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Papers by Konstantin Postnov

Monthly Notices of the Royal Astronomical Society, 2016

The Rossi X-ray Timing Explorer (RXTE) observed four outbursts of the accreting Xray binary trans... more The Rossi X-ray Timing Explorer (RXTE) observed four outbursts of the accreting Xray binary transient source, GX 304−1 in 2010 and 2011. We present results of detailed 3-100 keV spectral analysis of 69 separate observations, and report a positive correlation between cyclotron line parameters, as well as other spectral parameters, with power-law flux. The cyclotron line energy, width and depth versus flux, and thus luminosity, correlations show a flattening of the relationships with increasing luminosity, which are well described by quasi-spherical or disc accretion that yield the surface magnetic field to be ∼5 × 10 12 Gauss. Since HEXTE (High Energy X-ray Timing Experiment) cluster A was fixed aligned with the Proportional Counter Array field of view and cluster B was fixed viewing a background region 1. • 5 off of the source direction during these observations near the end of the RXTE mission, the cluster A background was estimated from cluster B events using HEXTEBACKEST. This made possible the detection of the ∼55 keV cyclotron line and an accurate measurement of the continuum. Correlations of all spectral parameters with the primary 2-10 keV power-law flux reveal it to be the primary driver of the spectral shape. The accretion is found to be in the collisionless shock braking regime.

AIP Conference Proceedings, 2008

Monthly Notices of the Royal Astronomical Society, 2016

We have characterized the typical temporal behaviour of the bright X-ray flares detected from the... more We have characterized the typical temporal behaviour of the bright X-ray flares detected from the three Supergiant Fast X-ray Transients showing the most extreme transient behaviour (XTE J1739-302, IGR J17544-2619, SAX J1818.6-1703). We focus here on the cumulative distributions of the waiting-time (time interval between two consecutive X-ray flares), and the duration of the hard X-ray activity (duration of the brightest phase of an SFXT outburst), as observed by INTEGRAL/IBIS in the energy band 17-50 keV. Adopting the cumulative distribution of waiting-times, it is possible to identify the typical timescale that clearly separates different outbursts, each composed by several single flares at ∼ks timescale. This allowed us to measure the duration of the brightest phase of the outbursts from these three targets, finding that they show heavy-tailed cumulative distributions. We observe a correlation between the total energy emitted during SFXT outbursts and the time interval covered by the outbursts (defined as the elapsed time between the first and the last flare belonging to the same outburst as observed by IN T EGRAL). We show that temporal properties of flares and outbursts of the sources, which share common properties regardless different orbital parameters, can be interpreted in the model of magnetized stellar winds with fractal structure from the OB-supergiant stars.

