Disentangling jet and disc emission from the 2005 outburst of XTE J1118+480

Abstract

The black hole X-ray transient, XTE J1118+480, has now twice been observed in outbursts – 2000 and 2005 – and on both occasions remained in the low/hard X-ray spectral state. Here we present radio, infrared, optical, soft X-ray and hard X-ray observations of the more recent outburst. We find that the light curves have very different morphologies compared with the 2000 event and the optical decay is delayed relative to the X-ray/radio. We attribute this lesser degree of correlation to contributions of emission from multiple components, in particular the jet and accretion disc. Whereas the jet seemed to dominate the broad-band spectrum in 2000, in 2005 the accretion disc seemed to be more prominent and we use an analysis of the light curves and spectra to distinguish between the jet and disc emission. There also appears to be an optically thin component to the radio emission in the 2005 data, possibly associated with multiple ejection events and decaying as the outburst proceeds. These results add to the discussion that the term ‘low/hard state’ covers a wider range of properties than previously thought, if it is to account for XTE J1118+480 during these two outbursts.