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Periodic Modulation : Newly emergent emission behaviour in Pulsars

Periodic modulations are seen in normal period pulsars ($P >$ 0.1 sec) over timescales ranging from a few seconds to several minutes. Such modulations have usually been associated with the phenomenon of subpulse drifting. A number of recent studies have shown subpulse drifting to exhibit very specific physical characteristics : i) drifting is seen only in conal components of the pulse profile and is absent in central core emission; ii) drifting pulsars are distributed over a narrow range of spin-down energy loss ($\dot{E}$), where pulsars with $\dot{E} < 2\times$10$^{32}$ erg s$^{-1}$ show this behaviour, iii) drifting periodicity ($P_3$) is anti-correlated with $\dot{E}$, such that pulsars with lower values of $\dot{E}$ tend to have longer $P_3$. These detailed characterisations of drifting behaviour on the other hand also revealed the presence of other distinct periodic modulations, which can be broadly categorised into two types, periodic nulling and periodic amplitude modulation. In contrast to drifting these periodic phenomena are seen across the entire profile in both the core and conal components simultaneously and are not restricted to any specific $\dot{E}$ range. In this work we have assembled an exhaustive list of around 70 pulsars which show such periodic modulations, 22 of which were newly detected using observations from the Giant Meterwave Radio Telescope and the remaining compiled from past publications. The presence of such a significant group in the pulsar population suggests that periodic nulling and periodic amplitude modulations are newly emergent phenomena in pulsars with their physical origin distinct from subpulse drifting.