EVLA radio detections of MAXI J1836-194 suggest it is a black hole X-ray binary

ATel #3628; J. C. A. Miller-Jones (ICRAR Curtin), G. R. Sivakoff (U. Alberta), M. Rupen (NRAO), , D. Altamirano (Amsterdam) and the JACPOT XRB collaboration
on 3 Sep 2011; 23:03 UT
Credential Certification: Gregory R Sivakoff (grs8g@virginia.edu)

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The newly detected X-ray transient MAXI J1836-194 (ATel #3611) has been identified as a potential Be/X-ray binary (Atel #3614) or black hole X-ray binary (ATel # 3618, #3619, #3626).

Following its identification as a black hole candidate, we observed MAXI J1836-194 with the EVLA on 2011 Sep 03 (02:52-03:23 UT; MJD 55807.13). We simultaneously observed at central frequencies of 4.6 and 7.9 GHz, with an observing bandwidth of 128 MHz at each frequency.

We detected a radio source at a position consistent with the Swift X-ray and UVOT localizations (ATel #3613) to within their 90% confidence intervals. Our best radio position (measured at 4.6 GHz) is:

RA (J2000) = 18:35:43.44587 ± 0.00009
Dec (J2000) = -19:19:10.462 ± 0.002.

The error bars here are purely statistical. Preliminary astrometric tests with the EVLA suggest that the true error could be some tens of milliarcseconds. A similar position was measured at 7.9 GHz.

The measured flux densities were 26.9 ± 0.1 mJy at 4.6 GHz and 39.6 ± 0.1 mJy at 7.9 GHz (statistical errors only). The spectral index, α (where S_ν ∝ nu^α), is 0.72 ± 0.01, consistent with optically thick emission.

X-ray spectral analyses (ATel #3626) show that the source likely made a transition from the hard intermediate state to the soft intermediate around 2011 Aug 31 (MJD 55804.5). RXTE data taken on on 2011 Sep 02 (11:25-13:20 UT; MJD 55806.52) are well modeled by a disk blackbody (kT ~ 1.2 keV)+ power-law (photon index Γ ~ 1.7) assuming the absorption column density of 2 × 10²¹ cm^-2 reported in Atel #3613. The total unabsorbed flux (2-10 keV) is ~ 1.1 × 10^-9 erg cm^-2 s^-1, with ~ 70% of the flux arising from the power-law component. This is consistent with the source still being in transition from the hard to soft state a day prior to the radio observation.

The X-ray and radio data taken together strongly suggest that MAXI is a black hole X-ray binary that has just ejected a jet when transitioning from the hard to the soft state. The strength of the radio emission indicates that MAXI is unlikely to be a neutron star X-ray binary.

The JACPOT XRB collaboration will continue monitoring this outburst with VLBA & EVLA observations, and will be triggering RXTE & Faulkes Telescope monitoring programs. The first VLBA observations are scheduled for 2011 Sep 4 01:00-05:00 UT.
Further multi-wavelength observations of this source are highly encouraged.

We thank the EVLA & VLBA staff for rapidly scheduling these observations.

JACPOT XRB Collaboration