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
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Subjects: X-ray, Transient
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.