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tokens: keV. As these features are not commonly observed in Galactic XRBs, they imply a new spectral regime, which is usually referred to as the ‘ultraluminous state' (Roberts 2007 ; Gladstone et al. 2009 ), and may be associated with super-Eddington accretion. A few luminous ULXs have very recently been observed with NuSTAR , resulting in spectra which demonstrate that the high-energy curvature persists above 10 keV (Bachetti et al. 2013 ; Walton et al. 2013a ; Rana et al. 2015 ), confirming this result. The spectral curvature may arise either from Comptonization in a cold, optically thick corona lying above an accretion disc (e.g. Stobbart et al. 2006 ; Gladstone et al. 2009 ); from a modified accretion disc, i.e. a slim disc (Mizuno et al. 2007 ); or the hot inner regions of the disc, which are highly distorted by advection, turbulence, self-heating and spin (Beloborodov 1998 ; Suleimanov et al. 2002 ; Kawaguchi 2003 ). The soft component is thought to originate from the photosphere of a radiatively driven wind that is ejected from the spherization radius of the accretion disc down to its innermost regions, which is expected to occur at super-Eddington accretion rates (e.g. Shakura Sunyaev 1973 ; Ohsuga Mineshige 2007 ; Poutanen et al. 2007 ; Ohsuga et al. 2009 ). Partially inhomogeneous winds in ULXs may explain the puzzling short-term variability reported in some ULXs (Heil, Vaughan Roberts 2009 ), through a combination of inclination angle and accretion rate (e.g. Middleton, Sutton Roberts 2011 ; Takeuchi, Ohsuga Mineshige 2013 ; Middleton et al. 2015 ). These same factors also determine the observed energy spectrum (e.g. Sutton, Roberts Middleton 2013b ; Pintore et al. 2014 ). On the other hand, HLX-1 in ESO 243-49 (Farrell et al. 2009 ), which has a peak luminosity of ∼10 42 erg s −1 , is observed with a strikingly different set of properties than standard ULXs. It displays a spectral evolutionary pattern similar to that of the Galactic BH X-ray transients (e.g. McClintock Remillard 2006 ; Fender Belloni 2012 ), undergoing periodic outbursts (Godet et al. 2014 ; Webb et al. 2014 ) in which the source switches from a high-luminosity soft state to a fainter hard state (Servillat et al. 2011 ). Multiwavelength emission has been detected from HLX-1 as it undergoes this outburst cycle. In particular, radio emission has been detected, which is associated with discrete jet ejection events (Webb et al. 2012 ), similar to those that have been observed during hard -to- soft state transitions state in Galactic BHs. By assuming that HLX-1 was in a similar state, the authors of that study were able to refine the BH mass estimates to ∼10 3 –10 4 M ⊙ . Other good IMBH candidates include: M82 X-1 (Feng, Rao Kaaret 2010 ), for which a BH mass of ∼400 M ⊙ has been suggested based on the tentative identification of twin-peak quasi periodic oscillations (Pasham, Strohmayer Mushotzky 2014 ); and an X-ray source in the galaxy NGC 2276 (Mezcua et al. 2015 ).