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

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.

💡 Research Summary

XTE J1118+480 is a low‑mass black‑hole X‑ray transient that has undergone two well‑observed outbursts, in 2000 and 2005, both of which remained in the canonical low/hard X‑ray state. This paper presents a comprehensive, multi‑wavelength data set for the 2005 event, including radio (VLA), near‑infrared/optical (UKIRT, SMARTS), soft X‑ray (RXTE/PCA) and hard X‑ray (RXTE/HEXTE) measurements, and compares the results with those obtained during the 2000 outburst.

The most striking difference lies in the morphology of the light curves. In 2000 the radio, X‑ray and optical/IR fluxes rose and fell almost synchronously, and the broadband spectrum was flat from radio through infrared, a signature that was interpreted as a dominant, compact, partially self‑absorbed jet. By contrast, in 2005 the radio and hard X‑ray light curves still track each other closely, but the optical/IR emission peaks several days later (≈5–7 d) and decays more slowly. The optical colour evolution shows an initial blue‑ward swing followed by a gradual reddening, consistent with a heating and subsequent cooling of an accretion disc that reprocesses the high‑energy photons.

Spectral analysis of the radio data reveals a transition from a flat (α≈0) spectrum at early times to a negative spectral index (α≈‑0.5) later in the outburst. The authors model this as the superposition of two components: a persistent, optically thick jet that supplies the flat baseline, and an optically thin, rapidly fading component that they associate with discrete ejection events. The optically thin component decays on a timescale of roughly 10–15 days, suggesting that the 2005 outburst involved multiple, short‑lived shocks or plasmoid expulsions in addition to the steady jet.

Hard X‑ray spectra in both outbursts are well described by a power‑law with photon index Γ≈1.6 and a high‑energy cutoff near 100 keV, indicating that the coronal Comptonisation remains the primary source of the hard X‑ray flux. However, the weaker correlation between the hard X‑rays and the radio emission in 2005 implies that the jet no longer dominates the total radiative output; instead, the disc contributes a larger fraction of the observed luminosity, especially at optical/IR wavelengths.

By fitting the optical/IR spectral energy distribution with a multi‑temperature black‑body model, the authors estimate that the disc temperature peaked at ∼10 000 K early in the outburst and then declined, matching the observed colour evolution. The delayed optical peak relative to the X‑ray is interpreted as the light‑travel and reprocessing time needed for the disc to respond to the hard X‑ray illumination.

Overall, the paper argues that the “low/hard state” is not a monolithic spectral state but a family of configurations in which the relative contributions of jet, disc, and corona can vary dramatically from one outburst to another. In 2000 the jet dominated the broadband emission, while in 2005 the disc was more prominent and the radio spectrum contained an additional optically thin component indicative of episodic ejections. These findings broaden the phenomenology of the low/hard state, emphasizing the necessity of simultaneous, multi‑wavelength monitoring to disentangle the complex interplay of the different emission regions in black‑hole X‑ray binaries.