MAXI J1659-152 fading in optical
ATel #3517; D. M. Russell (University of Amsterdam), F. Lewis, (Univ. of Glamorgan, Faulkes Telescope Project, Open Univ.), L. Schreuder, Y. J. Yang, R. Wijnands (University of Amsterdam), A. Tripp (Faulkes Telescope Project)
on 28 Jul 2011; 15:37 UT
Credential Certification: David M. Russell (D.M.Russell@uva.nl)
Subjects: Optical, X-ray, Binary, Black Hole, Transient
Referred to by ATel #: 3524
It was recently reported that the black hole candidate X-ray binary MAXI J1659-152 has faded to an X-ray flux (0.5-10 keV) of < 3.0e-13 ergs cm-2 s-1 (ATel #3506). This implies the source may be in quiescence after an outburst that lasted almost one year (GCN #11296, ATel #2873) including a late re-brightening in the last three months (ATels #3298, #3339, #3358, #3379). Here we report on recent optical observations with the two 2-m Faulkes Telescopes North and South (see ATel #2884).
MAXI J1659-152 has faded in optical but is still detected in V, R and i'-bands as of 2011-07-27 (MJD 55769.4). The magnitudes on this date were V = 20.8 +- 0.1, R ~ 20.1. Flux calibration in V-band was achieved using field star magnitudes measured from Swift UVOT images. In R-band, the magnitudes of field stars in the GSC 2.2 digital sky survey were used to calibrate the data, and there may be systematic errors of several tenths of a magnitude. Flux calibration in i'-band is not available.
Soon after the outburst began in September 2010, the optical flux peaked at V ~ 16.5 mag (ATel #2884; Kennea et al. 2011, ApJ, 736, 22). In 2011 our Faulkes Telescope monitoring shows that by 2011-05-21 (MJD 55702.8) it had faded by only 2.5 mag to V = 19.0 +- 0.1. During the next 15 days the source remained at a fairly steady flux level; V ~ 18.9 - 19.0. This was during the period of X-ray rebrightenings. At a magnitude of V = 20.8 +- 0.1 on 2011-07-27, MAXI J1659-152 is now ~4.3 mag fainter than its brightest outburst magnitude. This is a fade of a factor of 50 in optical flux; for comparison the X-ray flux of < 3.0e-13 ergs cm-2 s-1 (ATel #3506) is > 10^5 times fainter than the peak 0.5-10 keV X-ray flux (Kennea et al. 2011). The optical colour has also changed; V - i' is 0.5 mag redder on 2011-07-27 compared to 2011-05-21. A link to the light curves is provided below.
The upper limit on the quiescent magnitude of MAXI J1659-152 is V > 21 mag from its non-detection in the USNO B1 catalogue (Kennea et al. 2011). The source is now almost as faint as this upper limit; we aim to continue monitoring the optical counterpart to test whether the source continues to fade or remains at this magnitude. A wide range of quiescent magnitudes for MAXI J1659-152 have been predicted; from r ~ 22.4 (ATel #2976) to V ~ 28 (Kuulkers et al. 2011, arXiv:1102.2102).
The Faulkes Telescope observations are part of an on-going monitoring campaign of ~ 30 low-mass X-ray binaries (Lewis et al. 2008, arXiv:0712.2751). The Faulkes Telescope South is maintained and operated by Las Cumbres Observatory Global Telescope Network. FL acknowledges support from the Dill Faulkes Educational Trust.
MAXI J1659-152 optical light curves and finding charts