Mining the time axis with TRON - III. Discovery of a nulling pulsar towards the SMC with MeerKAT

Abstract

We report on the discovery of a nulling pulsar, PSR J0052$-$7551, in archival MeerKAT L-band observations of the Small Magellanic Cloud (SMC). The discovery was made using the image-plane transient detection pipeline, TRON, which creates a residual image for each time sample and uses a heuristic pruning strategy to identify promising candidates in the resulting temporal image cube. The TRON pipeline triggered on the approximately minute timescale variability associated with the nulling behaviour of a canonical pulsar. Follow-up MeerKAT UHF-band beamformer observations revealed a pulsar with a spin period of 0.55 s and a measured period derivative of 6.1(6)$\times$10$^{-17}$s$\cdot$s$^{-1}$. The source is seen in projection toward the periphery of the SMC, and has a dispersion measure value of 29.24(2) pc$\cdot$cm$^{-3}$, which is on the order of the expected Milky Way contribution along the line of sight. We use a Gaussian Mixture Model to fit the nulling behaviour which yielded a nulling fraction of $\sim 32$%. We find that the null lengths are consistent with an exponential distribution indicating a stochastic process. Additionally, power spectra of the UHF visibility data reveal an unexplained quasi-periodic feature at ~5$-$7 s that cannot be attributed to the source itself, RFI, or standard instrumental effects. Following on from previous image-plane detections of pulsars by the TRON approach, which relied on either scintillation or eclipsing behaviour, this is the first detection triggered by nulling. This discovery confirms TRON's sensitivity to a wide range of astrophysical phenomena of medium timescale duration.