arXiv subjects

G. G. Pavlov

Publications and source records attributed to G. G. Pavlov.

At least 109 records · Page 6Linked to original sources

arXiv2001

The compact central source in the RX J0852-4622 supernova remnant

The central region of the recently discovered supernova remnant RX J0852.0-4622 was observed with the ACIS detector aboard the Chandra X-ray Observatory. We found only one relatively bright source, about 4' north of the SNR center, with a flux of $\sim 2\times 10^{-12}$ erg s$^{-1}$ cm$^{-2}$ in the 0.5--10 keV band. The position of this point-like source, CXOU J085201.4-461753, rules out its association with the two bright stars in the field, HD 76060 and Wray 16-30. Observations of the field with the CTIO 0.9-m telescope show a star ($R\approx 17$, $B\approx 19$) at about 2\farcs4 from the nominal X-ray position. We consider association of this star with the X-ray source unlikely and estimate a limiting magnitude of the optical counterpart as $B \ge 22.5$ and $R \ge 21.0$. Based on the X-ray-to-optical flux ratio, we argue that the X-ray source is likely the compact remnant of the supernova explosion that created the RX J0852.0-4622 SNR. The observed X-ray spectrum of the source is softer than spectra of magnetospheric radiation of rotation-powered pulsars, but it is harder than spectra of cooling neutron stars emitting thermal radiation from the entire surface, similar to the central compact source of the Cas A SNR. We suggest that CXOU J085201.4-461753 belongs to the growing family of radio-quiet compact central sources, presumably neutron stars, recently discovered in a number of SNRs.

astro-ph↗

arXiv2001

Variability of the Vela Pulsar-wind Nebula Observed with Chandra

The observations of the pulsar-wind nebula (PWN) around the Vela pulsar with the Advanced CCD Imaging Spectrometer aboard the Chandra X-ray Observatory, taken on 2000 April 30 and November 30, reveal its complex morphology reminiscent of that of the Crab PWN. Comparison of the two observations shows changes up to 30% in the surface brightness of the PWN features. Some of the PWN elements show appreciable shifts, up to a few arcseconds (about 10^{16} cm), and/or spectral changes. To elucidate the nature of the observed variations, further monitoring of the Vela PWN is needed.

astro-ph↗

arXiv2001

The X-ray Spectrum of the Vela Pulsar Resolved with Chandra

We report the results of the spectral analysis of two observations of the Vela pulsar with the Chandra X-ray observatory. The spectrum of the pulsar does not show statistically significant spectral lines in the observed 0.25-8.0 keV band. Similar to middle-aged pulsars with detected thermal emission, the spectrum consists of two distinct components. The softer component can be modeled as a magnetic hydrogen atmosphere spectrum - for the pulsar magnetic field $B=3\times 10^{12}$ G and neutron star mass $M=1.4 M_\odot$ and radius $R^\infty =13$ km, we obtain $\tef^\infty =0.68\pm 0.03$ MK, $L_{\rm bol}^\infty = (2.6\pm 0.2)\times 10^{32}$ erg s$^{-1}$, $d=210\pm 20$ pc (the effective temperature, bolometric luminosity, and radius are as measured by a distant observer). The effective temperature is lower than that predicted by standard neutron star cooling models. A standard blackbody fit gives $T^\infty =1.49\pm 0.04$ MK, $L_{\rm bol}^\infty=(1.5\pm 0.4)\times 10^{32} d_{250}^2$ erg s$^{-1}$ ($d_{250}$ is the distance in units of 250 pc); the blackbody temperature corresponds to a radius, $R^\infty =(2.1\pm 0.2) d_{250}$ km, much smaller than realistic neutron star radii. The harder component can be modeled as a power-law spectrum, with parameters depending on the model adopted for the soft component - $γ=1.5\pm 0.3$, $L_x=(1.5\pm 0.4)\times 10^{31} d_{250}^2$ erg s$^{-1}$ and $γ=2.7\pm 0.4$, $L_x=(4.2\pm 0.6)\times 10^{31} d_{250}^2$ erg s$^{-1}$ for the hydrogen atmosphere and blackbody soft component, respectively ($γ$ is the photon index, $L_x$ is the luminosity in the 0.2--8 keV band). The extrapolation of the power-law component of the former fit towards lower energies matches the optical flux at $γ\simeq 1.35$--1.45.

