Tracey Ann Delaney

A MILLION-SECOND CHANDRA VIEW OF CASSIOPEIA A

We introduce a million second observation of the supernova remnant Cassiopeia A with the Chandra X-Ray Observatory. The bipolar structure of the Si-rich ejecta (northeast jet and southwest counterpart) is clearly evident in the new images, and their chemical similarity is confirmed by their spectra. These are most likely due to jets of ejecta as opposed to cavities in the circumstellar medium, since we can reject simple models for the latter. The properties of these jets and the Fe-rich ejecta will provide clues to the explosion of Cas A.

CAVITY OF MOLECULAR GAS ASSOCIATED WITH SUPERNOVA REMNANT 3C 397

3C 397 is a radio and X-ray bright Galactic supernova remnant (SNR) with an unusual rectangular morphology. Our CO observation obtained with the Purple Mountain Observatory at Delingha, Qinghai Province, China reveals that the remnant is well confined in a cavity of molecular gas and embedded at the edge of a molecular cloud (MC) at the local standard of rest systemic velocity of similar to 32 km s(-1). The cloud has a column density gradient increasing from southeast to northwest, perpendicular to the Galactic plane, in agreement with the elongation direction of the remnant. This systemic velocity places the cloud and SNR 3C 397 at a kinematic distance of similar to 10.3 kpc. The derived mean molecular density (similar to 10-30 cm(-3)) explains the high volume emission measure of the X-ray emitting gas. A (12)CO line broadening of the similar to 32 km s(-1) component is detected at the westmost boundary of the remnant, which provides direct evidence of the SNR MC interaction and suggests multi-component gas there with dense (similar to 10(4) cm(-3)) molecular clumps. We confirm the previous detection of an MC at similar to 38 km s(-1) to the west and south of the SNR and argue, based on H I self-absorption, that the cloud is located in the foreground of the remnant. A list of Galactic SNRs presently known and suggested to be in physical contact with environmental MCs is appended in this paper.

Cassiopeia A: dust factory revealed via submillimetre polarimetry

If Type II supernovae – the evolutionary end points of short-lived, massive stars – produce a significant quantity of dust (>0.1 M⊙) then they can explain the rest-frame far-infrared emission seen in galaxies and quasars in the first Gyr of the Universe. Submillimetre (submm) observations of the Galactic supernova remnant, Cas A, provided the first observational evidence for the formation of significant quantities of dust in Type II supernovae. In this paper, we present new data which show that the submm emission from Cas A is polarized at a level significantly higher than that of its synchrotron emission. The orientation is consistent with that of the magnetic field in Cas A, implying that the polarized submm emission is associated with the remnant. No known mechanism would vary the synchrotron polarization in this way and so we attribute the excess polarized submm flux to cold dust within the remnant, providing fresh evidence that cosmic dust can form rapidly. This is supported by the presence of both polarized and unpolarized dust emission in the north of the remnant where there is no contamination from foreground molecular clouds. The inferred dust polarization fraction is unprecedented (fpol∼ 30 per cent) which, coupled with the brief time-scale available for grain alignment (<300 yr), suggests that supernova dust differs from that seen in other Galactic sources (where fpol= 2−7 per cent) or that a highly efficient grain alignment process must operate in the environment of a supernova remnant.

