2012A&A...546A..25R

A compact steep spectrum radio source (J0535-0452) is located in the sky coincident with a bright optical rim in the HII region NGC 1977. J0535-0452 is observed to be ≤100mas in angular size at 8.44GHz. The spectrum for the radio source is steep and straight with a spectral index of -1.3 between 330 and 8440MHz. No 2µm IR counter part for the source is detected. These characteristics indicate that the source may be either a rare high redshift radio galaxy or a millisecond pulsar (MSP). We investigate whether the steep spectrum source is a millisecond pulsar. The optical rim is believed to be the interface between the HII region and the adjacent molecular cloud. If the compact source is a millisecond pulsar, it would have eluded detection in previous pulsar surveys because of the extreme scattering due to the HII region-molecular cloud interface. The limits obtained on the angular broadening along with the distance to the scattering screen are used to estimate the pulse broadening. The pulse broadening is shown to be less than a few msec at frequencies >5GHz. We therefore searched for pulsed emission from J0535-0452 at 14.8 and 4.8GHz with the Green Bank Telescope (GBT). No pulsed emission is detected to 55 and 30µJy level at 4.8 and 14.8GHz. Based on the parameter space explored by our pulsar search algorithm, we conclude that, if J0535-0452 is a pulsar, then it could only be a binary MSP of orbital period ≲5h.