Radio and X-ray monitoring of the accreting millisecond X-ray pulsar IGR J17591-2342 in outburst

TL;DR

This study presents extensive radio and X-ray observations of the 2018 outburst of the accreting millisecond X-ray pulsar IGR J17591-2342, revealing its unusually high radio brightness and variability, with implications for understanding neutron star binary behavior.

Contribution

First detailed radio and X-ray monitoring of IGR J17591-2342 during outburst, establishing its high radio luminosity and variability compared to other neutron star systems.

Findings

Radio luminosity varies significantly during outburst.

Radio luminosity is among the highest for NS-LMXBs.

No radio pulsations detected post-outburst.

Abstract

IGR J17591$-$2342 is a new accreting millisecond X-ray pulsar (AMXP) that was recently discovered in outburst in 2018. Early observations revealed that the source's radio emission is brighter than that of any other known neutron star low-mass X-ray binary (NS-LMXB) at comparable X-ray luminosity, and assuming its likely $\gtrsim 6$ kpc distance. It is comparably radio bright to black hole LMXBs at similar X-ray luminosities. In this work, we present the results of our extensive radio and X-ray monitoring campaign of the 2018 outburst of IGR J17591$-$2342. In total we collected 10 quasi-simultaneous radio (VLA, ATCA) and X-ray (Swift-XRT) observations, which make IGR J17591$-$2342 one of the best-sampled NS-LMXBs. We use these to fit a power-law correlation index $\beta = 0.37^{+0.42}_{-0.40}$ between observed radio and X-ray luminosities ( $L_\mathrm{R}\propto L_\mathrm{X}^{\beta}$). However, our monitoring revealed a large scatter in IGR J17591$-$2342's radio luminosity (at a similar X-ray luminosity, $L_\mathrm{X} \sim 10^{36}$ erg s$^{-1}$, and spectral state), with $L_\mathrm{R} \sim 4 \times 10^{29}$ erg s$^{-1}$ during the first three reported observations, and up to a factor of 4 lower $L_\mathrm{R}$ during later radio observations. Nonetheless, the average radio luminosity of IGR J17591$-$2342 is still one of the highest among NS-LMXBs, and we discuss possible reasons for the wide range of radio luminosities observed in such systems during outburst. We found no evidence for radio pulsations from IGR J17591$-$2342 in our Green Bank Telescope observations performed shortly after the source returned to quiescence. Nonetheless, we cannot rule out that IGR J17591$-$2342 becomes a radio millisecond pulsar during quiescence.