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

We present optical and infrared monitoring of the 2005 outburst of the halo black hole X-ray transient XTE J1118+480. We measured a total outburst amplitude of ~5.7+/-0.1 mag in the R band and ~5 mag in the infrared J, H, and K_s bands. The hardness ratio HR2 (5-12 keV:3-5 keV) from the RXTE ASM data is 1.53+/-0.02 at the peak of the outburst, indicating a hard spectrum. Both the shape of the light curve and the ratio L_X(1-10 keV)/L_opt resemble the minioutbursts observed in GRO J0422+32 and XTE J1859+226. During early decline, we find a 0.02 mag amplitude variation consistent with a superhump modulation, like the one observed during the 2000 outburst. Similarly, XTE J1118+480 displayed a double-humped ellipsoidal modulation distorted by a superhump wave when settled into a near-quiescence level, suggesting that the disk expanded to the 3:1 resonance radius after outburst, where it remained until early quiescence. The system reached quiescence at R=19.02+/-0.03, about 3 months after the onset of the outburst. The optical rise preceded the X-ray rise by at most 4 days. The spectral energy distributions (SEDs) at the different epochs during outburst are all quasi-power laws with F_ν~ν^α increasing toward the blue. At the peak of the outburst, we derived α=0.49+/-0.04 for the optical data alone and α=0.1+/-0.1 when fitting solely the infrared. This difference between the optical and the infrared SEDs suggests that the infrared is dominated by a different component (a jet?), whereas the optical is presumably showing the disk evolution.