arXiv subjects

R. A. Sunyaev

Publications and source records attributed to R. A. Sunyaev.

At least 55 records · Page 3Linked to original sources

arXiv2017

IGR J17445-2747 - yet another X-ray burster in the Galactic bulge

The discovery of a type I X-ray burst from the faint unidentified transient source IGR J17445-2747 in the Galactic bulge by the JEM-X telescope onboard the INTEGRAL observatory is reported. Type I bursts are believed to be associated with thermonuclear explosions of accreted matter on the surface of a neutron star with a weak magnetic field in a low-mass X-ray binary. Thus, this observation allows the nature of this source to be established.

astro-ph.HE↗

arXiv2016

The outburst of the X-ray nova GRS 1739-278 in September, 2016

During the scanning observations of the Galactic Center region in August - September 2016 we detected the new outburst of the historical X-ray nova GRS 1739-278, the black hole candidate LMXB system. In this letter we present results of INTEGRAL and Swift-XRT observations taken during the outburst. In hard X-ray band (20-60 keV) the flux from the source raised from $\sim$11 to $\sim$30 mCrab between 3 and 14 of September. For nearly 8 days the source has been observed at this flux level and then faded to $\sim$15 mCrab. The broadband quasi-simultaneous spectrum obtained during the outburst is well described by the absorbed powerlaw with the photon index $Γ=1.86\pm0.07$ in broad energy range 0.5-150 keV, with absorption corresponding to ${N_{H}}=2.3\times10^{22}$ cm$^{-2}$ assuming solar abundance. Based on this we can conclude that the source was in the low/hard state. From the lightcurve and spectra we propose that this outburst was `failed', i.e. amount of accreted matter was not sufficient to achieve the high/soft spectral state with dominant soft blackbody component as seen in normal outbursts of black hole candidates.

astro-ph.HE↗

arXiv2013

INTEGRAL observations of SS433: system's parameters and nutation of supercritical accretion disk

Based on multiyear INTEGRAL observations of SS433 in 2003-2011, a composite IBIS/ISGRI 18-60 keV orbital light curve is constructed around zero precessional phases psi_{pr}= 0 at the maximim accretion disk opening angle. It shows a peculiar shape with significant excess near the orbital phase phi_orb= 0.25, which is not seen in the softer 2-10 keV energy band. The 40-60 keV orbital light curve demonstrates two almost equal humps at phases \sim 0.25 and \sim 0.75, most likely due to nutation effects of the accretion disk. The nutational variability of SS433 in 15-50 keV with a period of 6.290 days is independently found from timing analysis of Swift/BAT data. The change of the off-eclipse 18-60 keV X-ray flux with the precessional phase shows a double-wave form with strong primary maximum at psi_{pr}= 0 and weak but significant secondary maximum at psi_{pr}= 0.6. A weak variability of the 18-60 keV flux in the middle of the orbital eclipse correlated with the disk precessional phase is also observed. The joint analysis of the broadband 18-60 keV orbital and precessional light curves confirms the presence of a hot extended corona in the central parts of the supercritical accretion disk and constrains the binary mass ratio in SS433 in the range 0.5>q>0.3, suggesting the black hole nature of the compact object.

astro-ph.HE↗

arXiv2012

SRG/eROSITA prospects for detection of GRB afterglows

We discuss the potential of the eROSITA telescope on board the \emph{Spectrum-X-Gamma} observatory to detect gamma-ray burst (GRB) X-ray afterglows during its 4-year all-sky survey. The expected rate of afterglows associated with long-duration GRBs without any information on the bursts proper that can be identified by a characteristic power-law light curve in the eROSITA data is 4--8 events per year. An additional small number, $\lesssim 2$ per year, of afterglows may be associated with short GRBs, ultra hard (GeV) GRBs and X-ray flashes. eROSITA can thus provide the first unbiased (unaffected by GRB triggering) sample of $\lesssim 40$ X-ray afterglows, which can be used for statistical studies of GRB afterglows and for constraining the shape of the GRB $\log N$--$\log S$ distribution at its low-fluence end. The total number of afterglows detected by eROSITA may be yet higher due to orphan afterglows and failed GRBs. The actual detection rate could thus provide interesting constraints on the properties of relativistic jets associated with collapse of massive stars. Finally, eROSITA can provide accurate ($\lesssim 30"$) coordinates of newly discovered afterglows within a day after the event, early enough for scheduling further follow-up observations.

