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Periodic Humps in XTE J1118+480 2005 Outburst

ATEL # 399; Y. Chou (National Central University), A. Chen (National Cheng Kung University), P. S. Chiang (National Cheng Kung University), A. C. Chen (National Central University), Z. Y. Lin (National Central University), Y. Chung (National Central University)
on 26 Jan 2005; 7:30 UT
Password Certification: Yi Chou (yichou@astro.ncu.edu.tw)

Subjects: Optical, Binaries, Black Holes, Transients, Variables

We report the follow-up observations of the outbursting black hole X-ray transient XTE J1118+480 (KV UMa, ATEL #383, #384, #385, #386, #387, #389, #393) made by Lulin One-meter Telescope (LOT), Taiwan. The observations were started on UT 2005 Jan 16.74 and the source is still monitoring. The brightness of the source is now declining with a rate of ~0.04 mag/d. The mean V band magnitudes detected each night by LOT were: Jan 16.814, 13.47; 17.784, 13.52; 18.758, 13.56; 19.781, 13.58; 20.779, 13.61; 21.792, 13.65; 22.882, 13.71 and 23.863, 13.82. A preliminary timing analysis shows that weak (amp ~0.02 mag) but significant periodic humps with a period of ~4 hrs, close to the one discovered in previous outburst (IAUC# 7418) , can be detected on the light curves between Jan 18.6 and 20.9. The false alarm probability of this signal obtained by Analysis of Variance (AoV) is less than 1e-5. The periodic humps may be owing to the orbital variations or the superhumps. More observations are planned for further investigations.

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