1,089 research outputs found
Chandra Observations of the X-ray Environs of SN 1998bw/GRB 980425
(Abrigded) We report X-ray studies of the environs of SN 1998bw and GRB
980425 using the Chandra X-Ray Observatory 1281 days after the GRB. Combining
our observation of the supernova with others of the GRB afterglow, a smooth
X-ray light curve, spanning ~1300 days, is obtained by assuming the burst and
supernova were coincident at 35.6 Mpc. When this X-ray light curve is compared
with those of the X-ray ``afterglows'' of ordinary GRBs, X-ray Flashes, and
ordinary supernovae, evidence emerges for at least two classes of lightcurves,
perhaps bounding a continuum. By three to ten years, all these phenomena seem
to converge on a common X-ray luminosity, possibly indicative of the supernova
underlying them all. This convergence strengthens the conclusion that SN 1998bw
and GRB 980425 took place in the same object. One possible explanation for the
two classes is a (nearly) standard GRB observed at different angles, in which
case X-ray afterglows with intermediate luminosities should eventually be
discovered. Finally, we comment on the contribution of GRB afterglows to the
ULX source population.Comment: 26 pages, 5 figues, submitted to Ap
Low energy polarization sensitivity of the Gas Pixel Detector
An X-ray photoelectric polarimeter based on the Gas Pixel Detector has been
proposed to be included in many upcoming space missions to fill the gap of
about 30 years from the first (and to date only) positive measurement of
polarized X-ray emission from an astrophysical source. The estimated
sensitivity of the current prototype peaks at an energy of about 3 keV, but the
lack of readily available polarized sources in this energy range has prevented
the measurement of detector polarimetric performances.
In this paper we present the measurement of the Gas Pixel Detector
polarimetric sensitivity at energies of a few keV and the new, light, compact
and transportable polarized source that was devised and built to this aim.
Polarized photons are produced, from unpolarized radiation generated with an
X-ray tube, by means of Bragg diffraction at nearly 45 degrees.
The employment of mosaic graphite and flat aluminum crystals allow the
production of nearly completely polarized photons at 2.6, 3.7 and 5.2 keV from
the diffraction of unpolarized continuum or line emission. The measured
modulation factor of the Gas Pixel Detector at these energies is in good
agreement with the estimates derived from a Monte Carlo software, which was up
to now employed for driving the development of the instrument and for
estimating its low energy sensitivity. In this paper we present the excellent
polarimetric performance of the Gas Pixel Detector at energies where the peak
sensitivity is expected. These measurements not only support our previous
claims of high sensitivity but confirm the feasibility of astrophysical X-ray
photoelectric polarimetry.Comment: 15 pages, 12 figures. Accepted for publication in NIM
Relating the Lorentzian and exponential: Fermi's approximation,the Fourier transform and causality
The Fourier transform is often used to connect the Lorentzian energy
distribution for resonance scattering to the exponential time dependence for
decaying states. However, to apply the Fourier transform, one has to bend the
rules of standard quantum mechanics; the Lorentzian energy distribution must be
extended to the full real axis instead of being bounded from
below (``Fermi's approximation''). Then the Fourier transform
of the extended Lorentzian becomes the exponential, but only for times , a time asymmetry which is in conflict with the unitary group time evolution
of standard quantum mechanics. Extending the Fourier transform from
distributions to generalized vectors, we are led to Gamow kets, which possess a
Lorentzian energy distribution with and have exponential
time evolution for only. This leads to probability predictions
that do not violate causality.Comment: 23 pages, no figures, accepted by Phys. Rev.
