arXiv subjects

E. V. Gotthelf

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

139 records · Page 8Linked to original sources

arXiv1998

The X-ray Expansion of the Supernova Remnant Cassiopeia A

We present the first X-ray expansion measurement of the SNR Cas A. We compared new ROSAT to archived Einstein HRI images with a separation of 17 years. The remnant is seen to be expanding, on average, at a rate of 0.20 %/yr, which is twice as fast as the expansion of the bright radio ring and two-thirds as fast as the ensemble of optical Fast Moving Knots. This argues that we are observing different hydrodynamical structures in each band, although all of these components are located on the same patchy ring. In addition, significant variations in expansion rate as a function of azimuth around the bright X-ray ring are found. These findings are discussed in the context of two classes of models, involving either an ambient medium with a uniform or monotonically decreasing density away from the center, or a pre-existing circumstellar shell.

astro-ph↗

arXiv1998

Evidence for Proportionate Partition Between the Magnetic Field and Hot Gas in Turbulent Cassiopeia A

We present a deep X-ray observation of the young Galactic supernova remnant Cas A, acquired with the ROSAT High Resolution Imager. This high dynamic range (232 ks) image reveals low-surface-brightness X-ray structure, which appears qualitatively similar to corresponding radio features. We consider the correlation between the X-ray and radio morphologies and its physical implications. After correcting for the inhomogeneous absorption across the remnant, we performed a point by point (4" resolution) surface brightness comparison between the X-ray and radio images. We find a strong (r = 0.75) log-log correlation, implying an overall relationship of $\log(Σ_{_{\rm X-ray}}) \propto (2.21\pm0.05) \times \log(Σ_{_{\rm radio}})$. This is consistent with proportionate partition (and possibly equipartition) between the local magnetic field and the hot gas --- implying that Cas A's plasma is fully turbulent and continuously amplifying the magnetic field.

astro-ph↗

arXiv1998

Discovery of a 7 Second Anomalous X-ray Pulsar in the Distant Milky Way

We report the serendipitous discovery of a 7-s X-ray pulsar using data acquired with the Advanced Satellite for Cosmology and Astrophysics. The pulsar is detected as an unresolved source located towards a region of the Galactic plane (l,b ~ 29.5, 0.08) that coincides with an overdensity of star-formation tracers. The signal suffers tremendous foreground absorption, equivalent to N_H ~ 10E23 cm^-2; the absorption correlates well with a line-of-sight that is tangential to the inner spiral arms and the 4-kpc molecular ring. The pulsar is not associated with any known supernova remnants or other cataloged objects in that direction. The near sinusoidal pulse (period P ~ 6.9712) is modulated at 35% pulsed amplitude, and the steep spectrum is characteristic of hot black-body emission with temperature kT ~ 0.65 keV. We characterize the source as an anomalous X-ray pulsar (AXP).

astro-ph↗

arXiv1998

Discovery of an Ultra-fast X-ray Pulsar in the Supernova Remnant N157B

We present the serendipitous discovery of 16 ms pulsed X-ray emission from the Crab-like supernova remnant N157B in the Large Magellanic Cloud. This is the fastest spinning pulsar associated with a supernova remnant (SNR). Observations with the Rossi X-ray Timing Explorer (RXTE), centered on the field containing SN1987A, reveal an X-ray pulsar with a narrow pulse profile. Archival ASCA X-ray data confirm this detection and locate the pulsar within 1' of the supernova remnant N157B, 14' from SN1987A. The pulsar manifests evidence for glitch(es) between the RXTE and ASCA observations which span 3.5 years; the mean linear spin-down rate is Pdot = 5.126 X 10E-14 s/s. The background subtracted pulsed emission is similar to other Crab-like pulsars with a power law of photon index of ~ 1.6. The characteristic spin-down age (~ 5000 years) is consistent with the previous age estimate of the SNR. The inferred B-field for a rotationally powered pulsar is ~ 1 X 10E12 Gauss. Our result confirms the Crab-like nature of N157B; the pulsar is likely associated with a compact X-ray source revealed by ROSAT HRI observations.

astro-ph↗

arXiv1997

X-Ray Emission from the Radio Pulsar PSR J1105-6107

We have detected significant X-ray emission from the direction of the young radio pulsar PSR J1105-6107 using the ASCA Observatory. The 11 sigma detection includes 460 background-subtracted source counts derived using data from all four ASCA detectors. The emission shows no evidence of pulsations; the pulsed fraction is less than 31%, at the 90% confidence level. The X-ray emission can be characterized by a power-law spectrum with photon index alpha = 1.8 +/- 0.4, for a neutral hydrogen column density Nh = 7 X 10E21 cm^-2. The unabsorbed 2 - 10 keV flux assuming the power-law model is (6.4 +/- 0.8) X 10E-13 ergs cm^-2 s^-1. The implied efficiency for conversion of spin-down luminosity to ASCA-band emission is (1.6 +/- 0.2) X 10E-3, assuming a distance of 7 kpc to the source. Within the limited statistics, the source is consistent with being unresolved. We argue that the X-rays are best explained as originating from a pulsar-powered synchrotron nebula.

