arXiv subjects

G. G. Pavlov

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

At least 91 records · Page 5Linked to original sources

arXiv2003

Ultraviolet Emission from the Millisecond Pulsar J0437-4715

We observed PSR J0437-4715 with the FUV-MAMA detector of the Hubble Space Telescope Imaging Spectrometer (STIS) to measure the pulsar's spectrum and pulsations. For the first time, UV emission from a millisecond pulsar is detected. The measured flux, $(2.0\pm 0.2)\times 10^{-15}$ erg s$^{-1}$ cm$^{-2}$ in the 1150-1700 Årange, corresponds to the luminosity $L_{\rm FUV}=(4.7\pm 0.5)\times 10^{27}$ erg s$^{-1}$, for the distance of 140 pc and negligible interstellar extinction. The shape of the observed spectrum suggests thermal emission from the neutron star surface with a surprisingly high temperature of about $1\times 10^5$ K, above the upper limit on the surface temperature of the younger ``ordinary'' pulsar J0108-1431. For the few-Gyr-old J0437-4715, such a temperature requires a heating mechanism to operate. The spectrum of J0437-4715 shows marginal evidence of an emission line at 1372 Å, which might be a gravitationally redshifted Zeeman component of the Hydrogen Ly$α$ line in a magnetic field $\sim 7\times 10^8$ G. No pulsations are detected, with a $3σ$ upper limit of 50% on pulsed fraction.

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arXiv2003

The jets of the Vela pulsar

Chandra observations of the Vela pulsar-wind nebula (PWN) have revealed a jet in the direction of the pulsar's proper motion, and a counter-jet in the opposite direction, embedded in diffuse nebular emission. The jet consists of a bright, 8''-long inner jet, between the pulsar and the outer arc, and a dim, curved outer jet that extends up to 100'' in approximately the same direction. From the analysis of thirteen Chandra observations spread over about 2.5 years we found that this outer jet shows particularly strong variability, changing its shape and brightness. We observed bright blobs in the outer jet moving away from the pulsar with apparent speeds (0.3-0.6)c and fading on time-scales of days to weeks. The spectrum of the outer jet fits a power-law model with a photon index of 1.3\pm0.1. The X-ray emission of the outer jet can be interpreted as synchrotron radiation of ultrarelativistic electrons/positrons. This interpretation allows one to estimate the magnetic field, ~100 microGauss, maximum energy of X-ray emitting electrons, ~2\times 10^{14} eV, and energy injection rate, ~8\times 10^{33} erg/s, for the outer jet. In the summed PWN image we see a dim, 2'-long outer counter-jet, which also shows a power-law spectrum with photon ined of 1.2-1.5. Southwest of the jet/counter-jet an extended region of diffuse emission is seen. Relativistic particles responsible for this radiation are apparently supplied by the outer jet.

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arXiv2003

Search for the Optical Counterpart of the Vela Pulsar X-ray Nebula

Observations of the Vela pulsar region with the Chandra X-ray observatory have revealed the fine structure of its synchrotron pulsar-wind nebula (PWN), which showed an overall similarity with the Crab PWN. However, contrary to the Crab, no firm detection of the Vela PWN in optical has been reported yet. To search for the optical counterpart of the X-ray PWN, we analyzed deep optical observations performed with different telescopes. We compared the optical images with those obtained with the Chandra ACIS to search for extended emission patterns which could be identified as counterparts of the X-ray nebula elements. Although some features are seen in the optical images, we find no correlation with the X-ray structure. Thus, we conclude that the diffuse optical emission is more likely associated with filaments in the host Vela SNR. The derived upper limits on the optical flux from the PWN are compatibile, within the uncertainties, with the values expected on the basis of the extrapolations of the X-ray data.