astro-ph.HE↗

arXiv2012

On the Change of the Inner Boundary of an Optically Thick Accretion Disk around White Dwarfs Using the Dwarf Nova SS Cyg as an Example

We present the results of our studies of the aperiodic optical flux variability for SS Cyg, an accreting binary systemwith a white dwarf. The main set of observational data presented here was obtained with the ANDOR/iXon DU-888 photometer mounted on the RTT-150 telescope, which allowed a record(for CCD photometers) time resolution up to 8 ms to be achieved. The power spectra of the source's flux variability have revealed that the aperiodic variability contains information about the inner boundary of the optically thick flow in the binary system. We show that the inner boundary of the optically thick accretion disk comes close to the white dwarf surface at the maximum of the source's bolometric light curve, i.e., at the peak of the instantaneous accretion rate onto the white dwarf, while the optically thick accretion disk is truncated at distances 8.5e9 cm ~10 R_{WD} in the low state. We suggest that the location of the inner boundary of the accretion disk in the binary can be traced by studying the parameters of the power spectra for accreting white dwarfs. In particular, this allows the mass of the accreting object to be estimated.

astro-ph.GA↗

arXiv2011

The evolution of CMB spectral distortions in the early Universe

The energy spectrum of the cosmic microwave background (CMB) allows constraining episodes of energy release in the early Universe. In this paper we revisit and refine the computations of the cosmological thermalization problem. For this purpose a new code, called CosmoTherm, was developed that allows solving the coupled photon-electron Boltzmann equation in the expanding, isotropic Universe for small spectral distortion in the CMB. We explicitly compute the shape of the spectral distortions caused by energy release due to (i) annihilating dark matter; (ii) decaying relict particles; (iii) dissipation of acoustic waves; and (iv) quasi-instantaneous heating. We also demonstrate that (v) the continuous interaction of CMB photons with adiabatically cooling non-relativistic electrons and baryons causes a negative mu-type CMB spectral distortion of DI_nu/I_nu ~ 10^{-8} in the GHz spectral band. We solve the thermalization problem including improved approximations for the double Compton and Bremsstrahlung emissivities, as well as the latest treatment of the cosmological recombination process. At redshifts z <~ 10^3 the matter starts to cool significantly below the temperature of the CMB so that at very low frequencies free-free absorption alters the shape of primordial distortions significantly. In addition, the cooling electrons down-scatter CMB photons introducing a small late negative y-type distortion at high frequencies. We also discuss our results in the light of the recently proposed CMB experiment Pixie, for which CosmoTherm should allow detailed forecasting. Our current computations show that for energy injection because of (ii) and (iv) Pixie should allow to improve existing limits, while the CMB distortions caused by the other processes seem to remain unobservable with the currently proposed sensitivities and spectral bands of Pixie.

astro-ph.CO↗

arXiv2011

Spread of Matter over a Neutron-Star Surface During Disk Accretion: Deceleration of Rapid Rotation