astro-ph↗

arXiv2000

Pulsar Astronomy: the HST Contribution

HST observations have contributed significantly to our knowledge on the behaviour of Isolated Neutron Stars (INSs) as optical emitters. First, HST has been instrumental both to discover new optical counterparts (PSR B1055-52, PSR B1929+10, PSR B0950+08) and to confirm proposed identifications (PSR B0656+14). Second, HST multicolor photometry provided useful information to characterize the optical emission mechanism(s) at work in middle-aged INSs like PSR B0656+14 and Geminga. Last, but not least, the superior angular resolution of the HST allowed both to perform a very accurate morphological study of the plerionic environments of young pulsars (e.g. the Crab and PSR B0540-69) and to perform very accurate astrometric measurements yielding proper motions (Crab, Vela, Geminga, PSR B0656+14) and parallaxes (Geminga).

astro-ph↗

arXiv2000

Optical photometry of the PSR B0656+14 and its neighborhood

We present the results of the broad-band photometry of the nearby middle-aged radio pulsar PSR B0656+14 and its neighborhood obtained with the 6-meter telescope of the SAO RAS and with the Hubble Space Telescope. The broad-band spectral flux $F_ν$ of the pulsar decreases with increasing frequency in the near-IR range and increases with frequency in the near-UV range. The increase towards UV can be naturally interpreted as the Rayleigh-Jeans tail of the soft thermal component of the X-ray spectrum emitted from the surface of the cooling neutron star. Continuation of the power-law component, which dominates in the high-energy tail of the X-ray spectrum, to the IR-optical-UV frequencies is consistent with the observed fluxes. This suggests that the non-thermal pulsar radiation may be of the same origin in a broad frequency range from IR to hard X-rays. We also studied 4 objects detected in the pulsar's 5" neighborhood.

astro-ph↗

arXiv2000

Discovery of 424 ms pulsations from the radio-quiet neutron star in the PKS 1209-52 supernova remnant

The central source of the supernova remnant PKS 1209-52 was observed with the Advanced CCD Imaging Spectrometer aboard Chandra X-ray observatory on 2000 January 6-7. The use of the Continuos Clocking mode allowed us to perform the timing analysis of the data with time resolution of 2.85 ms and to find a period P=0.42412927+/-2.3e-7 s. The detection of this short period proves that the source is a neutron star. It may be either an active pulsar with unfavorably directed radio beam or a truly radio-silent neutron star whose X-ray pulsations are caused by a nonuniform distribution of surface temperature. To infer the actual properties of this neutron star, the period derivative should be measured.

astro-ph↗

arXiv1999

The Compact Central Object in Cas A: A Neutron Star with Hot Polar Caps or a Black Hole?

The central pointlike X-ray source of the Cas A supernova remnant was discovered in the Chandra First Light Observation and found later in the archival ROSAT and Einstein images. The analysis of these data does not show statistically significant variability of the source. The power-law fit yields the photon index 2.6-4.1, and luminosity (2-60)e34 erg/s, for d=3.4 kpc. The power-law index is higher, and the luminosity lower, than those observed fromvery young pulsars. One can fit the spectrum equally well with a blackbody model with T=6-8 MK, R=0.2-0.5 km, L=(1.4-1.9)e33 erg/s. The inferred radii are too small, and the temperatures too high, for the radiationcould be interpreted as emitted from the whole surface of a uniformly heated neutron star. Fits with the neutron star atmosphere models increase the radius and reduce the temperature, but these parameters are still substantially different from those expected for a young neutron star. One cannot exclude, however, that the observed emission originates from hot spots on a cooler neutron star surface. Because of strong interstellar absorption, the possible low-temperature component gives a small contribution to the observed spectrum; an upper limit on the (gravitationally redshifted) surface temperature is < 1.9-2.3 MK. Amongst several possible interpretations, we favor a model of a strongly magnetized neutron star with magnetically confined hydrogen or helium polar caps on a cooler iron surface. Alternatively, the observed radiation may be interpreted as emitted by a compact object (more likely, a black hole) accreting from a fossil disk or from a late-type dwarf in a close binary.

