arXiv subjects

S. Sazonov

Publications and source records attributed to S. Sazonov.

At least 55 records · Page 3Linked to original sources

arXiv2008

Universal X-ray emissivity of the stellar population in early-type galaxies: unresolved X-ray sources in NGC 3379

We use deep Chandra observations to measure the emissivity of the unresolved X-ray emission in the elliptical galaxy NGC 3379. After elimination of bright, low-mass X-ray binaries with luminosities >10^{36 erg/sec, we find that the remaining unresolved X-ray emission is characterized by an emissivity per unit stellar mass L_x/M_stars ~8.2x10^{27} erg/s/M_sun in the 0.5-2 keV energy band. This value is in good agreement with those previousely determined for the dwarf elliptical galaxy M32, the bulge of the spiral galaxy M31 and the Milky Way, as well as with the integrated X-ray emissivity of cataclysmic variables and coronally active binaries in the Solar neighborhood. This strongly suggests that i) the bulk of the unresolved X-ray emission in NGC 3379 is produced by its old stellar population and ii) the old stellar populations in all galaxies can be characterized by a universal value of X-ray emissivity per unit stellar mass or per unit K band luminosity.

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arXiv2008

Discovery of heavily-obscured AGN among 7 INTEGRAL hard X-ray sources observed by Chandra

We observed 7 INTEGRAL sources with the Chandra X-ray Observatory to refine their localization to ~2 arcsec and to study their X-ray spectra. Two sources are inferred to have a Galactic origin: IGR J08390-4833 is most likely a magnetic cataclysmic variable with a white dwarf spin period ~1,450 s; and IGR J21343+4738 is a high-mass X-ray binary. Five sources (IGR J02466-4222, IGR J09522-6231, IGR J14493-5534, IGR J14561-3738, and IGR J23523+5844) prove to be AGN with significant intrinsic X-ray absorption along the line of sight. Their redshifts and hard X-ray (17-60 keV) luminosities range from 0.025 to 0.25 and from ~2x10^43 to ~2x10^45 erg/s, respectively, with the distance to IGR J14493-5534 remaining unknown. The sources IGR J02466-4222 and IGR J14561-3738 are likely Compton-thick AGN with absorption column densities NH>10^24 cm^-2, and the former further appears to be one of the nearest X-ray bright, optically-normal galaxies. With the newly-identified sources, the number of heavily-obscured (NH>10^24 cm^-2) AGN detected by INTEGRAL has increased to ~10. Therefore, such objects constitute 10-15% of hard X-ray bright, non-blazar AGN in the local Universe. The small ratio (<<1%) of soft (0.5-8.0 keV) to hard (17-60 keV) band fluxes (Chandra to INTEGRAL) and the non-detection of optical narrow-line emission in some of the Compton-thick AGN in our sample suggests that there is a new class of objects in which the central massive black hole may be surrounded by a geometrically-thick dusty torus with a narrow ionization cone.

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arXiv2008

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.

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arXiv2008

Low-mass X-ray binaries in the bulge of the Milky Way

We study the population of low-mass X-ray binaries (LMXBs) in the Galactic bulge using the deep survey of this region by the IBIS telescope aboard the INTEGRAL observatory. Thanks to the increased sensitivity with respect to previous surveys of this field, we succeeded to probe the luminosity function (LF) of LMXBs down to ~7e34 erg/sec in the 17-60keV energy band. The slope dlog N/dlog L=-0.96+/0.20 measured in the 1e35-1e37 erg/sec range confirms that the LMXB LF flattens below L_x<1e37 erg/sec with respect to higher luminosities. We discuss the origin of the observed LF flattening. We demonstrate that the spatial distribution of persistent LMXBs in the Galactic Center/Galactic bulge region is consistent with a model of stellar mass distribution that includes the nuclear stellar disk component in the innermost degree of the Galaxy. The spatial distribution of transient LMXBs detected in the Galactic Center region indicates an increased fraction of transient sources in the innermost degree of the Galaxy with respect to outer regions.

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arXiv2008

Cumulative hard X-ray spectrum of local AGN: a link to the cosmic X-ray background

