SPT-SV J222602.8-540533: South Pole Telescope Reports Detection of a Flaring Millimeter-Band Source

ATel #17781; Tarraneh Eftekhari (Northwestern U.) on behalf of the SPT-3G Collaboration
on 8 May 2026; 19:04 UT
Credential Certification: Tarraneh Eftekhari (teftekhari@northwestern.edu)

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The South Pole Telescope Collaboration reports: The 3rd-generation receiver on the South Pole Telescope (SPT-3G) detects a flaring millimeter point source at RA = 336.51170d, DEC = -54.09255 (estimated positional uncertainty of ~8 arcseconds). Detectable millimeter emission began 2026 April 13 UT rising to a peak flux density of 25.7 +/- 7.1 mJy (150 GHz) and 15.0 +/- 7.2 mJy (95 GHz) on 2026 April 22 UT. The average quiescent flux density at this position in 2025 measured via SPT-3G is 3.1 +/- 1.4 mJy (150 GHz) and 0.8 +/- 1.4 mJy (95 GHz), indicating a factor of >8 increase. At the time of this alert, the flux density remains above the quiescent level, reaching a maximum test-statistic of 88.2 on 2026 April 27 UT, corresponding to a 9.1-sigma significance (Guns et al. 2021, ApJ, 916, 98). We note that the inverted spectrum indicates the source has likely not yet peaked in the centimeter band. The 150 GHz light curve also shows some evidence of flaring in early 2020. All flux calibrations quoted here are preliminary and are believed to be accurate to 20%. SPT-3G will continue to monitor this source at hour- to day-long intervals.

The millimeter source is associated with the galaxy LEDA 422944 at a redshift z = 0.0571. The galaxy is detected in eROSITA at a flux of (3.3+-0.6)e-13 erg/sec (Merloni et al. 2024, A&A, 682, 34), yielding an X-ray luminosity of 2.6e42 erg/sec, near the border between low-luminosity AGN and standard Seyfert galaxies, but above the level for all but the most extreme star-forming galaxies. Inspection of the archival optical spectrum from 2dF (Colless et al. 2001, MNRAS, 328, 1039) indicates a likely LINER nature.

The South Pole Telescope is a 10-meter telescope designed for observations of the cosmic microwave background located at Amundsen-Scott South Pole Station and supported by the National Science Foundation and the US Dept. of Energy. The SPT online transient program providing data in this telegram is supported by NSF grants OPP-1852617 and OPP-2332483, and observes 1500 square degrees of the southern sky at 95, 150, and 220 GHz with an average revisit cadence of 12 hours. For more details on the SPT transient program and survey strategy, please see Guns et al. 2021, ApJ, 916, 98.