Ultraluminous X-ray Sources

Plot
Figure 1:Total (pn+MOS) count rate of M 82 X-1 during a 2004 XMM observation (top). Spectrogram of M 82 X-1 obtained from 2048-s long XMM data stretches (bottom). Darker grey corresponds to higher spectral density power, indicating the existence of a quasi periodic oscillation (from Mucciarelli et al. 2006, MNRAS, 365, 1123).

 When, at the beginning of the '80s, point-like, off-nuclear X-ray sources in the field of nearby galaxies were first detected (see, e.g., Fabbiano 89, ARA&A), it was immediately recognized that their luminosity was unusually large. If physically associated with their host galaxies, these sources should have had an isotropic luminosity in excess of the Eddington limit for 10 solar masses. The nature of most of these ultraluminous X-ray sources (ULXs) remains unclear. Several similarities with the properties of Galactic X-ray binaries, chiefly among them the variability, and the recent discovery of an orbital modulation in the optical light curve of NGC 1313 X-2 (Liu et al. 2009, ApJ, 690, L39) strongly suggest that the majority of them are X-ray binaries. The crucial question is then: What type of X-ray binary are they? How can they manage to emit such a huge luminosity? Two main interpretations have been proposed. Firstly ULXs could be relatively normal stellar-mass BHs (

Plot
Figure 2:Color-magnitude diagram for stars of 10, 15, 30 and 50 solar masses (black-solid, red-dotted, green-dashed, yellow-dot-dashed line, respectively) starting mass transfer onto a 100 solar masses BH during main sequence (Patruno & Zampieri 2008). The point C1 denotes the counterpart of NGC 1313 X-2. The binary tracks are plotted only during the contact phases. The starred symbol denotes the point where the main sequence ends and H-shell burning sets in.

 In the last few years, X-ray timing has provided a new opportunity to estimate BH masses in ULXs thanks in particular to the detection of broad band noise and quasi periodic oscillations (QPOs) in the power density spectrum of some ULXs (Figura 1). Recently, we proposed a new timing approach to assess BH masses in ULXs, which is based the so called ``variability plane'', populated by both Galactic black hole (BH) candidates and active galactic nuclei. Assuming that the accretion flow in ULXs behaves in a similar way (which remains an open question) and taking into account the uncertainty on the efficiency of the accretion disc, we find that the BH mass is in the interval 95-1300 solar masses for M 82 X-1 and 115-1300 solar masses for NGC 5408 X-1 (Casella et al. 2008). Comparison of stellar evolutionary tracks of ULXs with the photometric properties of their optical counterparts on the colour-magnitude diagram may also be used to constrain the masses of their donor stars and, if accurate photometry is available, to provide interesting clues to the BH mass. Once binary evolution and X-ray irradiation effects are taken into account, the photometric data of NGC 1313 X-2 are consistent with either a binary system composed by a H-shell burning star of 10-15 solar masses around a stellar mass BH or a ~12 solar masses donor dumping matter on a 100 solar masses BH during main sequence (Patruno & Zampieri 2008; Figure 2). Next year a significant effort will be devoted to activities in this area thanks also to the resources made available by a dedicated PRIN-INAF grant. We plan to work on different aspects of this problem. The study of the optical counterparts of ULXs is a primary target, as it will made possible the measurement of the mass function of ULX binary systems and will provide direct constraints on the BH masses of individual sources. To this end, we have been granted 5 nights at TNG to monitor the optical counterparts of two ULXs looking for their orbital period. At the same time, we are also working at the comparison of theoretical models of ULX binary systems with observations and at alternative scenarios for the formation of their compact remnants.

People:L. Zampieri, P. Mucciarelli, R. Falomo

Collaboration: T. Belloni, G. Trinchieri, A. Wolter (INAF OA Bologna), P. Casella, A. Patruno (Amsterdam Univ., The Netherlands), M. Colpi (University of Milano Bicocca), A. Lorenzin, R. Turolla (University of Padua), M. Mapelli (University of Zurich), T. P. Roberts (Durham University), A. Treves (Insubria University)  Publications: Patruno et al. (2008), MNRAS 386, 543; Casella et al. (2008), MNRAS 387, 1707; Mucciarelli et al. (2007), ApJ 658,999

Link: ULX web page

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