A NuSTAR Observation of the Gamma-Ray-Emitting X-ray Binary and Transitional Millisecond Pulsar Candidate 1RXS J154439.4-112820

TL;DR

This study presents a NuSTAR X-ray observation of the candidate transitional millisecond pulsar 1RXS J154439.4-112820, revealing its high-energy spectrum, variability, and long-term accretion behavior, supporting its classification as a low-luminosity accreting pulsar.

Contribution

First detailed NuSTAR observation of 1RXS J154439.4-112820, demonstrating its spectral properties and variability consistent with transitional millisecond pulsars, and analyzing its decade-long accretion history.

Findings

Detected up to ~30 keV with a power-law spectrum

Observed rapid large-amplitude variability between flux levels

Long-term light curves show stable flux, no prolonged radio pulsar state

Abstract

I present a 40 kilosecond Nuclear Spectroscopic Telescope Array (NuSTAR) observation of the recently identified low-luminosity X-ray binary and transitional millisecond pulsar (tMSP) candidate 1RXS J154439.4-112820, which is associated with the high-energy gamma-ray source 3FGL J1544.6--1125. The system is detected up to ~30 keV with an extension of the same power-law spectrum and rapid large-amplitude variability between two flux levels observed in soft X-rays. These findings provide further evidence that 1RXS J154439.4-112820 belongs to the same class of objects as the nearby bona fide tMSPs PSR J1023+0038 and XSS J12270-4859 and therefore almost certainly hosts a millisecond pulsar accreting at low luminosities. I also examine the long-term accretion history of 1RXS J154439.4-112820 based on archival optical, ultraviolet, X-ray, and $\gamma$-ray light curves covering the past $\sim$decade. Throughout this period, the source has maintained similar flux levels at all wavelengths, which is an indication that it has not experienced prolonged episodes of a non-accreting radio pulsar state but may spontaneously undergo such a state transformation in the future.