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Swift XRT and UVOT observations of renewed activity from MAXI J1836-194

ATel #3975; Y. J. Yang, R. Wijnands (University of Amsterdam) and J. A. Kennea (PSU)
on 18 Mar 2012; 23:06 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Transient

Referred to by ATel #: 4038, 4255

We report a pointed Swift observation of the black hole X-ray transient MAXI J1836-194 (ATel #3611, #3613). The source is suggested to be a black hole candidate based on its timing and spectral properties, and the radio detection during the rise of the previous outburst (ATels #3618, #3628). The source was first detected by MAXI/GSC on August 30, 2011 (ATel #3611), and it was on for about 3 months. Swift could not observe the source due to Sun constraints from mid-November, 2011 to mid-February, 2012. When the source became visible again, it has entered to its quiescence. On March 14, Krimm et al. (ATel #3966) reported an increase in the source flux as observed with the Swift/BAT. Our proposed Swift follow-up observation taken on March 18 shows that the source is actively accreting.

Preliminary results show that the average count rate of the source is around 7.6 +/- 0.3 c/s. The spectrum is best fitted with an absorbed power-law model with a column density NH = 2.5(+/-0.7)e+21 cm^-2, and a power-law photon index 1.63+/-0.17. This is consistent with a black hole in the low/hard state at beginning of an outburst. We obtained an unabsorbed flux (0.3-10 keV) of 4.7(+/- 0.4)e-10 erg cm^-2 s^-1.

In addition to the X-ray, the optical/UV counterparts were also detected (at the >5-sigma level) using the Swift UV/Optical telescope. The magnitudes are v = 16.05 +/- 0.07; b = 16.83 +/- 0.06; u = 16.38 +/- 0.06; uvw1 = 17.11 +/- 0.08; uvm2 = 17.78 +/- 0.12; uvw2 = 17.66 +/- 0.08. More Swift observations has been requested. Follow-up observations in other wavelengths are strongly encouraged.

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.