Radio quenching and subsequent flaring in Swift J1727.8-1613

ATel #16271; J. C.A. Miller-Jones (ICRAR-Curtin), A. Bahramian (ICRAR-Curtin), D. Altamirano (Southampton), J. Homan (Eureka Scientific), T. D. Russell (INAF), G. R. Sivakoff (Alberta)
on 6 Oct 2023; 17:18 UT
Credential Certification: James Miller-Jones (james.miller-jones@curtin.edu.au)

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The black hole candidate X-ray binary Swift J1727.8-1613 has spent the past several weeks in intermediate X-ray spectral states (ATel #16247), with a bright, compact radio jet (ATel #16211), whose flux density reached levels of 100-120 mJy (ATel #16228, #16231). If the source were to follow the standard evolution for a black hole X-ray binary, we would expect it to eventually make a transition to the soft state, leading to the quenching of the compact jets and the launching of relativistically-moving transient ejecta.

Radio monitoring with the Karl G. Jansky Very Large Array (VLA) showed the compact jet flux density to have quenched to a level of 7.2 ± 0.1 mJy at 5.25 GHz on 2023 October 5 (00:26-00:28 UT). The spectral index (defined as Sν∝ν^α) measured between 5.25 and 7.45 GHz was α= − 0.6 ± 0.1. The array was in the process of being reconfigured from its most extended A configuration to its least extended D configuration. The data were processed using the Common Astronomy Software Application v6.4.1. We used 3C286 as a flux density and bandpass calibrator, and J1733-1304 as a phase calibrator.

The following day, further VLA observations from 01:21-01:30 UT on 2023 October 6 showed the flux density to have risen dramatically, reaching 235.8 ± 1.1 mJy at 5.25 GHz, with a spectral index of −0.27 ± 0.01 between 5.25 and 11.0 GHz. This dramatic radio flare and flattening of the spectral index strongly suggests the ejection of transient, relativistic jets, and could potentially signify that a transition to the soft X-ray spectral state has taken place.

We strongly encourage observations at other bands, particularly at X-ray energies, to confirm whether a transition to the soft state has indeed occurred.

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.