The central point source in G76.9+1.0

Abstract

We describe the serendipitous discovery of a radio point source in a 618-MHz image of the supernova remnant(SNR) G76.9+1.0. The SNR has a bipolar structure and the point source is located near a faint bridge of emission joining the two lobes of emission. The point source was also detected in follow-up higher frequency (1170 MHz) observations. The spectral index for the point source obtained from the Giant Metrewave Radio Telescope (GMRT) observations is α=-2.1^-0.45_{+ 0.36}. The steep spectrum as well as the location of the point source near the centre of the SNR establishes the fact that it is indeed the pulsar J2022+3842 associated with this SNR. The weighted-mean radio position of this point source is α=20^h 22^m21^s.69 ±0^s.11, δ=38°42' 14".8 ± 1".7 J2000. Consistent with this, subsequent analysis of archival Chandra X-ray data shows a point source coincident with the radio point source, as well as diffuse extended X-ray emission surrounding the unresolved source. However, no pulsed emission was detected despite deep searches at both 610 and 1160 MHz, although pulsed emission has been seen at 2 GHz. It appears that the most likely reason for not detecting the pulsed signal at the GMRT is temporal broadening: for the estimated Dispersion Measure (DM) towards this SNR, the pulse broadening time could be as large as tens of milliseconds. The diffuse X-ray emission is elongated along the same direction as the bipolar structure seen in the radio. We interpret the radio lobes as having been formed from an equatorial wind. Although direct detection of pulsed signal has not been possible, we show convincingly that sensitive, high-resolution radio imaging at multiple frequencies is a useful method to search for pulsar candidates.