ATCA radio observation of MAXI J1836-194

ATel #4038; M. Coriat (Univ. Southampton), Tasso Tzioumis (ATNF), R. Fender (Univ. Southampton), S. Corbel (Univ. Paris Diderot & CEA Saclay), C. Brocksopp (MSSL)
on 12 Apr 2012; 23:25 UT
Credential Certification: Mickael Coriat (mickael@coriat.eu)

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Following the renewed activity of the black hole X-ray transient MAXI J1836-194 (ATel #3966, #3975), we conducted a radio observation on 2012 April 07 (21:12-00:00 UT; MJD 56024.94) with the Australia Telescope Compact Array in the H168 configuration. We detect a faint radio source at the following location:

RA(J2000) = 18h 35m (43.4 +/- 0.1)s

Dec(J2000) = -19d 19m (11.0 +/- 0.3)s

consistent with previous radio detection reported in Atel #3628. The source is consistent with being unresolved to the beamsize of 5.7 x 0.6 arcseconds (at 9 GHz), oriented 18 degrees E of N. The preliminary flux densities are 0.7 +/- 0.04 mJy at 5.5 GHz and 0.65 +/- 0.05 mJy at 9 GHz. The spectral index is -0.1 +/- 0.2 (errors on flux densities and spectral index are given at a 1 sigma level).

To compare the radio/X-ray luminosity ratio with other black hole and neutron star low-mass X-ray binaries (LMXBs), we analysed a RXTE observation of MAXI J1836-194 conducted simultaneously (< 24 hours) with the EVLA observation reported by Miller-Jones et al. 2011 (ATel #3628). We estimated a 1-10 keV unabsorbed flux of 1.9e-9 ergs/cm2/s using N_H = 2.8 x 10^21 cm-2 (Ferrigno et al. 2012, A&A, 537, L7).

The radio spectrum reported in ATel #3628 was consistent with optically thick emission. However, the EVLA observation was conducted during a hard to soft state transition. It is therefore unclear whether the emission originates from self-absorbed compact jets or represents the optically thick initial phase of a transient ejection. Assuming a compact jet origin and a distance > 1 kpc, the radio/X-ray luminosity ratio we obtain, locates the source within or above the scatter of the upper track of the radio/X-ray correlation of stellar-mass accreting black holes (see e.g. Corbel et al. 2003, A&A, 400, 1007; Gallo et al., 2003, MNRAS, 344, 60; Soleri & Fender 2011, MNRAS, 413, 2269; Coriat et al. 2011, MNRAS, 414, 677; Gallo et al. 2012, MNRAS in press.).

We thank Phillip Edwards and the ATCA staff for rapidly scheduling the observation.