Gamma-Ray Bursts in the Afterglow Era

Arguments are given favoring possible connection of GRBs with core collapse of massive Wolf-Rayet... more Arguments are given favoring possible connection of GRBs with core collapse of massive Wolf-Rayet stars. We analyze the observed properties of cosmic gamma-ray bursts, Wolf-Rayet (WR) stars and their CO-cores in the end of evolution. WR stars are deprived of their extended hydrogen envelopes, which makes it easier for the collapse energy to transform into observed gamma-ray emission. Presently, of ∼ 90 well-localized gamma-ray bursts, 21 ones are optically identified and for 16 of them redshifts are measured (z = 0.4 ÷ 4.5). The observed energy of gamma-ray bursts spans over a wide range from 3 × 10 51 to 2 × 10 54 ergs. There is some evidence that this distribution N (∆E) is bimodal if take into account GRB980425 associated with a peculiar type Ic supernova SN1998bw in a nearby galaxy ESO 184-G82 for which ∆E γ ≈ 10 48 ergs. These characteristics of gamma-ray bursts are similar to the distribution of the final masses of CO-cores of WR stars which is also wide and homogeneous: M CO = (1 − 2)M ÷ (20 − 44)M. A possible bimodality of the gamma-ray burst energy distribution (E 1 = 10 48 erg; ∆E 2 = 3×10 51 ÷2×10 54 erg) is in accord with the bimodal mass distribution of relativistic objects (M N S = (1.35 ± 0.15)M ; M BH = (4 ÷ 15)M). That the supernova SN1998bw is of the "peculiar Ic" type, atypical for WR collapses (type Ib/c), can be related to the rotation of the collapsing CO-core which can make the collapse longer and lead to the formation of a neutron star, the decrease of the gamma-ray burst energy, and the increase of the fraction of kinetic energy transported to the envelope. The expected collapse rate of CO-cores of most compact WR stars of type WO in the Galaxy is ∼ 10 −5 per year, which is only by one and a half order of magnitude higher than the average gamma-ray burst rate in one galaxy (∼ 10 −6 − 10 −7 per year). Two particular models that use WR stars as gamma-ray burst progenitors are considered: the hypernova model by Paczyński (1998) and the model of unstable CO-core collapse suggested by Gershtein (2000). In both models the allowance of a gamma-ray beaming or random outcome of the CO-core collapse due to some instabilities permits one to bring the rate of CO-core collapses in accordance with that of gamma-ray bursts. We argue that WR stars (most probably, of type WO) can be considered as progenitors of cosmic gamma-ray bursts. Two types of gamma-ray bursts are predicted in correspondence with the bimodal mass distribution of the relativistic objects. Three types of optical afterglows should appear depending on which CO-core is collapsing: of a single WR star, of a WR star in a WR+O or a hypothetic WR+(A-M) binary system. In addition, we briefly consider a model of gamma-ray bursts as a transient phenomenon occurring at early stages of galactic evolution (z > 1), when very massive 1 (M > 100M) low-metallicity stars could form. Such massive stars should also loose their hydrogen envelopes and become massive WR stars whose collapses could be accompanied by gamma-ray bursts. WR-galaxies can be most probable candidates for gamma-ray burst host galaxies.

Astronomical & Astrophysical Transactions, 1993

The distribution of old galactic neutron stars is computed for model galactic potential and initi... more The distribution of old galactic neutron stars is computed for model galactic potential and initial neutron star velocity distribution following from evolution scenario of binary systems. Neutron stars are supposed to get their initial velocities during (asymmetric) supernova explosion of a single massive (M,, > 10 Ma) star or a massive component of a binary system. Old neutron star population is shown to be capable of filling a torus-like area extending to a few tens kiloparsecs above the galactic plane. Statistical tests related to the spatial distribution of gamma-ray bursts are carried out for the distribution obtained.

The Astrophysical Journal, 1997

The prospects of detecting extragalactic supernovae (SNs) down to visual magnitudes 23È25 give us... more The prospects of detecting extragalactic supernovae (SNs) down to visual magnitudes 23È25 give us hope for observing them in the most distant parts of the universe. Using a Monte Carlo method of stellar population synthesis (the Scenario Machine), we compute, under standard assumptions of stellar evolution, the rates of SNs of various types in a model galaxy and evaluate the SN rates in the universe. The expected cumulative distribution log N [ m (number of events [ stellar magnitude) is calculated for various SN types and di †erent star formation histories in the universe. The results are also presented in terms of evolution of supernova units with redshifts. Recent observational data on the high-redshift SN Ia rate are in good agreement with our predictions for the relative density of baryons contained in stars at the present time () * \ 0.0057).

Astronomy & Astrophysics, 2005

We present the analysis of Rossi X-ray Timing Explorer (RXTE) observations of the turn-on phase o... more We present the analysis of Rossi X-ray Timing Explorer (RXTE) observations of the turn-on phase of a 35 day cycle of the X-ray binary Her X-1. During the early phases of the turn-on, the energy spectrum is composed of X-rays scattered into the line of sight plus heavily absorbed X-rays. The energy spectra in the 3-17 keV range can be described by a partial covering model, where one of the components is influenced by photoelectric absorption and Thomson scattering in cold material plus an iron emission line at 6.5 keV. In this paper we show the evolution of spectral parameters as well as the evolution of the pulse profile during the turn-on. We describe this evolution using Monte Carlo simulations which self-consistently describe the evolution of the X-ray pulse profile and of the energy spectrum.

