The slow heartbeats of an ultra-luminous X-ray source in NGC 3621

TL;DR

This paper reports on the unique variability of an ultra-luminous X-ray source in NGC 3621, showing heartbeat-like behavior similar to known black hole and neutron star systems, but does not conclusively identify the compact object.

Contribution

It provides the first detailed analysis of the spectral and timing properties of this ULX, highlighting its heartbeat-like variability and comparing it to other known systems.

Findings

The ULX exhibits a characteristic variability timescale of ~3500 s.

Its spectral and timing properties closely resemble those of GRS 1915+105 and other heartbeat sources.

Current data do not allow definitive identification of the compact object.

Abstract

We report on the results of X-ray observations of 4XMM J111816.0-324910, a transient ultra-luminous X-ray source located in the galaxy NGC 3621. This system is characterised by a transient nature and marked variability with characteristic time-scale of ~3500 s, differently from other ULXs, which in the vast majority show limited intra-observation variability. Such a behaviour is very reminiscent of the so-called heartbeats sometimes observed in the Galactic black hole binary GRS 1915+105, where the variability time-scale is ~10-1000 s. We study the spectral and timing properties of this object and find that overall, once the differences in the variability time-scales are taken into account, they match quite closely those of both GRS 1915+105, and of a number of objects showing heartbeats in their light-curves, including a confirmed neutron star and a super-massive black hole powering an active galactic nucleus. We investigate the nature of the compact object in 4XMM J111816.0-324910 by searching for typical neutron star signatures and by attempting a mass estimate based on different methods and assumptions. Based on the current available data, we are not able to unambiguously determine the nature of the accreting compact object responsible for the observed phenomenology.