ALMA + VLA observations of MAXI J1750-327

ATel #18007; A. J. Tetarenko, E. Lescure (ULethbridge), A. Bahramian, J. C.A. Miller-Jones (Curtin), P. Casella (INAF-OAR), N. Castro Segura (UWarwick), P. Gandhi (USouthampton), T. J. Maccarrone (Texas Tech), A. Marino (ICE-CSIC), T. D. Russell (INAF-IASF Palermo), J. van den Eijnden (UAmsterdam), and F. M. Vincentelli (UCoventry)
on 25 Aug 2026; 10:23 UT
Credential Certification: Alexandra Tetarenko (alexandra.tetarenko@uleth.ca)

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www.astronomerstelegram.org/?read=17963>17963, #17964). Since then, it has been extensively characterized through X-ray (ATels #17969, #17974, #17983, #17993, #18004), optical/near-infrared (ATels #17971, #17977, #17988, #17990), and radio (ATel #17979) observations.

As part of a larger multi-wavelength spectral-timing campaign, we carried out simultaneous radio and millimetre observations with the VLA in sub-array mode (central frequencies of 6 GHz, 10 GHz, and 13 GHz) and the ALMA (central frequency 233 GHz) on 2026 August 24 (MJD 61276). Both facilities observed between 01:00 - 05:00 UTC. For ALMA, we used J1924-2914 as a bandpass and flux calibrator, while J1744-3116 was used as a phase calibrator. For VLA, we used 3C286 as a flux calibrator, with J1751-253 and J1744-3116 as phase calibrators.

MAXI J1750-327 was clearly detected with both the VLA and ALMA. Preliminary analysis of the VLA data gave an average flux density of 84.6+/-0.7 mJy at 10 GHz. The ALMA QA0 quick look pipeline shows an average flux density of 12.5+/-0.1 mJy at 233 GHz. These flux densities suggest a radio-millimetre spectral index of ~ -0.6, consistent with optically thin synchrotron emission, which may suggest the compact jet is in the process of quenching and discrete jet ejecta are being launched. Detailed analysis of these observations is ongoing.

MAXI data indicate that in the X-rays the source may have softened slightly over the past few days. However, the overall trend and hardness are still consistent with a typical rising hard state observed during a black hole X-ray binary outburst (see the current Hardness Intensity Diagram generated from MAXI data, with a comparison to other recent black hole X-ray binary outbursts, linked below).

Further VLA + ALMA observations are planned, and we encourage those with access to additional multi-wavelength observing time to please contact us for coordination. We thank the observing teams at the VLA and ALMA for their efforts in obtaining these data.

Hardness Intensity Diagram for MAXI J1750-327