780 research outputs found
CHANDRA Observations of the X-ray Halo around the Crab Nebula
Two Chandra observations have been used to search for thermal X-ray emission
from within and around the Crab Nebula. Dead-time was minimized by excluding
the brightest part of the Nebula from the field of view. A dust-scattered halo
comprising 5% of the strength of the Crab is clearly detected with surface
brightness measured out to a radial distance of 18 arcminutes. Coverage is 100%
at 4 arcminutes, 50% at 12 arcminutes, and 25% at 18 arcminutes. The observed
halo is compared with predictions based on 3 different interstellar grain
models and one can be adjusted to fit the observation. This dust halo and
mirror scattering form a high background region which has been searched for
emission from shock-heated material in an outer shell. We find no evidence for
such emission. We can set upper limits a factor of 10-1000 less than the
surface brightness observed from outer shells around similar remnants. The
upper limit for X-ray luminosity of an outer shell is about 10e34 erg/s.
Although it is possible to reconcile our observation with an 8-13 solar mass
progenitor, we argue that this is unlikely.Comment: 26 pages, 12 figures, accepted by Ap
A New X-Ray Flare from the Galactic Nucleus Detected with the XMM-Newton Photon Imaging Cameras
Sgr A*, the compact radio source, believed to be the counterpart of the
massive black hole at the galactic nucleus, was observed to undergo rapid and
intense flaring activity in X-rays with Chandra in October 2000. We report here
the detection with XMM-Newton EPIC cameras of the early phase of a similar
X-ray flare from this source, which occurred on September 4, 2001. The source
2-10 keV luminosity increased by a factor about 20 to reach a level of 4
10^{34} erg s^{-1} in a time interval of about 900 s, just before the end of
the observation. The data indicate that the source spectrum was hard during the
flare. This XMM-Newton observation confirms the results obtained by Chandra and
suggests that, in Sgr A*, rapid and intense X-ray flaring is not a rare event.
This can constrain the emission mechanism models proposed for this source, and
also implies that the crucial multiwavelength observation programs planned to
explore the behaviour of the radio/sub-mm and hard X-ray/gamma-ray emissions
during the X-ray flares, have a good chance of success.Comment: 18 pages, 6 color figures, final version, accepted on October 24,
2002, to appear in ApJ, v584 n2 ApJ February 20, 2003 issu
A Deep Infrared Search for AXP 1E 1841-045
Multi-colour (JHKs) imaging and photometry of the field of the Anomalous
X-ray Pulsar AXP 1E 1841-045 is analysed in the light of new, accurate
coordinates from Chandra (Wachter et al, 2004). From excellentquality images,
we find multiple sources in and around the position error circle. Of these,
none can be confidently identified as the infrared counterpart. The limiting
magnitudes reached were J=22.1, H=20.7 and Ks=19.9$ (95% confidence).Comment: 8 pages LaTeX, 2 eps figures; ApJ accepte
Distance Determination of Variable Galactic Sources
We have developed a timing analysis method to determine the distances of
variable galactic X-ray sources based on the method advanced by Tr\"{u}mper and
Sch\"{o}nfelder in 1973. The light-curve of the halo produced by the scattering
of X-rays off the interstellar dust is delayed and smeared by the dust grains.
This method utilizes the differences between the power density spectra of the
point source and the halo. We present the details of this method and our first
applications of this method to the Chandra data of X-ray binary Cyg X-3.Comment: 5 page and 3 figure, The 6th pacific rim conference on steller
astrophysics -- a tribute to Helmut A. Ab
Fe Ka line emission from the Arches cluster region - evidence for ongoing particle bombardment?
We present the results of eight years of XMM-Newton observations of the
region surrounding the Arches cluster in the Galactic Center. We study the
spatial distribution and temporal behaviour of the Fe-Ka line emission with the
objective of identifying the likely source of the excitation. We investigate
the variability of the 6.4-keV line emission of four clouds through spectral
fitting of the EPIC MOS data with the use of a modelled background, which
avoids many of the systematics inherent in local background subtraction. We
also employ spectral stacking of both EPIC PN and MOS data to search for
evidence of an Fe-K edge feature imprinted on the underlying X-ray continuum.
The lightcurves of the Fe-Ka line from three bright molecular knots close to
the Arches cluster are found to be constant over the 8-year observation window.
West of the cluster, however, we found a bright cloud exhibiting the fastest
Fe-Ka variability yet seen in a molecular cloud in the Galactic Center region.
