Peculiarities of radio emission from new pulsars at 111 MHz

3.1 J0220+3622, pulsar with the subpulse drift

Subpulse drift is a sequence of individual pulse/subpulse shifting in phase from one edge of the mean pulse profile to the opposite, forming characteristic drift bands on the longitude-time diagram (Drake and Craft 1968). This phenomenon is characterized by two periods: second ( P 2 ) and third ( P 3 ) class. The value of P 2 is the horizontal drift band separation in time units. P 3 is a distance which is determined by the number of periods P on the ordinate. The subpulse drift phenomenon has been repeatedly investigated earlier by both observers and theorists. This effect is caused, probably, by the movement of emission regions in the pulsar magnetosphere and is associated with the radiation mechanism. A complete physical understanding of this phenomenon is still lacking, but the most famous explanation is the model of Ruderman and Sutherland (1975), which was subsequently extended by many authors (e.g., Filippenko and Radhakrishnan 1982, Deshpande and Rankin 1999, Gil et al. 2003), explaining the drift phenomenon by rotating the sub-beams around the magnetic axis (“carousel” model). The model describes the effect well for pulsars with single pulse profiles, since the subpulse drift is believed to be related to conal radiation, and it is much more complicated when the drift phenomenon is present in pulsars with complex, multicomponent pulse profiles (Rankin 1986).

The pulsar J0220+36 with the period P = 1.0297 s and the dispersion measure ( DM ) = 46 ± 1 pc/cm 3 (Tyul’bashev and Tyul’bashev 2015) has a very wide average pulse profile of ∼ 220 ms and narrow individual pulses (Figure 1). Pulsar has a complex multicomponent structure of individual pulses, drift of subpulses, as well as short bursts of radiation, when the drift becomes especially clear (Teplykh et al. 2020). During 1 of 222 observation days (3 November 2017), the pulsar showed the increase in emission activity (Figure 2). A three-component pulse structure (Figure 2c) is visible in the series of three consecutive pulses with the S/N from 8 to 16 (Figure 2b). The distances between subpulses are on average 82 and 64 ms. The analysis of time intervals between subpulses of individual pulses made it possible to measure the periods of the second and the third classes: P 2 = 70 ± 10 ms , P 3 = 7 ± 1 , as well as the drift rate D = 20 ± 5 ms per period. There is a new method to measure periods P 2 and P 3 (Malofeev and Tyul’bashev 2018). Figure 3 shows the summed power spectrum for J0220+3622 obtained by adding more than 500 daily Fourier spectra. The presence of a third class period is indicated by a significant substrate (pedestal) at the first two harmonics of the summed Fourier spectrum. As follows from the work of Malofeev and Tyul’bashev (2018), the period of the third class can be calculated from the position of the satellites near the harmonics. But since the summed power spectrum of this object does not show evident satellites, we can estimate only the low limit of P 3 ( P 3 > 6 ). This pulsar exhibits a rare phenomenon: a variable drift velocity, which has been first noted for PSR B0031-07 (Huguenin et al. 1970). Figure 4 demonstrates the subpulse drift for 2 days of observations.

Figure 1

The example of observations for pulsar J0220+36 (2 January 2018). From top to bottom: (a) integrated pulse profile, (b) dynamic spectrum, (c) variations of pulse intensity during one observation session (230 pulses), (d) example of individual pulse profile, and (e) dynamic spectrum for individual pulse. The abscissa axis for all graphs shows pulse period (one sample is equal to 2.46 ms).

Figure 2

(a) The integrated profile of the observed pulsar; (b) the pulse intensity dependence for the 100 periods, drift of the strong pulses is visible; (c) pulse profile of one of three strong individual pulses.

Figure 3

The summed Fourier spectrum of J0220+36 obtained by accumulation of more than 500 daily Fourier spectra.

Figure 4

The examples of the drift for J0220+36 on different observation days.

3.2 J0303+2248 pulsar with flare activity

The pulsar J0303+2248 (Tyul’bashev and Tyul’bashev 2015) discovered with P = 1.207 and DM = 20 ± 5 has irregular, flare activity. This type of sporadic signal amplification is observed in a number of radio pulsars (e.g., as giant pulses [Sutton et al. 1971], and as the characteristic of radio transients [McLaughlin et al. 2006]).

A more detailed study showed that the pulsar J0303+2248 exhibits powerful single pulses, and its emission is more similar to radiation from transient sources (RRATs), but with more frequent pulses. This pulsar has a two-component pulse structure with rare and weak inter-component emission. There is practically no radiation outside strong pulses. Figure 5 shows the sum of the eight strongest individual pulses. The signal-to-noise ratio (S/N) of this summed pulse is five times more than the S/N for sum of other 160 pulses. Some individual pulses are ten times stronger than the integrated pulse. The annual variations of the radio emission show that the signal has been recorded at the level S/N = 3–7 in most of the observation sessions. But we can see the increase in signal on some days. Also, this object shows a very rare phenomenon (one event during 146 days of observations). It is a flare in the inter-component space (Figure 6). A similar behavior was found earlier for the pulsar J0653 + 8041 (Malofeev et al. 2016), where the central component flares were observed very rarely in the three-component profile. We obtained more precise value of the DM for J0303+2248, it was 19 ± 1 pc/cm 3 .

Figure 5

The example of observations of J0303+22 (25 January 2019). The integrated pulse profile (top left) and intensity changes of the pulsar pulses during one observation session (bottom left). The pulse profiles of a pulsar obtained by summing eight strong pulses with S/N > 5 (top right) and by summing of all other pulses with S/N ≤ 5 (bottom right).

Figure 6

The inter-component emission detection from J0330+2248 (13 January 2018). (a) The integral pulse profile, (b) profile of the individual pulse (number 105) with S/N = 20.5 corresponding to inter-component emission, and (c) the intensity changes of the pulses during the observation session.

3.3 J0810+3725 – nulling pulsar

The nulling is a phenomenon when we can observe a temporary absence of pulsed emission from a neutron star (Backer 1970). Long-term studies of this phenomenon have shown that the percentage of nulling fraction (NF) can vary in range 1–90% (Wang et al. 2007, Gajjar et al. 2014, Burgay et al. 2019). Intermittent pulsars belong to the separate class of objects switching off their emission for a longer time (from several days to several years) (Kramer et al. 2006, Camilo et al. 2012).

The preliminary results of the J0810+3725 study were presented in previous study (Teplykh and Malofeev 2019). The pulsar with P = 1.2482 and DM = 17 (Tyul’bashev and Tyul’bashev 2015) has a wide emission window (the average half-width of the integral pulse is W 0.5 ∼ 25 ms ), while the individual pulses are very narrow ( W 0.5 ∼ 8 ms ) (Figure 7).

Figure 7

The example of the pulse profile of J0810+37 and the intensity of individual pulses during one observation session (left), and examples of individual pulse profiles (right).

Examples of nulling for the pulsar J0810+3725 are shown in Figure 8. In active state (during periods of “switch on”) the pulsar demonstrates a wide spread of nulling durations (10–90% of the observation session time) and in average is 40% (Teplykh and Malofeev 2019). During some days (Figures 7 and 8) we can see the allusion that this pulsar has a subpulse drift. We shall try to estimate P 2 and P 3 in the future. Pulsar manifests intermediate object between two classes: ordinary nullers (e.g., Wang et al. 2007) and “switch off” pulsars (Kramer et al. 2006, Camilo et al. 2012). We can make this conclusion on the basis of our investigation. This pulsar demonstrates the emission interruption for various intervals of the time: from several pulses to several days.

Figure 8

The examples of nulling for the pulsar J0810+37 on different observation days.