A possible radio counterpart to the neutron star X-ray binary 4U1608-52

ATel #5113; James C. A. Miller-Jones (ICRAR Curtin) and Simone Migliari (U. Barcelona)
on 7 Jun 2013; 09:36 UT
Credential Certification: James Miller-Jones (james.miller-jones@curtin.edu.au)

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Following the notification that the neutron star X-ray binary 4U1608-52 had left the hard state (ATel #5094), we made target of opportunity observations of the source with the Australia Telescope Compact Array (ATCA). We observed on 2013 June 2nd from 09:30-13:00 UT (MJD 56445.47), with the array in an extended 6km configuration. Using the Compact Array Broadband Backend (CABB), we made simultaneous observations at 5.5 and 9.0 GHz, with 2048 MHz of bandwidth at each frequency band.

We detected a 4 σ source at both frequencies, at a position marginally consistent with the known optical co-ordinates of the system. The measured flux densities were 53 ± 13 microJy/beam at 5.5 GHz and 64 ± 15 microJy/beam at 9.0 GHz, implying a spectral index of 0.4 ± 0.7.

Our best source position, from a 5.5 σ detection made when combining both frequency bands via multi-frequency synthesis, is
RA (J2000) = 16:12:43.059 ± 0.015 s (0.218 arcsec)
Dec (J2000) = -52:25:23.203 ± 0.501 arcsec

While this radio source is offset by 0.58 arcseconds from the nominal optical position of the X-ray binary (Grindlay & Liller 1978; J2000 RA 16:12:43, Dec -52:25:23), our uncertainties overlap the optical error circle of radius 0.5 arcseconds. Definitive confirmation of the association would require either a recent, accurate optical position, or detection of radio variability.

At the time of the radio observations, MAXI X-ray monitoring placed the source in the soft (banana) state with a 2-10 keV flux of 1.1 x 10^-8 erg cm^-2 s^-1 and a spectrum that could be fit well (reduced χ² of 0.99 for 17 d.o.f.) with a simple absorbed diskbb component with temperature kT = 2.0 ± 0.5. Should this source be the radio counterpart to 4U1608-52, it would be similar to 4U1820-30 and Ser X-1 in showing radio emission during a soft state (Migliari et al. 2004). The slightly inverted radio spectral index, similar to that seen during outbursts of MXB 1730-335 (Moore et al. 2000) and Aql X-1 (Miller-Jones et al. 2010), is inconsistent with transient, optically thin emission ejected at a state transition, and more likely to arise from a compact jet.

Further multi-wavelength monitoring of this outburst is encouraged.

We thank ATNF staff for the rapid response to our target-of-opportunity observing request.