VLA detection of a likely radio counterpart of IGR J17494-3030

ATel #14129; J. van den Eijnden (Univ. of Oxford), N. Degenaar, R. Wijnands, T. D. Russell, M. Stoop (Univ. of Amsterdam), M. Armas Padilla (Instituto de Astrofisica de Canarias), D. M. Russell (NYU Abu Dhabi), D. Maitra (Wheaton College), C. O. Heinke, G. R. Sivakoff (Univ. of Alberta), A. W. Shaw (Univ. of Nevada, Reno), T. J. Maccarone (Texas Tech Univ.), J. C. A. Miller-Jones, A. Bahramian (ICRAR-Curtin)
on 30 Oct 2020; 15:54 UT
Credential Certification: Jakob Van den Eijnden (a.j.vandeneijnden@uva.nl)

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IGR J17494-3030 is a transient X-ray binary that was recently reported to be in outburst by INTEGRAL (ATel #14119). Follow-up NICER observations have subsequently shown the presence of X-ray pulsations at 376 Hz (ATel #14124), confirming the neutron star nature of the compact object and identifying IGR J17494-3030 as an accreting millisecond X-ray pulsar (AMXP).

The VLA observed IGR J17494-3030 in the BnA configuration on 27 October 2020 from 21:40 to 22:40 UT. As primary and secondary calibrators, we used 3C 286 (1331+3030) and the nearby source 1751-253, respectively. We analysed the observation using the Common Astronomy Software Applications package (CASA) version 5.4.1 (McMullin et al. 2007, ASPC, 376, 127), imaging Stokes I using a Briggs weighting scheme and a robust parameter of 0. We observed with two sub-bands, centred at 4.5 and 7.5 GHz with 1 GHz of bandwidth each. While analysis at 7.5 GHz is still ongoing, we report our results at 4.5 GHz to ensure rapid distribution to the community.

At 4.5 GHz, a radio source is significantly detected at a best fit position of
RA (J2000) = 17h 49m 23.61s +/- 0.01s
Dec (J2000) = -30d 29' 59.1" +/- 0.1"

where the positional accuracy is limited by standard VLA systematics at 10% of the synthesized beam. This radio position is separated 2.2" from the Swift/XRT position of IGR J17494-3030 reported in ATel #3989, which has a 2.2" 90%-confidence region (we note that the position of the possible quiescent Chandra counterpart reported in ATel #3986 is inconsistent with the Swift/XRT, and our VLA, positions, as it is separated by 2').

We used the CASA task imfit to measure the source flux by fitting a 2D elliptical Gaussian in the image plane, using the FWHM and angle of the synthesized beam (1.1" x 1.0", 66.12 degrees E of N). We measured a flux density of 115 +/- 17 μJy at 4.5 GHz, which corresponds to a radio luminosity of L_R = 4E+28 (D/8 kpc)² erg s^-1. NICER X-ray observations on the same day measure an average unabsorbed flux between 0.5-10 keV of 1.68E-10 erg cm^-2 s^-1 (ATel #14124) , corresponding to a luminosity of L_X = 1.3E36 (D/8 kpc)² erg s^-1. This combination of X-ray and radio luminosity is consistent with the range seen in other jet-launching AMXPs (e.g. Bahramian et al. 2018; https://github.com/bersavosh/XRB-LrLx_pub).

Based on the positional consistency and expected X-ray and radio luminosity, we deem it likely that we have detected the radio counterpart of IGR J17494-3030. With continued radio monitoring and analysis of the VLA data at 7.5 GHz we will reassess this conclusion.

We thank the VLA operators for allowing and then rapidly performing these observations. Further X-ray and radio monitoring is ongoing, and we encourage multi-wavelength follow-up observations.