524 research outputs found
Earth-based remote sensing of planetary surfaces and atmospheres at radio wavelengths
Two reasons for remote sensing from the Earth are given: (1) space exploration, particularly below the surfaces or underneath cloud layers, is limited to only a very few planets; and (2) a program of regular monitoring, currently impractical with a limited number of space probes, is required. Reflected solar and nonthermal radiation are discussed. Relativistic electrons, trapped in large magnetospheres on Saturn and Jupiter, are discussed. These electrons produce synchrotron radiation and also interact with the ionosphere to produce bursts of low frequency emission. Because most objects are black-bodies, continuum radiometry is emphasized. Spectroscopic techniques and the measurement of nonthermal emission are also discussed
The clumpy circumstellar medium around young supernova remnants
Each of the youngest supernova remnants known in the Milky Way, Cas A, Kepler's SNR, and Tycho's SNR, shows a different morphological structure caused by different conditions in the progenitor stars and their surroundings. In all three cases, however, the observed shells have a thickness of about 1/4 the radius, a sharp outer edge, and significant brightness irregularities. These features require that the circumstellar medium be highly clumped. To investigate the phenomenon, models of the expansion have been constructed using a one-dimensional spherical hydrodynamic code. As a supernova shock moves down the external density gradient of the star, material behind the shock begins to go into free expansion. Then as surrounding material is encountered a reverse shock moving back into the ejectum will be formed. Until the expansion has swept up about eight times the ejected mass when the situation can be considered as a point explosion in its surroundings, the dynamics are controlled by conditions between the shocks. The region is also where the synchrotron radio emission from relativistic electrons trapped in magnetic fields arises. Initial particles and fields are accelerated and amplified by eddy motion at the interface between the ejected and swept-up material and at the boundaries of clumps. Polarimetry shows that these SNR have a net radial orientation of their magnetic fields apparently from stretching by Rayleigh-Taylor instabilities at the contact surfaces. Without clumps the observed shell is much too narrow and steep on the inside
Supernova Remnants in the Magellanic Clouds. IV. X-Ray Emission from the Largest SNR in the LMC
We present the first X-ray detection of SNR 0450-70.9 the largest known
supernova remnant (SNR) in the Large Magellanic Cloud. To study the physical
conditions of this SNR, we have obtained XMM-Newton X-ray observations, optical
images and high-dispersion spectra, and radio continuum maps. Optical images of
SNR 0450-70.9 show a large, irregular elliptical shell with bright filaments
along the eastern and western rims and within the shell interior. The interior
filaments have higher [S II]/Halpha ratios and form an apparent inner shell
morphology. The X-ray emission region is smaller than the full extent of the
optical shell, with the brightest X-ray emission found within the small
interior shell and on the western rim of the large shell. The expansion
velocity of the small shell is ~220 km/s, while the large shell is ~120 km/s.
The radio image shows central brightening and a fairly flat radio spectral
index over the SNR. However, no point X-ray or radio source corresponding to a
pulsar is detected and the X-ray emission is predominantly thermal. Therefore,
these phenomena can be most reasonably explained in terms of the advanced age
of the large SNR. Using hydrodynamic models combined with a nonequilibrium
ionization model for thermal X-ray emission, we derived a lower limit on the
SNR age of about 45,000 yr, well into the later stages of SNR evolution.
Despite this, the temperature and density derived from spectral fits to the
X-ray emission indicate that the remnant is still overpressured, and thus that
the development is largely driven by hot gas in the SNR interior.Comment: Accepted for publication in The Astrophysical Journa
Detection of an X-ray Pulsar Wind Nebula and Tail in SNR N157B
We report Chandra X-ray observations of the supernova remnant N157B in the
Large Magellanic Cloud, which are presented together with an archival HST
optical image and a radio continuum map for comparison. This remnant contains
the recently discovered 16 ms X-ray pulsar PSR J0537-6910, the most rapidly
rotating young pulsar known.
Using phase-resolved Chandra imaging, we pinpoint the location of the pulsar
to within an uncertainty of less than 1 arcsec. PSR J0537-6910 is not detected
in any other wavelength band. The X-ray observations resolve three distinct
features: the pulsar itself, a surrounding compact wind nebula which is
strongly elongated and a feature of large-scale diffuse emission trailing from
the pulsar. This latter comet tail-shaped feature coexists with enhanced radio
emission and is oriented nearly perpendicular to the major axis of the pulsar
wind nebula. We propose the following scenario to explain these features. The
bright, compact nebula is likely powered by a toroidal pulsar wind of
relativistic particles which is partially confined by the ram-pressure from the
supersonic motion of the pulsar. The particles, after being forced out from the
compact nebula (the head of the ``comet''), are eventually dumped into a bubble
(the tail), which is primarily responsible for the extended diffuse X-ray and
radio emission. The ram-pressure confinement also allows a natural explanation
for the observed X-ray luminosity of the compact nebula and for the unusually
small X-ray to spin-down luminosity ratio, compared to similarly energetic
pulsars. We estimate the pulsar wind Lorentz factor of N157B as about 4 times
10^6 (with an uncertainty of a factor about 2, consistent with that inferred
from the modeling of the Crab Nebula.Comment: 15 pages plus 4 figures. The postscript file of the whole paper is
available at http://xray.astro.umass.edu/wqd/papers/n157b/n157b.ps. accepted
for publication in Ap
Revealing New Physical Structures in the Supernova Remnant N63A through Chandra Imaging Spectroscopy
We present Chandra X-ray observations of the supernova remnant (SNR) N63A in
the Large Magellanic Cloud (LMC). N63A, one of the brightest LMC remnants, is
embedded in an H II region and probably associated with an OB association. The
optical remnant consists of three lobes of emission contained within the
approximately three times larger X-ray remnant. Our Chandra data reveal a
number of new physical structures in N63A. The most striking of these are the
several ``crescent''-shaped structures located beyond the main shell that
resemble similar features seen in the Vela SNR. In Vela, these have been
interpreted as arising from high speed clumps of supernova ejecta interacting
with the ambient medium. Another distinct feature of the remnant is a roughly
triangular ``hole'' in the X-ray emission near the location of the optical
lobes and the brightest radio emission. X-ray spectral analysis shows that this
deficit of emission is a result of absorption by an intervening dense cloud
with a mass of ~450 M_sun that is currently being engulfed by the remnant's
blast wave. We also find that the rim of the remnant, as well as the
crescent-shaped features, have considerably softer X-ray spectra than the
interior. Limits on hard X-ray emission rule out a young, energetic pulsar in
N63A, but the presence of an older or less active one, powering a wind nebula
with a luminosity less than ~4e10^34 erg/s, is allowed.Comment: 18 pages, 5 figures (2 color), accepted for publication in Ap
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
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 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
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