CXOM31 J004253.1+411422: the first ultraluminous X-ray transient in M 31

Context. We seek clarification of the nature of X-ray sources detected in M 31. Here we focus on CXOM31 J004253.1+411422, the brightness of which suggests that it belongs to the class of ultraluminous X-ray sources.
Aims: We determine the X-ray properties of sources detected in the XMM-Newton/Chandra monitoring program. We investigate spectral properties and search for periodic or quasi-periodic oscillations.
Methods: A multicomponent model was applied to the spectra obtained from XMM-Newton data to evaluate the relative contributions from thermal and nonthermal emission. The time dependence of this ratio was evaluated over a period of forty days. We simultaneously fit data from XMM-Newton EPIC-pn, MOS1, and MOS2 detectors with (nonthermal) powerlaw and (thermal) multicolored blackbody.
Results: The X-ray spectrum is best fit by the combination of a thermal component with kT ~ 1 keV and a powerlaw component with a photon index of approximately 2.6. From combined analysis of Chandra, Swift, and XMM-Newton data, the unabsorbed total luminosity of this source decreases from ~3.8 × 10³⁹ erg s^-1 in the first observation to ~0.5 × 10³⁹ erg s^-1 over a period of three months. The decay closely follows an exponential decline with a time constant of 32 days. The source spectrum evolves significantly, exhibiting a faster decline of the thermal component. We do not find evidence of any significant temporal features in the power density spectrum. The presence of a thermal component at kT ~ 1 keV in conjunction with a nonthermal high-energy tail, is also consistent with spectral properties of other ULXs in the "high state".
Conclusions: Our analysis indicates that the underlying source of this first ULX in M 31 is a black hole of mass, M ≥ 13M_⊙, accreting near the Eddington limit, that underwent a transient outburst followed by an exponential decay reminiscent of transients associated with galactic X-ray novae.

Based on observations obtained with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member States and NASA.