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X-ray and optical follow-up of ASAS 150946-2147.7

ATEL # 2030; M. Uemura, H. Takahashi, A. Arai, M. Sasada, M. Yamanaka, R. Ito (Hiroshima Univ.), T. Kato, D. Nogami, H. Maehara (Kyoto Univ.), K. Kinugasa, S. Honda, and O. Hashimoto (Gunma Astronomical Observatory)
on 24 Apr 2009; 8:54 UT
Password Certification: Hiromitsu Takahashi (

Subjects: Infra-Red, Optical, X-ray, Binaries, Black Holes, Transients

ASAS 150946-2147.7 is a suspected blackhole X-ray binary, currently in an optical outburst (CBET 1774). We performed an X-ray ToO observation and optical follow-up by Swift and "KANATA", respectively.

The Swift-XRT ToO observation started on 18 Apr. 2009 at 01:17:47 (UT) and ended at 04:08:33 (UT) with the total exposure time of 2.0 ks. The source was quite faint; the 1-10 keV average count-rate was about 0.014 counts/s. Assuming an absorbed power-law model with a hydrogen column density in the direction of the source (N_H=7.5 x 10^20 cm-2, Kalberla et al. 2005), we estimated the photon index of 2.5 (+/- 1.1) and the 1-10 keV flux of 6 (+7, -3) x 10^-13 erg cm-2 s-1. The 1-10 keV X-ray luminosity is ~7 x 10^31 erg s-1 assuming a distance of 1 kpc. It is almost comparable to the quiescent X-ray luminosity of blackhole X-ray binaries (e.g. Asai et al. 1998).

Optical and near-infrared follow-up observations were performed with the "KANATA" 1.5-m telescope of Hiroshima University from 9 to 22 Apr. The object has been fading monotonically with a rate of ~0.05 mag/day in V-band. A reddening trend in the V-J color index is associated with the fading. We also performed optical high-speed photometry with a time-resolution of 1 s for 6 nights between 11 and 22 Apr. No rapid fluctuation nor quasi-periodic oscillation was significantly detected in a frequency range of 0.01-0.5 Hz. Preliminary results from KANATA can be seen at:

We would like to thank the Swift Team for carrying out the ToO observation.

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