arXiv subjects

M. E. Prokhorov

Publications and source records attributed to M. E. Prokhorov.

At least 37 records · Page 2Linked to original sources

arXiv2001

Period distribution of old accreting isolated neutron stars

In this paper we present calculations of period distribution for old accreting isolated neutron stars (INSs). After few billion years of evolution low velocity INSs come to the stage of accretion. At this stage INS's period evolution is governed by magnetic braking and angular momentum accreted. Since the interstellar medium is turbulized accreted momentum can either accelerate or decelerate spin of an INS, therefore the evolution of period has chaotic character. Our calculations show that in the case of constant magnetic field accreting INSs have relatively long spin periods (some hours and more, depending on INS's spatial velocity, its magnetic field and density of the surrounding medium). Such periods are much longer than the values measured by {\emph ROSAT} for 3 radio-silent isolated neutron stars. Due to long periods INSs should have high spin up/down rates, $\dot p$, which should fluctuate on a time scale of $\sim 1$ yr.

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arXiv2001

Orbital Eccentricity Effects on the Stochastic Gravitational Wave Background from Coalescing Binary Neutron Stars

Unresolved coalescing binary neutron stars in the Galaxy and beyond produce a stochastic gravitational wave background within the LISA frequency band, which can be potentially dangerous for possible detection of fundamentally more interesting relic cosmological backgrounds. Here we address the question what effects the unavoidable eccentricity of orbits of these systems should have on the properties of this backgrounds. In particularly, we examine starting from which frequency one-year observations with LISA can be secure from contamination by the noise produced by galactic binary neutron stars in eccentric orbits. We come to the conclusion that harmonics from such binaries do not contribute significantly above $\sim 10^{-3}$ Hz.

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arXiv2001

Gravitational wave background from coalescing compact stars in eccentric orbits

Stochastic gravitational wave background produced by a stationary coalescing population of binary neutron stars in the Galaxy is calculated. This background is found to constitute a confusion limit within the LISA frequency band up to a limiting frequency $\NUlim{}\sim 10^{-3}$ Hz, leaving the frequency window $\sim 10^{-3}$--$10^{-2}$ Hz open for the potential detection of cosmological stochastic gravitational waves and of signals involving massive black holes out to cosmological distances.

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arXiv2001

Evolution of isolated neutron stars in globular clusters: number of Accretors

With a simple model from the point of view of population synthesis we try to verify an interesting suggestion made by Pfahl & Rappaport (2001) that dim sources in globular clusters (GCs) can be isolated accreting neutron stars (NSs). Simple estimates show, that we can expect about 0.5-1 accreting isolated NS per typical GC with $M=10^5 M_{\odot}$ in correspondence with observations. Properties of old accreting isolated NSs in GCs are briefly discussed. We suggest that accreting NSs in GCs experienced significant magnetic field decay.

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arXiv2001

Gravitational Wave Astronomy: in Anticipation of First Sources to be Detected

The first generation of long-baseline laser interferometric detectors of gravitational waves will start collecting data in 2001-2003. We carefully analyse their planned performance and compare it with the expected strengths of astrophysical sources. The scientific importance of the anticipated discovery of various gravitatinal wave signals and the reliability of theoretical predictions are taken into account in our analysis. We try to be conservative both in evaluating the theoretical uncertainties about a source and the prospects of its detection. After having considered many possible sources, we place our emphasis on (1) inspiraling binaries consisting of stellar mass black holes and (2) relic gravitational waves. We draw the conclusion that inspiraling binary black holes are likely to be detected first by the initial ground-based interferometers. We estimate that the initial interferometers will see 2-3 events per year from black hole binaries with component masses 10-15M_\odot, with a signal-to-noise ratio of around 2-3, in each of a network of detectors consisting of GEO, VIRGO and the two LIGOs. It appears that other possible sources, including coalescing neutron stars, are unlikely to be detected by the initial instruments. We also argue that relic gravitational waves may be discovered by the space-based interferometers in the frequency interval 2x10^{-3}-10^{-2} Hz, at the signal-to-noise ratio level around 3.