The problem of disk accretion onto the surface of a neutron star with a weak magnetic field at a luminosity exceeding several percent of Eddington is reduced to the problem of the braking of a hypersonic flow with a velocity that is 0.4-0.5 of the speed of light above the base of the spreading layer -- a dense atmosphere made up of previously fallen matter. We show that turbulent braking in the Prandtl-Karman model with universally accepted coefficients for terrestrial conditions and laboratory experiments and a ladder of interacting gravity waves in a stratified quasi-exponential atmosphere at standard Richardson numbers lead to a spin-up of the massive zone that extends to the ocean made up of a plasma with degenerate electrons. Turbulent braking in the ocean at the boundary with the outer solid crust reduces the rotation velocity to the solid-body rotation velocity of the star. This situation should lead to strong heating of deep atmospheric layers and to the switch-off of the explosive helium burning mechanism. Obviously, a more efficient mechanism for the dissipation of a fast azimuthal flow in the atmosphere should operate in X-ray bursters. We show that a giant solitary gravity wave in the atmosphere can lead to energy dissipation and to a sharp decrease in azimuthal velocity in fairly rarefied atmospheric layers above the zone of explosive helium burning nuclear reactions. We discuss the reasons why this wave, that has no direct analog in the Earth's atmosphere or ocean, appears and its stability. We pose the question as to whether neutron stars with massive atmospheres, spun up to high velocities by accreting matter from a disk, can exist among the observed Galactic X-ray sources.

astro-ph.HE↗

arXiv2011

Supermassive Black Hole in an Elliptical Galaxy: Accretion of a Hot Gas with a Low but Finite Angular Momentum

The accretion of hot slowly rotating gas onto a supermassive black hole is considered. Rotation velocities at the Bondi radius r_B are small in comparison with speed of sound c_s. The centrifugal barrier at a depth r_c = l^2/G M_BH << r_B hinders supersonic accretion. We take into account saturated electron thermal conductivity and Bremsstrahlung energy losses of two temperature plasma for density and temperature near the Bondi radius similar to those observed in M87 galaxy. Joint action of electron thermal conductivity and free-free radiation leads to the effective cooling of accreting plasma and formation of the subsonic settling of accreting gas above the zone of a centrifugal barrier. A toroidal condensation and a hollow funnel that separates the torus from the black hole emerge near the barrier. The barrier divides the flow into two regions: (1) the settling zone with slow subKeplerian rotation and (2) the zone with rapid supersonic nearly Keplerian rotation. Existence of the centrifugal barrier leads to significant decrease of the accretion rate dM/dt in comparison with the critical Bondi solution for gamma=5/3. Shear instabilities in the torus and related friction cause the gas to spread slowly in the equatorial plane in two directions. As a result, outer (r>r_c) and inner (r<r_c) disks are formed. The gas enters the zone of the internal ADAF flow along the accretion disk (r<r_c). Since the angular momentum is conserved, the outer disk removes outward an excess of angular momentum along with part of the matter falling into the torus. Such outer Keplerian disk was observed by Hubble Space Telescope around the nucleus of the M87 galaxy in the optical emission lines. Turbulence causes rotation. We discuss the characteristic times during which the turbulence should lead to the changes in the orientation of the torus, accretion disk and, possibly, of the jet.

astro-ph.HE↗

arXiv2011

Type Ia supernovae and stellar winds in AGN driven relativistic bubbles

We analyse behavior of stellar winds of evolved stars and the outcome of SN Ia explosions in the AGN driven relativistic bubble. We find that the expansion of wind shells is efficiently decelerated by the relativistic pressure; their bulk motion however is preserved so they cross the bubble together with the parent star. The wind material occupies a small fraction of bubble volume and does not affect substantially the expansion of SN remnants. The estimated maximal radius of a SN remnant in the bubble is 30-40 pc, if the envelope keeps its integrity and remains spherical. A fragmentation of SN shell due to Rayleigh-Taylor instability can alleviate the propagation of the SN material so the ejecta fragments are able to cross the relativistic bubble. Outside the bubble wind shells and supernova fragments are decelerated in the intracluster medium at close range off the bubble boundary. The deposited SNe Ia material can enrich the intracluster gas with metals in a thin layer at the boundary of the relativistic bubble. This process may lead to a rim of enhanced line emission. In the opposite limit, when the fragmentation of supernova remnant is moderate or absent, the SN Ia matter is advected by the relativistic plasma and may leave the central region of the bright cluster galaxy together with buoyantly moving bubbles.

