Swift XRT/UVOT follow-up of the Black Hole Candidate MAXI J1659-152 during a low luminosity state

ATel #3201; Y. J. Yang, R. Wijnands, N. Degenaar, D. M. Russell, D. Altamirano, M. Kalamkar (University of Amsterdam)
on 2 Mar 2011; 15:29 UT
Credential Certification: Nathalie Degenaar (degenaar@uva.nl)

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We report on a Swift follow-up observation of the X-ray transient MAXI J1659-152 (ATel #2873, ATel #2877, GCN #11296). This source is suggested to be a stellar-mass black hole candidate (Kalamkar et al. 2011, arXiv:1012.4330). The source also has a very short orbital period ~2.4 hours (Kuulkers et al. 2011, arXiv:1102.2102). The optical counterpart of MAXI J1659-152 has been identified (GCN #11298, ATel #2884), and the quiescent optical magnitude has been found to be 22.4 in r-band (ATel #2976).

MAXI J1659-152 is currently in a low X-ray luminosity state. The Swift observation was performed on 2011-02-28 15:40:12 UT for a duration of ~1.2 ks, and was observed in Photon Counting mode. Preliminary results show that the average count rate of this source is about > 0.2 c/s. We note that the source is located on a bad column of the CCD, and therefore the count rate might be slightly underestimated. The corrected spectrum is best fitted with an absorbed power-law model which resulted in a column density NH = 3.8e+21 cm^-2, and a power-law photon index 1.8+/-0.4. We obtained an unabsorbed flux (0.3-10 keV) 2.1e-11 erg cm^-2 s^-1, which is more than two orders of magnitude lower than the previous reported flux with RXTE in October 2010 (ATel #2927). Assuming a distance of 7 kpc (Kuulkers et. al, 2011, arXiv:1102.2102), we estimate the X-ray luminosity to be 1.2e+35 ergs s^-1.

In addition to the X-ray, the optical/UV counterpart of MAXI J1659-152 was also detected (at the >3-sigma level) using the Swift UV/Optical telescope. The magnitudes are v = 18.8 +/- 0.4; b = 19.2 +/- 0.2; u = 18.6 +/- 0.2; uvw1 = 19.2 +/- 0.3; uvm2 = 19.9 +/- 0.4. When assuming a flat optical spectrum, the source is still ~ 3 - 4 magnitudes brighter than the proposed quiescent level of r ~ 22.4 (ATel #2976). Clearly the source is still actively accreting and we encourage continued monitoring of this source at all wavelengths.

We thank the Swift team for their prompt arrangement of the observation. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.