astro-ph↗

arXiv1999

Polarization of Thermal X-rays from Isolated Neutron Stars

Since the opacity of a magnetized plasma depends on polarization of radiation, the radiation emergent from atmospheres of neutron stars with strong magnetic fields is expected to be strongly polarized. The degree of linear polarization, typically ~10-30%, depends on photon energy, effective temperature and magnetic field. The spectrum of polarization is more sensitive to the magnetic field than the spectrum of intensity. Both the degree of polarization and the position angle vary with the neutron star rotation period so that the shape of polarization pulse profiles depends on the orientation of the rotational and magnetic axes. Moreover, as the polarization is substantially modified by the general relativistic effects, observations of polarization of X-ray radiation from isolated neutron stars provide a new method for evaluating the mass-to-radius ratio of these objects, which is particularly important for elucidating the properties of the superdense matter in the neutron star interiors.

astro-ph↗

arXiv1998

X-ray Pulsations from the Central Source in Puppis A

There are several supernova remnants which contain unresolved X-ray sources close to their centers, presumably radio-quiet neutron stars. To prove that these objects are indeed neutron stars, to understand the origin of their X-ray radiation, and to explain why they are radio-quiet, one should know their periods and period derivatives. We searched for pulsations of the X-ray flux from the radio-quiet neutron star candidate RX J0822-4300 near the center of the Puppis A supernova remnant observed with the ROSAT PSPC and HRI. A standard timing analysis of the separate PSPC and HRI data sets does not allow one to detect the periodicity unequivocally. However, a thorough analysis of the two observations separated by 4.56 yr enabled us to find a statistically significant period $P\simeq 75.3$ ms and its derivative $\dot{P}\simeq 1.49\times 10^{-13}$ s s$^{-1}$. The corresponding characteristic parameters of the neutron star, age $τ=P/(2\dot{P})=8.0$ kyr, magnetic field $B=3.4\times 10^{12}$ G, and rotational energy loss $\dot{E}=1.4\times 10^{37}$ erg s$^{-1}$, are typical for young radio pulsars. Since the X-ray radiation has a thermal-like spectrum, its pulsations may be due to a nonuniform temperature distribution over the neutron star surface caused by anisotropy of the heat conduction in the strongly magnetized crust.

astro-ph↗

arXiv1998

The Thermal X-ray Spectra of Cen X-4, Aql X-1, and 4U 1608-522 in Quiescence

We re-analyze the available X-ray spectral data of the type I bursting neutron star transients Aql X-1, Cen X-4, and 4U 1608-522 using realistic hydrogen atmosphere models. Previous spectral fits assumed a blackbody spectrum; because the free-free dominated photospheric opacity decreases with increasing frequency, blackbody spectral fits overestimate the effective temperature and underestimate, by as much as two orders of magnitude, the emitting area. Hydrogen atmosphere spectral models, when fit to the available observational data, imply systematically larger emission area radii, consistent with the canonical 10 km radius of a neutron star. This suggests that a substantial fraction of the quiescent luminosity is thermal emission from the surface of the neutron star. The magnitude of the equivalent hydrogen column density toward these systems, however, presents a considerable systematic uncertainty, which can only be eliminated by high signal-to-noise X-ray spectral measurements (e.g., with AXAF or XMM) which would permit simultaneous determination of the equivalent hydrogen column density, emission area, and thermal temperature.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

