arXiv subjects

D. Sanwal

Publications and source records attributed to D. Sanwal.

18 recordsLinked to original sources

arXiv2006

The pulsar wind nebula of the Geminga pulsar

The superb spatial resolution of Chandra has allowed us to detect a 20''-long tail behind the Geminga pulsar, with a hard spectrum (photon index 1.0+/-0.2) and a luminosity (1.3+/-0.2) 10^{29} ergs/s in the 0.5 - 8 keV band, for an assumed distance of 200 pc. The tail could be either a pulsar jet, confined by a toroidal magnetic field of about 100 microGauss, or it can be associated with the shocked relativistic wind behind the supersonically moving pulsar confined by the ram pressure of the oncoming interstellar medium. We also detected an arc-like structure 5'' - 7'' ahead of the pulsar, extended perpendicular to the tail, with a factor of 3 lower luminosity. We see a 3-sigma enhancement in the Chandra image apparently connecting the arc with the southern outer tail that has been possibly detected with XMM-Newton. The observed structures imply that the Geminga's pulsar wind is intrinsically anisotropic.

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arXiv2005

Near-Infrared and Optical Limits for the Central X-ray Point Source in the Cassiopeia A Supernova Remnant

Near-infrared images of the center of the Cas A supernova remnant taken with the NICMOS 2 camera on HST using the F110W and F160W filters (~J & ~H bands) have magnitude limits of 26.2 and 24.6 respectively, but reveal no sources within a 1.2" radius of the Chandra detected central X-ray point source (XPS). The NICMOS data, taken together with broadband optical magnitude limits (R ~ 28 mag) obtained from a deep STIS CCD exposure taken with a clear filter (50CCD), indicate that the XPS luminosities are very low in the optical/NIR bands (e.g., L_H < 3 x E29 erg/s) with no optical, J, or H band counterpart to the XPS easily detectable by HST. We discuss the nature of the Cas A central compact object based upon these new near-infrared and optical flux limits.

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arXiv2004

Variations in the spin period of the radio-quiet pulsar 1E 1207.4-5209

The X-ray source 1E 1207.4-5209 is a compact central object in the G296.5+10.0 supernova remnant. Its spin period of 424 ms, discovered with the Chandra X-ray Observatory, suggests that it is a neutron star. The X-ray spectrum of this radio-quiet pulsar shows at least two absorption lines, first spectral features discovered in radiation from an isolated neutron star. Here we report the results of timing analysis of Chandra and XMM-Newton observations of this source showing a non-monotonous behavior of its period. We discuss three hypotheses which may explain the observational result. The first one assumes that 1E 1207.$-5209 is a glitching pulsar, with frequency jumps of Δf > 5 \muHz occurring every 1-2 years. The second hypothesis explains the deviations from a steady spin-down as due to accretion, with accretion rate varying from \sim 10^{13} to >10^{16} g s^{-1}, from a disk possibly formed from ejecta produced in the supernova explosion. Finally, the period variations could be explained assuming that the pulsar is in a wide binary system with a long period, P_orb \sim 0.2-6 yr, and a low-mass companion, M_2 < 0.3 M_\odot.

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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

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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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

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

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

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.

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arXiv2001

X-Ray Observations of the supernova remnant G21.5-0.9

We present the analysis of archival X-ray observations of the supernova remnant (SNR) G21.5-0.9. Based on its morphology and spectral properties, G21.5-0.9 has been classified as a Crab-like SNR. In their early analysis of the CHANDRA calibration data, Slane et al. (2000) discovered a low-surface-brightness, extended emission. They interpreted this component as the blast wave formed in the supernova (SN) explosion. In this paper, we present the CHANDRA analysis using a total exposure of ~150 ksec. We also include ROSAT and ASCA observations. Our analysis indicates that the extended emission is non-thermal -- a result in agreement with XMM observations. The entire remnant of radius ~ 2'.5 is best fitted with a power law model with a photon index steepening away from the center. The total unabsorbed flux in the 0.5-10 keV is 1.1E-10 erg/cm2/s with an 85% contribution from the 40" radius inner core. Timing analysis of the High-Resolution Camera (HRC) data failed to detect any pulsations. We put a 16% upper limit on the pulsed fraction. We derive the physical parameters of the putative pulsar and compare them with those of other plerions (such as the Crab and 3C 58). G21.5-0.9 remains the only plerion whose size in X-rays is bigger than in the radio. Deep radio observations will address this puzzle.

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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.

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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.

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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.

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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.

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