Searcharxiv⌕ Search

arXiv subjects

Vladimir V. Usov

Publications and source records attributed to Vladimir V. Usov.

25 records · Page 2Linked to original sources
arXiv1997

Colliding Winds in the Stellar Core at the Galactic Center: Some Implications

We point out that a high number density of stars in the core of a dense star cluster such as the central stellar cluster at the Galactic center, where many stars possess strong stellar winds, should result in collisions of those winds. The wind collisions in the dense stellar core at the Galactic center would result in production of strong X-ray flares with the rate of $\sim 10^{-4}~ (N_w/10^3)^2 / yr$ and duration of $\sim 1$ week, where $N_w$ is the number of the wind producing stars in the core. Presence of a massive black hole would enhance the stellar density around it and would make collisions of the winds in the core substantially more frequent. Collisions of the stellar winds in the cluster have also a number of interesting observable implications, including generation of $γ$-rays by particles accelerated by the shocks from the colliding winds. These processes are also expected to be relevant to compact regions of intense star formation elsewhere.

astro-ph↗
arXiv1997

Low-mass normal-matter atmospheres of strange stars and their radiation

The quark surface of a strange star has a very low emissivity for X-ray photons. I find that a small amount of normal matter at the quark surface with temperature in the range $10^7\la T_{_S}} \ll mc^2/k\simeq 6\times 10^9$ K is enough to produce X-rays with high luminosity, $L_X\simeq 10^{32}- 10^{34}(ΔM/10^{-22}M_\odot)^2 erg s^{-1}$. For the total atmosphere mass $ΔM\sim (10^{-20}-10^{-19})M_\odot$, this luminosity may be as high as the Eddington limit. The mean energy of X-ray photons which are radiated from such a low-mass atmosphere of a strange star is $\sim 10^2(T_S/10^8 K)^{0.45} \simeq 30-300$ times larger than the mean energy of X-ray photons which are radiated from the surface of both a neutron star and a strange star with a massive normal-matter envelope, $ΔM\sim 10^{-5}M_\odot$, for a fixed temperature at the stellar core. This raises the possibility that some black hole candidates with hard X-ray spectra are, in fact, such strange stars with a low-mass atmosphere. The X-ray emission from single strange stars is estimated.

astro-ph↗
arXiv1994

Hydrodynamics and High-Energy Physics of WR Colliding Winds

One of the main properties of Wolf-Rayet (WR) stars is a very intense outflow of gas. No less than 40\% \ of WR stars belong to binary systems. Young massive O and B stars are the secondary components of such systems. OB stars also have an intense stellar wind. If the intensities of the stellar winds of WR and OB stars are more or less comparable or if the distance between the components of the binary is large enough, the winds flowing out of WR and OB stars can collide and the shock waves are formed. In the shock the gas is heated to temperature $\sim 10^7$ K and generates X-ray emission. Stellar wind collision may be responsible not only for the X-ray emission of WR + OB binaries and for their radio, IR and $γ$-ray emision as well. Stellar wind collision, gas heating, particle acceleration, and generation of X-ray, $γ$-ray, radio and IR emission in WR + OB binaries are discussed.

astro-ph↗
arXiv1993

Glitches in the X-Ray Pulsar 1e 2259+586

Starquakes are considered for fast-rotating magnetic white dwarfs. The X-ray pulsar 1E 2259 + 586 may be such a white dwarf. It is shown that in this case starquakes may be responsible for the decrease of the mean spin-down rate which was observed for 1E 2259 + 586 between 1987 and 1990. The required mass of the white dwarf which is identified with 1E 2259 + 586 is $\sim 1.4 - 1.5 M_\odot$, making this X-ray pulsar the most massive white dwarf known.

astro-ph↗
arXiv1993

Radiation from Vela-Like Pulsars Near the Death Line

Radiation of both the outer gaps and the neutron star surface is considered for a Vela-like pulsar near the death line. It is shown that if such a pulsar is close enough to the death line, its optical, UV and X-ray emission has to increase. Using results of this consideration, it is argued that Geminga is not a close relative of Vela-like pulsars. The outer gap model of Geminga in which the main part of the outer gap volume operates as a Vela-like generator of $γ$-rays is ruled out. A Vela-like mechanism of $γ$-ray generation can operate only in a small region of the outer gap of Geminga. The length of this region along the magnetic field is an order of magnitude smaller than the outer gap dimensions. In the magnetosphere of Geminga the main mechanism of $γ$-ray generation at $\sim 10 - 10^4$ MeV is curvature radiation and not the synchrotron radiation as it was assumed for Vela-like pulsars.

astro-ph↗
arXiv1993

On the Nature of Nonthermal Radiation from Cosmological Gamma-Ray Bursters

Relativistic electron-positron winds with strong magnetic fields are considered as a source of radiation for cosmological $γ$-ray bursters. Such a wind is generated by a millisecond pulsar with a very strong magnetic field. An electron-positron plasma near the pulsar is optically thick and in quasi-thermodynamic equilibrium. It is shown that the main part of radiation from the pulsar wind is nonthermal and generates in the following way. Kinetic energy which is released in the process of deceleration of the neutron star rotation transforms mainly to the magnetic field energy. The magnetic field is frozen in the outflowing plasma if the distance to the pulsar is smaller than $\sim 10^{13}$ cm. This field transfers the energy from the pulsar environment to the region outside the $γ$-ray photosphere of the electron-positron wind. At a distance of more than $\sim 10^{13}$ cm the magnetohydrodynamic approximation for the pulsar wind is broken, and intense electromagnetic waves are generated. The frequency of these waves is equal to the frequency of the pulsar rotation. Outflowing particles are accelerated in the field of intense electromagnetic waves to Lorentz factors of the order of $10^6$ and generate nonthermal synchro-Compton radiation. The typical energy of nonthermal photons is $\sim 1$ MeV. A high-energy tail of the $γ$-ray spectrum may be up to $\sim 10^4$ MeV. Baryonic matter is ejected occasionally from the pulsar magnetosphere. The baryonic matter ejection and subsequent suppression of the $γ$-ray emission may be responsible for the time structure of $γ$-ray bursts.

astro-ph↗