Radio detection of MAXI J0556-332 with the ATCA

ATel #3119; M. Coriat (Univ. Southampton), Tasso Tzioumis (ATNF), S. Corbel (Univ. Paris Diderot & CEA Saclay), R. Fender (Univ. Southampton), C. Brocksopp (MSSL), J. Broderick (Univ. Southampton), P. Casella (Univ. Southampton), T. Maccarone (Univ. Southampton)
on 25 Jan 2011; 12:01 UT
Credential Certification: Piergiorgio Casella (casella@science.uva.nl)

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Following the discovery of the new X-Ray transient MAXI J0556-332 (ATel #3102, #3103, #3104, #3106, #3110, #3112, #3116), we conducted a radio observation on 19 January 2011 (MJD 55580.5) with the Australia Telescope Compact Array in the 6A configuration. We detect a faint radio source at the following location: RA(J2000) = 05h 56m (46.3 +/- 0.2)s Dec(J2000) = -33d 10m (26.3 +/- 0.5)s fully consistent with the optical counterpart reported on Atel #3104. The source is consistent with being unresolved to the beamsize of 3.6 x 0.8 arcseconds (at 9GHz), oriented 10 degrees W of N. The preliminary flux densities are 0.14 +/- 0.04 mJy at 5.5 GHz and 0.09 +/- 0.06 mJy at 9 GHz, giving a spectral index of -0.9 +/- 1.5 (errors on flux densities and spectral index are given at a 3 sigma level). The uncertainties on the spectral index do not allow us to constrain the nature (optically thick or thin) of the radio emission. 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 J0556-332 conducted simultaneously (MJD 55580.4) to our radio observation. We estimated a 2-10 keV unabsorbed flux of 1.65e-9 ergs/cm2/s using N_H = 9.8+/-0.6 x 10^20 cm-2 (ATel #3103). For a radio emission originating from self absorbed compact jets and a distance  < 20 kpc, the radio/X-ray luminosity ratio we obtain is typical of a neutron star LMXB (see e.g. Migliari & Fender 2006) and is not compatible with any known black hole source displaying compact jets. This is thus in agreement with the low optical/X-ray ratio of the source (Atel#3116). If the radio emission is optically thin, the radio/X-ray ratio is still consistent with a neutron star system but is also perfectly consistent with a black hole in transition from the hard to the soft state if the source distance is >~ 8 kpc (based on the typical X-ray luminosity of such transition). Given that MAXI J0556-332 is ~25 degrees out of the Galactic plane, such a distance could imply that it is located in the Galactic halo.