We determine the cumulative spectral energy distribution (SED) of local AGN in the 3-300 keV band and compare it with the spectrum of the cosmic X-ray background (CXB) in order to test the widely accepted paradigm that the CXB is a superposition of AGN and to place constraints on AGN evolution. We performed a stacking analysis of the hard X-ray spectra of AGN detected in two recent all-sky surveys, performed by the IBIS/ISGRI instrument aboard INTEGRAL and by the PCA instrument aboard RXTE, taking into account the space densities of AGN with different luminosities and absorption column densities. We derived the collective SED of local AGN in the 3-300 keV energy band. Those AGN with luminosities below 10^43.5 erg/s (17-60 keV) provide the main contribution to the local volume hard X-ray emissivity, at least 5 times more than more luminous objects. The cumulative spectrum exhibits (although with marginal significance) a cutoff at energies above 100-200 keV and is consistent with the CXB spectrum if AGN evolve over cosmic time in such a way that the SED of their collective high-energy emission has a constant shape and the relative fraction of obscured AGN remains nearly constant, while the AGN luminosity density undergoes strong evolution between z~1 and z=0, a scenario broadly consistent with results from recent deep X-ray surveys. The first direct comparison between the collective hard X-ray SED of local AGN and the CXB spectrum demonstrates that the popular concept of the CXB being a superposition of AGN is generally correct. By repeating this test using improved AGN statistics from current and future hard X-ray surveys, it should be possible to tighten the constraints on the cosmic history of black hole growth.

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arXiv2008

Earth X-ray albedo for cosmic X-ray background radiation in the 1--1000 keV band

We present calculations of the reflection of the cosmic X-ray background (CXB) by the Earth's atmosphere in the 1--1000 keV energy range. The calculations include Compton scattering and X-ray fluorescent emission and are based on a realistic chemical composition of the atmosphere. Such calculations are relevant for CXB studies using the Earth as an obscuring screen (as was recently done by INTEGRAL). The Earth's reflectivity is further compared with that of the Sun and the Moon -- the two other objects in the Solar system subtending a large solid angle on the sky, as needed for CXB studies.

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arXiv2007

Large-scale variations of the cosmic X-ray background and the X-ray emissivity of the local Universe

We study the cosmic X-ray background (CXB) intensity variations on large angular scales using slew data of the RXTE observatory. We detect intensity variations up to ~2% on angular scales of 20--40deg. These variations are partly correlated with the local large-scale structure, which allowed us to estimate the emissivity of the local Universe in the energy band 2--10 keV at 9+/-4 x 10^{38} ers/sec/Mpc^{3}. The spectral energy distribution of the large-angular-scale variations is hard and is compatible with that of the CXB, which implies that normal galaxies and clusters of galaxies, whose spectra are typically much softer, do not contribute more than 15% to the total X-ray emissivity of the local Universe. Most of the observed CXB anisotropy (after exclusion of point sources with fluxes >10^{-11} erg/s/cm^2) can be attributed to low-luminosity AGNs

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arXiv2007

GRB 070311: a direct link between the prompt emission and the afterglow

We present prompt gamma-ray, early NIR/optical, late optical and X-ray observations of the peculiar GRB 070311 discovered by INTEGRAL, in order to gain clues on the mechanisms responsible for the prompt gamma-ray pulse as well as for the early and late multi-band afterglow of GRB 070311. We fitted with empirical functions the gamma-ray and optical light curves and scaled the result to the late time X-rays. The H-band light curve taken by REM shows two pulses peaking 80 and 140 s after the peak of the gamma-ray burst and possibly accompanied by a faint gamma-ray tail. Remarkably, the late optical and X-ray afterglow underwent a major rebrightening between 3x10^4 and 2x10^5 s after the burst with an X-ray fluence comparable with that of the prompt emission extrapolated in the same band. Notably, the time profile of the late rebrightening can be described as the combination of a time-rescaled version of the prompt gamma-ray pulse and an underlying power law. This result supports a common origin for both prompt and late X-ray/optical afterglow rebrightening of GRB 070311 within the external shock scenario. The main fireball would be responsible for the prompt emission, while a second shell would produce the rebrightening when impacting the leading blastwave in a refreshed shock (abridged).

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arXiv2007

Hard X-ray emission of the Earth's atmosphere: Monte Carlo simulations

We perform Monte Carlo simulations of cosmic ray-induced hard X-ray radiation from the Earth's atmosphere. We find that the shape of the spectrum emergent from the atmosphere in the energy range 25-300 keV is mainly determined by Compton scatterings and photoabsorption, and is almost insensitive to the incident cosmic-ray spectrum. We provide a fitting formula for the hard X-ray surface brightness of the atmosphere as would be measured by a satellite-born instrument, as a function of energy, solar modulation level, geomagnetic cutoff rigidity and zenith angle. A recent measurement by the INTEGRAL observatory of the atmospheric hard X-ray flux during the occultation of the cosmic X-ray background by the Earth agrees with our prediction within 10%. This suggests that Earth observations could be used for in-orbit calibration of future hard X-ray telescopes. We also demonstrate that the hard X-ray spectra generated by cosmic rays in the crusts of the Moon, Mars and Mercury should be significantly different from that emitted by the Earth's atmosphere.

