MeerKAT monitoring reveals GX 339-4 to be in a radio quiescent state
ATel #18015; Miriam M. Nyamai (JIVE), Rob P. Fender, (Oxford/Cape Town), Sara Motta (INAF), Noa Grollimund (Univ. Paris Cite/CEA) for the X-KAT collaboration
on 27 Aug 2026; 17:43 UT
Credential Certification: Miriam Mumbua Nyamai (nyamai@jive.eu)
Subjects: Radio, Binary, Black Hole
As part of the X-KAT Large Survey programme with the MeerKAT telescope, the recurrent black hole X-ray binary GX 339-4 is observed weekly, usually at 1.284 GHz (L-band). The exposure time on target for each epoch is ten minutes. The latest radio data show that GX 339-4 has dropped below detection with MeerKAT since June 2026.
The latest hard state outburst activity in August 2025 and subsequent slow decay to low/hard state months later was reported in ATels #17327, #17330, #17484. Between the observation taken on 2026-06-01 18:39:23.3 UTC (61192.78) and the most recent observation taken on 2026-08-22 15:26:08.9 UTC (MJD 61274.64), the source has remained undetected with a 3 &sigma flux density upper limit of approximately 70 µJy in the latest epoch, corresponding to three times the rms of the image.
Prior to the drop below the detection limit, GX 339-4 was marginally detected on 2026-05-24 21:30:47.1 UTC (61184.9) with flux density of 103 +/- 43 µJy. The rms of the image is ~28 µJy, providing a 2.4 &sigma detection. This is consistent with quiescence flux density values presented by Tremou et al. (2020).
X-KAT is a large MeerKAT open-time programme to observe X-ray binaries in the radio band, performing weekly monitoring of bright, active systems, with capacity for higher cadence observations, and in coordination with large X-ray and optical monitoring programmes. For further information on this programme contact Rob Fender.