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arXiv2000

The 35-day cycle in Her X-1 as observational appearance of freely precessing neutron star and forcedly precessing accretion disk

A careful analysis of X-ray light curves and pulse profiles of Her X-1 obtained over more than 20 years strongly evidences for free precession of a magnetized neutron star with rotational axis inclined to the orbital plane as a central clock underlying the observed 35-day period. Strong asymmetric X-ray illumination of the optical star atmosphere leads to the formation of gaseous streams coming out of the orbital plane and forming a tilted accretion disk around the neutron star. Such a disk precesses due to tidal forces and dynamical action of gaseous streams from the secondary companion. The locking of these torques with neutron star precession makes the net disk precession period to be very close to that of the neutron star free precession.

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arXiv2000

Isolated Neutron Stars in the Galaxy

In this article we briefly review our recent results on evolution and properties of isolated neutron stars (INSs) in the Galaxy. As the first step we calculate a {\it census} of INSs in our Galaxy. We infer a lower bound for the mean kick velocity of NSs, $ \sim $(200-300) ${\rm km s^{-1}}$. The same conclusion is reached for both a constant magnetic field ($B\sim 10^{12}$ G) and for a magnetic field decaying exponentially with a timescale $\sim 10^9$ yr. These results, moreover, constrain the fraction of low velocity stars, which could have escaped pulsar statistics, to $\sim$few percents. Then we show that the range of minimum value of magnetic moment, $μ_b$: $\sim 10^{29.5}\ge μ_b \ge 10^{28} {\rm G} {\rm cm}^3$, and the characteristic decay time, $t_d$: $\sim 10^8\ge t_d \ge 10^7 {\rm yrs}$, can be excluded assuming the standard initial magnetic momentum, $μ_0=10^{30} {\rm G} {\rm cm}^3$, if accreting INSs are observed. For these parameters an INS would never reach the stage of accretion from the interstellar medium even for a low space velocity of the star and high density of the ambient plasma. The range of excluded parameters increases for lower values of $μ_0$. It is shown that old accreting INSs become more abundant than young cooling INSs at X-ray fluxes below $\sim 10^{-13}$ erg cm$^{-2}$ s$^{-1}$. We can predict that about one accreting INS per square degree should be observed at the {\it Chandra} and {\it Newton} flux limits of $\sim 10^{-16}$ erg cm$^{-2}$ s$^{-1}.$ The weak {\it ROSAT} sources, associated with INSs, can be young cooling objects, if the NSs birth rate in the solar vicinity during the last $\sim 10^6$ yr was much higher than inferred from radiopulsar observations.

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arXiv2000

Constrains on parameters of magnetic field decay for accreting isolated neutron stars

The influence of exponential magnetic field decay (MFD) on the spin evolution of isolated neutron stars is studied. The ROSAT observations of several X-ray sources, which can be accreting old isolated neutron stars, are used to constrain the exponential and power-law decay parameters. We show that for the exponential decay the ranges of minimum value of magnetic moment, $μ_b$, and the characteristic decay time, $t_d$, $\sim 10^{29.5}\ge μ_b \ge 10^{28} {\rm G} {\rm cm}^3$, $\sim 10^8\ge t_d \ge 10^7 {\rm yrs}$ are excluded assuming the standard initial magnetic moment, $μ_0=10^{30} {\rm G} {\rm cm}^3$. For these parameters, neutron stars would never reach the stage of accretion from the interstellar medium even for a low space velocity of the stars and a high density of the ambient plasma. The range of excluded parameters increases for lower values of $μ_0$. We also show, that, contrary to exponential MFD, no significant restrictions can be made for the parameters of power-law decay from the statistics of isolated neutron star candidates in ROSAT observations. Isolated neutron stars with constant magnetic fields and initial values of them less than $μ_0 \sim 10^{29} {\rm G} {\rm cm}^3$ never come to the stage of accretion. We briefly discuss the fate of old magnetars with and without MFD, and describe parameters of old accreting magnetars.

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arXiv2000

Restrictions on parameters of power-law magnetic field decay for accreting isolated neutron stars

In this short note we discuss the influence of power-law magnetic field decay on the evolution of old accreting isolated neutron stars. We show, that, contrary to exponential field decay (Popov & Prokhorov 2000), no additional restrictions can be made for the parameters of power-law decay from the statistics of isolated neutron star candidates in ROSAT observations. We also briefly discuss the fate of old magnetars with and without field decay, and describe parameters of old accreting magnetars.

