Subpulse Drifting, Nulling and Mode changing in PSR J1822$-$2256

We report a detailed observational study of the single pulses from the pulsar J1822$-$2256. The pulsar shows the presence of subpulse drifting, nulling as well as multiple emission modes. During these observations the pulsar existed primarily in two modes; mode A with prominent drift bands and mode B which was more disorderly without any clear subpulse drifting. A third mode C was also seen for a short duration with a different drifting periodicity compared to mode A. The nulls were present throughout the observations but were more frequent during the disorderly B mode. The nulling also exhibited periodicity with a clear peak in the fluctuation spectra. Before the transition from mode A to nulling the pulsar switched to a third drifting state with periodicity different from both mode A and C. The diversity seen in the single pulse behaviour of the pulsar J1822$-$2256 provides an unique window into the emission physics.

Interstellar Plasma Turbulence Spectrum Toward the Pulsars PSR B0809+74 and B0950+08

Interstellar scintillations of pulsars PSR B0809+74 and B0950+08 have been studied using observations at low frequencies (41, 62, 89, and 112 MHz). Characteristic temporal and frequency scales of diffractive scintillations at these frequencies have been determined. The comprehensive analysis of the frequency and temporal structure functions reduced to the same frequency has shown that the spectrum of interstellar plasma inhomogeneities toward both pulsars is described by a power law. The exponent of the spectrum of fluctuations of interstellar plasma inhomogeneities toward PSR B0950+08 (n = 3.00 +- 0.05) appreciably differs from the Kolmogorov exponent. Toward PSR B0809+74 the spectrum is a power law with an exponent n = 3.7 +- 0.1. A strong angular refraction has been detected toward PSR B0950+08. The distribution of inhomogeneities along the line of sight has been analyzed; it has been shown that the scintillations of PSR B0950+08 take place on a turbulent layer with enhanced electron density, which is localized at approximately 10 pc from the observer. For PSR B0809+74 the distribution of inhomogeneities is quasi-uniform. Mean-square fluctuations of electron density on inhomogeneities with a characteristic scale rho_0 = 10^7 m toward four pulsars have been estimated. On this scale the local turbulence level in the 10-pc layer is 20 times higher than in an extended region responsible for the scintillations of PSR B0809+74.

Subpulse Drifting, Nulling and Mode changing in PSR J2006$-$0807 with Core emission

We report a detailed analysis of the emission behaviour of the five component, core-double cone, pulsar J2006$-$0807 (B2003$-$08). The single pulses revealed the presence of the three major phenomena of subpulse drifting, nulling and mode changing. The pulsar switched between four different emission modes, two of which showed systematic drifting with prominent drift bands, and were classified as modes A and B respectively. The drifting was seen primarily in the conal components and exhibited the rare phenomenon of bi-drifting, where the drift direction in the second component was opposite to the fourth component. This made PSR J2006$-$0807 the only known example where systematic drift bands were seen around a central core emission. The emission showed a gradual decrease in intensity during mode A which stabilised to a relatively constant level in the subsequent mode B. The presence of a low frequency, weak and wide structure in the fluctuation spectra was also seen primarily in the core component during modes A and B. The core component vanished during mode C and was most prominent during the fourth mode D. Both these modes were frequently interspersed with null pulses. No detectable drifting was seen during modes C and D, but the pulsar showed short duration periodic nulling in the core as well as the conal components. In addition to the four emission modes the pulsar also nulled for long durations lasting up to hundred rotation periods.

Subpulse Drifting and Periodic Nulling in single pulse emission of PSR B2000+40

We have carried out a detailed study of single pulse emission from the pulsar B2000+40 (J2002+4050), observed at 1.6 GHz frequencies using the Effelsberg radio telescope. The pulsar has three components which are not well separated, with the central component resembling core emission. We have investigated modulations in single pulse behaviour using the fluctuation spectral analysis which showed presence of two prominent periodicities, around 2.5$P$ and 40$P$, respectively. The shorter periodicity was associated with the phenomenon of subpulse drifting and was seen to be absent in central core component. Drifting showed large phase variations in conal components. Additionally, the periodic modulations had significant evolution with time, varying between very sharp and highly diffuse features. In addition to drifting the pulsar also had presence of nulling in the single pulse sequence. The longer periodic feature in the fluctuation spectra was associated with nulling behaviour. The pulsar joins a select group which shows the presence of phase modulated drifting as well as periodic nulling in the presence of core emission. This provides further evidence for the two phenomena to be distinct from each other with different physical origin.