Discovery of PSR J0523-7125 as a Circularly Polarized Variable Radio Source in the Large Magellanic Cloud

TL;DR

We discovered a highly circularly polarized, variable pulsar in the Large Magellanic Cloud with extreme rotation measure and steep spectrum, highlighting the potential of continuum surveys to find unusual and distant pulsars.

Contribution

This is the first identification of a pulsar with such high circular polarization and extreme RM in the LMC, demonstrating new detection capabilities in radio surveys.

Findings

Pulsar PSR J0523-7125 has a period of 322.5 ms and DM of 157.5 pc cm$^{-3}$.

The pulsar exhibits a high fractional circular polarization of about 20%.

It has an extremely steep spectral index of approximately -3.

Abstract

We report the discovery of a highly circularly polarized, variable, steep-spectrum pulsar in the Australian Square Kilometre Array Pathfinder (ASKAP) Variables and Slow Transients (VAST) survey. The pulsar is located about $1^\circ$ from the center of the Large Magellanic Cloud, and has a significant fractional circular polarization of $\sim$20%. We discovered pulsations with a period of 322.5 ms, dispersion measure (DM) of 157.5 pc cm$^{-3}$, and rotation measure (RM) of $+456$ rad m$^{-2}$ using observations from the MeerKAT and the Parkes telescopes. This DM firmly places the source, PSR J0523$-$7125, in the Large Magellanic Cloud (LMC). This RM is extreme compared to other pulsars in the LMC (more than twice that of the largest previously reported one). The average flux density of $\sim$1 mJy at 1400 MHz and $\sim$25 mJy at 400 MHz places it among the most luminous radio pulsars known. It likely evaded previous discovery because of its very steep radio spectrum (spectral index $\alpha \approx -3$, where $S_\nu \propto \nu^\alpha$) and broad pulse profile (duty cycle $\gtrsim35$%). We discuss implications for searches for unusual radio sources in continuum images, as well as extragalactic pulsars in the Magellanic Clouds and beyond. Our result highlighted the possibility of identifying pulsars, especially extreme pulsars, from radio continuum images. Future large-scale radio surveys will give us an unprecedented opportunity to discover more pulsars and potentially the most distant pulsars beyond the Magellanic Clouds.