We present simultaneous multicolor infrared and optical photometry of the black hole X-ray transient XTE J1118+480 during its short 2005 January outburst , supported by simultaneous X-ray observations .
The variability is dominated by short timescales , \sim 10 s , although a weak superhump also appears to be present in the optical .
The optical rapid variations , at least , are well correlated with those in X-rays .
Infrared JHK _ { s } photometry , as in the previous outburst , exhibits especially large amplitude variability .
The spectral energy distribution ( SED ) of the variable infrared component can be fitted with a power-law of slope \alpha = -0.78 \pm 0.07 where F _ { \nu } \propto \nu ^ { \alpha } .
There is no compelling evidence for evolution in the slope over five nights , during which time the source brightness decayed along almost the same track as seen in variations within the nights .
We conclude that both short-term variability , and longer timescale fading , are dominated by a single component of constant spectral shape .
We can not fit the SED of the IR variability with a credible thermal component , either optically thick or thin .
This IR SED is , however , approximately consistent with optically thin synchrotron emission from a jet .
These observations therefore provide indirect evidence to support jet-dominated models for XTE J1118+480 and also provide a direct measurement of the slope of the optically thin emission which is impossible based on the average spectral energy distribution alone .