Support ATel At Patreon

[ Previous | Next | ADS ]

Radio emission detected from Aql X-1 following the recent state transition

ATel #5148; James C. A. Miller-Jones (ICRAR Curtin) and Gregory R. Sivakoff (U. Alberta)
on 20 Jun 2013; 02:21 UT
Credential Certification: James Miller-Jones (james.miller-jones@curtin.edu.au)

Subjects: Radio, Binary, Neutron Star, Transient

Referred to by ATel #: 5158

Ongoing optical and X-ray observations have detected a new outburst of the neutron star X-ray binary Aql X-1 (ATel #5114, ATel #5117, ATel #5129), with the hard-to-soft X-ray spectral state transition beginning on MJD 56457 (ATel #5136). Since the state transition has previously been associated with transient radio emission, we made a 30-minute VLA observation on 2013 June 19th from 04:20-04:50 UT (MJD 56462.196 ± 0.004).

We observed in two 1024-MHz basebands centred at 5.25 and 7.45 GHz, with the array in the relatively compact C configuration. Aql X-1 was significantly detected in both frequency bands, with measured flux densities of 263 ± 15 and 199 ± 10 microJy/beam at 5.25 and 7.45 GHz respectively. The quoted uncertainties are the statistical errors from point source fits in the image plane, and do not take into account systematic uncertainties in the flux density scale, which are believed to be of order 5-10%.

The derived spectral index α (defined as Sν ∝ να) is -0.80 ± 0.35, which is the steepest radio spectrum observed to date from Aql X-1 (see Miller-Jones et al. 2010, ApJ, 716, L109). Although still consistent with a flat spectrum at the 3σ level, an intrinsically steep spectrum could suggest the presence of discrete ejecta, as recently detected from the neutron star X-ray binary Cygnus X-2 (Spencer et al., arXiv:1306.0599). High-resolution VLBI follow up may resolve such ejecta, and is strongly encouraged over the next few days, before the radio emission fades below the detection threshold.

We thank the NRAO staff for rapid evaluation and execution of this Director's Discretionary Time request.