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We present optical and infrared monitoring of the 2005 outburst of the haloblack hole X-ray transient XTE J1118+480. We measured a total outburstamplitude of ~5.7+-0.1 mag in the R band and ~5 mag in the infrared J, H andK_s bands. The hardness ratio HR2 (5-12 keV/3-5 keV) from the RXTE/ASM data is1.53+-0.02 at the peak of the outburst indicating a hard spectrum. Both theshape of the light curve and the ratio L_X (1-10 keV)/L_opt resemble themini-outbursts observed in GRO J0422+32 and XTE J1859+226. During earlydecline, we find a 0.02-mag amplitude variation consistent with a superhumpmodulation, like the one observed during the 2000 outburst. Similarly, XTEJ1118+480 displayed a double-humped ellipsoidal modulation distorted by asuperhump wave when settled into a near-quiescence level, suggesting that thedisk expanded to the 3:1 resonance radius after outburst where it remaineduntil early quiescence. The system reached quiescence at R=19.02+-0.03 aboutthree months after the onset of the outburst. The optical rise preceded theX-ray rise by at most 4 days. The spectral energy distributions (SEDs) at thedifferent epochs during outburst are all quasi-power laws with F_nuproportional to nu^alpha increasing toward the blue. At the peak of theoutburst we derived alpha=0.49+-0.04 for the optical data alone andalpha=0.1+-0.1 when fitting solely the infrared. This difference between theoptical and the infrared SEDs suggests that the infrared is dominated by adifferent component (a jet?) whereas the optical is presumably showing the diskevolution.Comment: Accepted for publication in The Astrophysical Journa

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