astro-ph↗

arXiv1997

An Unusual X-ray Burst from the Globular Cluster M28

We report the discovery of an unusual X-ray burst from the direction of the Globular Cluster M28 using data acquired with the ASCA Observatory. The burst was recorded by all four ASCA telescopes and displays a fast (~ 70 ms) rise followed by an exponential decay (t = 7.5 s) and a steady afterglow which lasts between 800 - 3250 s. The image of the burst is consistent with an ASCA point source and is centered on quiescent X-ray emission from the core of M28. The burst temporal profile is similar to Type-I bursts emitted by accreting neutron stars of low mass X-ray binaries (LMXB). We argue that the burst arises from an LMXB that is located in the core of M28. The burst is unique in two ways: it is intrinsically sub-luminous, ~ 0.02 L_Edd and more importantly, originates from a source whose quiescent luminosity is fainter than that of the known cluster bursters by three orders of magnitude. We suggest that this burst is from a highly magnetized neutron star accreting at a low rate. These accreting systems may account for the mysterious low luminosity X-ray sources in globular clusters.

astro-ph↗

arXiv1997

Evidence of X-ray Synchrotron Emission from Electrons Accelerated to 40 TeV in the Supernova Remnant Cassiopeia A

We present the 2-60 keV spectrum of the supernova remnant Cassiopeia A measured using the Proportional Counter Array and the High Energy X-ray Timing Experiment on the Rossi X-ray Timing Explorer satellite. In addition to the previously reported strong emission-line features produced by thermal plasmas, the broad-band spectrum has a high-energy "tail" that extends to energies at least as high as 120 keV. This tail may be described by a broken power law that has photon indices of 1.8 +0.5/-0.6 and 3.04 +0.15/-0.13 and a break energy of 15.9 +0.3/-0.4 keV. We argue that the high-energy component, which dominates the spectrum above about 10 keV, is produced by synchrotron radiation from electrons that have energies up to at least 40 TeV. This conclusion supports the hypothesis that Galactic cosmic rays are accelerated predominantly in supernova remnants.

astro-ph↗

arXiv1997

The nature of the radio-quiet compact X-ray source in SNR RCW 103

We consider the nature of the elusive neutron star candidate 1E~161348-5055 using X-ray observations obtained with the ASCA Observatory. The compact X-ray source is centered on the shell-type Galactic supernova remnant RCW103 and has been interpreted as a cooling neutron star associated with the remnant. The X-ray spectrum of the remnant shell can be characterized by a non-equilibrium ionization (NEI) thermal model for a shocked plasma of temperature kT ~ 0.3 keV. The spectrum falls off rapidly above 3 keV to reveal a point source in the spectrally-resolved images, at the location of 1E~161348-5055. A black-body model fit to the source spectrum yields a temperature kT = 0.6 keV, with an unabsorbed 0.5-10 keV luminosity of Lx ~ 10E34 erg/s (for an assumed distance of 3.3 kpc, both of which are at least a factor of 2 higher than predicted by cooling neutron star models. Alternatively, a power-law model for the source continuum gives a steep photon index of ~ 3.2, similar to that of other radio-quiet, hard X-ray point sources associated with supernova remnants. 1E~161348-5055 may be prototypical of a growing class of radio-quiet neutron stars revealed by ASCA; we suggest that these objects account for previously hidden neutron stars associated with supernova remnants.

astro-ph↗

arXiv1997

The Discovery of an Anomalous X-ray Pulsar in the Supernova Remnant Kes 73

We report the discovery of pulsed X-ray emission from the compact source 1E 1841-045, using data obtained with the Advanced Satellite for Cosmology and Astrophysics. The X-ray source is located in the center of the small-diameter supernova remnant (SNR) Kes 73 and is very likely to be the compact stellar-remnant of the supernova which formed Kes 73. The X-rays are pulsed with a period of ~ 11.8 s, and a sinusoidal modulation of roughly 30 %. We interpret this modulation to be the rotation period of an embedded neutron star, and as such would be the longest spin period for an isolated neutron star to-date. This is especially remarkable since the surrounding SNR is very young, at ~ 2000 yr old. We suggest that the observed characteristics of this object are best understood within the framework of a neutron star with an enormous dipolar magnetic field, B ~ 8x10^14 G.

astro-ph↗

arXiv1997

Kes 73: A Young Supernova Remnant with an X-ray Bright, Radio-quiet Central Source

