The Slow Heartbeats of an Ultraluminous X-Ray Source in NGC 3621

We report on the results of X-ray observations of 4XMM J111816.0-324910, a transient ultraluminous X-ray source located in the galaxy NGC 3621. This system is characterized by a transient nature and marked variability with a characteristic timescale of 3500 s, in contract with other ultraluminous X-ray sources, which in the vast majority show limited intra-observation variability. Such a behavior is very reminiscent of the so-called heartbeats sometimes observed in the Galactic black hole binary GRS 1915+105, where the variability timescale is similar to 10-1000 s. We study the spectral and timing properties of this object and find that overall, once the differences in the variability timescales 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 supermassive 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.