346 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
The G292.0+1.8 pulsar wind nebula in the mid-infrared
G292.0+1.8 is a Cas A-like supernova remnant that contains the young pulsar
PSR J1124-5916 powering a compact torus-like pulsar wind nebula visible in
X-rays. A likely counterpart to the nebula has been detected in the optical VRI
bands. To confirm the counterpart candidate nature, we examined archival
mid-infrared data obtained with the Spitzer Space Telescope. Broad-band images
taken at 4.5, 8, 24, and 70 microns were analyzed and compared with available
optical and X-ray data. The extended counterpart candidate is firmly detected
in the 4.5 and 8 micron bands. It is brighter and more extended in the bands
than in the optical, and its position and morphology agree well with the
coordinates and morphology of the torus-like pulsar wind nebula in X-rays. The
source is not visible in 24 and 70 micron images, which are dominated by bright
emission from the remnant shell and filaments. We compiled the infrared fluxes
of the nebula, which probably contains a contribution from an unresolved pulsar
in its center, with the optical and X-ray data. The resulting unabsorbed
multiwavelength spectrum is described by power laws of significantly steeper
slope in the infrared-optical than in X-rays, implying a double-knee spectral
break between the optical and X-rays. The 24 and 70 microns flux upper limits
suggest a second break and a flatter spectrum at the long wavelength limit.
These features are common to two other pulsar wind nebulae associated with the
remnants B0540-69.3 and 3C 58 and observed in all three ranges. The position,
morphology, and spectral properties of the detected source allow us to comfirm
that it is the infrared-optical counterpart to both the pulsar and its wind
nebula system in the G292.0+1.8 supernova remnant.Comment: 5 pages, 2 figure
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
Radio Polarization Observations of the Snail: A Crushed Pulsar Wind Nebula in G327.1-1.1 with a Highly Ordered Magnetic Field
Pulsar wind nebulae (PWNe) are suggested to be acceleration sites of cosmic
rays in the Galaxy. While the magnetic field plays an important role in the
acceleration process, previous observations of magnetic field configurations of
PWNe are rare, particularly for evolved systems. We present a radio
polarization study of the "Snail" PWN inside the supernova remnant G327.1-1.1
using the Australia Telescope Compact Array. This PWN is believed to have been
recently crushed by the supernova (SN) reverse shock. The radio morphology is
composed of a main circular body with a finger-like protrusion. We detected a
strong linear polarization signal from the emission, which reflects a highly
ordered magnetic field in the PWN and is in contrast to the turbulent
environment with a tangled magnetic field generally expected from
hydrodynamical simulations. This could suggest that the characteristic
turbulence scale is larger than the radio beam size. We built a toy model to
explore this possibility, and found that a simulated PWN with a turbulence
scale of about one-eighth to one-sixth of the nebula radius and a pulsar wind
filling factor of 50--75% provides the best match to observations. This implies
substantial mixing between the SN ejecta and pulsar wind material in this
system.Comment: 13 pages, 10 figures, Accepted for publication in Ap
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