First results of observations of the low mass X-ray binary Her X-1/HZ Her performed by the INTEGR... more First results of observations of the low mass X-ray binary Her X-1/HZ Her performed by the INTEGRAL satellite in July-August 2005 are presented. A significant part of one 35 day main-on state was covered. The cyclotron line in the X-ray spectrum is well observed and its position and shape, as well as its variability with time and phase of the 1.24 s pulsation are explored. X-ray pulse profiles for different energy bands are studied throughout the observation. The pulse period is found to vary on short time scales revealing a dynamical spin-up/spin-down behavior. Results of simultaneous optical observations of HZ Her are also discussed. Key words: X-ray binaries; accretion disks; neutron stars.

arXiv: Instrumentation and Methods for Astrophysics, 2020

The new SAI MSU observatory 2.5-meter telescope and capabilities of its current instrumentation a... more The new SAI MSU observatory 2.5-meter telescope and capabilities of its current instrumentation are described. The facility operates actively since 2014 in parallel to the engineering works. It has delivered a number of prominent results in the field of optical and near-infrared photometry and spectroscopy as well as newly developed observational techniques like differential speckle polarimetry.

Accretion Flows in Astrophysics

A theory of quasi-spherical subsonic accretion onto slowly rotating magnetized neutron stars is p... more A theory of quasi-spherical subsonic accretion onto slowly rotating magnetized neutron stars is presented. In this regime, the accreted matter settles with subsonic velocities onto the rotating magnetosphere forming an extended quasi-spherical shell. The accretion rate in the shell is determined by the ability of the plasma to enter the magnetosphere due to the Rayleigh-Taylor instability with account for cooling. This accretion regime may be established for moderate X-ray luminosities, corresponding to accretion rates \\(\\dot M< \\dot M^\\dag \\simeq 4\\times 10^{16}\\) g s−1. For higher accretion rates a free-fall gap appears, due to strong Compton cooling of the flow above the magnetosphere, and accretion becomes highly non-stationary. Observations of spin-up and spin-down in equilibrium wind-fed X-ray pulsars with known orbital periods (like GX 301-2 and Vela X-1) enable the determination of the basic dimensionless model parameters and estimation of the neutron star magnetic field. In equilibrium pulsars with independently measured magnetic fields, the model enables the stellar wind velocity to be independently estimated. For non-equilibrium pulsars, there exists a maximum spin-down rate of the accreting neutron star. The model can also explain bright flares in Supergiant Fast X-ray Transients if stellar winds of the O-supergiant companions are magnetized.

Proceedings of The 34th International Cosmic Ray Conference — PoS(ICRC2015), Aug 18, 2016

Theories of galaxy formation predict the existence of extended gas halo around spiral galaxies. I... more Theories of galaxy formation predict the existence of extended gas halo around spiral galaxies. If there are 10-100 nG magnetic fields at several ten kpc distances from the galaxies, extended galactic cosmic ray (CR) haloes could also exist. Galactic CRs can interact with the tenuous hot halo gas to produce observable gamma-rays. We have performed search for a gamma-ray halo around the M31 galaxy-the closest large spiral galaxy. Our analysis of almost 7 years of the Fermi LAT data revealed the presence of a spatially extended diffuse emission excess around M31. The data can be fitted using the simplest morphology of a uniformly bright circle. The best fit gave a 4.7σ significance for a 0.9 • (12 kpc) halo with a photon flux of ∼ (3.2 ± 1.0) × 10 −9 cm −2 s −1 and a luminosity of (4.0±1.5)×10 38 erg s −1 in the energy range 0.3-100 GeV. Our results also imply a low level of the flux from the disc of the M31 galaxy (3.3±1.0)×10 −10 cm −2 s −1. The corresponding gamma-ray luminosity, 5×10 37 erg s −1 is several times smaller than the luminosity of the Milky Way. This difference could be explained by a lower star formation rate in M31: there are less CRs and the level of the ISM turbulence is lower, which in turn leads to a shorter time of CR containment.

Physical Review D

Note that the requisite p mF axion is much less than the QCD scale; hence this is not the QCD axion.

