S. A. Grebenev

Catalog of short gamma-ray transients detected in the SPI/INTEGRAL experiment

We analyzed the data obtained by the SPI telescope onboard the INTEGRAL observatory to search for short transient events with a duration from 1 ms to a few tens of seconds. An algorithm for identifying gamma-ray events against the background of a large number of charged particle interactions with the detector has been developed. The classification of events was made. Apart from the events associated with cosmic gamma-ray bursts (GRBs) confirmed by other space experiments and the activity of known soft gamma repeaters (for example, SGR 1806-20), previously unreported GRBs have been found. GRB candidates and short gamma-ray events probably associated with the activity of known SGRs and AXPs have been selected. The spectral evolution of 28 bright GRBs from the catalog has been studied extensively. A new method for investigating the spectral evolution is proposed. The energy dependence of the spectral lag for bursts with a simple structure of their light curves and for individual pulses of multipulse events is shown to be described by a logarithmic function, lag ∼ Alog(E). It has been established that the parameter A depends on the pulse duration, with the dependence being universal for all of the investigated GRBs. No negative spectral lags have been detected for bursts with a simple structure of their light curves.

Deep hard X-ray survey of the Large Magellanic Cloud

Results of the deep survey of the Large Magellanic Cloud (LMC), performed with the INTEGRAL observatory, are presented. The long exposure (∼ 7 Ms) allowed us to detect twenty one sources in this sky region: ten belonging to the LMC itself (7 HMXBs, 2 PSRs, 1 LMXB), six of extragalactic origin and three belonging to other galaxies from the Local Group — the Milky Way (2 sources) and Small Magellanic Cloud (1 source). Four new hard X-ray sources of these 21 ones were discovered during the survey in addition to IGR J05414-6858 reported earlier; two of them were identified with extragalactic objects. We report also for the first time the detection of a hard Xray emission from the Crab-like pulsar PSRJ0537-6910 and identification of the hard X-ray source IGRJ05305-6559 with the high-mass X-ray binary EXO053109-6609.

Second outburst of the black hole candidate MAXI J1836-194 from SWIFT and INTEGRAL observationshard X-ray sources from Chandra and XMM-Newton data

Details of the observations of a new (second) outburst of the X-ray transientMAXI J1836-194 discovered late in August 2011, a suspected black hole in a low-mass binary system, with the instruments of the SWIFT and INTEGRAL orbiting observatories are presented. The outburst was weaker than the first one; the source had a power-law spectrum in a wide X-ray (0.3–400 keV) energy range without any clear evidence for the presence of a soft (blackbody) component related to the emission from the outer accretion disk regions. This shows that the outburst was a “failed” one: the source did not pass through the sequence of spectral states characteristic of X-ray novae. The observed optical emission from the source whose variability was strongly correlated with its X-ray variability seems to have also been an extension of the power-law spectrum. Spectrum uniformity is, on the whole, unusual for other sources containing a black hole and raises the question about the nature of the emission from MAXI J1836-194 (disk or jet).

Broadband spectrum of the X-ray nova SWIFT J174510.8-262411 at the decaying phase of its outburst

Results of quasi-simultaneous SWIFT and RTT-150 observations for the X-ray nova SWIFT J174510.8-262411 in May–June 2013 at the decaying phase of its outburst are presented. It is shown that the nova spectrum can be fitted in a very wide energy range (from the infrared z and i bands to hard X-rays) by a single power law attenuated due to absorption but without any traces of the presence of a soft (blackbody) component. The presence of such a component is suggested by the generally accepted models of disk accretion onto a black hole in a binary system. The observation of a single power-law spectrum may imply that synchrotron radiation from the source’s relativistic jets makes a major contribution to its flux or that the accretion disk is everywhere hot, optically thin, and radiates nonthermally.

Supergiant Fast X-ray Transients observed by INTEGRAL

We review X-ray properties of the Supergiant Fast X-ray Transients following from their observations with INTEGRAL and show that a compact object in these systems is a neutron star with strong magnetic field accreting from the stellar wind of a donor star. We show that presence of a centrifugal barrier at the magnetospheric boundary of the neutron star may be a key to understanding of abrupt short X-ray outbursts of these transients and long intervals of their quiescence.

X-ray nova MAXI J1828-249. Evolution of the broadband spectrum during its 2013–2014 outburst

Based on data from the SWIFT, INTEGRAL, MAXI/ISS orbital observatories, and the ground-based RTT-150 telescope, we have investigated the broadband (from the optical to the hard X-ray bands) spectrum of the X-ray nova MAXI J1828-249 and its evolution during the outburst of the source in 2013–2014. The optical and infrared emissions from the nova are shown to be largely determined by the extension of the power-law component responsible for the hard X-ray emission. The contribution from the outer cold regions of the accretion disk, even if the X-ray heating of its surface is taken into account, turns out to be moderate during the source’s “high” state (when a soft blackbody emission component is observed in the X-ray spectrum) and is virtually absent during its “low” (“hard”) state. This result suggests that much of the optical and infrared emissions from such systems originates in the same region of main energy release where their hard X-ray emission is formed. This can be the Compton or synchro-Compton radiation from a high-temperature plasma in the central accretion disk region puffed up by instabilities, the synchrotron radiation from a hot corona above the disk, or the synchrotron radiation from its relativistic jets.

