The 2005 Outburst of the Halo Black Hole X‐Ray Transient XTE J1118+480

Abstract

We present optical and infrared monitoring of the 2005 outburst of thehalo black hole X-ray transient XTE J1118+480. We measured a totaloutburst amplitude of ~5.7+/-0.1 mag in the R band and ~5 mag in theinfrared J, H, and K_{s} bands. The hardness ratio HR2 (5-12keV:3-5 keV) from the RXTE ASM data is 1.53+/-0.02 at the peak of theoutburst, indicating a hard spectrum. Both the shape of the light curveand the ratio L_{X}(1-10 keV)/L_{opt} resemble theminioutbursts observed in GRO J0422+32 and XTE J1859+226. During earlydecline, we find a 0.02 mag amplitude variation consistent with asuperhump modulation, like the one observed during the 2000 outburst.Similarly, XTE J1118+480 displayed a double-humped ellipsoidalmodulation distorted by a superhump wave when settled into anear-quiescence level, suggesting that the disk expanded to the 3:1resonance radius after outburst, where it remained until earlyquiescence. The system reached quiescence at R=19.02+/-0.03, about 3months after the onset of the outburst. The optical rise preceded theX-ray rise by at most 4 days. The spectral energy distributions (SEDs)at the different epochs during outburst are all quasi-power laws withF_{nu}~{ν}^{alpha} increasing toward the blue. Atthe peak of the outburst, we derived {α}=0.49+/-0.04 for the opticaldata alone and {α}=0.1+/-0.1 when fitting solely the infrared. Thisdifference between the optical and the infrared SEDs suggests that theinfrared is dominated by a different component (a jet?), whereas theoptical is presumably showing the disk evolution.