astro-ph.HE↗

arXiv2011

Fast optical variability of SS 433

We study the optical variability of the peculiar Galactic source SS 433 using the observations made with the Russian Turkish 1.5-m telescope (RTT150). A simple technique which allows to obtain high-quality photometric measurements with 0.3-1 s time resolution using ordinary CCD is described in detail. Using the test observations of nonvariable stars, we show that the atmospheric turbulence introduces no significant distortions into the measured light curves. Therefore, the data obtained in this way are well suited for studying the aperiodic variability of various objects. The large amount of SS 433 optical light curve measurements obtained in this way allowed us to obtain the power spectra of its flux variability with a record sensitivity up to frequencies of ~0.5 Hz and to detect its break at frequency =~2.4e-3 Hz. We suggest that this break in the power spectrum results from the smoothing of the optical flux variability due to a finite size of the emitting region. Based on our measurement of the break frequency in the power spectrum, we estimated the size of the accretion-disk photosphere as 2e12 cm. We show that the amplitude of the variability in SS 433 decreases sharply during accretion-disk eclipses, but it does not disappear completely. This suggests that the size of the variable optical emission source is comparable to that of the normal star whose size is therefore R_O \approx 2e12 cm \approx 30 R_sun. The decrease in flux variability amplitude during eclipses suggests the presence of a nonvariable optical emission component with a magnitude m_R=~13.2.

astro-ph.HE↗

arXiv2010

The ISW-tSZ cross correlation: ISW extraction out of pure CMB data

If Dark Energy introduces an acceleration in the universal expansion then large scale gravitational potential wells should be shrinking, causing a blueshift in the CMB photons that cross such structures (Integrated Sachs-Wolfe effect, [ISW]). Galaxy clusters are known to probe those potential wells. In these objects, CMB photons also experience inverse Compton scattering off the hot electrons of the intra-cluster medium, and this results in a distortion with a characteristic spectral signature of the CMB spectrum (the so-called thermal Sunyaev-Zel'dovich effect, [tSZ]). Since both the ISW and the tSZ effects take place in the same potential wells, they must be spatially correlated. We present how this cross ISW-tSZ signal can be detected in a CMB-data contained way by using the frequency dependence of the tSZ effect in multi frequency CMB experiments like {\it Planck}, {\em without} requiring the use of external large scale structure tracers data. We find that by masking low redshift clusters, the shot noise level decreases significantly, boosting the signal to noise ratio of the ISW--tSZ cross correlation. We also find that galactic and extragalactic dust residuals must be kept at or below the level of ~0.04 muK^2 at l=10, a limit that is a factor of a few below {\it Planck}'s expectations for foreground subtraction. If this is achieved, CMB observations of the ISW-tSZ cross correlation should also provide an independent probe for the existence of Dark Energy and the amplitude of density perturbations.

astro-ph.CO↗

arXiv2010

New fast X-ray transient IGR J18462-0223 discovered by the INTEGRAL observatory

Details of the discovery of a new X-ray source, IGR J18462-0223, on October 12, 2007, during a short (several hours), intense (~ 35 mCrab at the peak) outburst of hard radiation by the IBIS/ISGRI gamma-ray telescope onboard the INTEGRAL observatory are given. The detection of another earlier outburst from this source occurred on April 28, 2006, in the archival data of the telescope is reported. We present the results of the source's localization and our spectral/timing analysis of the observational data. The source may turn out to be yet another representative of the continuously growing population of fast X-ray transients, which are the focus of attention because of the identification of their optical counterparts with early-type supergiants.

astro-ph.HE↗

arXiv2010

Photometry and spectroscopy of GRB 060526: A detailed study of the afterglow and host galaxy of a z=3.2 gamma-ray burst

