The ultraluminous X-ray source population of NGC 4485/4490

Abstract

We report the results of spectral and temporal variability studies of the ultraluminous X-ray sources (ULXs) contained within the interacting pair of galaxies NGC 4485/4490, combining Chandra and XMM-Newton observations. Each of the four separate observations provide at least modest quality spectra and light curves for each of the six previously identified ULXs in this system; we also note the presence of a new transient ULX in the most recent observation. No short-term variability was observed for any ULX within our sample, but three out of five sources show correlated flux/spectral changes over longer timescales, with two others remaining stable in spectrum and luminosity over a period of at least five years. We model the spectra with simple power-law and multi-colour disc black body models. Although the data is insufficient to statistically distinguish models in each epoch, those better modelled (in terms of their chi^2 fit) by a multi-colour disc black body appear to show a disc-like correlation between luminosity and temperature, whereas those modelled by a power-law veer sharply away from such a relationship. The ULXs with possible correlated flux/spectral changes appear to change spectral form at ~2 x 10^39 erg s^{-1}, suggestive of a possible change in spectral state at high luminosities. If this transition is occurring between the very high state and a super-Eddington ultraluminous state, it indicates that the mass of the black holes in these ULXs is around 10-15 M_solar.