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arXiv2007

On the contribution of point sources to the Galactic ridge X-ray emission

We analyzed deep Chandra observations of the Galactic plane region centered at l=28.55, b=-0.03 with the aim to obtain the best possible constraints on the contribution of weak point sources to the Galactic ridge X-ray emission (GRXE) in this region. We demonstrate that the vast majority of the detected sources are Galactic in origin and are probably cataclysmic variables and coronally active stars. We use the number-flux function of detected sources to constrain the luminosity function of Galactic X-ray sources in the range 1e30--1e32 erg/s and find good agreement with the luminosity function of sources in the Solar vicinity. The fraction of the total flux at energies 1--7 keV resolved into point sources at the current sensitivity level is ~25%. Excluding the expected contribution of extragalactic sources, ~19% of the GRXE is due to point Galactic sources with interstellar absorption corrected fluxes higher than 1.2e-15 erg/s/cm^2in the energy band 1-7 keV.

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arXiv2007

X-ray emission from the stellar population in M32

Using Chandra observations, we study the X-ray emission of the stellar population in the compact dwarf elliptical galaxy M32. The proximity of M32 allows one to resolve all bright point sources with luminosities higher than 8e33 erg/s in the 0.5--7 keV band. The remaining (unresolved) emission closely follows the galaxy's optical light and is characterized by an emissivity per unit stellar mass of ~4.3e27 erg/s/M_sun in the 2--10 keV energy band. The spectrum of the unresolved emission above a few keV smoothly joins the X-ray spectrum of the Milky Way's ridge measured with RXTE and INTEGRAL. These results strongly suggest that weak discrete X-ray sources (accreting white dwarfs and active binary stars) provide the bulk of the ``diffuse'' emission of this gas-poor galaxy. Within the uncertainties, the average X-ray properties of the M32 stars are consistent with those of the old stellar population in the Milky Way. The inferred cumulative soft X-ray (0.5--2 keV) emissivity is however smaller than is measured in the immediate Solar vicinity in our Galaxy. This difference is probably linked to the contribution of young (age <1Gyr) stars, which are abundant in the Solar neighborhood but practically absent in M32. Combining Chandra, RXTE and INTEGRAL data, we obtain a broad-band (0.5--60 keV) X-ray spectrum of the old stellar population in galaxies.

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arXiv2007

INTEGRAL observations of the cosmic X-ray background in the 5-100 keV range via occultation by the Earth

We study the spectrum of the cosmic X-ray background (CXB) in energy range $\sim$5-100 keV. Early in 2006 the INTEGRAL observatory performed a series of four 30ksec observations with the Earth disk crossing the field of view of the instruments. The modulation of the aperture flux due to occultation of extragalactic objects by the Earth disk was used to obtain the spectrum of the Cosmic X-ray Background(CXB). Various sources of contamination were evaluated, including compact sources, Galactic Ridge emission, CXB reflection by the Earth atmosphere, cosmic ray induced emission by the Earth atmosphere and the Earth auroral emission. The spectrum of the cosmic X-ray background in the energy band 5-100 keV is obtained. The shape of the spectrum is consistent with that obtained previously by the HEAO-1 observatory, while the normalization is $\sim$10% higher. This difference in normalization can (at least partly) be traced to the different assumptions on the absolute flux from the Crab Nebulae. The increase relative to the earlier adopted value of the absolute flux of the CXB near the energy of maximum luminosity (20-50 keV) has direct implications for the energy release of supermassive black holes in the Universe and their growth at the epoch of the CXB origin.

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arXiv2007

INTEGRAL/IBIS all-sky survey in hard X-rays

We present results of an all-sky hard X-ray survey based on almost four years of observations with the IBIS telescope on board the INTEGRAL observatory. The dead time-corrected exposure of the survey is ~33 Ms. Approximately 12% and 80% of the sky have been covered to limiting fluxes lower than 1 and 5 mCrab, respectively. Our catalog of detected sources includes 400 objects, 339 of which exceed a 5 sigma detection threshold on the time-averaged map of the sky and the rest were detected in various subsamples of exposures. Among the identified sources, 213 are Galactic (87 low-mass X-ray binaries, 74 high-mass X-ray binaries, 21 cataclysmic variables, 6 coronally active stars, and other types) and 136 are extragalactic, including 131 active galactic nuclei (AGNs) and 3 clusters of galaxies. We obtained number-flux functions for AGNs and Galactic sources. The logN-logS relation of AGNs (excluding blazars) is based on 69 sources with fluxes higher than S_lim=1.1 x 10^{-11} erg/s/cm^2 (~0.8 mCrab) in the 17-60 keV energy band. The cumulative number-flux function of AGNs located at Galactic latitudes $|b|>5^\circ$, where the survey is characterized by high identification completeness, can be described by a power law with a slope of 1.62 +/- 0.15 and normalization of (5.7 +/- 0.7) x 10^{-3} sources per deg^2 at fluxes >1.43 x 10^{-11} erg/s/cm^2 (>1 mCrab). AGNs with fluxes higher than S_lim make up ~1% of the cosmic X-ray background at 17-60 keV. We present evidence of strong inhomogeneity in the spatial distribution of nearby (<70 Mpc) AGNs, which reflects the large-scale structure in the local Universe.

