New Frontiers in Black Hole Astrophysics Proceedings IAU Symposium No. 324, 2016 Andreja Gomboc, ed. c  International Astronomical Union 2017 doi:10.1017/S1743921316012850 Investigating ultraluminous X-ray sources through their multiwavelength variability and broadband spectra Luca Zampieri 1 , Elena Ambrosi 2 and Amid Nayerhoda 2,3 1 INAF-Astronomical Observatory of Padova Vicolo dell’Osservatorio 5, 35122 Padova, Italy email: luca.zampieri@oapd.inaf.it 2 Department of Physics and Astronomy, University of Padova, Vicolo Osservatorio 3, 35122 Padova, Italy 3 Institute of Nuclear Physics PAN, ul. Radzikowskiego 152, 31-342 Krak´ ow, Poland Abstract. The population of ultraluminous X-ray sources encompasses accreting compact ob- jects with significantly different masses, from black holes of intermediate mass, to black holes of stellar origin, to neutron stars. Investigating these sources will help us answering crucial ques- tions on the distribution of black hole masses and on mass accretion above the Eddington limit in the local Universe, that have potential implications in other astrophysical areas. In order to perform a detailed investigation of ultraluminous X-ray sources, an accurate modeling of their evolution and multiwavelength emission properties is needed. We report some preliminary results of the activities that we are carrying out at present in this area. Keywords. accretion, accretion disks, methods: numerical, X-rays: binaries 1. Introduction Ultra-Luminous X-ray sources (ULXs) are point-like off-nuclear X-ray sources in nearby galaxies, intrinsically powerful but observationally rather faint. Their luminosity exceeds (although not necessarily all the time) the Eddington limit for spherical accretion onto a ∼ 10 M ⊙ black hole (L> 10 39 erg s −1 ). Hundreds of sources have been discovered and reported in various surveys/catalogues to date. The observational evidence points towards the identification of the majority of ULXs (∼ 60 - 70%) with accreting binaries (see e.g. Feng & Soria 2011 for a review). Two dynamical mass measurements of the compact objects in ULXs have been recently performed through dedicated spectral and/or photometric monitorings of their optical counterparts. M 101 ULX-1 has a black hole (BH) with mass M BH > 5 M ⊙ (Liu et al. 2013), while the mass of the compact object in NGC 7793 P13 is < 15 M ⊙ (Motch et al. 2014). The latter system was very recently discovered to show X-ray pulsations and then hosts a neutron star (Fuerst et al. 2016, Israel et al. 2016a). Other two pulsating neutron stars (NSs) have been identified among ULXs (M 82 X-1, Bachetti et al. 2014; NGC 5907 ULX-1, Israel et al. 2016b), suggesting that pulsar ULXs may not be uncommon. On the other hand, a few extremely luminous or hyperluminous sources, most notably HLX-1 in ESO 243-49 (with a peak luminosity of 10 42 erg s −1 ; Farrell et al. 2009), have been identified as very good candidates intermediate mass BHs. For HLX-1, BH mass estimates based on X-ray spectroscopy and radio jet emission are in the ballpark of 10 4 - 10 5 M ⊙ (Servillat et al. 2011, Webb et al. 2012). 31 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921316012850 Downloaded from https://www.cambridge.org/core. IP address: 54.224.216.183, on 14 Dec 2021 at 16:24:06, subject to the Cambridge Core terms of use, available at