RADIO SPECTRAL INDEX VARIATIONS AND PHYSICAL CONDITIONS IN KEPLER'S SUPERNOVA REMNANT

A new epoch of VLA measurements of Keplers supernova remnant (SNR) was obtained to make accurate measurements of the radio spectral index variations and polarization. We have compared these new radio images with Hα, infrared (IR), and X-ray data to better understand the three-dimensional structure and dynamics of Keplers SNR and to better understand the physical relationships between the various nonthermal and thermal plasmas. Spatial variations in the radio spectral index from -0.85 to -0.6 are observed between 6 and 20 cm. The mean spectral index is -0.71. The mean percent polarization is 3.5% at 20 cm and 6% at 6 cm. There is a strong correspondence between the radial and azimuthal profiles of the radio, X-ray, Hα, and IR emission in different locations around the remnant, although there is no single, global pattern. Spectral tomography shows that the flat- and steep-spectrum radio emissions have distinct structures. The flat-spectrum radio emission is found at either a larger radius than or coincident with the steep-spectrum emission. We interpret these spectral components as tracing forward- and reverse-shocked material, respectively. The flat-spectrum radio emission can alternatively be interpreted as the bow shocked material (reshocked by the forward shock) from the progenitors motion through the interstellar medium. The Hα and IR images are very similar. Their leading edges are coincident and are either in front of or coincident with the leading edges of the X-ray and radio emission. The X-ray emission matches the Hα and IR emission in places and, in other places, traces the steep-spectrum radio emission. In the north, there is also an anticorrelation in the azimuthal profiles around the remnant of the flat-spectrum radio emission and the thermal X-ray, Hα, and IR emissions. We suggest that this could be due to a relative weakening of the particle acceleration at the forward shock due to Alfven wave damping in regions of high density.

The First Measurement of Cassiopeia A’s Forward Shock Expansion Rate

We have obtained a second-epoch observation of the Cassiopeia A supernova remnant (SNR) with the Chandra X-Ray Observatory to measure detailed X-ray proper motions for the first time. Both observations are 50 ks exposures of the ACIS-S3 chip, and they are separated by 2 years. Measurements of the thin X-ray continuum-dominated filaments located around the edge of the remnant (which are identified with the forward shock) show expansion rates from 0.02% to 0.33% yr-1. Many of these filaments are therefore significantly decelerated. Their median value of 0.21% yr-1 is equal to the median expansion of the bright ring (0.21% yr-1) as measured with Einstein and ROSAT. This presents a conundrum if the motion of the bright ring is indicative of the reverse shock speed. We have also reevaluated the motion of the radio bright ring with emphasis on angle-averaged emissivity profiles. Our new measurement of the expansion of the angle-averaged radio bright ring is 0.07% ± 0.03% yr-1, somewhat slower than the previous radio measurements of 0.11% yr-1, which were sensitive to the motions of small-scale features. We propose that the expansion of the small-scale bright ring features in the optical, X-ray, and radio do not represent the expansion of the reverse shock, but rather represent a brightness-weighted average of ejecta passing through and being decelerated by the reverse shock. The motion of the reverse shock itself is then represented by the motion of the angle-averaged emissivity profile of the radio bright ring.

Spitzer spectral mapping of supernova remnant cassiopeia A

We present the global distribution of fine-structure infrared line emission in the Cassiopeia A supernova remnant using data from the Spitzer Space Telescope’s infrared spectrograph. We identify emission from ejecta materials in the interior, prior to their encounter with the reverse shock, as well as from the postshock bright ring. The global electron density increases by ≳ 100 at the shock to ~ 10^4 cm^(−3), providing evidence for strong radiative cooling. There is also a dramatic change in ionization state at the shock, with the fading of emission from low-ionization interior species like [Si ii] giving way to [S iv] and, at even further distances, highenergy X-rays from hydrogenic silicon. Two compact, crescent-shaped clumps with highly enhanced neon abundance are arranged symmetrically around the central neutron star. These neon crescents are very closely aligned with the “kick” direction of the compact object from the remnant’s expansion center, tracing a new axis of explosion asymmetry. They indicate that much of the apparent macroscopic elemental mixing may arise from different compositional layers of ejecta now passing through the reverse shock along different directions.

Spitzer IRAC Images and Sample Spectra of Cassiopeia A's Explosion

We present Spitzer IRAC images and representative 5.27-38.5 μm IRS spectra of the Cas A SNR. We find that various IRAC channels are each most sensitive to a different spectral and physical component. Channel 1 (3.6 μm) provides an excellent match to the radio synchrotron images. Where channel 1 is strong with respect to the other IRAC channels, the longer wavelength spectra show a broad continuum gently peaking around 26 μm, with weak or no lines. We suggest that this is due to unenriched progenitor circumstellar dust behind the outer shock. Where channel 4 (8 μm) is relatively brightest, the long-wavelength spectra show a strong, 2-3 μm wide peak at 21 μm, likely due to silicates and protosilicates. Strong ionic lines of [Ar II], [Ar III], [S IV], and [Ne II] also appear in these regions. We suggest that in these locations, the dust and ionic emission originate from the explosions O-burning layers. The regions where channels 2 (4.5 μm) and 3 (5.6 μm) are strongest relative to channel 4 show a spectrum that rises gradually to 38 μm, becoming flatter longward of 21 μm, along with higher ratios of [Ne II] to [Ar II]. We suggest that the dust and ionic emission in these locations arise primarily from the C- and Ne-burning layers. All of these findings are consistent with asymmetries deep in the explosion, producing variations in the velocity structure in different directions, but generally preserving the nucleosynthetic layering. At each location, the dust and ionic lines in the mid-infrared and the hotter and more highly ionized optical and X-ray emission are then dominated by the layer currently encountering the reverse shock in that direction.

