Probable intermediate‐mass black holes in NGC 4559: XMM–Newton spectral and timing constraints

We have examined X‐ray and optical observations of two ultraluminous X‐ray sources, X7 and X10 in NGC 4559, using XMM–Newton , Chandra and the Hubble Space Telescope ( HST ). The ultraviolet/X‐ray luminosity of X7 exceeds 2.1 × 10 40 erg s −1 in the XMM–Newton observation, and that of X10 is > 1.3 × 10 40 erg s −1 . X7 has both thermal and power‐law spectral components, The characteristic temperature of the thermal component is 0.12 keV. The power‐law components in the two sources both have slopes with photon index ≃2.1. A timing analysis of X7 indicates a break frequency at 28 MHz in the power spectrum, while that for X10 is consistent with an unbroken power law. The luminosity of the blackbody component in the X‐ray spectrum of X7 and the nature of its time‐variability provides evidence that this object is an intermediate‐mass black hole accreting at sub‐Eddington rates, but other scenarios which require high advection efficiencies from a hollowed‐out disc might be possible. The emission from X10 can be characterized by a single power‐law. This source can be interpreted either as an intermediate‐mass black hole, or as a stellar mass black hole with relativistically beamed Comptonized emission. There are four optical counterparts in the error circle of X7. No counterparts are evident in the error circle for X10.