Swift follow-up observations of the flaring blazar TXS 0646-176

ATel #14987; F. D'Ammando (INAF-IRA Bologna)
on 20 Oct 2021; 08:45 UT
Credential Certification: Filippo D'Ammando (dammando@ira.inaf.it)

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Following the gamma-ray flaring activity from the flat spectrum radio quasar TXS 0646-176 observed by Fermi-LAT and AGILE-GRID (ATel #14980, ATel #14981), Swift target of opportunity observations were performed on 2021 October 18.

Swift/XRT data were taken in Photon Counting mode for a net exposure of about 2.6 ksec. The X-ray spectrum (0.3-10 keV) can be fit by an absorbed power-law model with a HI column density consistent with the Galactic value in the direction of the source (n_H = 2.37 x 10^21 cm^-2, Ben Bekhti et al., 2016, A&A, 594, A116) and a photon index of 1.25 +/- 0.15. The corresponding observed 0.3-10 keV flux is (9.4 +/- 0.9)e-12 erg cm^-2 s^-1. By comparison, the (observed) 0.3-10 keV flux estimated in the previous XRT observations performed between 2018 December 3 and 28 (at the time of a previous gamma-ray flaring activity observed by Fermi-LAT, ATel #12327) ranges between (5.2-9.1)e-12 erg cm^-2 s^-1. This indicates an increase of the X-ray activity in 2021 October, in agreement with the observed gamma-ray flaring activity.

Simultaneous Swift/UVOT observations were performed on 2021 October 18. The observed magnitudes are V = 18.09 +/- 0.22, B = 18.91 +/- 0.18, U = 18.56 +/- 0.19, W1 = 19.15 +/- 0.24, M2 = 19.82 +/- 0.29, and W2 > 20.12. The observed magnitudes on 2018 December 3 are V = 17.97 +/- 0.15, B = 18.80 +/- 0.14, U = 18.49 +/- 0.15, W1 = 19.23 +/- 0.24, M2 > 19.96, and W2 > 20.38, comparable to the values observed on 2021 October 18 within the uncertainties. Upper limits are estimated if the source is detected with a significance < 3-sigma.

In consideration of the ongoing activity of this source, we encourage multi-wavelength observations. Further observations with Swift and NICER are planned in the next days.

We would like to thank the Swift Team, Observatory Duty Scientists, and PI for making these observations possible.