Simultaneous multi-frequency single pulse observations of pulsars

Aims. We report on simultaneous multi-frequency single pulse observations of a sample of pulsars with previously reported, frequency dependent subpulse drift inferred from non-simultaneous and short observations. We aim to clarify if the frequency dependence is a result of multiple drift modes in these pulsars. Methods. We performed simultaneous observations at 326.5 MHz with the Ooty Radio Telescope and at 326, 610, and 1308 MHz with the Giant Meterwave Radio Telescope for a sample of 12 pulsars, where frequency dependent single pulse behaviour was reported. The single pulse sequences were analysed with three types of fluctuation analysis techniques, namely longitude-resolved fluctuation spectrum technique, two-dimensional fluctuation spectrum technique and sliding two-dimensional fluctuation spectrum technique. The first two techniques are sensitive to average fluctuation properties of the pulses, whereas the last technique is used for examining the temporal behaviour of the pulses. Results. We report subpulse drifting in PSR J0934−5249 for the first time. We also report pulse nulling measurements in PSRs J0934−5249, B1508+55, J1822−2256, B1845−19, and J1901−0906 for the first time. Our measurements of subpulse drifting and pulse nulling for the rest of the pulsars are consistent with previously reported values. Contrary to previous belief, we find no evidence for a frequency dependent drift pattern in PSR B2016+28 as reported in previous studies. In PSRs B1237+25, J1822−2256, J1901−0906, and B2045−16, our longer and more sensitive observations reveal multiple drift rates with distinct P3. We increase the sample of pulsars showing concurrent nulling across multiple frequencies by more than 100 percent, adding four more pulsars to this sample. Our results confirm and further strengthen the understanding that the subpulse drifting and pulse nulling are consistent in the broadband with previous studies and are closely tied to physics of polar gap.