Bright X-ray Transients in M31: 2004 July Xmm-newton Observations

We present the results of X-ray observations of four bright transients sources detected in the July 2004 XMM-Newton observations of the central bulge of M31. Two X-ray sources, XMMU J004315.5+412440 and XMMU J004144.7+411110, were discovered for the first time. Two other sources, CXOM31 J004309.9+412332 and CXOM31 J004241.8+411635, were previously detected by Chandra. The properties of the sources suggest their identification with accreting binary systems in M31. The X-ray spectra and variability of two sources, XMMU J004144.7+411110 and CXOM31 J004241.8+411635, are similar to that of the Galactic black hole transients, making them a good black hole candidates. The X-ray source XMMU J004315.5+412440 demonstrates a dramatic decline of the X-ray flux on a time scale of three days, and a remarkable flaring behavior on a short time scales. The X-ray data on XMMU J004315.5+412440 and CXOM31 J004309.9+412332 suggest that they can be either black hole or neutron star systems. Combining the results of 2000-2004 XMM observations of M31, we estimate a total rate of the bright transient outbursts in the central region of M31 to be 6-12 yr −1 , in agreement with previous studies. 1. INTRODUCTION Bright X-ray transient sources provide a unique opportunity to study the properties of the accretion onto stellar-mass compact objects. The observations of the Galactic X-ray novae (XRNe) show that they can be either high-mass (HMXB) and low-mass (LMXB) binaries (Tanaka & Shibazaki 1996), but most of confirmed black-hole binaries are low-mass systems (McClintock & Remillard 2005, and references therein). Until recently, the detailed study of X-ray transient sources has been mostly limited to our Galaxy. The advent of a new generation of X-ray telescopes (Chandra and XMM-Newton) has allowed the study of the spectral and temporal properties of XRNe located in the nearby galaxies and compare them to their Galactic counterparts. The relative proximity and favorable orientation of M31 make it a prime target for the study of an extragalactic XRNe population. Recent Chandra and XMM-Newton observations of M31 have led to the discovery of several dozen bright transient X-ray sources with luminosities between ∼ 10 36 and ∼ 10 38 ergs s b, c). The follow-up observations of X-ray transients with HST ensured the identification and study of optical counterparts to some of these systems (Williams et al. 2004; Williams et al. 2005a, b). Here we present the results of spectral and timing analysis of four bright transient sources detected in the 2004 July …