The time-averaged spectra of the molecular clouds reveal no convincing evidence
of the 7.1-keV edge feature. The EW of the 6.4-keV line emitted by the clouds
near the cluster is found to be ~1.0 keV. The observed Fe-Ka line flux and the
high EW suggest the fluorescence has a photoionization origin, although
excitation by cosmic-ray particles is not specifically excluded. For the three
clouds nearest to the cluster, an identification of the source of
photo-ionizing photons with an earlier outburst of Sgr A* is however at best
tentative. The hardness of the nonthermal component associated with the 6.4-keV
line emission might be best explained in terms of bombardment by cosmic-ray
particles from the Arches cluster itself. The relatively short-timescale
variability seen in the 6.4-keV line emission from the cloud to the West of the
cluster is most likely the result of illumination by a nearby transient X-ray
source.Comment: 13 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
Echo Emission From Dust Scattering and X-Ray Afterglows of Gamma-Ray Bursts
We investigate the effect of X-ray echo emission in gamma-ray bursts (GRBs).
We find that the echo emission can provide an alternative way of understanding
X-ray shallow decays and jet breaks. In particular, a shallow decay followed by
a "normal" decay and a further rapid decay of X-ray afterglows can be together
explained as being due to the echo from prompt X-ray emission scattered by dust
grains in a massive wind bubble around a GRB progenitor. We also introduce an
extra temporal break in the X-ray echo emission. By fitting the afterglow light
curves, we can measure the locations of the massive wind bubbles, which will
bring us closer to finding the mass loss rate, wind velocity, and the age of
the progenitors prior to the GRB explosions.Comment: 25 pages, 3 figures, 2 tables. Accepted for publication in Ap
The Broad-Band Spectrum and Infrared Variability of the Magnetar AXP 1E1048.1-5937
We present photometry of the Anomalous X-ray pulsar 1E1048.1-5937 in the
infrared and optical, taken at Magellan and the VLT. The object is detected in
the I, J and Ks bands under excellent conditions. We find that the source has
varied greatly in its infrared brightness and present these new magnitudes. No
correlation is found between the infrared flux and spin-down rate, but the
infrared flux and X-ray flux may be anti-correlated. Assuming nominal reddening
values, the resultant spectral energy distribution is found to be inconsistent
with the only other AXP SED available (for 4U0142+61). We consider the effect
of the uncertainty in the reddening to the source on its SED. We find that
although both the X-ray and infrared fluxes have varied greatly for this
source, the most recent flux ratio is remarkably consistent with what is is
found for other AXPs. Finally, we discuss the implications of our findings in
the context of the magnetar model.Comment: 21 pages, 5 eps figures. Submitted to Ap
A Search for Fallback Disks in Four Young Supernova Remnants
We report on our search for the optical/infrared counterparts to the central
compact objects in four young supernova remnants: Pup A, PKS 1209-52, RCW 103,
and Cas A. The X-ray point sources in these supernova remnants are excellent
targets for probing the existence of supernova fallback disks, since
irradiation of a disk by a central X-ray source should lead to an infrared
excess. We used ground-based optical and near-infrared imaging and Spitzer
Space Telescope mid-infrared imaging to search for optical/infrared
counterparts at the X-ray point source positions measured by the Chandra X-Ray
Observatory. We did not detect any counterparts, and hence find no evidence for
fallback disks around any of these sources. In PKS 1209-52, we are able to
exclude a nearby optical/infrared candidate counterpart. In RCW 103, a blend of
3 faint stars at the X-ray source position prevents us from deriving useful
limits. For the other targets, the upper limits on the infrared/X-ray flux
ratio are as deep as (1.0--1.7). Comparing these limits to the
ratio of measured for 4U 0142+61 (a young pulsar
recently found with an X-ray irradiated dust disk), we conclude that the
non-detection of any disks around young neutron stars studied here are
consistent with their relatively low X-ray luminosities, although we note that
a similar dust disk around the neutron star in Pup A should be detectable by
deeper infrared observations.Comment: 9 pages, 5 figures, revised to address referee's comments, and
accepted for publication in Ap
Soft X-ray Scattering and Halos from Dust
Small angle scatterings of X-rays by interstellar dust particles create halos
around x-ray sources. By observing these halos with sufficient angular
resolution it is possible to derive information about both the spatial
distribution of the dust along the line of sight to the source and some of the
physical properties of the scattering dust particles. Previous analyses of
halos have been used to infer that the scattering dust particles must be
fluffy. These fluffy dust particles have also been suggested as a possible
solution to the interstellar carbon "crisis." This crisis results from the
discrepancy between the available ISM carbon abundance and the amount required
to be in dust to account for the average ISM extinction. Dwek (1997), however,
pointed out that fluffy dust particles may not solve the carbon crisis. In
light of this, we re-examine the evidence for fluffy dust in the ISM from x-ray
scattering data.
We find that previous studies overestimated the intensity of the halos by
using the RG approximation, which fails for energies below 1 keV. Using the
exact Mie solution we find that below 1 keV the scattering becomes much less
efficient. Applying the Mie calculations to the x-ray halo observed around Nova
Cygni 1992, we conclude that the data can be explained by scattering from
regular dust, and does not require interstellar dust particles to be fluffy.Comment: latex, uses aaspp4.sty, 21 pages + 10 figures. Submitted to the
Astrophysical Journa
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