We clarify the nature of the small-diameter supernova remnant (SNR) Kes 73 and its central compact source, 1E 1841-045, using X-ray data acquired with the ASCA Observatory. We introduce a spatio-spectral decomposition technique necessary to disentangle the ASCA spectrum of the compact source from the barely resolved shell-type remnant. The source spectrum (1 - 8 keV) is characterized by an absorbed power-law with a photon index ~ 3.4 and N_H ~ 3.0E22 cm^-2, possibly non-thermal in nature. This bright X-ray source is likely a slowly spinning pulsar, whose detection is reported in our companion paper (Vasisht & Gotthelf 1997). The SNR spectrum is characteristic of a thermal plasma, with kT ~ 0.7 keV, and emission lines typical of a young remnants. The element Mg and possibly O and Ne are found to be over-abundant, qualitatively suggesting an origin from a massive progenitor. We find that Kes 73 is a young (~ 2000 yr) type II/Ib SNR containing a neutron star pulsar spinning anomalously slow for its age. Kes 73 is yet another member of a growing class of SNRs containing radio-quiet compact sources with a hard spectral signature.

astro-ph↗

arXiv1997

A Possible Site of Cosmic Ray Acceleration in the Supernova Remnant IC 443

We present evidence for shock acceleration of cosmic rays to high energies (about 10 TeV) in the supernova remnant IC 443. X-ray imaging spectroscopy with ASCA reveals two regions of particularly hard emission: an unresolved source embedded in an extended emission region, and a ridge of emission coincident with the southeastern rim. Both features are located on part of the radio shell where the shock wave is interacting with molecular gas, and together they account for a majority of the emission at 7 keV. Though we would not have noticed it a priori, the unresolved feature is coincident with one resolved by the ROSAT HRI. Because this feature overlaps a unique region of flat radio spectral index (alpha < 0.24), has about equal light-crossing and synchrotron loss times, and a power law spectrum with a spectral index of 2, we conclude that the hard X-ray feature is synchrotron radiation from a site of enhanced particle acceleration. Evidence against a plerion includes a lack of observed periodicity (the pulsed fraction upper limit is 33%), the spectral similarity with the more extended hard region, the location of the source outside the 95% error circle of the nearby EGRET source, the fact that it is nestled in a bend in the molecular cloud ring with which IC 443 is interacting, and the requirement of an extremely high transverse velocity (>5,000 km/s). We conclude that the anomalous feature is most likely tracing enhanced particle acceleration by shocks that are formed as the supernova blast wave impacts the ring of molecular clouds.

astro-ph↗

arXiv1996

The EINSTEIN Observatory Detection of Faint X-Ray Flashes

We report on the result of an extensive search for X-ray counterparts to Gamma Ray Bursts (GRB) using data acquired with the Imaging Proportional Counter (IPC) on-board the Einstein Observatory. We examine background sky fields from all pointed observations for short timescale (~10 sec) transient X-ray phenomena not associated a-priori with detectable point sources. A total of 1.5 X 10^7 seconds of exposure time was searched on arc-minute spatial scales down to a limiting sensitivity of 10^-11 erg/cm^2/sec in the 0.2-3.5 keV IPC band. Forty-two highly significant X-ray flashes (Poisson probability < 10^-7 of being produced by statistical fluctuations) are discovered, of which eighteen have spectra consistent with an extragalactic origin and lightcurves similar to the Ginga-detected X-ray counterparts to GRB. Great care is taken to identify and exclude instrumental and observational artifacts; we develop a set of tests to cull events which may be associated with spacecraft or near-Earth space backgrounds. The flashes are found to be distributed isotropically on the sky and have an approximately Euclidean number-size relation. They are not associated with any known sources and, in particular, they do not correlate with the nearby galaxy distribution. Whether or not these flashes are astrophysical and/or associated with GRB, the limits imposed by the search described herein produces important constraints on GRB models. In this paper, we discuss possible origins for these flashes; in a companion paper, Hamilton, Gotthelf, and Helfand (1996) we use the results of our search to constrain strongly all halo models for GRB.

astro-ph↗

arXiv1996

The Absence of X-ray Flashes from Nearby Galaxies and the Gamma-Ray Burst Distance Scale

If typical gamma-ray bursts (GRBs) have X-ray counterparts similar to those detected by Ginga, then sensitive focusing X-ray telescopes will be able to detect GRBs three orders of magnitude fainter than the detection limit of the Burst and Transient Source Experiment (BATSE). If a substantial portion of the burst population detected by BATSE originates in a Galactic halo at distances greater than or equal to 150 kpc, existing X-ray telescopes will be able to detect GRBs in external galaxies out to a distance of at least 4.5 Mpc. As reported in Gotthelf, Hamilton, & Helfand (1996) the Imaging Proportional counter (IPC) on board the Einstein Observatory detected 42 transient events with pointlike spatial characteristics and timescales of less than 10 seconds. These events are distributed isotropically on the sky; in particular, they are not concentrated in the directions of nearby external galaxies. For halo models of the BATSE bursts with radii of 150 kpc or greater, we would expect to see several burst events in observations pointed towards nearby galaxies. We see none. We therefore conclude that if the Ginga detections are representative of the population of GRBs sampled by BATSE, GRBs cannot originate in a Galactic halo population with limiting radii between 150 kpc and 400 kpc. Inasmuch as halos with limiting radii outside of this range have been excluded by the BATSE isotropy measurements, our result indicates that all halo models are excluded. This result is independent of whether the flashes we do detect have an astronomical origin.

astro-ph↗