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arXiv2003

The Variable Jet of the Vela Pulsar

Observations of the Vela pulsar-wind nebula (PWN) with the Chandra X-ray Observatory have revealed a complex, variable PWN structure, including inner and outer arcs, a jet in the direction of the pulsar's proper motion, and a counter-jet in the opposite direction, embedded in diffuse nebular emission. From the analysis of thirteen Chandra observations spread over about 2.5 years we found that this outer jet shows particularly strong variability, changing its shape and brightness. We observed bright blobs in the outer jet moving away from the pulsar with apparent speeds (0.3-0.6)c and fading on time-scales of days to weeks. The spectrum of the outer jet fits a power-law model with a photon index of 1.3. The X-ray emission of the outer jet can be interpreted as synchrotron radiation of ultrarelativistic electrons/positrons. This interpretation allows one to estimate the magnetic field, ~100 microGauss, maximum energy of X-ray emitting electrons, $2 \times 10^{14}$ eV, and energy injection rate, $8\times 10^{33}$ erg/s, for the outer jet. In the summed PWN image, we see a faint, strongly bent, extension of the outer jet. The more extreme bends closer to the pulsar, as well as the apparent side motions of the outer jet, can be associated with kink instabilities of a magnetically confined, pinched jet flow. Another feature found in the summed image is a dim, 2'-long outer counter-jet, which also shows a power-law spectrum with photon index of 1.2-1.5. Southwest of the jet/counter-jet (i.e., approximately perpendicular to the direction of pulsar's proper motion), an extended region of diffuse emission is seen. Relativistic particles responsible for this radiation are apparently supplied by the outer jet.

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arXiv2003

Thermal Radiation from Cooling Neutron Stars

Observations of thermal radiation from neutron stars allow one to measure the surface temperatures and confront them with cooling scenarios. Detection of gravitationally redshifted spectral lines can yield the mass-to-radius ratio. In the few cases when the distance is known, one can measure the neutron star radius, which is particularly important to constrain the equation of state of the superdense matter in the neutron star interiors. Finally, one can infer the chemical composition of the neutron star surface layers, which provides information about formation of neutron stars and their interaction with the environments. We present the observational results on thermal radiation from active pulsars and radio-quiet neutron stars, with emphasis on the results obtained with the Chandra X-ray Observatory and discuss some implications of these results. In particular, we present a new interpretation of the thermal emission detected in the broad energy range (from optical to X-rays) from the puzzling object RX J1856.5-3754

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arXiv2003

Chandra X-Ray Observatory observations of the globular cluster M28 and its millisecond pulsar B1821-24

We report here the results of the first Chandra X-Ray Observatory observations of the globular cluster M28 (NGC 6626). 46 X-ray sources are detected, of which 12 lie within one core radius of the center. We show that the apparently extended X-ray core emission seen with the ROSAT HRI is due to the superposition of multiple discrete sources for which we determine the X-ray luminosity function down to a limit of about 6xE30 erg/s. For the first time the unconfused phase-averaged X-ray spectrum of the 3.05-ms pulsar B1821--24 is measured and found to be best described by a power law with photon index ~ 1.2. Marginal evidence of an emission line centered at 3.3 keV in the pulsar spectrum is found, which could be interpreted as cyclotron emission from a corona above the pulsar's polar cap if the the magnetic field is strongly different from a centered dipole. The unabsorbed pulsar flux in the 0.5--8.0 keV band is ~3.5xE-13 ergs/s/cm^2. Spectral analysis of the 5 brightest unidentified sources is presented. Based on the spectral parameters of the brightest of these sources, we suggest that it is a transiently accreting neutron star in a low-mass X-ray binary, in quiescence. Fitting its spectrum with a hydrogen neutron star atmosphere model yields the effective temperature T_eff^\infty = 90^{+30}_{-10} eV and the radius R_NS^\infty = 14.5^{+6.9}_{-3.8} km. In addition to the resolved sources, we detect fainter, unresolved X-ray emission from the central core of M28. Using the Chandra-derived positions, we also report on the result of searching archival Hubble Space Telescope data for possible optical counterparts.

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arXiv2003

HST and VLT observations of Isolated Neutron Stars

New results of HST and VLT observations of isolated pulsars and their environments are presented. We present the first deep optical observations of the nearby pulsar PSR J0108-1431, the identification evidence of PSR 1929+10 based on the proper motion measurement of its optical counterpart and a deep investigation at optical wavelengths of the X-ray PWN around the Vela pulsar.

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arXiv2002

The Milky Way - Pulsars and Isolated Neutron Stars

CONTENTS The Milky Way -- Pulsars and Isolated Neutron Stars / Introduction: Historical Overview / Physics and Astrophysics of Isolated Neutron Stars / Rotation-powered Pulsars: The Magnetic Braking Model / High-energy Emission Models / Magnetospheric Emission Models / Thermal Evolution of Neutron Stars / Photospheric Emission from Cooling Neutron Stars / The Current Picture of High-Energy Emission Properties of Isolated Neutron Stars / Young Neutron Stars in Supernova Remnants / Crab-like Pulsars / Vela-like pulsars / Radio-silent Neutron Stars in Supernova Remnants / Anomalous X-ray Pulsars and Soft Gamma-Ray Repeaters / Thermal Emission from Middle-Aged Pulsars / Old Nearby Radio Pulsars / Isolated Radio-quiet Neutron Stars / Recycled Millisecond Pulsars / Impressive Achievements and Great Expectations / References

