Search for Off-pulse Emission in Long-period Pulsars

We have revisited the problem of off-pulse emission in pulsars, where detailed search for the presence of low level radio emission outside the pulse window is carried out. The presence of off-pulse emission was earlier reported in two long period pulsars, PSR B0525+21 and B2046-16 at frequencies below 1 GHz using the Giant Meterwave Radio Telescope (GMRT). However, subsequent studies did not detect off-pulse emission from these pulsars at higher radio frequencies (> 1 GHz). We have carefully inspected the analysis scheme used in the earlier detections and found an anomaly with data editing routines used, which resulted in leakage of signal from the on-pulse to the off-pulse region. We show that the earlier detections from PSR B0525+21 and B2046-16 were a result of this leakage. The above analysis scheme has been modified and offline-gating has been used to search for off-pulse emission in 21 long period pulsars (P > 1.2 sec) at different observing frequencies of GMRT. The presence of low level off-pulse emission of peak flux 0.5 mJy was detected in the brightest pulsar in this list PSR 0B0628-28, with off-pulse to average pulsar flux ratio of 0.25%. We suggest that coherent radio emission resulting due to cyclotron resonance near the light cylinder can be a possible source for the off-pulse emission in this pulsar.