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

• Published in: The Astrophysical journal 2006-06, Vol.644 (1), p.432-438
• Main Authors: Zurita, C, Torres, M. A. P, Steeghs, D, Rodríguez-Gil, P, Muñoz-Darias, T, Casares, J, Shahbaz, T, Martínez-Pais, I. G, Zhao, P, Garcia, M. R, Piccioni, A, Bartolini, C, Guarnieri, A, Bloom, J. S, Blake, C. H, Falco, E. E, Szentgyorgyi, A, Skrutskie, M
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/503286

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 65.7 c 0.1 mag in the R band and 65 mag in the infrared J, H, and K sub(s) bands. The hardness ratio HR2 (5-12 keV: 3-5 keV) from the RXTE ASM data is 1.53 c 0.02 at the peak of the outburst, indicating a hard spectrum. Both the shape of the light curve and the ratio L sub(X)(l-10 keV)/L sub(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 Jl118+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 c 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 sub(u) 8 u a increasing toward the blue. At the peak of the outburst, we derived or = 0.49 c 0.04 for the optical data alone and a = 0.1 c 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.