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arXiv2002

Discovery of absorption features in the X-ray spectrum of an isolated neutron star

We observed 1E 1207.4--5209, a neutron star in the center of the supernova remnant PKS 1209--51/52, with the ACIS detector aboard the Chandra X-ray observatory and detected two absorption features in the source spectrum. The features are centered near 0.7 keV and 1.4 keV, their equivalent widths are about 0.1 keV. We discuss various possible interpretations of the absorption features and exclude some of them. A likely interpretation is that the features are associated with atomic transitions of once-ionized helium in the neutron star atmosphere with a strong magnetic field. The first clear detection of absorption features in the spectrum of an isolated neutron star provides an opportunity to measure the mass-to-radius ratio and constrain the equation of state of the superdense matter.

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arXiv2002

Thermal Radiation from Neutron Stars: Chandra Results

The outstanding capabilities of the Chandra X-ray observatory have greatly increased our potential to observe and analyze thermal radiation from the surfaces of neutron stars (NSs). Such observations allow one to measure the surface temperatures and confront them with the predictions of the NS cooling models. Detection of gravitationally redshifted spectral lines can yield the NS mass-to-radius ratio. In rare cases when the distance is known, one can measure the NS radius, which is particularly important to constrain the equation of state of the superdense matter in the NS interiors. Finally, one can infer the chemical composition of the NS surface layers, which provides information about formation of NSs and their interaction with the environment. We overview the recent Chandra results on the thermal radiation from various types of NSs -- active pulsars, young radio-quiet neutron stars in supernova remnants, old radio-silent ``dim'' neutron stars -- and discuss their implications.

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arXiv2002

Modeling Neutron Star Atmospheres

Models of thermal emission of neutron stars, presumably formed in their atmospheres, are needed to infer the surface temperatures, magnetic fields, chemical composition, and neutron star masses and radii from the observational data. This information, supplemented with model equations of state and neutron star cooling models, is expected to move us further in understanding the fundamental properties of the superdense matter in the neutron star interiors. The neutron star atmospheres are very different from those of usual stars due to the immense gravity and huge magnetic fields. In this presentation we review the current status of the neutron star atmosphere modeling and present most important results.

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arXiv2002

1E 1207.4-5209: The puzzling pulsar at the center of the PKS 1209-51/52 supernova remnant

Second Chandra observation of 1E 1207.4-5209, the central source of the supernova remnant PKS 1209-51/52, allowed us to confirm the previously detected period of 424 ms and, assuming a uniform spin-down, estimate the period derivative, Pdot (0.7-3)e-14 s/s. The corresponding characteristic age of the pulsar, P/2Pdot~200-900 kyr, is much larger than the estimated age of the SNR, \~7 kyr. The values of the spin-down luminosity, Edot (0.4-1.6)e34 erg/s, and conventional magnetic field, B (2-4)e12 G, are typical for a middle-aged radio pulsar, although no manifestations of pulsar activity have been observed. If 1E 1207.4-5209 is indeed the neutron star formed in the same supernova explosion that created PKS 1209-51/52, such a discrepancy in ages could be explained either by a long initial period, close to its current value, or, less likely, by a very large braking index of the pulsar. Alternatively, the pulsar could be a foreground object unrelated to the supernova remnant, but the probability of such a coincidence is very low.

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arXiv2001

X-ray Radiation from the Millisecond Pulsar J0437-4715

We report on spectral and timing observations of the nearest millisecond pulsar J0437-4715 with Chandra. The pulsar spectrum, detected up to 7 keV, cannot be described by a simple one-component model. We suggest that it consists of two components, a nonthermal power-law spectrum generated in the pulsar magnetosphere, with a photon index about 2, and a thermal spectrum emitted by heated polar caps, with a temperature decreasing outwards from 2 MK to 0.5 MK. The lack of spectral features in the thermal component suggests that the neutron star surface is covered by a hydrogen (or helium) atmosphere. The timing analysis shows one X-ray pulse per period, with a pulsed fraction of about 40% and the peak at the same pulse phase as the radio peak. No synchrotron pulsar-wind nebula is seen in the X-rays.