Aims: With this paper we want to investigate the highly variable afterglow light curve and environment of gamma-ray burst (GRB) 060526 at $z=3.221$. Methods: We present one of the largest photometric datasets ever obtained for a GRB afterglow, consisting of multi-color photometric data from the ultraviolet to the near infrared. The data set contains 412 data points in total to which we add additional data from the literature. Furthermore, we present low-resolution high signal-to-noise spectra of the afterglow. The afterglow light curve is modeled with both an analytical model using broken power law fits and with a broad-band numerical model which includes energy injections. The absorption lines detected in the spectra are used to derive column densities using a multi-ion single-component curve-of-growth analysis from which we derive the metallicity of the host of GRB 060526. Results: The temporal behaviour of the afterglow follows a double broken power law with breaks at $t=0.090\pm0.005$ and $t=2.401\pm0.061$ days. It shows deviations from the smooth set of power laws that can be modeled by additional energy injections from the central engine, although some significant microvariability remains. The broadband spectral-energy distribution of the afterglow shows no significant extinction along the line of sight. The metallicity derived from \ion{S}{II} and \ion{Fe}{II} of [S/H] = --0.57 $\pm$0.25 and [Fe/H] = --1.09$\pm$0.24 is relatively high for a galaxy at that redshift but comparable to the metallicity of other GRB hosts at similar redshifts. At the position of the afterglow, no host is detected to F775W(AB) = 28.5 mag with the HST, implying an absolute magnitude of the host M(1500 Å)$>$--18.3 mag which is fainter than most long-duration hosts, although the GRB may be associated with a faint galaxy at a distance of 11 kpc.

astro-ph↗

arXiv2010

Fine-structure infrared lines from the Cassiopeia A knots

Aims: Archival observations of infrared fine-structure lines of the young Galactic supernova remnant Cassiopeia A allow us to test existing models and determine the physical parameters of various regions of the fast-moving knots (FMKs), which are metal-dominated clouds of material ejected by the supernova explosion. Methods: The fluxes of the far-infrared [O i] and [O iii] lines are extracted from the previously unpublished archival ISO data. The archival Spitzer data are used to determine the fluxes of the O, Ne, Si, S, Ar and Fe ion fine-structure lines originating in the FMKs. The ratios of these line fluxes are used for the plasma diagnostics. We also determine the infrared line flux ratios to the optical [O iii] 5007 A line in the knots having previously measured reddening. Additionally, we analyze several optical and near-infrared observations of the FMKs to obtain clearer insight into the post-shock photoionized region (PIR) structure. Results: We show that the infrared oxygen line flux predictions of all existing theoretical models are correct only to within a factor of several. For the models to reproduce the observations it is essential to include the electron conductivity and effects of the dust. Detailed analysis of the diagnostic line flux ratios allows us to qualitatively confirm the general model of the FMK emission and to determine observationally the physical conditions in the PIR after the shock front. We infer that the pre-shock cloud densities most probably constitute several hundred particles per cm^3. We also determine the Cas A luminosities in the infrared continuum and lines. We show that accounting for the charge exchange processes in the post-shock PIR allows us to reproduce most of the relevant spectral line ratios even in the frame of a single-temperature model of this region. We also estimate its plasma parameters, thickness, and carbon abundance.

astro-ph↗

arXiv2010

Polarization of X-ray lines from galaxy clusters and elliptical galaxies - a way to measure tangential component of gas velocity

We study the impact of gas motions on the polarization of bright X-ray emission lines from the hot intercluster medium (ICM). The polarization naturally arises from resonant scattering of emission lines owing to a quadrupole component in the radiation field produced by a centrally peaked gas density distribution. If differential gas motions are present then a photon emitted in one region of the cluster will be scattered in another region only if their relative velocities are small enough and the Doppler shift of the photon energy does not exceed the line width. This affects both the degree and the direction of polarization. The changes in the polarization signal are in particular sensitive to the gas motions perpendicular to the line of sight. We calculate the expected degree of polarization for several patterns of gas motions, including a slow inflow expected in a simple cooling flow model and a fast outflow in an expanding spherical shock wave. In both cases, the effect of non-zero gas velocities is found to be minor. We also calculate the polarization signal for a set of clusters, taken from large-scale structure simulations and evaluate the impact of the gas bulk motions on the polarization signal. We argue that the expected degree of polarization is within reach of the next generation of space X-ray polarimeters.