Proceedings of 7th INTEGRAL Workshop — PoS(Integral08)

, which reached a maximum X-ray flux of 400mCrab in the 5-100keV range. The outburst was observed... more , which reached a maximum X-ray flux of 400mCrab in the 5-100keV range. The outburst was observed by INTEGRAL and RXTE. The energy of the fundamental cyclotron line has been measured with INTEGRAL and RXTE at ∼45keV. Flaring activity was observed during the rise to the peak of the outburst. RXTE observations during one of these flares found the energy of the fundamental cyclotron line shifted to a significantly higher position than during the rest of the outburst, where it remains constant. Also, the energy-dependent pulse profiles during the flare differ significantly from the rest of the outburst. These differences have been interpreted with the presence of magnetospheric instabilities at the onset of the accretion. A decomposition method is applied to A 0535+26 pulse profiles. Basic assumptions of the method are that the asymmetry observed in the pulse profiles is caused by non-antipodal magnetic poles, and that the emission regions have axisymmetric beam patterns. Using pulse profiles obtained from RXTE observations, the contribution of the two emission regions has been disentangled. Constraints on geometry of the pulsar and a possible solution of the beam pattern are given. First results of the comparison of the reconstructed beam pattern with a geometrical model that includes relativistic light deflection are presented.

Proceedings of the International Astronomical Union, 2015

We perform a Bayesian analysis of pulsar-timing residuals from the NANOGrav pulsar-timing array t... more We perform a Bayesian analysis of pulsar-timing residuals from the NANOGrav pulsar-timing array to search for a specific form of stochastic narrow-band signal produced by oscillating gravitational potential (Gravitational Potential Background) in the Galactic halo. Such oscillations arise in models of warm dark matter composed of an ultralight massive scalar field (m ≃ 10−23 eV). The propagation of an electromagnetic signal from a pulsar through the time-dependent spacetime will leave an imprint in the pulsar timing, much like a gravitational wave. From the physical point of view, this is the classical Sachs-Wolfe effect. A distinctive feature of the pulsar-timing residuals due to GBP produced by a variable scalar field is that the amplitude of the TOA residuals should be independent of the pulsar location in the sky. In the monochromatic approximation, the stringent upper limit (95% C.L.) on the variable gravitational potential amplitude is found to be (Ψ c <1.14 × 10−15), corre...

Astronomy and Astrophysics, Apr 1, 1987

Assuming binary neutron star mergings as a standard-candle model for GRBs, an independent estimat... more Assuming binary neutron star mergings as a standard-candle model for GRBs, an independent estimate is obtained for the redshift of GRB970228 and GRB970508 with optical counterparts, using mean statistical properties of GRBs observed by BATSE. We derive z = 0.7 ± 0.1 and z = 1.9 ± 0.1 for GRB970228 and GRB970508 respectively, depending on the power-law index of the GRB spectrum s = −1.1 ± 0.3 and the value of the redshift of the initial star formation z * = 3 − 10 in a flat Ω = 1 Universe with a cosmological term Λ = 0.7.

Astron Rep, Dec 22, 1998

In this article we use "Scenario Machine"-the population synthesis simulatorto calculate the evol... more In this article we use "Scenario Machine"-the population synthesis simulatorto calculate the evolution of the populations of the selected types of X-ray sources after the starformation burst with the total mass in binaries 1.5 • 10 6 M ⊙ during the first 20 Myr after the burst. Sources of the four types were calculated: transient sources-accreting neutron stars with Be-stars; accreting neutron stars in pair with supergiants; Cyg X-1-like sources-black holes with supergiants; superaccreting black holes. We used two values of the α − coef f icient in the mass-function: 2.35 (Salpeter's function) and 1.01 ("flat spectrum"). The calculations were made for two values of the upper limit of the mass-function: 120 and 30 M ⊙. For the flat spectrum the number of sources of all types significantly increased. Decreasing of the upper mass limit below the critical mass of a black hole formation increase (for the "flat spectrum") the number of transient sources with neutron stars up to ≈ 300. We give approximating formulae for the time dependence of source numbers.