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arXiv2001

The puzzling compact sources in supernova remnants

X-ray images of some young supernova remnants show bright point sources which have not been detected in radio, optical and gamma-ray bands. Despite the similarity of the X-ray spectra of these objects, they show a variety of temporal properties. Most likely, they are neutron stars whose properties (spin periods? magnetic fields? environments?) are different from those of radio and/or gamma-ray pulsars. We present an overview of observational results on several objects of this class -- the central sources of Cassiopeia A, RX J0852-4622, RCW 103, Puppis A, and PKS 1209-51/52 -- with emphasis on the recent Chandra observations.

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arXiv2001

X-ray Spectrum and Pulsations of the Vela Pulsar

We report the results of the spectral and timing analysis of observations of the Vela pulsar with the Chandra X-ray Observatory. The spectrum shows no statistically significant spectral lines in the observed 0.25--8.0 keV band. It consists of two distinct continuum components. The softer component can be modeled as either a magnetic hydrogen atmosphere spectrum with kT = 59 +- 3 eV, R = 15.5 +- 1.5 km, or a standard blackbody with kT = 129 +- 4 eV, R = 2.1 +- 0.2 km (the radii are for a distance of 250 pc). The harder component, modeled as a power-law spectrum, gives photon indices depending on the model adopted for the soft component: gamma = 1.5 +- 0.3 for the magnetic atmosphere soft component, and gamma = 2.7 +- 0.4 for the blackbody soft component. Timing analysis shows three peaks in the pulse profile, separated by about 0.3 in phase. Energy-resolved timing provides evidence for pulse profile variation with energy. The higher energy (E > 1.8 keV) profile shows significantly higher pulsed fraction.

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arXiv2001

Chandra reveals the Dynamic Structure of the Inner Crab Nebula

We present a series of monitoring observations of the Crab Nebula with the Chandra X-ray Observatory, focusing on the temporal evolution of the structure. This series of 8 observations, spanning a period of approximately six months, shows the dynamic nature of the inner X-ray structures. We detected outward moving ``wisps'' from the recently discovered inner ring seen in optical observations. We also find that the inner ring itself shows temporal variations in structure. The torus also appears to be expanding. Such temporal variations generally match the canonical scenario that an expanding synchrotron nebula injected from the pulsar is confined by the supernova ejecta.

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arXiv2001

The Intriguing Plerionic Supernova Remnant: G21.5-0.9

G21.5-0.9 is a plerionic supernova remnant (SNR) whose properties hint at the presence of a pulsar -- yet no pulsations have been found at any wavelength. Early observations with Chandra led to the discovery of an extended component, making the SNR at least twice as big as originally thought. Our analysis indicates that this low-surface brightness extended component is non-thermal. We perform a spatially resolved spectroscopy using a 72 ksec exposure with ACIS-S, and find no evidence of line emission. The 5-arcminute diameter remnant is well fitted with a power law with a photon index steepening from 1.5 (central core) to 2.7 (extended component) and a column density N_H=2.2 10^22 cm^-2. Using a 76 ksec exposure with the HRC, we derive an upper limit of 16% on the pulsed fraction from a putative pulsar. We also infer the parameters of the `hidden pulsar' in G21.5-0.9 in comparison with the Crab and 3C 58 pulsars. This remnant remains unique and intriguing since it is, to date, the only candidate whose size is bigger in X-rays than in the radio.

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arXiv2001

Simulating CCDs for the Chandra Advanced CCD Imaging Spectrometer

We have implemented a Monte Carlo algorithm to model and predict the response of various kinds of CCDs to X-ray photons and minimally-ionizing particles and have applied this model to the CCDs in the Chandra X-ray Observatory's Advanced CCD Imaging Spectrometer. This algorithm draws on empirical results and predicts the response of all basic types of X-ray CCD devices. It relies on new solutions of the diffusion equation, including recombination, to predict the radial charge cloud distribution in field-free regions of CCDs. By adjusting the size of the charge clouds, we can reproduce the event grade distribution seen in calibration data. Using a model of the channel stops developed here and an insightful treatment of the insulating layer under the gate structure developed at MIT, we are able to reproduce all notable features in ACIS calibration spectra. The simulator is used to reproduce ground and flight calibration data from ACIS, thus confirming its fidelity. It can then be used for a variety of calibration tasks, such as generating spectral response matrices for spectral fitting of astrophysical sources, quantum efficiency estimation, and modeling of photon pile-up.

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