David Moffett

Morphological evidence for azimuthal variations of the cosmic ray ion acceleration at the blast wave of SN 1006

Using radio, X-ray, and optical observations, we present evidence for morphological changes due to efficient cosmic ray ion acceleration in the structure of the southeastern region of the supernova remnant SN 1006. SN 1006 has an apparent bipolar morphology in both the radio and high-energy X-ray synchrotron emission. In the optical, the shock front is clearly traced by a filament of Balmer emission in the southeast. This optical emission enables us to trace the location of the blast wave (BW) even in places where the synchrotron emission from relativistic electrons is either absent or too weak to detect. The contact discontinuity (CD) is traced using images in the low-energy X-rays (oxygen band), which we argue reveals the distribution of shocked ejecta. We interpret the azimuthal variations of the ratio of radii between the BW and CD plus the X-ray and radio synchrotron emission at the BW using CR-modified hydrodynamic models. We assumed different azimuthal profiles for the injection rate of particles into the acceleration process, magnetic field, and level of turbulence. We found that the observations are consistent with a model in which these parameters are all azimuthally varying, being largest in the brightest regions.

EVIDENCE FOR PARTICLE ACCELERATION TO THE KNEE OF THE COSMIC RAY SPECTRUM IN TYCHO'S SUPERNOVA REMNANT

Supernova remnants (SNRs) have long been assumed to be the source of cosmic rays (CRs) up to the knee of the CR spectrum at 1015 eV, accelerating particles to relativistic energies in their blast waves by the process of diffusive shock acceleration (DSA). Since CR nuclei do not radiate efficiently, their presence must be inferred indirectly. Previous theoretical calculations and X-ray observations show that CR acceleration significantly modifies the structure of the SNR and greatly amplifies the interstellar magnetic field. We present new, deep X-ray observations of the remnant of Tychos supernova (SN 1572, henceforth Tycho), which reveal a previously unknown, strikingly ordered pattern of non-thermal high-emissivity stripes in the projected interior of the remnant, with spacing that corresponds to the gyroradii of 1014-1015 eV protons. Spectroscopy of the stripes shows the plasma to be highly turbulent on the (smaller) scale of the Larmor radii of TeV energy electrons. Models of the shock amplification of magnetic fields produce structure on the scale of the gyroradius of the highest energy CRs present, but they do not predict the highly ordered pattern we observe. We interpret the stripes as evidence for acceleration of particles to near the knee of the CR spectrum in regions of enhanced magnetic turbulence, while the observed highly ordered pattern of these features provides a new challenge to models of DSA.

The expansion of the radio remnant of the supernova of 1006 AD

We have reobserved the remnant of SN 1006 AD with the Very Large Array of the National Radio Astronomy Observatory at 1370 and 1665 Mhz. We compare the resulting images with those reconstructed from observations made in 1983-1984 by Reynolds & Gilmore and find that expansion has occurred in the eight intervening years. The mean expansion in this period is 3.5±1 arcsec, implying an expansion rate of 0.049% yr −1 or R∞t 0.48±0.13 , consistent with Sedov expansion, or with a forward/reverse shock pair moving into constant-density material. There is some evidence that the expansion rate is not uniform around the shell, being slightly smaller in the southwest quadrant

ON THE RADIO POLARIZATION SIGNATURE OF EFFICIENT AND INEFFICIENT PARTICLE ACCELERATION IN SUPERNOVA REMNANT SN 1006

Radio polarization observations provide essential information on the degree of order and orientation of magnetic fields, which themselves play a key role in the particle acceleration processes that take place in supernova remnants (SNRs). Here we present a radio polarization study of SN 1006, based on combined Very Large Array and Australia Telescope Compact Array observations at 20 cm that resulted in sensitive images with an angular resolution of 10 arcsec. The fractional polarization in the two bright radio and X-ray lobes of the SNR is measured to be 0.17, while in the southeastern sector, where the radio and non-thermal X-ray emission are much weaker, the polarization fraction reaches a value of 0.6 ± 0.2, close to the theoretical limit of 0.7. We interpret this result as evidence of a disordered, turbulent magnetic field in the lobes, where particle acceleration is believed to be efficient, and a highly ordered field in the southeast, where the acceleration efficiency has been shown to be very low. Utilizing the frequency coverage of our observations, an average rotation measure of ~12 rad m–2 is determined from the combined data set, which is then used to obtain the intrinsic direction of the magnetic field vectors. While the orientation of magnetic field vectors across the SNR shell appear to be radial, a large fraction of the magnetic vectors lie parallel to the Galactic plane. Along the highly polarized southeastern rim, the field is aligned tangent to the shock, and therefore also nearly parallel to the Galactic plane. These results strongly suggest that the ambient field surrounding SN 1006 is aligned with this direction (i.e., from northeast to southwest) and that the bright lobes are due to a polar cap geometry. Our study establishes that the most efficient particle acceleration and generation of magnetic turbulence in SN 1006 is attained for shocks in which the magnetic field direction and shock normal are quasi-parallel, while inefficient acceleration and little to no generation of magnetic turbulence are obtained for the quasi-perpendicular case.