arXiv1997

BVRI observations of PSR B0656+14 with the 6-meter telescope

We observed the middle-aged radio pulsar 0656+14 with a CCD detector at the 6-m telescope. Broadband BVRI images show the following magnitudes of the pulsar counterpart: B = 24.85 (+0.19, -0.16), V = 24.90 (+0.16, -0.14), R = 24.52 (+0.12, -0.11), I = 23.81 (+0.27, -0.21). We fitted the UV-optical (space + ground-based) data with a two-component model which combines a power law (non-thermal component) with a thermal spectrum emitted by the neutron star surface. The power law component, with the energy power-law index α=1.5 (+1.1,-1.2), dominates in the observed range. Constraints on the thermal component correspond to the Rayleigh-Jeans parameter G\equiv T_6(R_{10}/d_{500})^2=4.1 (+2.1, -4.1), where $T=10^6T_6$ K is the brightness temperature, R_\infty = 10 R_{10}$ km is the neutron star radius as seen by a distant observer, and $d=500 d_{500}$ pc is the distance. The shape of the optical-UV spectrum of PSR 0656+14 differs considerably from those observed from other pulsars: the middle-aged Geminga and young Crab, Vela.

astro-ph↗

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.

astro-ph↗

arXiv1997

Photoionization of hydrogen in atmospheres of magnetic neutron stars

The strong magnetic fields (B ~ 10^{12} - 10^{13} G) characteristic of neutron stars make all the properties of an atom strongly dependent on the transverse component K_\perp of its generalized momentum. In particular, the photoionization process is modified substantially: (i) threshold energies are decreased as compared with those for an atom at rest, (ii) cross section values are changed significantly, and (iii) selection rules valid for atoms at rest are violated by the motion so that new photoionization channels become allowed. To calculate the photoionization cross sections, we, for the first time, employ exact numerical treatment of both initial and final atomic states. This enables us to take into account the quasi-bound (autoionizing) atomic states as well as coupling of different ionization channels. We extend the previous consideration, restricted to the so-called centered states corresponding to relatively small values of K_\perp, to arbitrary states of atomic motion. We fold the cross sections with the thermal distribution of atoms over K. For typical temperatures of neutron star atmospheres, the averaged cross sections differ substantially from those of atoms at rest. In particular, the photoionization edges are strongly broadened by the thermal motion of atoms; this "magnetic broadening" exceeds the usual Doppler broadening by orders of magnitude. The decentered states of the atoms give rise to the low-energy component of the photoionization cross section. This new component grows significantly with increasing temperature above 10^{5.5} K and decreasing density below 1 g/cm^3, i.e., for the conditions expected in atmospheres of middle-aged neutron stars.

astro-ph↗

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.

astro-ph↗

arXiv1996

Ionization of the hydrogen atom in strong magnetic fields: beyond the adiabatic approximation

High magnetic fields in neutron stars, B ~ 10^{11} - 10^{13} G, substantially modify the properties of atoms and their interaction with radiation. In particular, the photoionization cross section becomes anisotropic and strongly polarization dependent. In a number of previous works based on the adiabatic approximation the conclusion was drawn that the transverse cross section vanishes for frequencies smaller than the electron cyclotron frequency. In other works (which employed a different form of the interaction operator) appreciable finite values were obtained. An adequate interpretation of the neutron star thermal-like radiation requires a resolution of this controversy. In this work the atomic wave functions for both discrete and continuum states are calculated by solving the coupled channel equations allowing the admixture between different Landau levels, which provides much higher accuracy than the adiabatic approximation. This enables to resolve the above contradiction in favour of the finite transverse cross sections. The non-adiabatic treatment of the continuum includes coupling between closed and open channels, which leads to the autoionization of quasi-bound energy levels associated with the electron cyclotron excitations and gives rise to Beutler - Fano resonances of the photoionization cross section. Autoionization widths of these quasi-bound levels are calculated and compared with the radiative widths. The results are important for investigations of the radiation emergent from the surface layers of neutron stars.

atom-ph↗

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.

astro-ph↗