28 research outputs found
Accelerated Electrons in Cassiopeia A: An Explanation for the Hard X-ray Tail
We propose a model for the hard X-ray (> 10 keV) emission observed from the
supernova remnant Cas A. Lower hybrid waves are generated in strong (mG)
magnetic fields, generally believed to reside in this remnant, by shocks
reflected from density inhomogeneities. These then accelerate electrons to
energies of several tens of keV. Around 4% of the x-ray emitting plasma
electrons need to be in this accelerated distribution, which extends up to
electron velocities of order the electron Alfven speed, and is directled along
magnetic field lines. Bremsstrahlung from these electrons produces the observed
hard x-ray emission. Such waves and accelerated electrons have been observed in
situ at Comet Halley, and we discuss the viability of the extrapolation from
this case to the parameters relevant to Cas A.Comment: 20 pages, 3 figures, aasTeX502, accepted in Ap
Near-Infrared Synchrotron Emission from Cas A
High energy observations of Cas A suggested the presence of synchrotron
radiation, implying acceleration of cosmic rays by young supernova remnants. We
detect synchrotron emission from Cas A in the near-infrared using Two Micron
All Sky Survey (2MASS) and Palomar 200 inch PFIRCAM observations. The remnant
is detected in J, H, and Ks bands, with Ks band brightest and J faint. In the J
and H bands, bright [Fe II] lines (1.24um and 1.64um) are detected
spectroscopically. The Palomar observations include Ks continuum, narrow-band
1.64um (centered on [Fe II]) and 2.12um (centered on H2(1-0)) images. While the
narrow-band 1.64um image shows filamentary and knotty structures, similar to
the optical image, the Ks image shows a relatively smooth, diffuse shell,
remarkably similar to the radio image. The broad-band near-infrared fluxes of
Cas A are generally consistent with, but a few tens of percent higher than, an
extrapolation of the radio fluxes. The hardening to higher frequencies is
possibly due to nonlinear shock acceleration and/or spectral index variation
across the remnant. We show evidence of spectral index variation. The presence
of near-infrared synchrotron radiation requires the roll-off frequency to be
higher than 1.5e14 Hz, implying that electrons are accelerated to energies of
at least 0.2 TeV. The morphological similarity in diffuse emission between the
radio and Ks band images implies that synchrotron losses are not dominant. Our
observations show unambiguous evidence that the near-infrared Ks band emission
of Cas A is from synchrotron emission by accelerated cosmic-ray electrons.Comment: accepted by Ap
Nonthermal Emission from a Supernova Remnant in a Molecular Cloud
In evolved supernova remnants (SNRs) interacting with molecular clouds, such
as IC 443, W44, and 3C391, a highly inhomogeneous structure consisting of a
forward shock of moderate Mach number, a cooling layer, a dense radiative shell
and an interior region filled with hot tenuous plasma is expected. We present a
kinetic model of nonthermal electron injection, acceleration and propagation in
that environment and find that these SNRs are efficient electron accelerators
and sources of hard X- and gamma-ray emission. The energy spectrum of the
nonthermal electrons is shaped by the joint action of first and second order
Fermi acceleration in a turbulent plasma with substantial Coulomb losses.
Bremsstrahlung, synchrotron, and inverse Compton radiation of the nonthermal
electrons produce multiwavelength photon spectra in quantitative agreement with
the radio and the hard emission observed by ASCA and EGRET from IC 443. We
distinguish interclump shock wave emission from molecular clump shock wave
emission accounting for a complex structure of molecular cloud. Spatially
resolved X- and gamma- ray spectra from the supernova remnants IC 443, W44, and
3C391 as might be observed with BeppoSAX, Chandra XRO, XMM, INTEGRAL and GLAST
would distinguish the contribution of the energetic lepton component to the
gamma-rays observed by EGRET.Comment: 14 pages, 4 figure, Astrophysical Journal, v.538, 2000 (in press
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.Comment: 10 pages with embedded figures; accepted by the Ap.J; author's web
pages at http://lheawww.gsfc.nasa.gov/users/jonathan/ ,
http://lheawww.gsfc.nasa.gov/users/rob/Vita/petre.html ,
http://lheawww.gsfc.nasa.gov/users/evg/evg.htm
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.Comment: 10 pages of text, 3 figures, accepted for Astrophys. J. Letter
A study of Tycho's SNR at TeV energies with the HEGRA CT-System
Tycho's supernova remnant (SNR) was observed during 1997 and 1998 with the
HEGRA Cherenkov Telescope System in a search for gamma-ray emission at
energies above
~1 TeV. An analysis of these data, ~65 hours in total, resulted in no
evidence for TeV gamma-ray emission. The 3sigma upper limit to the gamma-ray
flux (>1 TeV) from Tycho is estimated at 5.78x10^{-13} photons cm^{-2} s^{-1},
or 33 milli-Crab. We interpret our upper limit within the framework of the
following scenarios:
(1) that the observed hard X-ray tail is due to synchrotron emission. A lower
limit on the magnetic field within Tycho may be estimated B>=22 microG,
assuming that the RXTE-detected
X-rays were due to synchrotron emission. However, using results from a
detailed model of the ASCA emission, a more conservative lower limit B>=6
microG is derived.
