Probing the variable nature of a new Ultraluminous X-Ray Source in NGC 3621

A new ULX was recently discovered in the nearby galaxy NGC 3621 in thearchival XMM-Newton data, which shows extremely marked variability -unique among the known ULXs - reminiscent of the so-called heartbeats,a type of variability pattern observed in only a handful of systems. Thearchival data yielded contradictory information on the nature of thecompact object powering XMM J111816.0-324910, partly due to the limitedquality of the data, and to the limited understanding of the heartbeatphenomenon. We propose a Target of Opportunity observation of XMMJ111816.0-324910 for a total of 120ks, to obtain a better understandingof the properties of J1118 by means of detailed spectral and timinganalysis. This ToO will be triggered based on approved Swift observations.