Re-brightening of the BL Lac object PKS 0735+178 observed by Swift
ATel #15130; Filippo D'Ammando (INAF-IRA Bologna)
on 21 Dec 2021; 11:36 UT
Credential Certification: Filippo D'Ammando (dammando@ira.inaf.it)
Subjects: Optical, Ultra-Violet, X-ray, AGN, Black Hole, Blazar
Following the strong activity observed in radio (ATel #15105), optical (ATel #15098), X-rays (ATel #15102, #15109) and gamma-rays (ATel #15099) from the BL Lac object PKS 0735+178, as part of the follow-up observations of the region around the IceCube event IC211208A (GCN #31191), the Swift monitoring of the source has continued and further observations were performed on 2021 December 14 and 17.
Swift/XRT data were taken during 2021 December 14 and 17 in Photon Counting mode for a net exposure of about 1.2 and 2.8 ksec, respectively. Preliminary analysis has been performed fitting the 0.3-10 keV X-ray spectrum with an absorbed power-law with the Galactic absorption corresponding to a hydrogen column density of n_H = 3.67 x 10^20 cm^-2 (Ben Bekhti N. et al., 2016, A&A, 594, A116) and a photon index of 2.24 +/- 0.25 and 2.74 +/- 0.12 for 2021 December 14 and 17, respectively. The corresponding unabsorbed 0.3-10 keV flux is (3.9 +/- 0.6)e-12 and (8.2 +/- 0.5)e-12 erg cm^-2 s^-1 on December 14 and 17, respectively. The flux observed on 2021 December 17 is slightly lower than the maximum flux observed so far from this source on 2021 December 10 (ATel #15109), with a significant softening of the X-ray spectrum. This behaviour is in agreement with the enhanced gamma-ray activity observed on 2021 December 19 (ATel #15129).
Simultaneous Swift/UVOT observations were performed on 2021 December 14 and 17. The observed magnitudes are: B = 15.19 +/- 0.05, U = 14.31 +/- 0.05, W1 = 14.24 +/- 0.06, M2 = 14.22 +/- 0.06, W2 = 14.34 +/- 0.05 for 2021 December 14; B = 15.32 +/- 0.05, U = 14.40 +/- 0.05, W1 = 14.37 +/- 0.06, M2 = 14.40 +/- 0.06, W2 = 14.48 +/- 0.05 for 2021 December 17. These values confirm a re-brightening of the source in optical and UV bands with respect to UVOT observations performed during 2006-2012 and marginally to the values observed on 2021 December 12 and 13 (ATel #15109), in agreement with the behaviour observed in X-rays.
In consideration of the re-brightening of the source, confirmed also by the Swift observations, we encourage further multi-wavelength observations. Additional Swift observations are planned.
We would like to thank the Swift Team, the Swift ODS, and the PI for making these observations possible.