ALMA and VLT HAWK-I observations of BW Cir in Outburst
ATel #17611; A. J. Tetarenko (ULethbridge), F. M. Vincentelli (UCoventry), M. Del Santo (INAF-IASF Palermo), P. Casella (INAF-OAR), R. Ma (USouthampton), F. Carotenuto (INAF-OAR), S. Corbel (CEA Saclay), N. Grollimund (CEA Saclay), E. Lescure (ULethbridge), T. D. Russell (INAF-IASF Palermo), and C. Salvaggio (INAF-OAB)
on 19 Jan 2026; 16:17 UT
Credential Certification: Alexandra Tetarenko (alexandra.tetarenko@uleth.ca)
Subjects: Radio, Millimeter, Sub-Millimeter, Infra-Red, X-ray, Black Hole, Transient
The black hole X-ray binary BW Cir (GS 1354-64) entered a new outburst in late December 2025 (ATel #17563), and has since been detected in the radio (ATel #17582) and optical (ATel #17586). As part of a larger multi-wavelength spectral-timing campaign, we carried out simultaneous millimetre and near-infrared observations with ALMA (central frequency 233 GHz) and VLT HAWK-I (Ks band, 2.2 um) on 18 January 2026 (MJD 61058). ALMA observed between 06:30 - 09:00 UTC (approximately 94 min on-source) and VLT observed between 07:44 - 08:32 UTC (approximately 50 min on source). For ALMA, we used J1107-4449 as a bandpass and flux calibrator, while J1337-6509 was used as a phase calibrator. The HAWK-I near-infrared observations were taken with 0.125s time resolution, and the target count time series was normalized using the nearby reference star 2MASS 13581525-6444291 (Ks magnitude of 10.39).
BW Cir was clearly detected with both ALMA and VLT HAWK-I. The ALMA QA0 quick look pipeline showed an average flux density of 7.1+/-0.1 mJy at 233 GHz. The VLT HAWK-I observations showed evidence of rapid flaring activity, with an rms amplitude of 20%. The average magnitude was found to be 13.2, marking the highest infrared flux reported for the source so far (e.g., IAU Circ., No. 6781). Detailed analysis of these observations is ongoing.
Quasi-simultaneous Swift/XRT observations were also executed between 14:34 - 14:47 UTC on 18 January 2026. Fitting the XRT spectrum with a simple absorbed power-law model yields a photon index of Gamma = 1.44+/-0.03 and a column density of N_H = 0.74 (+/-0.02) e+22 cm^-2. The resulting unabsorbed flux is 6e-9 erg/cm^2/s in the 0.5-10 keV energy range. This is consistent with a typical rising hard state observed during a black hole X-ray binary outburst.
Further ALMA + VLT observations are planned, and we encourage those with access to additional multi-wavelength observing time to please contact us if they wish to coordinate. We thank the observing teams at ALMA and VLT for their efforts in obtaining these data, and we thank the Swift/XRT team for scheduling the X-ray observations.
