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Register a new userOptical detection and spectroscopic confirmation of supernova remnant G213.0-0.6 (now re-designated as G213.3-0.4)
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During a detailed search for optical counterparts of known Galactic supernova remnants (SNRs) using the Anglo Australian Observatory/United Kingdom Schmidt Telescope (AAO/UKST) HAlpha survey of the southern Galactic plane we have found characteristic optical HAlpha filaments and associated emission in the area of SNR G213.0-0.6. Although this remnant was previously detected in the radio as a non-thermal source, we also confirm emission at 4850 MHz in the Parkes-MIT-NRAO (PMN) survey and at 1400 MHz in the NRAO/VLA Sky Survey (NVSS). There is an excellent match in morphological structure between the optical (HAlpha) and radio emission. We subsequently obtained optical spectroscopy of selected HAlpha filaments using the South African Astronomical Observatory 1.9-m telescope which confirmed shock excitation typical of supernova remnants. Our discovery of HAlpha emission and the positional match with several radio frequency maps led us to reassign G213.0-0.6 as G213.3-0.4 as these co-ordinates more accurately reflect the actual centre of the SNR shell and hence the most probable place of the original supernova explosion. Support for this new SNR ID comes from the fact that the X-ray source 1RXS J065049.7-003220 is situated in the centre of this new remnant and could be connected with the supernova explosion.
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A combination of archival multi-frequency radio observations with narrow-band HAlpha optical imagery and new confirmatory optical spectroscopy have shown that candidate supernova remnant G6.31+0.54 can now be confirmed as part of a Galactic supernova remnant (SNR). It has non-thermal emission, an optical emission line spectrum displaying shock excitation and standard SNR line ratios, fine filamentary structures in HAlpha typical of optical remnants and closely overlapping radio and optical footprints. An X-ray ROSAT source 1RXS J175752.1-231105 was also found that matches the radio and optical emission though a definite association is not proven. Nevertheless, taken together, all these observed properties point to a clear SNR identification for this source. We provide a rough estimate for the kinematic distance to G6.31+0.54 of ~4.5kpc. The detected optical filaments are some ~10arcminutes in extent (or about 13 pc at the assumed distance). However, as only a partial arcuate structure of the SNR can be seen (and not a full shell) the full angular extent of the SNR is unclear. Hence the physical extent of the observed partial shell is also difficult to estimate. If we assume an approximately circular shell then a conservative fit to the optical arc shaped filaments gives an angular diameter of ~20 arcminutes corresponding to a physical diameter of ~26 pc that shows this to be an evolved remnant.
We present a multi-wavelength study of the radio source G296.7-0.9. This source has a bilateral radio morphology, a radio spectral index of -0.5 +/- 0.1, sparse patches of linear polarisation, and thermal X-rays with a bright arc near the radio boundary. Considering these characteristics, we conclude that G296.7-0.9 is a supernova remnant (SNR). The age and morphology of the SNR in the context of its environment suggest that the source is co-located with an HII region, and that portions of the shock front have broken out into a lower density medium. We see no evidence for a neutron star or pulsar wind nebula associated with SNR G296.7-0.9.
We report the detection of carbon monoxide (CO) emission from the young supernova remnant Cassiopeia A (Cas A) at wavelengths corresponding to the fundamental vibrational mode at 4.65 micron. We obtained AKARI Infrared Camera spectra towards 4 positions which unambiguously reveal the broad characteristic CO ro-vibrational band profile. The observed positions include unshocked ejecta at the center, indicating that CO molecules form in the ejecta at an early phase. We extracted a dozen spectra across Cas A along the long 1 arcmin slits, and compared these to simple CO emission models in Local Thermodynamic Equilibrium to obtain first-order estimates of the excitation temperatures and CO masses involved. Our observations suggest that significant amounts of carbon may have been locked up in CO since the explosion 330 years ago. Surprisingly, CO has not been efficiently destroyed by reactions with ionized He or the energetic electrons created by the decay of the radiative nuclei. Our CO detection thus implies that less carbon is available to form carbonaceous dust in supernovae than is currently thought and that molecular gas could lock up a significant amount of heavy elements in supernova ejecta.
We report the results from a spectrophotometric study sampling the roughly 300 candidate supernova remnants (SNRs) in M83 identified through optical imaging with Magellan/IMACS and HST/WFC3. Of the 118 candidates identified based on a high [S II] $lambdalambda$ 6716,6731 to H$alpha$ emission ratio, 117 show spectroscopic signatures of shock-heated gas, confirming them as SNRs---the largest uniform set of SNR spectra for any galaxy. Spectra of 22 objects with a high [O III] 5007 $lambda$ to H$alpha$ emission ratio, selected in an attempt to identify young ejecta-dominated SNRs like Cas A, reveal only one (previously reported) object with the broad (over 1000 km/s) emission lines characteristic of ejecta-dominated SNRs, beyond the known SN1957D remnant. The other 20 [O III]-selected candidates include planetary nebulae, compact H II regions, and one background QSO. Although our spectroscopic sample includes 22 SNRs smaller than 11 pc, none of the other objects shows broad emission lines; instead their spectra stem from relatively slow (< 200 km/s) radiative shocks propagating into the metal-rich interstellar medium of M83. With six SNe in the past century, one might expect more of M83s small-diameter SNRs to show evidence of ejecta; this appears not to be the case. We attribute their absence to several factors, including that SNRs expanding into a dense medium evolve quickly to the ISM-dominated phase, and that SNRs expanding into regions already evacuated by earlier SNe are probably very faint.
We present spectroscopic confirmation of the Pisces Overdensity, also known as Structure J, a photometric overdensity of RR Lyrae stars discovered by the Sloan Digital Sky Survey (SDSS) at an estimated photometric distance of ~85kpc. We measure radial velocities for 8 RR Lyrae stars within Pisces. We find that 5 of the 8 stars have heliocentric radial velocities within a narrow range of -87 km/s < v < -67 km/s, suggesting that the photometric overdensity is mainly due to a physically associated system, probably a dwarf galaxy or a disrupted galaxy. Two of the remaining 3 stars differ from one another by only 9 km/s, but it would be premature to identify them as a second system.
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