GRB 070912—A gamma-ray burst recorded from the direction to the galactic center

The detection of GRB 070912 recorded in the field of view of the SPI, IBIS/ISGRI, and JEMX telescope on September 12, 2007, at 07h32m19s (UT) when analyzing the INTEGRAL archival data is reported. The burst is one of the well-localized events closest to the direction toward the Galactic center (less than from the source Sgr A*) over the entire history of burst observations. Since it was not promptly revealed by the INTEGRAL Burst Alert System (IBAS), no information about its coordinates was disseminated and no search for optical and soft X-ray afterglows was conducted. The 3–200 keV fluence was 2.8 × 10−6 erg cm−2 and the peak flux was 1.8 × 10−7 erg cm−2 s−1 (1.9 ph cm−2 s−1). The burst was also observed in the KONUS/WIND experiment in the background mode, although it was not included in the list of recorded bursts. GRB 070912 is among a limited number of events for which a broadband (3 keV-2 MeV) spectrum of X-ray and gamma-ray emission has been obtained and their evolution from the first instants to complete decay has been traced. It shows how the fast evolution of its spectrum gives rise to absorption features at energies of ∼100 keV.Within the first seconds after the onset of the burst, its spectrum was a power law with a photon index of ∼0.8, but it exhibited a noticeable deficit of photons at energies below 20 keV. Such an initial deficit (a delay in appearance) of X-ray photons can be explained by their “high-latitude” origin relative to the line of sight. The spectrum rapidly softened and at the decay phase was well described by a blackbody (or Wien) law. This allows the distance (redshift) to the burst source to be estimated.

JEM-X/INTEGRAL X-ray survey of the galactic center region

An X-ray survey of the Galactic center region with a radius of ~20◦ has been performed using the data obtained with the JEM-X telescope onboard the INTEGRAL observatory over ~10 years of observations (2003–2013). The exposure at the field center directly toward the Galactic center has reached 4.8 Ms. We have constructed sky maps in the 5–10 and 10–25 keV energy bands and compiled a catalog of detected sources. Together with 83 sources revealed on the integral sky maps, it includes 22 transients that are absent on them but are confidently detected during outbursts with a duration of several days. One of the persistent sources, IGR J17452-2909, has never been observed previously. In contrast to the catalog of sources detected in the harder energy band by the IBIS/ISGRI telescope onboard the INTEGRAL observatory, most of the sources in this catalog are low-mass X-ray binaries (73 of the 105 sources) and only 18 + 3 are high-mass X-ray binaries and cataclysmic variables. Out of the Galactic sources, there are also the black hole candidate XTE J1652-453, the peculiar X-ray burster XMMJ174457-2850.3, and the soft gamma repeater SGR 1806–20 in the catalog; out of the extragalactic sources, there are three active galactic nuclei and a galaxy cluster (Oph CL). The nature of four sources, including the newly discovered one, still remains unknown. We have constructed the luminosity function for the low-mass X-ray binaries from the catalog and considered other statistical properties of their sample.

Luminosity function of high-mass X-ray binaries and anisotropy in the distribution of active galactic nuclei toward the Large Magellanic Cloud

In 2003–2012, the INTEGRAL observatory has performed long-term observations of the Large Magellanic Cloud (LMC). At present, this is one of the deepest hard X-ray (20–60 keV) surveys of extragalactic fields in which more than 20 sources of different natures have been detected. We present the results of a statistical analysis of the population of high-mass X-ray binaries in the LMC and active galactic nuclei (AGNs) observed in its direction. The hard X-ray luminosity function of high-mass X-ray binaries is shown to be described by a power law with a slope α 1.8, that in agreement with the luminosity function measurements both in the LMC itself, but made in the soft X-ray energy band, and in other galaxies. At the same time, the number of detected AGNs toward the LMC turns out to be considerably smaller than the number of AGNs registered in other directions, in particular, toward the source 3C 273. The latter points to the nonuniformity of the distribution of matter in the local Universe.

Low-Frequency Quasi-Periodic Oscillations in the X-ray Nova MAXI J1535-571 at the Initial Stage of Its 2017 Outburst

We report the discovery of low-frequency quasi-periodic oscillations (QPOs) in the power spectrum of the X-ray nova MAXI J1535-571 at the initial stage of its outburst in September 2017. Based on data from the SWIFT and INTEGRAL instruments, we have traced the evolution of the QPO parameters (primarily their frequency) with time and their correlation with changes in the X-ray spectrum of the source (changes in the emission flux and hardness). We place constraints on the theoretical QPO generation models.