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arXiv1999

IMF and evolution of close binaries after starformation bursts

This paper is a continuation and development of our previous articles (Popov et al., 1997, 1998). We use `Scenario Machine'' (Lipunov et al., 1996b) -- the population synthesis simulator (for single binary systems calculations the program is available in WWW: http://xray.sai.msu.ru/ (Nazin et al., 1998)) -- to calculate evolution of populations of several types of X-ray sources during the first 20 Myrs after a starformation burst. We examined the evolution of 12 types of X-ray sources in close binary systems (both with neutron stars and with black holes) for different parameters of the IMF -- slopes: $α=1$, $α=1.35$ and $α=2.35$ and upper mass limits, $M_{up}$: 120 $M_{\odot}$, 60 $M_{\odot}$ and 40 $M_{\odot}$. Results, especially for sources with black holes, are very sensitive to variations of the IMF, and it should be taken into account when fitting parameters of starformation bursts. Results are applied to several regions of recent starformation in different galaxies: Tol 89, NGC 5253, NGC 3125, He 2-10, NGC 3049. Using known ages and total masses of starformation bursts (Shaerer at al., 1998) we calculate expected numbers of X-ray sources in close binaries for different parameters of the IMF. Usually, X-ray transient sources consisting of a neutron star and a main sequence star are most abundant, but for very small ages of bursts (less than $\approx 4$ Myrs) sources with black holes can become more abundant.

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arXiv1999

Population synthesis of old neutron stars in the Galaxy

The paucity of old isolated accreting neutron stars in ROSAT observations is used to derive a lower limit on the mean velocity of neutron stars at birth. The secular evolution of the population is simulated following the paths of a statistical sample of stars for different values of the initial kick velocity, drawn from an isotropic Gaussian distribution with mean velocity $0\leq < V>\leq 550$ ${\rm km s^{-1}}$. The spin-down, induced by dipole losses and the interaction with the ambient medium, is tracked together with the dynamical evolution in the Galactic potential, allowing for the determination of the fraction of stars which are, at present, in each of the four possible stages: Ejector, Propeller, Accretor, and Georotator. Taking from the ROSAT All Sky Survey an upper limit of $\sim 10$ accreting neutron stars within $\sim 140$ pc from the Sun, we infer a lower bound for the mean kick velocity, $ < V>\ga 200-300$ ${\rm km s^{-1}}$. The same conclusion is reached for both a constant ($B\sim 10^{12}$ G) and a magnetic field decaying exponentially with a timescale $\sim 10^9$ yr. Present results, moreover, constrain the fraction of low velocity stars, which could have escaped pulsar statistics, to $\la 1%$.

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arXiv1999

The Neutron Stars Census

The paucity of old isolated accreting neutron stars in ROSAT observations is used to derive a lower limit on the mean velocity of neutron stars at birth. The secular evolution of the population is simulated following the paths of a statistical sample of stars for different values of the initial kick velocity, drawn from an isotropic Gaussian distribution with mean velocity $0\leq < V>\leq 550$ ${\rm km s^{-1}}$. The spin--down, induced by dipole losses and the interaction with the ambient medium, is tracked together with the dynamical evolution in the Galactic potential, allowing for the determination of the fraction of stars which are, at present, in each of the four possible stages: Ejector, Propeller, Accretor, and Georotator. Taking from the ROSAT All Sky Survey an upper limit of $\sim 10$ accreting neutron stars within $\sim 140$ pc from the Sun, we infer a lower bound for the mean kick velocity, $ < V>\gtrsim 200-300$ ${\rm km s^{-1}},$ corresponding to a velocity dispersion $σ_V\gtrsim 125-190$ km s$^{-1}$. The same conclusion is reached for both a constant magnetic field ($B\sim 10^{12}$ G) and a magnetic field decaying exponentially with a timescale $\sim 10^9$ yr. Such high velocities are consistent with those derived from radio pulsar observations. Present results, moreover, constrain the fraction of low velocity stars, which could have escaped pulsar statistics, to less than 1%.

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arXiv1999

ROSAT X-ray sources and exponential field decay in isolated neutron stars

In this paper we semianalyticaly evaluate influence of the exponential decay of magnetic field on the fate of isolated neutron stars. The fact of ROSAT observations of several X-ray sources, which can be accreting old isolated neutron stars gives us an opportunity to put some limits on the parameters of the exponential decay. We argue, that, if most part of neutron stars have approximately the same decay and initial parameters, then the combinations of the bottom magnetic momentum, $μ_b$, in the range $\sim 10^{28}-10^{29.5} {\rm G} {\rm cm}^3$ and characteristic time scale, $t_d$, in the range $\sim 10^7-10^8 {\rm yrs}$ for standard initial magnetic momentum, $μ_0=10^{30} {\rm G} {\rm cm}^3$, can be excluded, because for that sets of parameters neutron stars never come to the stage when accretion of the interstellar medium on their surfaces is possible even for low velocity of neutron stars and relatively high density of the interstellar medium. The region of excluded parameters increases with $μ_0$ decreasing.

