Intermediate-Mass Black Holes or Beamed Super-Eddington Accretors - the Puzzle of Two Ultraluminous X-ray Sources in NGC 1365

Iskra Strateva, MPI for Extraterrestrial Physics

We present the X-ray and optical properties of 23 point sources discovered with Chandra in the inner region of the giant barred spiral galaxy NGC 1365. The majority of these sources are high-mass X-ray binaries, containing a neutron star or a black hole accreting from a luminous companion at a sub-Eddington rate. But the X-ray luminosities of two of these point sources are in excess of 10^40 erg/s and suggest more exotic accretion modes: sub-Eddington accretion onto intermediate-mass black holes or beamed super-Eddington accretion onto stellar-mass compact remnants. Using repeat Chandra, XMM-Newton, and optical observations, we discuss in detail the natures of these highly variable ultraluminous X-ray sources.