RXTE Observations of MAXI J1836-194

ATel #3618; T. E. Strohmayer (NASA/GSFC), E. A. Smith (Wyle IS & NASA/GSFC)
on 31 Aug 2011; 17:28 UT
Credential Certification: Tod Strohmayer (tod.strohmayer@nasa.gov)

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RXTE observed the recently discovered X-ray transient MAXI J1836-194 (Atel #3611, #3613, #3614) beginning at 11:08:01 (UT) on August 31, 2011, for a total of 6 ksec of good exposure over two RXTE orbits.

We detect the source at a count rate of approximately 140 cts/sec (1PCU), and variability is evident with the eye. A power spectral study shows significant flat-topped, band-limited noise breaking to a power-law below 5 Hz with evidence for a weak QPO at 0.5 Hz, above the break. The total fractional rms variability in the range from 0.05 to 5 Hz is 18%. There is no strong evidence for coherent pulsations. These timing characteristics are typical of black holes beginning a transition from the low hard state to a hard intermediate state, and suggests that MAXI J1836-194 may be an accreting black hole binary.

The energy spectral continuum measured with the PCA is reasonably well fit by a power-law with an index of 1.84, again consistent with an accreting black hole interpretation. Fits with the power-law continuum only result in significant residuals in the Fe band, suggestive of an Fe k-alpha line and absorption edge. Inclusion of a gaussian line and smeared edge improves the fit to a statistically acceptable level. The fitted line energy of 6.3 +- 0.2 keV and edge energy of 7.2 +- 0.2 keV appear consistent with this interpretation. The RXTE/PCA spectrum does not significantly constrain the absorption column to the source. The derived flux in the 3 - 20 keV band is 9.8 e-10 erg/cm^2/s.

These results suggest that MAXI J1836-194 is likely an accreting black hole binary and may be evolving into a hard intermediate state. Further RXTE observations are being planned. Higher resolution X-ray spectroscopy would be valuable, particularly given the evidence for emission and absorption features in the Fe band.