Optical variability in MAXI J1659-152

ATel #2884; D. M. Russell (Univ. of Amsterdam), F. Lewis (Faulkes Telescope Project, Open Univ., Univ. of Glamorgan), D. Bersier and Z. Cano (Liverpool JMU), P. Gandhi (ISAS, Japan Aerospace Exploration Agency), A. Patruno, M. Kalamkar, Y. J. Yang, D. Altamirano (Univ. of Amsterdam), P. Casella (Univ. of Southampton), M. Linares (MIT), M. Armas Padilla, Y. Cavecchi, N. Degenaar, R. Kaur, M. van der Klis, A. Watts and R. Wijnands (Univ. of Amsterdam), N. Rea (CSIC-IEEC)
on 29 Sep 2010; 16:52 UT
Credential Certification: David M. Russell (D.M.Russell@uva.nl)

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The newly identified black hole candidate X-ray binary MAXI J1659-152 (ATel #2873, #2881, GCN #11307), originally designated GRB100925A (GCN #11296) has now been detected at radio (ATel #2874), sub-mm (GCN #11304), optical (GCN #11298, #11301, #11302, #11305, #11306, #11307, #11308, #11314) and X-ray (GCN #11296, ATel #2875, #2877) wavelengths. Here we report a brightening of the optical counterpart since the outburst began and short-timescale (minutes) variability characteristic of a black hole transient in outburst.

The field of MAXI J1659-152 was imaged with the 2-m Faulkes Telescope North on 26th September at 05:42 - 06:22 UT (MJD 55465.3) in V and i'-bands, 22 hours after the initial Swift slew to the new transient (to detect the afterglow of the proposed gamma-ray burst). The optical counterpart was bright compared with digital sky survey images; V = 16.8 (calibrating using field star magnitudes measured by Swift UVOT). The point source is consistent with the 0.61 arcsec error circle estimated from Swift UVOT (GCN #11298) and close to the recent Swift XRT localization in ATel #2877. We then imaged the field on the following two days with the 2-m Faulkes Telescope South (three images around MJD 55466.4 and 16 around MJD 55467.4). Follow the link below for finding charts and light curves.

The optical counterpart has brightened by ~ 0.4 mag in V and i' over 2.1 days and is also variable on timescales of a few minutes. The variability spanned ~0.1 mag on 26th September in V (from 4 images) and i'-bands (from 5 images) and on 28th September in i'-band (from 14 images). The relative errors are 0.01 mag. A comparison star of similar magnitude spanned 0.01 mag in i', consistent with having constant flux. We have not yet performed flux calibration for R and i'-bands, their magnitudes will have a systematic error. The time resolution was 5.5 mins and 3.0 mins on 26th and 28th September, respectively.

We have also analyzed Swift UVOT V and U-band observations, which have been made at least once per day since the first detection of the transient. The V and U-band light curves confirm the optical increase in flux over the first four days of the outburst.

Rapid variability studies in optical and infrared are encouraged. Ultraviolet observations are also encouraged, as this transient appears to have low foreground extinction. The Faulkes Telescope observations are part of an optical monitoring project of low-mass X-ray binaries (Lewis et al. 2008). The Faulkes Telescope Project is an educational and research arm of the Las Cumbres Observatory Global Telescope Network (LCOGTN). DMR acknowledges support from a Netherlands Organisation for Scientific Research (NWO) Veni Fellowship. FL acknowledges support from the Dill Faulkes Educational Trust.

Optical light curves and finding chart for MAXI J1659-152