Recurrent outbursts revealed in 3XMM J031820.8-663034

TL;DR

This study analyzes decades of X-ray data from 3XMM J031820.8-663034, revealing four recurrent outbursts with consistent luminosity and duration, suggesting the source is powered by an accreting stellar-mass black hole with outbursts triggered by thermal-viscous instability.

Contribution

It provides the first detailed long-term analysis of this transient ULX, characterizing its outburst cycle, spectral evolution, and proposing the accreting black hole mechanism.

Findings

Four outbursts observed since 1992 with ~1800-day recurrence

Peak X-ray luminosity around 1.5 x 10^{39} erg/s

Outburst duration of approximately 240-300 days

Abstract

3XMM J031820.8-663034, first detected by ROSAT in NGC 1313, is one of a few known transient ultraluminous X-ray sources (ULXs). In this paper, we present decades of X-ray data of this source from ROSAT, XMM-Newton, Chandra and the Neil Gehrels Swift Observatory. We find that its X-ray emission experienced four outbursts since 1992, with a typical recurrent time $\sim$ 1800 days, an outburst duration $\sim 240-300$ days, and a nearly constant peak X-ray luminosity $\sim 1.5 \times 10^{39}$ erg/s. The upper limit of X-ray luminosity at the quiescent state is $\sim 5.6 \times 10^{36}$ erg/s, and the total energy radiated during one outburst is $\sim 10^{46}$ erg. The spectra at the high luminosity states can be described with an absorbed disk black-body, and the disk temperature increases with the X-ray luminosity. We compare its outburst properties with other known transient ULXs including ESO 243-49 HLX-1. As its peak luminosity only marginally puts it in the category of ULXs, we also compare it with normal transient black hole binaries. Our results suggest that the source is powered by an accreting massive stellar-mass black hole, and the outbursts are triggered by the thermal-viscous instability.