548 research outputs found
The magnetic field of the Large Magellanic Cloud revealed through Faraday rotation
We have measured the Faraday rotation toward a large sample of polarized
radio sources behind the Large Magellanic Cloud (LMC), to determine the
structure of this galaxy's magnetic field. The magnetic field of the LMC
consists of a coherent axisymmetric spiral of field strength ~1 microgauss.
Strong fluctuations in the magnetic field are also seen, on small (<0.5
parsecs) and large (~100 parsecs) scales. The significant bursts of recent star
formation and supernova activity in the LMC argue against standard dynamo
theory, adding to the growing evidence for rapid field amplification in
galaxies.Comment: 15 pages, including 3 embedded EPS figures (1 color, 2 b/w) plus
supporting on-line material; uses scicite.sty. To appear in Science, vol 307,
number 5715 (11 March 2005
The high energy gamma-ray emission expected from Tycho's supernova remnant
A nonlinear kinetic model of cosmic ray (CR) acceleration in supernova
remnants (SNRs) is used to describe the properties of Tycho's SNR. Observations
of the expansion characteristics and of the nonthermal radio and X-ray emission
spectra, assumed to be of synchrotron origin, are used to constrain the overall
dynamical evolution and the particle acceleration parameters of the system, in
addition to what is known from independent estimates of the distance and
thermal X-ray observations. It is shown that a very efficient production of
nuclear cosmic rays, leading to strong shock modification, and a large
downstream magnetic field strength B_d approx 240muG are required to reproduce
the observed synchrotron emission from radio to X-ray frequencies. This field
strength is still well within the upper bound for the effective magnetic field,
consistent with the acceleration process. The pi^0-decay gamma-ray flux turns
out to be somewhat greater than the inverse Compton (IC) flux off the Cosmic
Microwave Background at energies below 1 TeV, dominating it strongly at 10 TeV.
The predicted TeV gamma-ray flux is consistent with but close to the very low
upper limit recently obtained by HEGRA. A future detection at E_gamma ~ 10 TeV
would clearly indicate hadronic emission.Comment: 8 pages, 6 figures. Accepted in Astronomy and Astrophyic
An X-Ray Study of the Supernova Remnant G290.1-0.8
G290.1-0.8 (MSH 11-61A) is a supernova remnant (SNR) whose X-ray morphology
is centrally bright. However, unlike the class of X-ray composite SNRs whose
centers are dominated by nonthermal emission, presumably driven by a central
pulsar, we show that the X-ray emission from G290.1-0.8 is thermal in nature,
placing the remnant in an emerging class which includes such remnants as W44,
W28, 3C391, and others. The evolutionary sequence which leads to such X-ray
properties is not well understood. Here we investigate two scenarios for such
emission: evolution in a cloudy interstellar medium, and early-stage evolution
of a remnant into the radiative phase, including the effects of thermal
conduction. We construct models for these scenarios in an attempt to reproduce
the observed center-filled X-ray properties of G290.1-0.8, and we derive the
associated age, energy, and ambient density conditions implied by the models.
We find that for reasonable values of the explosion energy, the remnant age is
of order (1 - 2) x 10^{4} yr. This places a fairly strong constraint on any
association between G290.1-0.8 and PSR J1105-610, which would require an
anomalously large velocity for the pulsar.Comment: 7 pages, 7 figures, ApJ, accepte
Discovery of Radio/X-ray/Optical Resolved Supernova Remnants in the Center of the Andromeda Galaxy
We have detected a spatially resolved supernova remnant (SNR) in the center
of the Andromeda Galaxy, in radio, X-ray, and optical wavelengths. These
observations provide the highest spatial resolution imaging of a
radio/X-ray/optical SNR in that galaxy to date. The multi-wavelength
morphology, radio spectral index, X-ray colors, and narrow-band optical imaging
are consistent with a shell-type SNR. A second SNR is also seen resolved in
both radio and X-ray. By comparing the morphological sturcture of the SNRs in
different wavelengths and with that in our own Galaxy, we can study the shock
morphologies of SNRs in the Andromeda Galaxy. The proximity of the SNRs to the
core suggests high interstellar medium density in the vicinity of the SNRs in
the center of the Andromeda Galaxy.Comment: 5 pages, 3 figures, accepted for publication in ApJ
An Optical and X-ray Examination of Two Radio Supernova Remnant Candidates in 30 Doradus
The giant HII region 30 Doradus is known for its violent internal motions and
bright diffuse X-ray emission, suggesting the existence of supernova remnants
(SNRs), but no nonthermal radio emission has been detected. Recently, Lazendic
et al. compared the H-alpha/H-beta and radio/H-alpha ratios and suggested two
small radio sources to be nonthermal and thus SNR candidates; however, no
optical or X-ray counterparts were detected. We have used high-resolution
optical images and high-dispersion spectra to examine the morphological,
spectral, and kinematic properties of these two SNR candidates, and still find
no optical evidence supporting their identification as SNRs. We have also
determined the X-ray luminosities of these SNR candidates, and find them 1-3
