2006/02/09 by C. Zurita, M. A. P. Torres, D. Steeghs +18 · 2 citations
Physics and Astronomy · #Astrophysical Phenomena and Observations #Astrophysics and Cosmic Phenomena #Relativity and Gravitational Theory #astro-ph
paper · pdf · doi:10.1086/503286
published as Astrophys.J. 644 (2006) 432-438 · Accepted for publication in The Astrophysical Journal
arxiv created 2006/02/09 · openalex publication_date 2006/06/10 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/31
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.