Astronomy & Astrophysics
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 ≥13 M, accreting near the Eddington limit, that underwent a transient outburst followed by an exponential decay reminiscent of transients associated with galactic X-ray novae.
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