Galactic supernova remnant candidates discovered by THOR

Abstract

Context. There is a considerable deficiency in the number of known supernova remnants (SNRs) in the Galaxy compared to that expected. This deficiency is thought to be caused by a lack of sensitive radio continuum data. Searches for extended low-surface brightness radio sources may find new Galactic SNRs, but confusion with the much larger population of H II regions makes identifying such features challenging. SNRs can, however, be separated from H II regions using their significantly lower mid-infrared (MIR) to radio continuum intensity ratios. Aims. Our goal is to find missing SNR candidates in the Galactic disk by locating extended radio continuum sources that lack MIR counterparts. Methods. We use the combination of high-resolution 1-2 GHz continuum data from The HI, OH, Recombination line survey of the Milky Way (THOR) and lower-resolution VLA 1.4 GHz Galactic Plane Survey (VGPS) continuum data, together with MIR data from the Spitzer GLIMPSE, Spitzer MIPSGAL, and WISE surveys to identify SNR candidates. To ensure that the candidates are not being confused with H II regions, we exclude radio continuum sources from the WISE Catalog of Galactic H II Regions, which contains all known and candidate H II regions in the Galaxy. Results. We locate 76 new Galactic SNR candidates in the THOR and VGPS combined survey area of 67.4 • > > 17.5 • , | b | ≤ 1.25 • and measure the radio flux density for 52 previously-known SNRs. The candidate SNRs have a similar spatial distribution to the known SNRs, although we note a large number of new candidates near 30 • , the tangent point of the Scutum spiral arm. The candidates are on average smaller in angle compared to the known regions, 6.4 ± 4.7 versus 11.0 ± 7.8 , and have lower integrated flux densities.Conclusions. The THOR survey shows that sensitive radio continuum data can discover a large number of SNR candidates, and that these candidates can be efficiently identified using the combination of radio and MIR data. If the 76 candidates are confirmed as true SNRs, for example using radio polarization measurements or by deriving radio spectral indices, this would more than double the number of known Galactic SNRs in the survey area. This large increase would still, however, leave a discrepancy between the known and expected SNR populations of about a factor of two.

Investigating Galactic Supernova Remnant Candidates Using LOFAR

“…G53.07+0.49: Candidate G53.07+0.49 has a small angular size in the VLA observations, but the peak flux density in the WSRT and LOFAR observations is offset from the SNR candidate location suggested by Anderson et al (2017). As such, we do not measure WSRT or LOFAR flux densities for this candidate, and because there are no X-ray observations available, further investigation using X-ray or higher resolution low-frequency observations is required to comment on the nature of this candidate.…”

“…Using the region size of 18 7 (Anderson et al 2017) we estimate the X-ray count rate with the 2σ upper limit to be 1.5×10 −2 counts s −1 in the ROSAT 0.4-2.4 keV energy band.…”

“…The 1 GHz surface brightness was obtained using the 1.4 GHz flux density measured by Anderson et al (2017) and a spectral index of α=−0.6 (see Table 1). Using the relation between diameter and surface brightness (the Σ-D relation) in Pavlovic et al (2014) gives yet another distance estimate of 8 kpc.…”

“…About 90% of SNRs and SNR candidates have been found in radio surveys (Anderson et al 2017), but it is thought that there could be many missing SNRs (e.g., Li et al 1991;Tammann et al 1994;Gerbrandt et al 2014). Obtaining a more complete record of the SNR population, including confirming or rejecting the nature of SNR candidates, is important because it leads to better estimates of the Galactic supernova rate, the maximum ages of SNRs, and because SNRs are obvious locations for searching for young pulsars.…”

“…These SNR candidates were identified in a study of THOR +VGPS observations by Anderson et al (2017) and are G51.21+0.11, G52.37-0.70, G53.07+0.49, G53.41+0.03, G53.84-0.75, and G54.1+0.3. In particular, we present a multifrequency analysis of SNR candidate G53.41+0.03.…”