74 research outputs found
The Importance of Physical Models for Deriving Dust Masses and Grain Size Distributions in Supernova Ejecta I: Radiatively Heated Dust in the Crab Nebula
Recent far-infrared (IR) observations of supernova remnants (SNRs) have
revealed significantly large amounts of newly-condensed dust in their ejecta,
comparable to the total mass of available refractory elements. The dust masses
derived from these observations assume that all the grains of a given species
radiate at the same temperature, regardless of the dust heating mechanism or
grain radius. In this paper, we derive the dust mass in the ejecta of the Crab
Nebula, using a physical model for the heating and radiation from the dust. We
adopt a power-law distribution of grain sizes and two different dust
compositions (silicates and amorphous carbon), and calculate the heating rate
of each dust grain by the radiation from the pulsar wind nebula (PWN). We find
that the grains attain a continuous range of temperatures, depending on their
size and composition. The total mass derived from the best-fit models to the
observed IR spectrum is 0.019-0.13 solar masses, depending on the assumed grain
composition. We find that the power-law size distribution of dust grains is
characterized by a power-law index of 3.5-4.0 and a maximum grain size larger
than 0.1 microns. The grain sizes and composition are consistent with what is
expected for dust grains formed in a Type IIP SN. Our derived dust mass is at
least a factor of two less than the mass reported in previous studies of the
Crab Nebula that assumed more simplified two-temperature models. The results of
this study show that a physical model resulting in a realistic distribution of
dust temperatures can constrain the dust properties and affect the derived dust
masses. Our study may also have important implications for deriving grain
properties and mass estimates in other SNRs and for the ultimate question of
whether SNe are major sources of dust in the Galactic interstellar medium (ISM)
and in external galaxies.Comment: 9 pages, 2 tables, 8 figures, Accepted to The Astrophysical Journa
High-Energy Emission from the Composite Supernova Remnant MSH 15-56
MSH 15-56 (G326.3-1.8) is a composite supernova remnant (SNR) that consists
of an SNR shell and a displaced pulsar wind nebula (PWN) in the radio. We
present XMM-Newton and Chandra X-ray observations of the remnant that reveal a
compact source at the tip of the radio PWN and complex structures that provide
evidence for mixing of the supernova (SN) ejecta with PWN material following a
reverse shock interaction. The X-ray spectra are well fitted by a non-thermal
power-law model whose photon index steepens with distance from the presumed
pulsar, and a thermal component with an average temperature of 0.55 keV. The
enhanced abundances of silicon and sulfur in some regions, and the similar
temperature and ionization timescale, suggest that much of the X-ray emission
can be attributed to SN ejecta that have either been heated by the reverse
shock or swept up by the PWN. We find one region with a lower temperature of
0.3 keV that appears to be in ionization equilibrium. Assuming the Sedov model,
we derive a number of SNR properties, including an age of 16,500 yr. Modeling
of the gamma-ray emission detected by Fermi shows that the emission may
originate from the reverse shock-crushed PWN.Comment: 11 pages, 3 tables, 8 figures, accepted for publication in The
Astrophysical Journa
Infrared and X-Ray Spectroscopy of the KES 75 Supernova Remnant Shell: Characterizing the Dust and Gas Properties
We present deep Chandra observations and Spitzer Space Telescope infrared (IR) spectroscopy of the shell in the composite supernova remnant (SNR) Kes 75 (G29.7-0.3). The remnant is composed of a central pulsar wind nebula and a bright partial shell in the south that is visible at radio, IR, and X-ray wavelengths. The X-ray emission can be modeled by either a single thermal component with a temperature of ~ 1.5 keV, or with two thermal components with temperatures of 1.5 and 0.2 keV. Previous studies suggest that the hot component may originate from reverse-shocked SN ejecta. However, our new analysis shows no definitive evidence for enhanced abundances of Si, S, Ar, Mg, and Fe, as expected from supernova (SN) ejecta, or for the IR spectral signatures characteristic of confirmed SN condensed dust, thus favoring a circumstellar or interstellar origin for the X-ray and IR emission. The X-ray and ill emission in the shell are spatially correlated, suggesting that the dust particles are collisionally heated by the X-ray emitting gas. The IR spectrum of the shell is dominated by continuum emission from dust with little, or no line emission. Modeling the IR spectrum shows that the dust is heated to a temperature of ~ 140 K by a relatively dense, hot plasma, that also gives rise to the hot X-ray emission component. The density inferred from the IR emission is significantly higher than the density inferred from the X-ray models, suggesting a low filling factor for this X-ray emitting gas. The total mass of the warm dust component is at least 1.3 x 10(exp -2) solar mass, assuming no significant dust destruction has occurred in the shell. The IR data also reveal the presence of an additional plasma component with a cooler temperature, consistent with the 0.2 keV gas component. Our IR analysis therefore provides an independent verification of the cooler component of the X-ray emission. The complementary analyses of the X-ray and IR emission provide quantitative estimates of density and filling factors of the clumpy medium swept up by the SNR
Supernova Remnant Kes 17: Efficient Cosmic Ray Accelerator inside a Molecular Cloud
Supernova remnant Kes 17 (SNR G304.6+0.1) is one of a few but growing number
of remnants detected across the electromagnetic spectrum. In this paper, we
analyze recent radio, X-ray, and gamma-ray observations of this object,
determining that efficient cosmic ray acceleration is required to explain its
broadband non-thermal spectrum. These observations also suggest that Kes 17 is
expanding inside a molecular cloud, though our determination of its age depends
on whether thermal conduction or clump evaporation is primarily responsible for
its center-filled thermal X-ray morphology. Evidence for efficient cosmic ray
acceleration in Kes 17 supports recent theoretical work that the strong
magnetic field, turbulence, and clumpy nature of molecular clouds enhances
cosmic ray production in supernova remnants. While additional observations are
needed to confirm this interpretation, further study of Kes 17 is important for
understanding how cosmic rays are accelerated in supernova remnants.Comment: 13 pages, 6 figures, 4 table
A Deep X-ray View of the Synchrotron-Dominated Supernova Remnant G330.2+1.0
We present moderately deep (125 ks) {\it XMM-Newton} observations of
supernova remnant G330.21.0. This remnant is one of only a few known that
fall into "synchrotron-dominated" category, with the emission almost entirely
dominated by a nonthermal continuum. Previous X-ray observations could only
characterize the spectra of a few regions. Here, we examine the spectra from
fourteen regions surrounding the entire rim, finding that the spectral
properties of the nonthermal emission do not vary significantly in any
systematic way from one part of the forward shock to another, unlike several
other remnants of this class. We confirm earlier findings that the power-law
index, , ranges from about 2.1-2.5, while the absorbing column density
is generally between 2.0-2.6 cm. Fits with the {\it
srcut} model find values of the roll-off frequency in the range of 10 Hz, implying energies of accelerated electrons of TeV.
These values imply a high shock velocity of km s, favoring a
young age of the remnant. Diffuse emission from the interior is nonthermal in
origin as well, and fits to these regions yield similar values to those along
the rim, also implying a young age. Thermal emission is present in the east,
and the spectrum is consistent with a km s shock wave
encountering interstellar or circumstellar material with a density of
cm.Comment: Accepted for publication by ApJ. Manuscript produced with emulateapj.
10 pages, 8 figure
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