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MAXI J1836-194 has returned to quiescence

ATel #4255; Y. J. Yang (UvA), P. Curran (CEA-Saclay), J. A. Kennea (PSU), A. K.H. Kong (NTHU)
on 12 Jul 2012; 18:05 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)

Subjects: X-ray, Black Hole, Transient

We report our most recent Swift observations of the black hole candidate MAXI J1836-194. After the observed renewed activity in March 2012 (ATels #3966, #3975), the source steadily decreased in luminosity with some minor fluctuations.

Our latest two observations taken on July 5 and July 10 (~1.7 ks in total) yield a non-detection in both X-ray and optical bands, suggesting that the source has returned to quiescence. Given the non-detection, we estimated an upper limit using small number statistics described in Gehrels (1986; ApJ, 303, 336). Assuming a column density of 2.3e21 cm-2 (Dickey & Lockman) and a power-law model with a photon index of 2, PIMMs predicts a flux of 2e-13 ergs cm-2 s-1 (in 0.5-10 keV), yielding an upper limit (3 sigma) of luminosity ~1.5e33 erg/s (assuming a distance of 8 kpc).

The source was not detected in all UV/optical bands during all two observations. The obtained upper limits from the last observation are: b >20.1; m2 >20.3; u >19.8; v >19.0; w1 >20.0 and w2 >20.7.

Further optical and NIR observations are encouraged to identify the quiescent counterpart of MAXI J1836-194 (i.e. for search of the periodic modulation and quiescent magnitude of the system).

We thank the Swift team for their scheduling of these observations.