astro-ph.HE↗

arXiv2009

Cosmological recombination: feedback of helium photons and its effect on the recombination spectrum

In this paper we consider the re-processing of high frequency photons emitted by HeII and HeI during the epoch of cosmological recombination by HeI and HI. We demonstrate that, in comparison to computations which neglect all feedback processes, the number of cosmological recombination photons that are related to the presence of helium in the early Universe could be increased by ~40%-70%. Our computations imply that per helium nucleus ~3-6 additional photons could be produced. Therefore, a total of ~12-14 helium-related photons are emitted during cosmological recombination. This is an important addition to cosmological recombination spectrum which in the future may render it slightly easier to determine the primordial abundance of helium using differential measurements of the CMB energy spectrum. Also, since these photons are the only witnesses of the feedback process at high redshift, observing them in principle offers a way to check our understanding of the recombination physics. Here most interestingly, the feedback of HeII photons on HeI leads to the appearance of several additional, rather narrow spectral features in the HeI recombination spectrum at low frequencies. Consequently, the signatures of helium-related features in the CMB spectral distortion due to cosmological recombination at some given frequency can exceed the average level of ~17% several times. We find that in particular the bands around nu ~10GHz, ~35GHz, ~80GHz, and ~200GHz seem to be affected strongly. In addition, we computed the changes in the cosmological ionization history, finding that only the feedback of primary HeI photons on the dynamics of HeII-->HeI recombination has an effect, producing a change of DN_e/N_e ~+ 0.17% at z~2300. This result seems to be ~2-3 times smaller than the one obtained in earlier computations for this process (abridged).

astro-ph.CO↗

arXiv2009

Signals From the Epoch of Cosmological Recombination

The physical ingredients to describe the epoch of cosmological recombination are amazingly simple and well-understood. This fact allows us to take into account a very large variety of physical processes, still finding potentially measurable consequences for the energy spectrum and temperature anisotropies of the Cosmic Microwave Background (CMB). In this contribution we provide a short historical overview in connection with the cosmological recombination epoch and its connection to the CMB. Also we highlight some of the detailed physics that were studied over the past few years in the context of the cosmological recombination of hydrogen and helium. (abrigded)

astro-ph.CO↗

arXiv2009

Analysis of hard X-ray eclipse in SS433 from INTEGRAL observations

The analysis of hard X-ray INTEGRAL observations (2003-2008) of superaccreting galactic microquasar SS433 at precessional phases of the source with the maximum disk opening angle is carried out. It is found that the shape and width of the primary X-ray eclipse is strongly variable suggesting additional absorption in dense stellar wind and gas outflows from the optical A7I-component and the wind-wind collision region. The independence of the observed hard X-ray spectrum on the accretion disk precessional phase suggests that hard X-ray emission (20-100 keV) is formed in an extended, hot, quasi-isothermal corona, probably heated by interaction of relativistic jet with inhomogeneous wind outflow from the precessing supercritical accretion disk. A joint modeling of X-ray eclipsing and precessional hard X-ray variability of SS433 revealed by INTEGRAL by a geometrical model suggests the binary mass ratio $q=m_x/m_v\simeq 0.25÷0.5$. The absolute minimum of joint orbital and precessional $χ^2$ residuals is reached at $q\simeq 0.3$. The found binary mass ratio range allows us to explain the substantial precessional variability of the minimum brightness at the middle of the primary optical eclipse. For the mass function of the optical star $f_v=0.268 M_\odot$ as derived from Hillwig & Gies data, the obtained value of $q\simeq 0.3$ yields the masses of the components $m_x\simeq 5.3 M_\odot$, $m_v\simeq 17.7 M_\odot$, confirming the black hole nature of the compact object in SS433.

astro-ph.HE↗