Deep VLA imaging of pulsar-powered nebulae and the beaming fraction of young pulsars

We have made sensitive, high-resolution images of the fields of seven pulsar-powered nebulae with the VLA at 20 and 6 cm. Our goal was to resolve out the extended emission from the nebula and to look for any remaining point sources that could be candidates for a central pulsar powering the nebular emission. Several point sources were found but no compelling pulsar candidates were identified on the basis of their integrated properties (i.e., polarization, spectral index, scintillation, and location). In the remnant 3C 58 we report the discovery of an elongated wisp near the vicinity of a compact X-ray source. We argue that this feature is a termination shock from the relativistic wind of an unseen pulsar

High-resolution radio observations of the supernova remnant 3C 391: possible breakout morphology

We present radio observations of the bright compact SNR 3C 391 (G31.9+0.0) made with the VLA2 at 6 arcsec resolution. The remnants morphology suggests expansion into a medium with a very strong density gradient. We interpret radio and X-ray morphologies as evidence for a breakout of the expanding material into a region of abruptly lower density. The CO survey of Sanders et al. (1986), though not fully sampled, shows the presence of molecular gas at the position of the remnant, with a radial velocity of 90-110 km/s similar to the highest velocity absorption features seen in H I toward 3C 391. This gas may thus be near the remnant, though direct evidence of interaction is lacking. More detailed CO observations, and searches for evidence of shocked molecular gas such as 2.12 micron emission from H2, might confirm or refute the tantalizing possibility that 3C 391 is interacting with an observable molecular cloud.

A Broadband Study of the Emission from the Composite Supernova Remnant MSH 11-62

MSH 11-62 (G291.0-0.1) is a composite supernova remnant for which radio and X-ray observations have identified the remnant shell as well as its central pulsar wind nebula. The observations suggest a relatively young system expanding into a low-density region. Here, we present a study of MSH 11-62 using observations with the Chandra, XMM -Newton, and Fermi observatories, along with radio observations from the Australia Telescope Compact Array. We identify a compact X-ray source that appears to be the putative pulsar that powers the nebula, and show that the X-ray spectrum of the nebula bears the signature of synchrotron losses as particles diffuse into the outer nebula. Using data from the Fermi Large Area Telescope, we identify ?-ray emission originating from MSH 11-62. With density constraints from the new X-ray measurements of the remnant, we model the evolution of the composite system in order to constrain the properties of the underlying pulsar and the origin of the ?-ray emission.

Correlation Between X-Ray Light-Curve Shape and Radio Arrival Time in the Vela Pulsar

We report the results of simultaneous observations of the Vela pulsar in X-rays and radio from the RXTE satellite and the Mount Pleasant Radio Observatory in Tasmania.We sought correlations between Vela’s X-ray emission and radio arrival times on a pulse-by-pulse basis. At a confidence level of 99.8% we have found significantly higher flux density in Vela’s main X-ray peak during radio pulses that arrived early. This excess flux shifts to the ‘‘trough’’ following the second X-ray peak during radio pulses that arrive later. Our results suggest that the mechanism producing the radio pulses is intimately connected to the mechanism producing X-rays. Current models using resonant absorption of radio emission in the outer magnetosphere as a cause of the X-ray emission are explored as a possible explanation for the correlation.

G291.0–0.1: Powered by a pulsar?

As part of a continuing study of plerionic supernova remnants, we have closely examined the SNR G291.0–0.1 (MSH 11–62) at radio wavelengths following ASCA X-ray observations of this source. The high angular resolution, full-polarization radio observations at 13 and 20 cm support its plerionic nature; the remnant has an interior dominated by a bright, linearly-shaped, polarized central feature, and has an exterior composed of filamentary structure. No compact source suggestive of a pulsar or associated wind nebula was found. In addition, we observed HI absorption toward the remnant, which places it at or beyond 3.5 kpc.

The VSOP 5 GHz Active Galactic Nucleus Survey. II. Data Calibration and Imaging

The VLBI Space Observatory Programme (VSOP) mission is a Japanese-led project to study radio sources with sub-milliarcsecond angular resolution using an orbiting 8 m telescope, HALCA, and global arrays of Earth-based telescopes. Approximately 25% of the observing time has been devoted to a survey of compact active galactic nuclei (AGNs) at 5 GHz that are stronger than 1 Jy—the VSOP AGN Survey. This paper, the second in a series, describes the data calibration, source detection, self-calibration, imaging, and modeling and gives examples illustrating the problems specific to space VLBI. The VSOP Survey Web site, which contains all results and calibrated data, is described.