VLA detection of the radio counterpart of the BeXRB 1A 0535+262

ATel #14193; J. van den Eijnden (Univ. of Oxford), N. Degenaar, R. Wijnands (Univ. of Amsterdam), T. D. Russell (INAF/IASF Palermo), G. R. Sivakoff (Univ. of Alberta), J. C. A. Miller-Jones (ICRAR-Curtin), A. Rouco Escorial (Northwestern), J. Hernandez Santisteban (Univ. of St Andrews), M. Reynolds (Univ. of Michigan)
on 18 Nov 2020; 09:54 UT
Credential Certification: Jakob Van den Eijnden (a.j.vandeneijnden@uva.nl)

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After the announcement of a new outburst of the transient Be/X-ray binary and X-ray pulsar 1A 0535+262 (Atel #14157), and its continuously increasing X-ray flux since then (Atel #14170, Atel #14171, Atel #14173, Atel #14179), we have observed this source with the Karl G. Jansky Very Large Array (VLA) to search for a radio counterpart. Radio observations during its 2010 and 2011 outbursts previously returned non-detections (Atel #2798, Atel #3198).

We observed 1A 0535+262 with the VLA on 10 and 15 November 2020, with the array in the BnA and the BnA->A configurations, respectively. In both observations we observed at C band, centred at 6 GHz and a bandwidth of 4 GHz. The primary calibrators were 3C 48 and 3C 286, respectively, while we used J0547+2721 as the secondary calibrator in both observations. We analysed the observations using the Common Astronomy Software Applications package (CASA) version 5.4.1 (McMullin et al. 2007, ASPC, 376, 127), imaging the observations using a Briggs weighting scheme and a robust parameter of 1. We then used the imfit tool to measure the source flux density by fitting, in the image plane, a 2D elliptical Gaussian with FWHM and angle equal to the synthesized beam.

In the first observation, 1A 0535+262 was marginally detected at a flux density of 13 +/- 4 μJy. In the second observation, the radio counterpart had brightened to 39 +/- 4 μJy. Swift/BAT monitoring showed that, between these days, the source evolved in X-rays (15-50 keV) from 0.34 to 1.67 cts cm^-2 s^-1, or approximately 1.5 to 7.5 Crab. Assuming the Gaia distance of 2.13 kpc to the source (Bailer-Jones et al. 2018, AJ, 156, 58; Treuz et al. 2018, arxiv:1806.11397), the radio flux densities correspond to luminosities of 4.2E26 erg s^-1 and 1.3E27 erg s^-1, respectively. The measured flux densities are below the deepest previous radio upper limit of 160 μJy, when the source's Swift/BAT flux was 3 Crab, during the 2011 outburst (Atel #3198).

We measure a best-fit radio position in the second observation of
RA (J2000) = 05h 38m 54.571s +/- 0.008s
Dec (J2000) = 26d 18' 56.79" +/- 0.09"

where the uncertainties correspond to 10% of the beam size due to standard VLA systematics uncertainties in astrometry. This position is consistent within its uncertainties with the Gaia position of 1A 0535+262, with an angular separation of 0.07". The position of the radio source in the first observation is also consistent with the Gaia source.

The coupled increase in X-ray and radio flux of 1A 0535+262 show that the radio emission is directly related to the current accretion state, similar to the behaviour seen in the transient BeXRB Swift J0243.6+6124 (Van den Eijnden et al. 2018, Nature, 562, 233). With continued monitoring in X-ray and radio during the outburst rise, peak, and decay, we will further test and study this connection.

We thank the VLA operators for rapidly performing these observations, especially in the light of current operational challenges. We strongly encourage multi-wavelength follow-up observations.