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

Abstract

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 × 10⁴⁰ erg s^-1 in the XMM-Newton observation, and that of X10 is > 1.3 × 10⁴⁰ 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.