arXiv subjects

D. A. Green

Publications and source records attributed to D. A. Green.

79 records · Page 5Linked to original sources

arXiv2005

Some statistics of Galactic SNRs

The selection effects applicable to the identification of Galactic supernova remnants (SNRs) at radio wavelengths are discussed. Low surface brightness remnants are missing, as are those with small angular sizes (including young but distant SNRs). Several statistical properties of Galactic SNRs are discussed, including the surface-brightness/diameter (Sigma-D) relation. The wide range of intrinsic properties of Galactic remnants with known distances, and the observational selection effects, means that the Sigma-D relation is of limited use to derive diameters and hence distances for individual SNRs, or for statistical studies.

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arXiv2004

Galactic Supernova Remnants: an Updated Catalogue and Some Statistics

A catalogue of 231 Galactic supernova remnants (SNRs) is presented, and the selection effects applicable to the identification of remnants at radio wavelengths are discussed. In addition to missing low surface brightness remnants, small angular size -- i.e. young but distant -- remnants are also missing from the current catalogue of Galactic SNRs. Several statistical properties of Galactic SNRs are discussed, including the surface-brightness/diameter (Sigma-D) relation. It is concluded that the wide range of intrinsic properties of Galactic remnants with known distances, together with the observational selection effects, means that use of the Sigma-D relation to derive diameters and hence distances for individual SNRs, or for statistical studies, is highly uncertain. The observed distribution of bright SNRs, which are thought to be largely free from selection effects, is also used to derive a simple model for the distribution of SNRs with Galactocentric radius.

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arXiv2004

Far-infrared and sub-mm observations of the Crab nebula

We present FIR and sub-mm images of the Crab nebula, taken with ISOPHOT on board ISO and with the SCUBA on the JCMT. The ISOPHOT observations were made at 60, 100, 170 micron, with FWHM of equivalent area Gaussian beams of 44, 47 and 93 arcsec, respectively. The 850 micron SCUBA image has a Gaussian FWHM of 17 arcsec. The 60 and 100 micron images show clear excess of emission above that expected from an extrapolation of the synchrotron spectrum of the Crab nebula from lower frequencies. The ISOPHOT images reveal that about half this excess is attributable to two peaks, separated by approximately 80 arcsec. We also present spectra taken using the LWS on board ISO, which show that the FIR excess is not due to line emission. The lower resolution 170 micron image does not show any excess emission. These findings are consistent with a picture in which the FIR excess is due to emission from a small amount of warm dust which radiates predominantly at 60 and 100 micron micron but not at 170 micron, and which is superimposed on a synchrotron spectrum which gradually steepens towards shorter wavelengths throughout the FIR and MIR spectral range. The dust geometry is consistent with a torus of diameter approximately 0.8 pc created by the supernova progenitor, superimposed upon a broadly distributed component which may be supernova condensates in the filaments. The dust-to-gas ratio in the filaments is comparable to the interstellar value. To study spectral index variations in the radio synchrotron emission we compared the 850-micron image with a 20-cm VLA image. These images are very similar, implying that there is little variation in spectral index across the face of the remnant between these wavelengths.

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arXiv2004

Radio and near-infrared observations of the steep spectrum Galactic plane radio source WKB 0314+57.8

Radio and near-infared observations towards the steep spectrum Galactic plane radio source WKB 0314+57.8 are presented, in order to clarify the nature of this source. The radio observations include archival and survey data, together with new Giant Metrewave Radio Telescope observations at 617 MHz. The near-infrared observations are in the J and K bands, from the Gemini instrument on the Shane 3-m telescope. The radio observations show that WKB 0314+57.8 is extended, with an very steep spectrum (with flux density proportional to frequency to -2.5 power between 40 MHz and 1.5 GHz). The colour--magnitude diagram constructed from near-infrared observations of the field suggests the presence of a z approx 0.08 galaxy cluster behind the Galactic plane, reddened by about 6 magnitudes of visual extinction. Although the steep spectrum source has no obvious identification, two other radio sources in the field covered by the near-infrared observations have tentative identifications with galaxies. These observations indicate that WKB 0314+57.8 is a relic source in a cluster of galaxies, not a pulsar.

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arXiv2003

The Historical Supernovae

The available historical records of supernovae occurring in our own Galaxy over the past two thousand years are reviewed. These accounts include the well-recorded supernovae of AD 1604 (Kepler's SN), 1572 (Tycho's SN), 1181, 1054 (which produced the Crab Nebula) and 1006, together with less certain events dating back to AD 185. In the case of the supernovae of AD 1604 and 1572 it is European records that provide the most accurate information available, whereas for earlier supernovae records are principally from East Asian sources. Also discussed briefly are several spurious supernova candidates, and the future prospects for studies of historical supernovae.

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arXiv2001

The Crab Nebula at 850 microns

Observations of the Crab Nebula at 850 micron made with the "Submillimetre Common-User Bolometer Array" (SCUBA) on the James Clerk Maxwell Telescope are presented. A variety of chop-throws and scanning directions was used in these observations, to well sample structure over a range of scales and directions. The resulting image, which was restored using a MEM algorithm, has a resolution of 17 arcsec, and has been compared with a 20-cm VLA image of the Crab Nebula to study spectral index variations. The 850-micron and 20-cm images are very similar, implying that there is little variation in spectral index across the face of the remnant between these wavelengths.

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arXiv2000

The supernova remnant RX J0852.0-4622: radio characteristics and implications for SNR statistics

We present new radio observations of the recently identified, young Galactic supernova remnant (SNR) RX J0852.0-4622 (G266.2-01.2) made at 1.40 GHz with a resolution of 14.9 arcmin. These results, along with other radio observations from the literature, are used to derive the extent, morphology and radio spectrum of the remnant. The possible age and distance to this remnant are discussed, along with the consequences of its properties - especially its low radio surface brightness - for statistical studies of Galactic SNRs. The extended features identified by Combi et al. (1999) are considered, and we conclude that these are probably unrelated to the new remnant. If RX J0852.0-4622 is nearby, as is suggested by the available gamma-ray data, then the range of intrinsic radio luminosities for SNRs of the same diameter may be much larger than was previously thought.

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