arXiv subjects

V. E. Zavlin

Publications and source records attributed to V. E. Zavlin.

45 records · Page 3Linked to original sources

arXiv1997

The Neutron Star in the Supernova Remnant PKS 1209-52

We re-analyzed soft X-ray data collected with the ROSAT and ASCA observatories on a candidate neutron star (NS) near the center of the supernova remnant PKS 1209-52. We fitted the observed spectra with NS atmosphere models. The hydrogen atmosphere fits yield more realistic parameters of the NS and the intervening hydrogen column than the traditional blackbody fit. In particular, for a NS of mass 1.4 M_\odot and radius 10 km, we obtained the NS surface temperature T_{eff}=(1.4-1.9)*10^6 K and distance d=1.6-3.3 kpc versus T=(4.2-4.6)*10^6 K and (implausible) d=11-13 kpc for the blackbody fit, at a 90% confidence level. Our fits suggest that the surface magnetic field is either very weak, B < 10^{10} G, or it exceeds 2*10^{12} G. The hydrogen column density inferred from the atmosphere fits, n_H=(0.7-2.2)*10^{21} cm^{-2}, agrees fairly well with independent estimates obtained from UV observations of nearby stars, radio data, and X-ray spectrum of the shell of the supernova remnant, whereas the blackbody and power-law fits give considerably lower and greater values, n_H=(0.2-0.4)*10^{21} and (5.2-7.0)*10^{21} cm^{-2}, respectively. The inferred NS surface temperature is consistent with standard NS cooling models.

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arXiv1997

Mass-to-Radius Ratio for the Millisecond Pulsar J0437-4715

Properties of X-ray radiation emitted from the polar caps of a radio pulsar depend not only on the cap temperature, size, and position, but also on the surface chemical composition, magnetic field, and neutron star's mass and radius. Fitting the spectra and the light curves with neutron star atmosphere models enables one to infer these parameters. As an example, we present here results obtained from the analysis of the pulsed X-ray radiation of a nearby millisecond pulsar J0437-4715. In particular, we show that stringent constraints on the mass-to-radius ratio can be obtained if orientations of the magnetic and rotation axes are known, e.g., from the radio polarization data.

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arXiv1997

Soft X-rays from polar caps of the millisecond pulsar J0437--4715

We show that the soft X-ray spectra and light curves observed with the ROSAT and EUVE from the closest known millisecond pulsar J0437--4715 can be interpreted as thermal radiation from two hot polar caps whose emitting layers (atmospheres) are comprised of hydrogen. The simplest model yields a uniform temperature of $(0.8-0.9)\times 10^6 K$ within a cap radius of $0.7-0.9 km$. The spectral fits indicate that the temperature may be nonuniformly distributed along the cap surface. The distribution can be approximated by a central core heated up to $(1-2)\times 10^6 K$ within a radius of $0.2-0.4 km$, surrounded by a colder rim with temperatures $(3-5)\times 10^5 K$ extending out to $2-6 km$. The polar cap interpretation implies low column densities, $(1-3)\times 10^{19} cm^{-2}$, and a high degree of ionization, $> 20$%, of the interstellar hydrogen towards the pulsar. The inferred bolometric luminosity of the polar caps, $(1.0-1.6)\times 10^{30} erg s^{-1}$, is in excellent agreement with the predictions of the slot-gap model of radio pulsars developed by Arons and his coworkers. Similar polar cap radiation should be emitted by other millisecond pulsars, although in some of them (e.~g., PSR B1821--24) the soft X-ray flux is dominated by the nonthermal radiation from pulsar magnetospheres.

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arXiv1997

Constrains on the mass and radius of pulsars from X-ray observations of their polar caps

The properties of X-ray radiation from the polar caps predicted by the radio pulsar models depend on the surface chemical composition, magnetic field and star's mass and radius as well as on the cap temperature, size and position. Fitting the radiation spectra and light curves with the neutron star atmosphere models enables one to infer these parameters. We present here results obtained from the analysis of the soft X-ray radiation of PSR J0437-4715. In particular, with the aid of radio polarization data, we put constraints on the pulsar mass-to-radius ratio.