(2) the hadronic model of Drury, Aharonian & Voelk, and (3) the more recent
time-dependent kinetic theory of Berezhko & Voelk.
Our upper limit lies within the range of predicted values of both hadronic
models, according to uncertainties in physical parameters of Tycho, and shock
acceleration details. In the latter case, the model was scaled to suit the
parameters of Tycho and re-normalised to account for a simplification of the
original model.
We find that we cannot rule out Tycho as a potential contributor at an
average level to the Galactic cosmic-ray flux.Comment: 9 pages, 6 figures. Accepted for publication in Astronomy and
Astrophysic
Separating Thermal and Nonthermal X-Rays in Supernova Remnants I: Total Fits to SN 1006 AD
The remnant of SN 1006 has an X-ray spectrum dominated by nonthermal
emission, and pre-ASCA observations were well described by a synchrotron
calculation with electron energies limited by escape. We describe the results
of a much more stringent test: fitting spatially integrated ASCA GIS (0.6-8
keV) and RXTE PCA (3-10 keV) data with a combination of the synchrotron model
SRESC newly ported to XSPEC and a new thermal shock model VPSHOCK. The new
model can describe the continuum emission above 2 keV well, in both spatial
distribution and spectrum. We find that the emission is dominantly nonthermal,
with a small but noticeable thermal component: Mg and Si are clearly visible in
the integrated spectrum. The synchrotron component rolls off smoothly from the
extrapolated radio spectrum, with a characteristic rolloff frequency of 3.1E17
Hz, at which the spectrum has dropped about a factor of 6 below a powerlaw
extrapolation from the radio spectrum. Comparison of TeV observations with new
TeV model images and spectra based on the X-ray model fits gives a mean
post-shock magnetic field strength of about 9 microGauss, implying (for a
compression ratio of 4) an upstream magnetic field of 3 microGauss, and fixing
the current energy content in relativistic electrons at about 7E48 ergs,
resulting in a current electron-acceleration efficiency of about 5%. This total
energy is about 100 times the energy in the magnetic field. Our results
indicate that joint thermal and nonthermal fitting, using sophisticated models,
will be required for analysis of most supernova-remnant X-ray data in the
future.Comment: 26 pages + 12 figures, accepted by Ap
The Relation Between the Surface Brightness and the Diameter for Galactic Supernova Remnants
In this work, we have constructed a relation between the surface brightness
() and diameter (D) of Galactic C- and S-type supernova remnants
(SNRs). In order to calibrate the -D dependence, we have carefully
examined some intrinsic (e.g. explosion energy) and extrinsic (e.g. density of
the ambient medium) properties of the remnants and, taking into account also
the distance values given in the literature, we have adopted distances for some
of the SNRs which have relatively more reliable distance values. These
calibrator SNRs are all C- and S-type SNRs, i.e. F-type SNRs (and S-type SNR
Cas A which has an exceptionally high surface brightness) are excluded. The
Sigma-D relation has 2 slopes with a turning point at D=36.5 pc: (at 1
GHz)=8.4 D
WmHzster (for
WmHzster and D36.5 pc) and (at 1
GHz)=2.7 10 D
WmHzster (for
WmHzster and D36.5 pc). We discussed the theoretical
basis for the -D dependence and particularly the reasons for the change
in slope of the relation were stated. Added to this, we have shown the
dependence between the radio luminosity and the diameter which seems to have a
slope close to zero up to about D=36.5 pc. We have also adopted distance and
diameter values for all of the observed Galactic SNRs by examining all the
available distance values presented in the literature together with the
distances found from our -D relation.Comment: 45 pages, 2 figures, accepted for publication in Astronomical and
Astrophysical Transaction
Supernova remnants: the X-ray perspective
Supernova remnants are beautiful astronomical objects that are also of high
scientific interest, because they provide insights into supernova explosion
mechanisms, and because they are the likely sources of Galactic cosmic rays.
X-ray observations are an important means to study these objects.And in
particular the advances made in X-ray imaging spectroscopy over the last two
decades has greatly increased our knowledge about supernova remnants. It has
made it possible to map the products of fresh nucleosynthesis, and resulted in
the identification of regions near shock fronts that emit X-ray synchrotron
radiation.
In this text all the relevant aspects of X-ray emission from supernova
remnants are reviewed and put into the context of supernova explosion
properties and the physics and evolution of supernova remnants. The first half
of this review has a more tutorial style and discusses the basics of supernova
remnant physics and thermal and non-thermal X-ray emission. The second half
offers a review of the recent advances.The topics addressed there are core
collapse and thermonuclear supernova remnants, SN 1987A, mature supernova
remnants, mixed-morphology remnants, including a discussion of the recent
finding of overionization in some of them, and finally X-ray synchrotron
radiation and its consequences for particle acceleration and magnetic fields.Comment: Published in Astronomy and Astrophysics Reviews. This version has 2
column-layout. 78 pages, 42 figures. This replaced version has some minor
language edits and several references have been correcte