We have identified a new supernova remnant ( SNR ) , G51.04+0.07 , using observations at 74 MHz from the Very Large Array Low-Frequency Sky Survey Redux .
Earlier , higher frequency radio continuum , recombination line , and infrared data had correctly inferred the presence of nonthermal radio emission within a larger , complex environment including ionised nebulae and active star formation .
However , our observations have allowed us to redefine at least one SNR as a relatively small source ( 7 ^ { \prime } .5 \times 3 ^ { \prime } in size ) located at the southern periphery of the originally defined SNR candidate G51.21+0.11 .
The integrated flux density of G51.04+0.07 at 74 MHz is 6.1 \pm 0.8 Jy , while its radio continuum spectrum has a slope \alpha = - 0.52 \pm 0.05 ( S _ { \nu } \propto \nu ^ { \alpha } ) , typical of a shell-type remnant .
We also measured spatial variations in the spectral index between 74 and 1400 MHz across the source , ranging from a steeper spectrum ( \alpha = - 0.50 \pm 0.04 ) coincident with the brightest emission to a flatter component ( \alpha = - 0.30 \pm 0.07 ) in the surrounding fainter region .
To probe the interstellar medium into which the redefined SNR is likely evolving , we have analysed the surrounding atomic and molecular gas using the 21 cm neutral hydrogen ( HI ) and ^ { 13 } CO ( J=1 - 0 ) emissions .
We found that G51.04+0.07 is confined within an elongated HI cavity and that its radio emission is consistent with the remains of a stellar explosion that occurred \sim 6300 yr ago at a distance of 7.7 \pm 2.3 kpc .
Kinematic data suggest that the newly discovered SNR lies in front of HII regions in the complex , consistent with the lack of a turnover in the low frequency continuum spectrum .
The CO observations revealed molecular material that traces the central and northern parts of G51.04+0.07 .
The interaction between the cloud and the radio source is not conclusive and motivates further study .
The relatively low flux density ( \sim 1.5 Jy at 1400 MHz ) of G51.04+0.07 is consistent with this and many similar SNRs lying hidden along complex lines of sight towards inner Galactic emission complexes .
It would also not be surprising if the larger complex studied here hosted additional SNRs .