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

TL;DR

This study analyzes the 2005 outburst of XTE J1118+480 across multiple wavelengths, revealing differences from the 2000 event and highlighting the complex interplay of jet and disc emissions in the low/hard state.

Contribution

It provides a detailed multi-wavelength analysis of the 2005 outburst, distinguishing jet and disc contributions and showing variability in emission mechanisms within the low/hard state.

Findings

Optical decay delayed relative to X-ray/radio

Disc emission more prominent in 2005 compared to 2000

Presence of an optically thin radio component possibly from ejection events

Abstract

The black hole X-ray transient, XTE J1118+480, has now twice been observed in outburst - 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 lightcurves 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 broadband spectrum in 2000, in 2005 the accretion disc seems to be more prominent and we use an analysis of the lightcurves 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.