Magnetic fields of isolated neutron stars: evidence for decay

We show, that different types of isolated neutron stars (INSs) show evidence in favor of magnetic field decay in these objects, and discuss how observations of INSs can help to constrain models of field decay.Comment: 3 pages, no figures, to appear in proceedings of the conference Variable Stars - 2001 in Odessa Astron. Pub

Spatial distribution of accreting isolated neutron stars

We present here the computer model of the distribution of the luminosity, produced by old isolated neutron stars (OINSs) accreting from the interstellar medium (ISM). We show, that for different mean velocities of OINSs the distribution of the luminosity has a torus-like structure, with the maximum at $\approx 5 kpc$.Comment: 8 pages, 9 PostScript figures, to be presented as a poster at the conference "Current problems in astrophysics", Moscow, September 23-28, 1996. Short version will be published in Astronomical and Astrophysical Transaction

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: $\alpha=1$, $\alpha=1.35$ and $\alpha=2.35$ 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.Comment: 4 pages including 3 PostScript figures. To be published in the proceedings of the conference "The Universe of Gamow: Original Ideas in Astrophysics and Cosmology

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: $\alpha=1, \alpha=1.35$ and $\alpha=2.35$ 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.Comment: 15 pages, including 9 PostScript figures and 9 tables, to be presented as a poster at the conference "Astronomy at the border of the centuries", Pushchino, May 17-22, 199

Evolution of close binaries after the burst of starformation for different IMFs

We use "Scenario Machine" -- the population synthesis simulator -- to calculate the evolution of populations of selected types of X-ray sources after a starformation burst with the total mass in binaries (1--1.5) \cdot 10^6 M_{\odot}$during the first 20 Myr after a burst. We present here the results of two sets of runs of the program. In the first set we examined the following types of close binaries: transient sources-- neutron stars with Be- stars; ``X-ray pulsars''-- neutron stars in pairs with supergiants; Cyg X-1-like sources-- black holes with supergiants; ``SS443-like sources''-- superaccreting black holes. We used two values of the exponent$\alpha$in the initial mass function: 2.35 (Salpeter's function) and 1 (``flat spectrum''). The calculations were made for the folowing values of the upper limit of the mass function: 120 and 30$M_{\odot}$. For the ``flat spectrum'', suggested in (Contini et al, 1995), the number of sources of all types significantly increased. With the ``flat spectrum'' and with the upper mass limit 120$M_{\odot}$we obtained hundreds of sources of all calculated types. Decreasing of the upper mass limit below the critical mass of a black hole formation increase the number of transient sources with neutron stars up to$\approx 300$. In the second set we examined the evolution of of 12 other types of X-ray sources for$\alpha=1$,$\alpha=1.35$and$\alpha=2.35$and for three upper mass limits: 120$M_{\odot}$, 60$M_{\odot}$, 40$M_{\odot}$ (see Perez-Olia & Colina 1995 for the reasons for such upper limits) on the same time scale 20 Myr after a star formation burst.Comment: 12 pages, 4 PostScript figures, to be presented as a poster at the conference "Cosmology: from COBE to galaxy formation", 1-7 December 1997, Copenhage

Young close isolated compact objects

We suggest that the seven radio-quiet isolated neutron stars observed with ROSAT are young cooling objects associated to recent near-by supernova explosions which formed runaway stars and the Local Bubble, affecting the topology of the interstellar medium in the vicinity of the Sun (within a few hundred parsecs). In the aftermath of these explosions, a few black holes might have been formed, according to the local initial mass function. We thus discuss the possibility of determining approximate positions of close-by isolated black holes using data on runaway stars and simple calculations of binary evolution and disruption.Comment: 4 pages, no figures, to appear in proceedings of the conference ICGA-V in Gravitation & Cosmology (2002

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 $\alpha - coefficient$ 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.Comment: 6 pages, 2 PostScript figures, in the replaced version reference to the russian variant of the article was adde

The Magnificent Seven: Close-by Cooling Neutron Stars?

We model Galactic populations of accreting and cooling isolated neutron stars in the attempt to explore their link with a new class of dim soft X-ray sources revealed by ROSAT. For accretors we follow the magneto-rotational and dynamical evolution in the Galactic potential and a realistic large scale distribution of the interstellar medium is used. Under standard assumptions old neutron stars enter the accretor stage only if their magnetic field exceeds $\approx 10^{11}$--$10^{12}$ G. We predict about 1 source per square degree for fluxes $\approx 10^{-15}$--$10^{-16}$ erg cm$^{-2}$s$^{-1}$ in the energy range 0.5-2 keV. Cooling neutron stars are explored within a simpler model of local sources, including however interstellar absorption. They are found to be significantly less abundant at low fluxes, $<0.1$ sources per square degree, but dominate over accretors at higher flux levels ($\approx 10^{-12}$--$10^{-11}$ erg cm$^{-2}$s$^{-1}$). We suggest that the faint sources observed by ROSAT may be young cooling neutron stars with typical age \la 10^6 yrs, if the total number of young neutron stars in the Solar proximity is $\sim 10$ times higher than inferred from radiopulsars statistics.Comment: 6 pages, 1 PostScript figure, To appear in Proc. of "X-ray astronomy 2000" (Palermo Sep. 2000), Eds. R. Giacconi, L. Stella, S. Serio; ASP Conf. Series, in pres

Binary Evolution in World Wide Web

We present a WWW-version of the {\it Scenario Machine} - a computer code designed to calculate the evolution of close binary stellar systems. The Internet users can directly access to the code and calculate binary evolutionary tracks with parameters at the user's will. The program is running on the {\it Pentium} server of the Division of the Relativistic Astrophysics of the Sternberg Astronimical Institute (http://xray.sai.msu.su/ ). The results are presented both in the form of tables and graphic diagrams. The work is always in progress. More possibilities for Internet users are intended to become available in the near future.Comment: 7 pages in Latex, figures are available at http://xray.sai.msu.su/sciwork/scenario.html; to be published in the proceedings of the COSMION-96 conferenc

Magnetars origin and progenitors with enhanced rotation

Among a dozen known magnetar candidates there are no binary objects. As an estimate of a fraction of binary neutron stars is about 10% it is reasonable to address the question of solitarity of magnetars, to estimate theoretically the fraction of binary objects among them, and to mark o probable companions. We present population synthesis calculations of binary systems. Our goal is to estimate the number of neutron stars originated from progenitors with enhanced rotation, as such compact objects can be expected to have large magnetic fields, ie. they can be magnetars. The fraction of such neutron stars in our calculations is about 13-16%. Most of these objects are isolated due to coalescences of components prior to a neutron star formation, or due to a system disruption after a supernova explosion. The fraction of such neutron stars in survived binaries is about 1% or lower. Their most numerous companions are black holes.Comment: 11 pages, no figures (resubmitted to MNRAS