Monthly Notices of the Royal Astronomical Society, 2016

The Rossi X-ray Timing Explorer (RXTE) observed four outbursts of the accreting Xray binary trans... more The Rossi X-ray Timing Explorer (RXTE) observed four outbursts of the accreting Xray binary transient source, GX 304−1 in 2010 and 2011. We present results of detailed 3-100 keV spectral analysis of 69 separate observations, and report a positive correlation between cyclotron line parameters, as well as other spectral parameters, with power-law flux. The cyclotron line energy, width and depth versus flux, and thus luminosity, correlations show a flattening of the relationships with increasing luminosity, which are well described by quasi-spherical or disc accretion that yield the surface magnetic field to be ∼5 × 10 12 Gauss. Since HEXTE (High Energy X-ray Timing Experiment) cluster A was fixed aligned with the Proportional Counter Array field of view and cluster B was fixed viewing a background region 1. • 5 off of the source direction during these observations near the end of the RXTE mission, the cluster A background was estimated from cluster B events using HEXTEBACKEST. This made possible the detection of the ∼55 keV cyclotron line and an accurate measurement of the continuum. Correlations of all spectral parameters with the primary 2-10 keV power-law flux reveal it to be the primary driver of the spectral shape. The accretion is found to be in the collisionless shock braking regime.

AIP Conference Proceedings, 2008

Monthly Notices of the Royal Astronomical Society, 2016

We have characterized the typical temporal behaviour of the bright X-ray flares detected from the... more We have characterized the typical temporal behaviour of the bright X-ray flares detected from the three Supergiant Fast X-ray Transients showing the most extreme transient behaviour (XTE J1739-302, IGR J17544-2619, SAX J1818.6-1703). We focus here on the cumulative distributions of the waiting-time (time interval between two consecutive X-ray flares), and the duration of the hard X-ray activity (duration of the brightest phase of an SFXT outburst), as observed by INTEGRAL/IBIS in the energy band 17-50 keV. Adopting the cumulative distribution of waiting-times, it is possible to identify the typical timescale that clearly separates different outbursts, each composed by several single flares at ∼ks timescale. This allowed us to measure the duration of the brightest phase of the outbursts from these three targets, finding that they show heavy-tailed cumulative distributions. We observe a correlation between the total energy emitted during SFXT outbursts and the time interval covered by the outbursts (defined as the elapsed time between the first and the last flare belonging to the same outburst as observed by IN T EGRAL). We show that temporal properties of flares and outbursts of the sources, which share common properties regardless different orbital parameters, can be interpreted in the model of magnetized stellar winds with fractal structure from the OB-supergiant stars.