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arXiv1996

Multiwavelength observations of isolated neutron stars as a tool to probe the properties of their surfaces

We show that an analysis of multiwavelength observations of isolated neutron stars based on neutron star atmosphere models can be used not only to evaluate the neutron star effective temperature, but also to determine chemical composition of its surface. To demonstrate how this new method can be applied to a specific object, we chose the old isolated neutron star candidate RXJ1856.5-3754, whose soft X-ray radiation has been studied recently by Walter et al. (1996). We fitted the soft X-ray spectrum of this object with neutron star atmosphere models of different chemical compositions and used these fits to calculate the source spectrum over a broad wavelength range. We showed, in particular, that the optical/UV flux expected from this object depends drastically on the composition of its surface. In particular, the neutron star covered with hydrogen would be 5-6 magnitudes brighter than the neutron star with an iron surface. The object should also be observable with $EUVE$; the EUV flux is expected to be almost twice higher for the iron surface than for the hydrogen one. Thus, multiwavelength observations of this object would enable one to examine, for the first time, chemical composition of the neutron star surface. The method proposed can be applied to other nearby isolated neutron stars.

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arXiv1996

Model neutron star atmospheres with low magnetic fields. 1. Atmospheres in radiative equilibrium

We present a detailed investigation of neutron star atmospheres with low magnetic fields, $B < 10^8 -10^{10}$ G, which do not affect opacities and equation of state of the atmospheric matter. We compute the atmospheric structure, emergent spectral fluxes and specific intensities for hydrogen, helium and iron atmospheres in a wide domain of effective temperatures and gravitational accelerations expected for neutron stars. The iron atmospheres are computed with the opacities and equations of state from the OPAL opacity library. We show that the model atmosphere spectra are substantially different from the blackbody spectra. For light element atmospheres, the flux is greater than the blackbody flux, and the spectrum is harder, at high photon energies, whereas at low energies the spectral flux follows the Rayleigh-Jeans law with a (surface) temperature lower than the effective temperature. The spectra of iron atmospheres display prominent spectral features in the soft X-ray range. The emergent specific intensity is anisotropic, with the anisotropy depending on energy. These properties of the atmospheric radiation should be taken into account for the proper interpretation of the thermal component of the neutron star radiation detectable at X-ray through UV energies. In particular, fitting of the thermal component with the blackbody model may result in substantially incorrect parameters of the radiating regions. We discuss an application of our atmosphere models to the nearby millisecond pulsar PSR J0437-4715.

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arXiv1996

Temperature, distance and cooling of the Vela pulsar

We use models of magnetized hydrogen atmospheres to fit the soft X-ray thermal spectrum of the Vela pulsar obtained by ROSAT. The distance and hydrogen column density required by our fits are in good agreement with other independents estimates and can be considered as an argument for the presence of hydrogen at the surface of this pulsar. The low temperature obtained, T = 7.85 +/- 0.25 10^5 K, is below the predictions of the `standard' model of neutron star cooling and strengthens previous claims for the necessity of the occurrence of fast neutrino cooling in this neutron star.

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arXiv1995

X-ray spectra from convective photospheres of neutron stars

We present first results of modeling convective photospheres of neutron stars. We show that in photospheres composed of the light elements convection arises only at relatively low effective temperatures (< 3-5*10^4 K), whereas in the case of iron compositon it arises at < 3*10^ K. Convection changes the depth dependence of the photosphere temperature and the shapes of the emergent spectra. Thus, it should be taken into account for the proper interpretation of EUV/soft-X-ray observations of the thermal radiation from neutron stars.

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arXiv1995

GEMINGA'S SOFT X-RAY EMISSION AND THE STRUCTURE OF ITS SURFACE

We present a model to explain the decrease in the amplitude of the pulse profile with increasing energy observed in Geminga's soft X-ray surface thermal emission. We assume the presence of plates surrounded by a surface with very distinct physical properties: these two regions emit spectra of very distinct shapes which present a crossover, the warm plates emitting a softer spectrum than the colder surrounding surface. The strongly pulsed emission from the plates dominates at low energy while the surroundings emission dominates at high energy, producing naturally a strong decrease in the pulsed fraction. In our illustrative example the plates are assumed to be magnetized while the rest of the surface is field free. This plate structure may be seen as a schematic representation of a continuous but very nonuniform distribution of the surface magnetic field or as a quasi realistic structure induced by past tectonic activity on Geminga.

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