Chandra Phase-Resolved Spectroscopy of the Crab Pulsar
We present the first phase-resolved study of the X-ray spectral properties of
the Crab Pulsar that covers all pulse phases. The superb angular resolution of
the Chandra X-ray Observatory enables distinguishing the pulsar from the
surrounding nebulosity, even at pulse minimum. Analysis of the pulse-averaged
spectrum measures interstellar X-ray extinction due primarily to photoelectric
absorption and secondarily to scattering by dust grains in the direction of the
Crab Nebula. We confirm previous findings that the line-of-sight to the Crab is
underabundant in oxygen, although more-so than recently measured. Using the
abundances and cross sections from Wilms, Allen & McCray (2000) we find [O/H] =
(3.33 +/-0.25) x 10**-4. Analysis of the spectrum as a function of pulse phase
measures the low-energy X-ray spectral index even at pulse minimum -- albeit
with large statistical uncertainty -- and we find marginal evidence for
variations of the spectral index. The data are also used to set a new (3-sigma)
upper limit to the temperature of the neutron star of log T(infinity) < 6.30.Comment: 20 Pages including 7 figures. Accepted for publication in the
Astrophysical Journa
High Spatial Resolution X-Ray Spectroscopy of the IC443 Pulsar Wind Nebula and Environs
Deep Chandra ACIS observations of the region around the putative pulsar, CXOU
J061705.3+222127, in the supernova remnant IC443 reveal an
~5-radius ring-like structure surrounding the pulsar and a
jet-like feature oriented roughly north-south across the ring and through the
pulsar's location at 06175.200
+222127.52 (J2000.0 coordinates). The
observations further confirm that (1) the spectrum and flux of the central
object are consistent with a rotation-powered pulsar, (2) the non-thermal
spectrum and morphology of the surrounding nebula are consistent with a pulsar
wind and, (3) the spectrum at greater distances is consistent with thermal
emission from the supernova remnant. The cometary shape of the nebula,
suggesting motion towards the southwest, appears to be subsonic: There is no
evidence either spectrally or morphologically for a bow shock or contact
discontinuity; the nearly circular ring is not distorted by motion through the
ambient medium; and the shape near the apex of the nebula is narrow. Comparing
this observation with previous observations of the same target, we set a 99%
confidence upper limit to the proper motion of CXOU J061705.3+222127 to be less
than 44 mas/yr (310 km/s for a distance of 1.5 kpc), with the best-fit (but not
statistically significant) projected direction toward the west.Comment: Accepted for publication in the Astrophysical Journa
Collisional perturbation of radio-frequency E1 transitions in an atomic beam of dysprosium
We have studied collisional perturbations of radio-frequency (rf)
electric-dipole (E1) transitions between the nearly degenerate opposite-parity
levels in atomic dysprosium (Dy) in the presence of 10 to 80 Torr of
H, N, He, Ar, Ne, Kr, and Xe. Collisional broadening and
shift of the resonance, as well as the attenuation of the signal amplitude are
observed to be proportional to the foreign-gas density with the exception of
H and Ne, for which no shifts were observed. Corresponding rates and cross
sections are presented. In addition, rates and cross sections for O are
extracted from measurements using air as foreign gas. The primary motivation
for this study is the need for accurate determination of the shift rates, which
are needed in a laboratory search for the temporal variation of the
fine-structure constant [A. T. Nguyen, D. Budker, S. K. Lamoreaux, and J. R.
Torgerson, Phys. Rev. A \textbf{69}, 22105 (2004)].Comment: 11 pages, 8 figure
Characterization of the Inner Knot of the Crab: The Site of the Gamma-ray Flares?
One of the most intriguing results from the gamma-ray instruments in orbit
has been the detection of powerful flares from the Crab Nebula. These flares
challenge our understanding of pulsar wind nebulae and models for particle
acceleration. We report on the portion of a multiwavelength campaign using
Keck, HST, and Chandra concentrating on a small emitting region, the Crab's
inner knot, located a fraction of an arcsecond from the pulsar.
We find that the knot's radial size, tangential size, peak flux, and the
ratio of the flux to that of the pulsar are correlated with the projected
distance of the knot from the pulsar. A new approach, using singular value
decomposition for analyzing time series of images, was introduced yielding
results consistent with the more traditional methods while some uncertainties
were substantially reduced.