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arXiv2006

Resolving the Galactic X-ray background

We use Chandra deep observations of the Galactic Center (GC) region to improve the constraints on the unresolved fraction of the Galactic X-ray background (also known as the Galactic ridge X-ray emission). We emphasize the importance of correcting the measured source counts at low fluxes for bias associated with Poisson noise. We find that at distances of 2'-4' from Sgr A* at least ~40% of the total X-ray emission in the energy band 4-8 keV originates from point sources with luminosities L(2-10 keV)> 10^{31} erg/sec. From a comparison of the source number-flux function in the GC region with the known luminosity function of faint X-ray sources in the Solar vicinity, we infer that Chandra has already resolved a large fraction of the cumulative contribution of cataclysmic variables to the total X-ray flux from the GC region. This comparison further indicates that most of the yet unresolved ~60% of the X-ray flux from the GC region is likely produced by weak cataclysmic variables and coronally active stars with L(2-10 keV)<10^{31} erg/sec. We conclude that the bulk of the Galactic X-ray background is produced by discrete sources.

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arXiv2006

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 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.

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arXiv2006

Map of the Galaxy in the 6.7 keV emission line

We study the two dimensional surface brightness distribution of the Galactic X-ray background emission outside the central degree around Sgr A* in the 6.7 keV line as measured by the PCA spectrometer of the RXTE observatory. The use of the emission line instead of continuum (3-20 keV) radiation and application of time variability filtering to the long data set allows us to strongly suppress the contamination of the GRXE map by bright point sources. The surface brightness in the 6.7 keV line demonstrates very good correspondence with the near-infrared surface brightness over the whole Galaxy, supporting the notion that the GRXE consists mostly of integrated emission from weak Galactic X-ray sources. We find compatible linear correlations between near-infrared and 6.7 keV surface brightness for the bulge and disk of the Galaxy. This indicates that the populations of weak X-ray sources making up the GRXE in the disk and bulge are not significantly different.

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arXiv2006

Constraints on the late X-ray emission from the low-energy GRB 031203: INTEGRAL data

Comparison of the INTEGRAL upper limits on the hard X-ray flux before and after the low-energy GRB 031203 with the XMM measurements of the dust-scattered radiation at lower energies suggests that a significant fraction of the total burst energy could be released in the form of soft X-rays at an early afterglow stage with a characteristic duration of ~100-1000s. The overall time evolution of the GRB 031203 afterglow may have not differed qualitatively from the behavior of standard (i.e., more intense) bursts studied by the SWIFT observatory. The available data also admit the possibility that the dust-scattered radiation was associated with an additional soft component in the spectrum of the gamma-ray burst itself.

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arXiv2006

Hard X-ray emission from the Galactic ridge

We present results of a study of the Galactic ridge X-ray emission (GRXE) in hard X-rays performed with the IBIS telescope aboard INTEGRAL. The imaging capabilities of this coding aperture telescope make it possible to account for the flux from bright Galactic point sources whereas the wide field of view permits to collect large flux from the underlying GRXE. Extensive study of the IBIS/ISGRI detector background allowed us to construct a model that predicts the detector count rate with $\sim1-2$% accuracy in the energy band 17-60 keV. The derived longitude and latitude profiles of the ridge emission are in good agreement with the Galactic distribution of stars obtained from infrared observations. This, along with the measured hard X-ray spectrum of the Galactic ridge emission strongly indicates its stellar origin. The derived unit stellar mass emissivity of the ridge in the energy band 17-60 keV, $(0.9 - 1.2)\times 10^{27}$\lummass (assuming a bulge mass of $1-1.3 \times 10^{10} M_\odot$) agrees with that of local (in the Solar neigborhood) accreting magnetic white dwarf binaries - dominant contributors to the GRXE at these energies. In addition, the shape of the obtained GRXE spectrum can be used to determine the average mass of white dwarfs in such systems in the Galaxy as $\sim0.5 M_{\sun}$. The total hard X-ray luminosity of the GRXE is $L_{\rm 17-60 keV} =(3.7\pm0.2)\times10^{37}$\lum in the 17--60 keV band. At energies 70--200 keV no additional contribution to the total emission of the Galaxy apart from the detected point sources is seen.

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