orders of magnitude lower than those commonly seen in young SNRs. High
extinction can obscure optical and X-ray signatures of an SNR, but would
prohibit the use of a high radio/H-alpha ratio to identify nonthermal radio
emission. We suggest that the SNR candidate MCRX J053831.8-690620 is associated
with a young star forming region; while the radio emission originates from the
obscured star forming region, the observed optical emission is dominated by the
foreground. We suggest that the SNR candidate MCRX J053838.8-690730 is
associated with a dust/molecular cloud, which obscures some optical emission
but not the radio emission.Comment: 13 pages, 2 figures, accepted for publication in the ApJ, Nov 10,
200
The population of X-ray supernova remnants in the Large Magellanic Cloud
We present a comprehensive X-ray study of the population of supernova
remnants (SNRs) in the LMC. Using primarily XMM-Newton, we conduct a systematic
spectral analysis of LMC SNRs to gain new insights on their evolution and the
interplay with their host galaxy. We combined all the archival XMM observations
of the LMC with those of our Very Large Programme survey. We produced X-ray
images and spectra of 51 SNRs, out of a list of 59. Using a careful modelling
of the background, we consistently analysed all the X-ray spectra and measure
temperatures, luminosities, and chemical compositions. We investigated the
spatial distribution of SNRs in the LMC and the connection with their
environment, characterised by various SFHs. We tentatively typed all LMC SNRs
to constrain the ratio of core-collapse to type Ia SN rates in the LMC. We
compared the X-ray-derived column densities to HI maps to probe the
three-dimensional structure of the LMC. This work provides the first
homogeneous catalogue of X-ray spectral properties of LMC SNRs. It offers a
complete census of LMC SNRs exhibiting Fe K lines (13% of the sample), or
revealing contribution from hot SN ejecta (39%). Abundances in the LMC ISM are
found to be 0.2-0.5 solar, with a lower [/Fe] than in the Milky Way.
The ratio of CC/type Ia SN in the LMC is , lower than in local SN surveys and galaxy clusters.
Comparison of X-ray luminosity functions of SNRs in Local Group galaxies
reveals an intriguing excess of bright objects in the LMC. We confirm that 30
Doradus and the LMC Bar are offset from the main disc of the LMC, to the far
and near sides, respectively. (abridged)Comment: Accepted for publication in Astronomy and Astrophysics. 54 pages, 18
figures, 12 tables. The resolution of the figures has been reduced compared
to the journal version; v2: New title, minor text edits; v3: Correct version
New Results of Observations of the Only Supernova Remnant in the IC1613 Galaxy
The new results of a study of the kinematics of the supernova remnant S8 in
the IC1613 galaxy are reported. The expansion velocity of the bright optical
nebula is determined based on observations made with the 6-m telescope of the
Special Astrophysical Observatory of the Russian Academy of Sciences using MPSF
field spectrograph and SCORPIO focal reducer operating in the scanning
Fabry--Perot interferometer mode. An analysis of 21-cm line VLA observations of
the galaxy corroborates our earlier proposed model of a SN exploding inside a
cavern surrounded by a dense shell and S8 colliding with the wall of the HI
shell.Comment: 8 pages, 7 figures; accepted for publication in Astronomy Letter
In which shell-type SNRs should we look for gamma-rays and neutrinos from p-p collisions?
We present a simple analytic model for the various contributions to the
non-thermal emission from shell type SNRs, and show that this model's results
reproduce well the results of previous detailed calculations. We show that the
\geq 1 TeV gamma ray emission from the shell type SNRs RX J1713.7-3946 and RX
J0852.0-4622 is dominated by inverse-Compton scattering of CMB photons (and
possibly infra-red ambient photons) by accelerated electrons. Pion decay (due
to proton-proton collisions) is shown to account for only a small fraction,
\lesssim10^-2, of the observed flux, as assuming a larger fractional
contribution would imply nonthermal radio and X-ray synchrotron emission and
thermal X-ray Bremsstrahlung emission that far exceed the observed radio and
X-ray fluxes. Models where pion decay dominates the \geq 1 TeV flux avoid the
implied excessive synchrotron emission (but not the implied excessive thermal
X-ray Bremsstrahlung emission) by assuming an extremely low efficiency of
electron acceleration, K_ep \lesssim 10^-4 (K_ep is the ratio of the number of
accelerated electrons and the number of accelerated protons at a given energy).
We argue that observations of SNRs in nearby galaxies imply a lower limit of
K_ep \gtrsim 10^-3, and thus rule out K_ep values \lesssim 10^-4 (assuming that
SNRs share a common typical value of K_ep). It is suggested that SNRs with
strong thermal X-ray emission, rather than strong non-thermal X-ray emission,
are more suitable candidates for searches of gamma rays and neutrinos resulting
from proton-proton collisions. In particular, it is shown that the neutrino
flux from the SNRs above is probably too low to be detected by current and
planned neutrino observatories (Abridged).Comment: 13 pages, 1 figure, accepted for publication in JCAP, minor revision
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