We exploit the characterization of the knot to discuss constraints on
standard shock-model parameters that may be inferred from our observations
assuming the inner knot lies near to the shocked surface. These include
inferences as to wind magnetization, shock shape parameters such as incident
angle and poloidal radius of curvature, as well as the IR/optical emitting
particle enthalpy fraction. We find that while the standard shock model gives
good agreement with observation in many respects, there remain two puzzles: (a)
The observed angular size of the knot relative to the pulsar--knot separation
is much smaller than expected; (b) The variable, yet high degree of
polarization reported is difficult to reconcile with a highly relativistic
downstream flow.Comment: 46 pages, 14 figures, submitted to the Astrophysical Journa
High resolution radio study of the Pulsar Wind Nebula within the Supernova Remnant G0.9+0.1
We have conducted a radio study at 3.6, 6 and 20 cm using ATCA and VLA and
reprocessed XMM-Newton and Chandra data of the pulsar wind nebula (PWN) in the
supernova remnant (SNR) G0.9+0.1. The new observations revealed that the
morphology and symmetry suggested by Chandra observations (torus and jet-like
features) are basically preserved in the radio range in spite of the rich
structure observed in the radio emission of this PWN, including several arcs,
bright knots, extensions and filaments. The reprocessed X-ray images show for
the first time that the X-ray plasma fills almost the same volume as the radio
PWN. Notably the X-ray maximum does not coincide with the radio maximum and the
neutron star candidate CXOU J174722.8-280915 lies within a small depression in
the radio emission. From the new radio data we have refined the flux density
estimates, obtaining S(PWN) ~ 1.57 Jy, almost constant between 3.6 and 20 cm.
For the whole SNR (compact core and shell), a flux density S(at 20 cm)= 11.5 Jy
was estimated. Based on the new and the existing 90 cm flux density estimates,
we derived alpha(PWN)=-0.18+/-0.04 and alpha(shell)=-0.68+/- 0.07. From the
combination of the radio data with X-ray data, a spectral break is found near
nu ~ 2.4 x 10^(12) Hz. The total radio PWN luminosity is L(radio)=1.2 x 10^(35)
erg s^(-1) when a distance of 8.5 kpc is adopted. By assuming equipartition
between particle and magnetic energies, we estimate a nebular magnetic field B
= 56 muG. The associated particle energy turns out to be U(part)=5 x 10^(47)
erg and the magnetic energy U(mag)=2 x 10^(47) erg. Based on an empirical
relation between X-ray luminosity and pulsar energy loss rate, and the
comparison with the calculated total energy, a lower limit of 1100 yr is
derived for the age of this PWN.Comment: 10 pages,8 figures, accepted for publication in A&A, June 13 200
A Chandra Search for Coronal X Rays from the Cool White Dwarf GD 356
We report observations with the Chandra X-ray Observatory of the single,
cool, magnetic white dwarf GD 356. For consistent comparison with other X-ray
observations of single white dwarfs, we also re-analyzed archival ROSAT data
for GD 356 (GJ 1205), G 99-47 (GR 290 = V1201 Ori), GD 90, G 195-19 (EG250 = GJ
339.1), and WD 2316+123 and archival Chandra data for LHS 1038 (GJ 1004) and GD
358 (V777 Her). Our Chandra observation detected no X rays from GD 356, setting
the most restrictive upper limit to the X-ray luminosity from any cool white
dwarf -- L_{X} < 6.0 x 10^{25} ergs/s, at 99.7% confidence, for a 1-keV
thermal-bremsstrahlung spectrum. The corresponding limit to the electron
density is n_{0} < 4.4 x 10^{11} cm^{-3}. Our re-analysis of the archival data
confirmed the non-detections reported by the original investigators. We discuss
the implications of our and prior observations on models for coronal emission
from white dwarfs. For magnetic white dwarfs, we emphasize the more stringent
constraints imposed by cyclotron radiation. In addition, we describe (in an
appendix) a statistical methodology for detecting a source and for constraining
the strength of a source, which applies even when the number of source or
background events is small.Comment: 27 pages, 4 figures, submitted to the Astrophysical Journa
Typing Supernova Remnants Using X-ray Line Emission Morphologies
We present a new observational method to type the explosions of young
supernova remnants (SNRs). By measuring the morphology of the Chandra X-ray
line emission in seventeen Galactic and Large Magellanic Cloud SNRs with a
multipole expansion analysis (using power ratios), we find that the
core-collapse SNRs are statistically more asymmetric than the Type Ia SNRs. We
show that the two classes of supernovae can be separated naturally using this
technique because X-ray line morphologies reflect the distinct explosion
mechanisms and structure of the circumstellar material. These findings are
consistent with recent spectropolarimetry results showing that core-collapse
SNe are intrinsically more asymmetric.Comment: 4 pages, 1 figure, accepted for publication in ApJ
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