5 with a characteristic luminosity of log L*=43.40+/-0.28. The estimated local luminosity density due to AGN with log Lx>40 is (1.4+/-0.3) 10^39 erg/s/Mpc^3 (17-60 keV). We demonstrate that the spectral shape and amplitude of the CXB are consistent with the simple scenario in which the NH distribution of AGN (for a given Lx/L*(z) ratio has not changed significantly since z~1.5, while the AGN luminosity function has experienced pure luminosity evolution.