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