The Three-Dimensional Structure of Interior Ejecta in Cassiopeia A at High Spectral Resolution

We used the Spitzer Space Telescopes Infrared Spectrograph to create a high-resolution spectral map of the central region of the Cassiopeia A (Cas A) supernova remnant, allowing us to make a Doppler reconstruction of its three-dimensional structure. The ejecta responsible for this emission have not yet encountered the remnants reverse shock or the circumstellar medium, making it an ideal laboratory for exploring the dynamics of the supernova explosion itself. We observe that the O, Si, and S ejecta can form both sheet-like structures and filaments. Si and O, which come from different nucleosynthetic layers of the star, are observed to be coincident in velocity space in some regions, and separated by 500 km s–1 or more in others. Ejecta traveling toward us are, on average, ~900 km s–1 slower than the material traveling away from us. We compare our observations to recent supernova explosion models and find that no single model can simultaneously reproduce all the observed features. However, models of different supernova explosions can collectively produce the observed geometries and structures of the interior emission. We use the results from the models to address the conditions during the supernova explosion, concentrating on asymmetries in the shock structure. We also predict that the back surface of Cas A will begin brightening in ~30 years, and the front surface in ~100 years.

Kinematics of X-Ray-Emitting Components in Cassiopeia A

We present high-resolution X-ray proper-motion measurements of Cassiopeia A using Chandra X-Ray Observatory observations from 2000 and 2002. We separate the emission into four spectrally distinct classes: Si-dominated, Fe-dominated, low-energy-enhanced, and continuum-dominated. These classes also represent distinct spatial and kinematic components. The Si- and Fe-dominated classes are ejecta and have a mean expansion rate of 0.2% yr-1. This is the same as for the forward shock filaments but less than the 0.3% yr-1 characteristic of optical ejecta. The low-energy-enhanced spectral class possibly illuminates a clumpy circumstellar component and has a mean expansion rate of 0.05% yr-1. The continuum-dominated emission likely represents the forward shock and consists of diffuse circumstellar material, which is seen as a circular ring around the periphery of the remnant as well as projected across the center.

Time Variability in the X-Ray Nebula Powered by Pulsar B1509–58

We use new and archival Chandra and ROSAT data to study the time variability of the X-ray emission from the pulsar wind nebula (PWN) powered by PSR B1509−58 on timescales of one week to twelve years. There is variability in the size, number, and brightness of compact knots appearing within 20 ′′ of the pulsar, with at least one knot showing a possible outflow velocity of ∼ 0.6c (assuming a distance to the source of 5.2 kpc). The transient nature of these knots may indicate that they are produced by turbulence in the flows surrounding the pulsar. A previously identified prominent jet extending 12 pc to the southeast of the pulsar increased in brightness by 30% over 9 years; apparent outflow of material along this jet is observed with a velocity of ∼ 0.5c. However, outflow alone cannot account for the changes in the jet on such short timescales. Magnetohydrodynamic sausage or kink instabilities are feasible explanations for the jet variability with timescale of ∼ 1.3−2 years. An arc structure, located 30 ′′ −45 ′′ north of the pulsar, shows transverse structural variations and appears to have moved inward with a velocity of ∼ 0.03c over three years. The overall structure and brightness of the diffuse PWN exterior to this arc and excluding the jet has remained the same over the twelve year span. The photon indices of the diffuse PWN and possibly the jet steepen with increasing radius, likely indicating synchrotron cooling at X-ray energies.