Gamma-Ray Bursts in the Afterglow Era

Arguments are given favoring possible connection of GRBs with core collapse of massive Wolf-Rayet... more Arguments are given favoring possible connection of GRBs with core collapse of massive Wolf-Rayet stars. We analyze the observed properties of cosmic gamma-ray bursts, Wolf-Rayet (WR) stars and their CO-cores in the end of evolution. WR stars are deprived of their extended hydrogen envelopes, which makes it easier for the collapse energy to transform into observed gamma-ray emission. Presently, of ∼ 90 well-localized gamma-ray bursts, 21 ones are optically identified and for 16 of them redshifts are measured (z = 0.4 ÷ 4.5). The observed energy of gamma-ray bursts spans over a wide range from 3 × 10 51 to 2 × 10 54 ergs. There is some evidence that this distribution N (∆E) is bimodal if take into account GRB980425 associated with a peculiar type Ic supernova SN1998bw in a nearby galaxy ESO 184-G82 for which ∆E γ ≈ 10 48 ergs. These characteristics of gamma-ray bursts are similar to the distribution of the final masses of CO-cores of WR stars which is also wide and homogeneous: M CO = (1 − 2)M ÷ (20 − 44)M. A possible bimodality of the gamma-ray burst energy distribution (E 1 = 10 48 erg; ∆E 2 = 3×10 51 ÷2×10 54 erg) is in accord with the bimodal mass distribution of relativistic objects (M N S = (1.35 ± 0.15)M ; M BH = (4 ÷ 15)M). That the supernova SN1998bw is of the "peculiar Ic" type, atypical for WR collapses (type Ib/c), can be related to the rotation of the collapsing CO-core which can make the collapse longer and lead to the formation of a neutron star, the decrease of the gamma-ray burst energy, and the increase of the fraction of kinetic energy transported to the envelope. The expected collapse rate of CO-cores of most compact WR stars of type WO in the Galaxy is ∼ 10 −5 per year, which is only by one and a half order of magnitude higher than the average gamma-ray burst rate in one galaxy (∼ 10 −6 − 10 −7 per year). Two particular models that use WR stars as gamma-ray burst progenitors are considered: the hypernova model by Paczyński (1998) and the model of unstable CO-core collapse suggested by Gershtein (2000). In both models the allowance of a gamma-ray beaming or random outcome of the CO-core collapse due to some instabilities permits one to bring the rate of CO-core collapses in accordance with that of gamma-ray bursts. We argue that WR stars (most probably, of type WO) can be considered as progenitors of cosmic gamma-ray bursts. Two types of gamma-ray bursts are predicted in correspondence with the bimodal mass distribution of the relativistic objects. Three types of optical afterglows should appear depending on which CO-core is collapsing: of a single WR star, of a WR star in a WR+O or a hypothetic WR+(A-M) binary system. In addition, we briefly consider a model of gamma-ray bursts as a transient phenomenon occurring at early stages of galactic evolution (z > 1), when very massive 1 (M > 100M) low-metallicity stars could form. Such massive stars should also loose their hydrogen envelopes and become massive WR stars whose collapses could be accompanied by gamma-ray bursts. WR-galaxies can be most probable candidates for gamma-ray burst host galaxies.

Astronomical & Astrophysical Transactions, 1993

The distribution of old galactic neutron stars is computed for model galactic potential and initi... more The distribution of old galactic neutron stars is computed for model galactic potential and initial neutron star velocity distribution following from evolution scenario of binary systems. Neutron stars are supposed to get their initial velocities during (asymmetric) supernova explosion of a single massive (M,, > 10 Ma) star or a massive component of a binary system. Old neutron star population is shown to be capable of filling a torus-like area extending to a few tens kiloparsecs above the galactic plane. Statistical tests related to the spatial distribution of gamma-ray bursts are carried out for the distribution obtained.

The Astrophysical Journal, 1997

The prospects of detecting extragalactic supernovae (SNs) down to visual magnitudes 23È25 give us... more The prospects of detecting extragalactic supernovae (SNs) down to visual magnitudes 23È25 give us hope for observing them in the most distant parts of the universe. Using a Monte Carlo method of stellar population synthesis (the Scenario Machine), we compute, under standard assumptions of stellar evolution, the rates of SNs of various types in a model galaxy and evaluate the SN rates in the universe. The expected cumulative distribution log N [ m (number of events [ stellar magnitude) is calculated for various SN types and di †erent star formation histories in the universe. The results are also presented in terms of evolution of supernova units with redshifts. Recent observational data on the high-redshift SN Ia rate are in good agreement with our predictions for the relative density of baryons contained in stars at the present time () * \ 0.0057).

Astronomy & Astrophysics, 2005

We present the analysis of Rossi X-ray Timing Explorer (RXTE) observations of the turn-on phase o... more We present the analysis of Rossi X-ray Timing Explorer (RXTE) observations of the turn-on phase of a 35 day cycle of the X-ray binary Her X-1. During the early phases of the turn-on, the energy spectrum is composed of X-rays scattered into the line of sight plus heavily absorbed X-rays. The energy spectra in the 3-17 keV range can be described by a partial covering model, where one of the components is influenced by photoelectric absorption and Thomson scattering in cold material plus an iron emission line at 6.5 keV. In this paper we show the evolution of spectral parameters as well as the evolution of the pulse profile during the turn-on. We describe this evolution using Monte Carlo simulations which self-consistently describe the evolution of the X-ray pulse profile and of the energy spectrum.

