Observed Re-brightening of SPT-S J051116-5341.9, a Potential Inverted Flaring AGN
ATel #17678; Thomas Crawford (University of Chicago) on behalf of the SPT-3G Collaboration
on 12 Feb 2026; 16:59 UT
Credential Certification: Thomas Crawford (tmcrawfo@uchicago.edu)
Subjects: Radio, Millimeter, AGN
We report the re-brightening at millimeter wavelengths of the source SPT-S J051116-5341.9 (R.A. 77.819d, decl. -53.700d). The source was originally reported in Vieira et al. (2010) as a candidate high-redshift dusty galaxy, with flux densities of 16.3 +/- 3.7 mJy at 1.4 mm and 5.9 +/- 1.3 mJy at 2.0 mm using observations taken in 2008 with the SPT-SZ camera on the South Pole Telescope. In more recent (2019-2023) observations with the SPT-3G camera, the source was observed to have quiescent flux densities of only ~3 mJy at 1.4, 2.0, and 3.2 mm (with uncertainties of roughly 1, 0.5, and 0.5 mJy, respectively). However, in late December 2025 and early January 2026, the source was seen in SPT-3G data to re-brighten to a state similar (in brightness and spectral behavior) to the original 2008 state, with average flux densities between 01 January 2026 and 19 January 2026 of 14 +/- 4 mJy, 8 +/- 1 mJy, and 6 +/- 1 mJy at 1.4 mm, 2.0 mm, and 3.2 mm. A multi-year light curve of this object from SPT-SZ and SPT-3G observations is viewable here.
While the source was originally identified, from the millimeter-wave spectrum and the lack of counterparts in other catalogs, as a candidate high-redshift dusty galaxy, two pieces of information have since made that classification unlikely: 1) the observed variability at millimeter wavelengths; 2) the identification of a radio counterpart (separation 6 arcseconds, coordinates R.A. 77.818241d, decl. -53.698783d) in the ASKAP RACS survey. The RACS counterpart is detected at ~5 sigma in RACS-low, RACS-mid, and RACS-high, with mean flux densities of roughly 2.6, 2.4, and 1.7 mJy in those three catalogs. There is also a faint optical counterpart (~21 mag, 3 arcsecond separation) detected in DES and VHS. The variability and radio counterpart indicate that this source is likely an AGN, but the highly inverted millimeter spectrum in the flaring state (in both 2008 and 2026) is unusual.
We have obtained ATCA radio data on this source (through ATCA project C3646, PI: Järvelä), though due to technical issues the data may not be usable. We have also obtained an optical spectrum of the source with the Magellan LDSS3 spectrograph at LCO. Analysis of the spectrum is ongoing. The SPT was off the sky for upgrades from approximately 20 January 2026 to 05 February 2026, and the data available in the US since resuming observations is not yet deep enough to confirm the source is still in the flaring state.
