VLA radio detection of the new black hole X-ray binary candidate Swift J1727.8-1613

ATel #16211; J. C.A. Miller-Jones (Curtin), G. R. Sivakoff (Alberta), A. Bahramian (Curtin), T. D. Russell (INAF), on behalf of the JACPOT-XRB collaboration
on 26 Aug 2023; 10:25 UT
Credential Certification: James Miller-Jones (james.miller-jones@curtin.edu.au)

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Following the X-ray (ATel #16205, #16206, #16207, #16210) and optical (GCN #34543, ATel #16208, #16209) detections of the new black hole X-ray binary candidate Swift J1727.8-1613, we triggered follow-up radio observations with the Karl G. Jansky Very Large Array (VLA). The array was in its most-extended A-configuration. We observed on 2023 August 25.98, in two 1024-MHz bands, centered at 5.25 and 7.45 GHz. We used 3C286 as a primary calibrator and J1733−1304 as a secondary calibrator. The on-source time was 220s. We processed the data using standard procedures and the VLA pipeline within the Common Astronomy Software Application v6.4.1 (CASA Team et al. 2022). We made images with Briggs weighting and a robustness parameter of 1. We employed a single round of phase-only self-calibration, with a solution interval of 30s, which led to images with residuals consistent with Gaussian noise.

We detected a radio source at both frequencies, consistent with the position of the optical counterpart. Our best radio position (at 7.45 GHz, and prior to self-calibration) was:
R.A. (J2000): 17:27:43.31 ± 0.04s
Dec. (J2000): −16:12:19.23 ± 0.02”,
where the uncertainties are dominated by systematics, set to 10% of the synthesized beam size.

By fitting a point source in the image plane, we measured flux densities of 17.64±0.03 mJy at 5.25 GHz, and 18.88±0.04 mJy at 7.45 GHz, which translates to a spectral index of α=0.19±0.07 (defined as S_ν ∝ ν^α). Such a slightly inverted spectral index would be consistent with a compact jet from a black hole X-ray binary in its hard X-ray spectral state.

The VLA Sky Survey (Lacy et al. 2020) observed this field at 3 GHz on 2022 February 7th. Quick-look images show no source at the above radio position, to a 3σ upper limit of 0.45 mJy. This confirms the transient nature of the radio source.

We thank the VLA schedulers for facilitating these observations. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

References:
CASA Team, et al. 2022, PASP, 134, 114501
Lacy et al. 2020, PASP, 132, 035001