First results of observations of the low mass X-ray binary Her X-1/HZ Her performed by the INTEGR... more First results of observations of the low mass X-ray binary Her X-1/HZ Her performed by the INTEGRAL satellite in July-August 2005 are presented. A significant part of one 35 day main-on state was covered. The cyclotron line in the X-ray spectrum is well observed and its position and shape, as well as its variability with time and phase of the 1.24 s pulsation are explored. X-ray pulse profiles for different energy bands are studied throughout the observation. The pulse period is found to vary on short time scales revealing a dynamical spin-up/spin-down behavior. Results of simultaneous optical observations of HZ Her are also discussed. Key words: X-ray binaries; accretion disks; neutron stars.

arXiv: Instrumentation and Methods for Astrophysics, 2020

The new SAI MSU observatory 2.5-meter telescope and capabilities of its current instrumentation a... more The new SAI MSU observatory 2.5-meter telescope and capabilities of its current instrumentation are described. The facility operates actively since 2014 in parallel to the engineering works. It has delivered a number of prominent results in the field of optical and near-infrared photometry and spectroscopy as well as newly developed observational techniques like differential speckle polarimetry.

Accretion Flows in Astrophysics

A theory of quasi-spherical subsonic accretion onto slowly rotating magnetized neutron stars is p... more A theory of quasi-spherical subsonic accretion onto slowly rotating magnetized neutron stars is presented. In this regime, the accreted matter settles with subsonic velocities onto the rotating magnetosphere forming an extended quasi-spherical shell. The accretion rate in the shell is determined by the ability of the plasma to enter the magnetosphere due to the Rayleigh-Taylor instability with account for cooling. This accretion regime may be established for moderate X-ray luminosities, corresponding to accretion rates \\(\\dot M< \\dot M^\\dag \\simeq 4\\times 10^{16}\\) g s−1. For higher accretion rates a free-fall gap appears, due to strong Compton cooling of the flow above the magnetosphere, and accretion becomes highly non-stationary. Observations of spin-up and spin-down in equilibrium wind-fed X-ray pulsars with known orbital periods (like GX 301-2 and Vela X-1) enable the determination of the basic dimensionless model parameters and estimation of the neutron star magnetic field. In equilibrium pulsars with independently measured magnetic fields, the model enables the stellar wind velocity to be independently estimated. For non-equilibrium pulsars, there exists a maximum spin-down rate of the accreting neutron star. The model can also explain bright flares in Supergiant Fast X-ray Transients if stellar winds of the O-supergiant companions are magnetized.

Proceedings of The 34th International Cosmic Ray Conference — PoS(ICRC2015), Aug 18, 2016

Theories of galaxy formation predict the existence of extended gas halo around spiral galaxies. I... more Theories of galaxy formation predict the existence of extended gas halo around spiral galaxies. If there are 10-100 nG magnetic fields at several ten kpc distances from the galaxies, extended galactic cosmic ray (CR) haloes could also exist. Galactic CRs can interact with the tenuous hot halo gas to produce observable gamma-rays. We have performed search for a gamma-ray halo around the M31 galaxy-the closest large spiral galaxy. Our analysis of almost 7 years of the Fermi LAT data revealed the presence of a spatially extended diffuse emission excess around M31. The data can be fitted using the simplest morphology of a uniformly bright circle. The best fit gave a 4.7σ significance for a 0.9 • (12 kpc) halo with a photon flux of ∼ (3.2 ± 1.0) × 10 −9 cm −2 s −1 and a luminosity of (4.0±1.5)×10 38 erg s −1 in the energy range 0.3-100 GeV. Our results also imply a low level of the flux from the disc of the M31 galaxy (3.3±1.0)×10 −10 cm −2 s −1. The corresponding gamma-ray luminosity, 5×10 37 erg s −1 is several times smaller than the luminosity of the Milky Way. This difference could be explained by a lower star formation rate in M31: there are less CRs and the level of the ISM turbulence is lower, which in turn leads to a shorter time of CR containment.

Physical Review D

Note that the requisite p mF axion is much less than the QCD scale; hence this is not the QCD axion.