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arXiv1999

GRB as explosions with standard power

We show that the distribution of observed energies of GRB with known redshifts can be explained by the hypothesis of the standard energy release E_0=5 10^51 ergs. Two situations are possible, either the beaming angle differs from burst to burst, or there is a universal emssion diagram in each burst, the observed difference being due to different viewing angles.

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arXiv1999

Populations of close binaries in galaxies with recent bursts of starformation

This paper is a continuation and development of our previous articles (Popov et al., 1997, 1998). We use ``Scenario Machine'' (Lipunov et al., 1996b) -- the population synthesis simulator (for single binary systems calculations the program is available in WWW at: http://xray.sai.msu.su/sciwork/scenario.html (Nazin et al., 1998) -- to calculate evolution of populations of several types of X-ray sources during the first 20 Myrs after a starformation burst. We examined the evolution of 12 types of X-ray sources in close binary systems (both with neutron stars and with black holes) for different parameters of the IMF -- slopes: $α=1, α=1.35$ and $α=2.35$ and upper mass limits: 120 M_{\odot}, 60 M_{\odot} and 40 M_{\odot}. Results, especially for sources with black holes, are very sensitive to variations of the IMF, and it should be taken into account when fitting parameters of starformation bursts. Results are applied to several regions of recent starformation in different galaxies: Tol 89, NGC 5253, NGC 3125, He 2-10, NGC 3049. Using known ages and total masses of starformation bursts (Shaerer at al., 1998) we calculate expected numbers of X-ray sources in close binaries for different parameters of the IMF. Usially, X-ray transient sources consisting of a neutron star and a main sequence star are most abundant, but for very small ages of bursts (less than $\approx 4$ Myrs) sources with black holes can become more abundant.

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arXiv1999

Binary Black Hole Formation and Mergings

The formation and evolution of binary black holes (BH) is studied using the modern evolutionary scenario for very massive stars with high mass loss (Vanbeveren et al. 1998). Main sequence stars with masses $M>35 M_\odot$ are assumed to form a BH in the end of their nuclear evolution. The mass of BH formed is parametrized as $M_{bh}=k_{bh}\times M_{SN}$, where $M_{SN}$ is the mass of the pre-supernova star taken from evolutionary calculations, $k_{bh}\le 1$. The possibility is explored that a newly formed BH acquires a kick velocity 0-250 km/s. Binary BH are found to merge within the Hubble time at an appreciable rate only for non-zero kick velocities. We calculate the galactic merging rates of binary BH systems, their detection rate by the initial laser interferometers, and the distributions of merging binary BH over orbital eccentricities at different frequencies. The distribution of angles between BH spins and the orbital angular momentum is also presented.

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arXiv1999

On the origin of X-ray dips in Her X-1

A strong X-ray illumination of the optical star atmosphere in Her X-1, asymmetric because of a partial shadowing by the tilted twisted accretion disk around central neutron star, leads to the formation of matter flows coming out of the orbital plane and crossing the line of sight before entering the disk. We suggest that the absorption of X-ray emission by this flow leads to the formation of pre-eclipse and anomalous dips of type I. These dips are observed during several orbits after turn-on both in the main-on and short-on state. Almost coherent action of tidal torques and matter streams enhances the disk wobbling which causes the disk edge to shield the X-ray source after the turn-on. Anomalous dips of type II and post-eclipse recovery appear due to this process only on the first orbit after turn-on.

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arXiv1999

The Influence of the Initial Mass Function on Populations of X-ray Binaries After a Burst of Star Formation

In this article we use "Scenario Machine" - the population synthesis simulator- to calculate the evolution of the populations of the selected types of X-ray sources after the starformation burst with the total mass in binaries $1.5\cdot10^6 M_{\odot}$ 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 $α- coefficient$ 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 $M_{\odot}$. 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 $\approx 300$. We give approximating formulae for the time dependence of source numbers.

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