Swift follow-up observations of the flaring millimeter-band source SPT-SJ010023-4357.1
ATel #14914; Marcos Santander (UAlabama), Greg Sivakoff (UAlberta), Nathan Whitehorn (MSU), RileyAnne Sharpe (UAlabama)
on 13 Sep 2021; 22:26 UT
Credential Certification: Marcos Santander (jmsantander@ua.edu)
Subjects: Millimeter, Ultra-Violet, X-ray, AGN, Transient
The Neil Gehrels Swift Observatory performed follow-up observations of the flaring mm-band point source reported by SPT-3G in ATel #14875 on Aug 26, 2021. Swift observations were scheduled between Sept 2 and Sept 9, accumulating a total exposure of 1.2 ks over three epochs.
The observations collected with the Swift X-ray Telescope (XRT) in photon-counting mode were analyzed to characterize the emission state of the candidate counterpart to the mm-band transient. No new X-ray source was detected in the error region of the mm source, or emission from the candidate counterpart suggested in ATel #14875, the X-ray source XMMSL2 J010025.1-435716, also catalogued as WISEA J010025.17-435713.5. We therefore place a 3-sigma upper limit on the X-ray count rate for this previously detected X-ray source of < 0.017 cts/s in the 0.3-10 keV band, which corresponds to a flux of 7.1 x 10^-13 erg cm^-2 s^-1 assuming an absorbed power-law spectrum with photon index of 1.7 and Galactic n_H = 1.76 x 10^20 cm^-2. This upper limit is a factor of ~2.4 lower than the flux value reported in the XMMSL2 catalog of (1.73 +/- 0.85) x 10^-12 erg cm^-2 s^-1 in a similar energy band (0.2-12 keV).
Optical/UV observations were collected simultaneously with the Swift UV/Optical Telescope (UVOT) in three bands: U, W1, and W2. The only UVOT detection consistent with the position of the mm source was that associated with XMMSL2 J010025.1-435716. We found no evidence of UV/optical variability for this source, with the fluxes in each band being consistent within uncertainties over the three epochs. The UVOT flux levels are also consistent with archival GALEX observations obtained from NED.
The average flux for each band is listed below. These values have been corrected for Galactic extinction using IRSA tables and R_lambda values calculated using the York Extinction Solver.
Band Flux
U (5.1 +/- 0.5) x 10^-13 erg cm^-2 s^-1
W1 (6.7 +/- 0.6) x 10^-13 erg cm^-2 s^-1
W2 (8.0 +/- 0.7) x 10^-13 erg cm^-2 s^-1
The rising optical/UV flux in nu F_nu can be interpreted as a synchrotron component peaking above the UV range (> 10^15 Hz), which would also be consistent with the SPT-3G flux values, although the X-ray upper limit constrain synchrotron peak values to be between ~10^15 and ~10^17 Hz in this scenario.
As no new sources were detected in the X-ray or UV/optical bands in the error region of SPT-SJ010023-4357.1, the source XMMSL2 J010025.1-435716 remains as its most likely candidate counterpart.
We wish to thank the Swift Team for making these observations possible, in particular J. Kennea as the Swift Observatory Duty Scientist and Brad Cenko for approving our request for follow-up observations.