Proceedings of 7th INTEGRAL Workshop — PoS(Integral08)

, which reached a maximum X-ray flux of 400mCrab in the 5-100keV range. The outburst was observed... more , which reached a maximum X-ray flux of 400mCrab in the 5-100keV range. The outburst was observed by INTEGRAL and RXTE. The energy of the fundamental cyclotron line has been measured with INTEGRAL and RXTE at ∼45keV. Flaring activity was observed during the rise to the peak of the outburst. RXTE observations during one of these flares found the energy of the fundamental cyclotron line shifted to a significantly higher position than during the rest of the outburst, where it remains constant. Also, the energy-dependent pulse profiles during the flare differ significantly from the rest of the outburst. These differences have been interpreted with the presence of magnetospheric instabilities at the onset of the accretion. A decomposition method is applied to A 0535+26 pulse profiles. Basic assumptions of the method are that the asymmetry observed in the pulse profiles is caused by non-antipodal magnetic poles, and that the emission regions have axisymmetric beam patterns. Using pulse profiles obtained from RXTE observations, the contribution of the two emission regions has been disentangled. Constraints on geometry of the pulsar and a possible solution of the beam pattern are given. First results of the comparison of the reconstructed beam pattern with a geometrical model that includes relativistic light deflection are presented.

Proceedings of the International Astronomical Union, 2015

We perform a Bayesian analysis of pulsar-timing residuals from the NANOGrav pulsar-timing array t... more We perform a Bayesian analysis of pulsar-timing residuals from the NANOGrav pulsar-timing array to search for a specific form of stochastic narrow-band signal produced by oscillating gravitational potential (Gravitational Potential Background) in the Galactic halo. Such oscillations arise in models of warm dark matter composed of an ultralight massive scalar field (m ≃ 10−23 eV). The propagation of an electromagnetic signal from a pulsar through the time-dependent spacetime will leave an imprint in the pulsar timing, much like a gravitational wave. From the physical point of view, this is the classical Sachs-Wolfe effect. A distinctive feature of the pulsar-timing residuals due to GBP produced by a variable scalar field is that the amplitude of the TOA residuals should be independent of the pulsar location in the sky. In the monochromatic approximation, the stringent upper limit (95% C.L.) on the variable gravitational potential amplitude is found to be (Ψ c <1.14 × 10−15), corre...

Astronomy and Astrophysics, Apr 1, 1987

Assuming binary neutron star mergings as a standard-candle model for GRBs, an independent estimat... more Assuming binary neutron star mergings as a standard-candle model for GRBs, an independent estimate is obtained for the redshift of GRB970228 and GRB970508 with optical counterparts, using mean statistical properties of GRBs observed by BATSE. We derive z = 0.7 ± 0.1 and z = 1.9 ± 0.1 for GRB970228 and GRB970508 respectively, depending on the power-law index of the GRB spectrum s = −1.1 ± 0.3 and the value of the redshift of the initial star formation z * = 3 − 10 in a flat Ω = 1 Universe with a cosmological term Λ = 0.7.

Astron Rep, Dec 22, 1998

In this article we use "Scenario Machine"-the population synthesis simulatorto calculate the evol... more In this article we use "Scenario Machine"-the population synthesis simulatorto calculate the evolution of the populations of the selected types of X-ray sources after the starformation burst with the total mass in binaries 1.5 • 10 6 M ⊙ during the first 20 Myr after the burst. Sources of the four types were calculated: transient sources-accreting neutron stars with Be-stars; accreting neutron stars in pair with supergiants; Cyg X-1-like sources-black holes with supergiants; superaccreting black holes. We used two values of the α − coef f icient in the mass-function: 2.35 (Salpeter's function) and 1.01 ("flat spectrum"). The calculations were made for two values of the upper limit of the mass-function: 120 and 30 M ⊙. For the flat spectrum the number of sources of all types significantly increased. Decreasing of the upper mass limit below the critical mass of a black hole formation increase (for the "flat spectrum") the number of transient sources with neutron stars up to ≈ 300. We give approximating formulae for the time dependence of source numbers.