Swift monitoring of the BL Lac object PKS 0735+178 during a bright state

ATel #15109; Filippo D'Ammando (INAF-IRA Bologna)
on 14 Dec 2021; 08:07 UT
Credential Certification: Filippo D'Ammando (dammando@ira.inaf.it)

Tweet

Following the recent strong activity observed in optical (ATel #15021) and gamma-rays (ATel #15099) from the BL Lac object PKS 0735+178, located in the vicinity of the IC211208A best-fit position, although outside the 90% uncertainty region reported by IceCube for this event, Swift target of opportunity observations were performed on 2021 December 10 (see preliminary results in ATel #15102), 12, and 13.

Swift/XRT data were taken during 2021 December 10, 12, and 13 in Photon Counting mode for a net exposure of about 2.2, 1.0, and 2.7 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 2.66 +/- 0.13, 2.51 +/- 0.22, and 2.57 +/- 0.20, respectively. The corresponding unabsorbed 0.3-10 keV flux is (8.7 +/- 0.6)e-12, (5.6 +/- 0.8)e-12, and (3.1 +/- 0.4)e-12 erg cm^-2 s^-1 for 2021 December 10, 12, and 13, respectively.

As a comparison, the XRT spectra collected during 2006-2012 can be well fitted by an absorbed power-law with a photon index that varies between 1.2 and 2.2. The corresponding unabsorbed 0.3-10 keV flux estimated during 2006-2012 ranges between (0.8-5.6)e-12 erg cm^-2 s^-1, indicating an increase of flux (up to a factor of 10) and a softening of the X-ray spectrum during 2021 December with respect to past Swift observations, with a decreasing flux from December 10 to 13. The X-ray fluxes observed by Swift/XRT during 2021 December are also significantly higher than the value observed by SRG/eROSITA during 2021 October 22-26 (ATel #15108).

Simultaneous Swift/UVOT observations were performed on 2021 December 10, 12, and 13. UVOT found PKS 0735+178 significantly brighter in optical and UV bands with respect to previous UVOT observations, e.g. the Swift observations performed at the time of the lowest (2010 November 2) and highest (2011 October 13) X-ray flux estimated during 2006-2012 (see Table below). The peak of the activity in optical and UV has been observed on 2021 December 10, in agreement with the behaviour observed in X-rays.

The observed magnitudes are:

Band	2010-11-02	2011-10-13	2021-12-10	2021-12-12	2021-12-13
B	16.95 +/- 0.06	16.89 +/- 0.08	14.88 +/- 0.05	15.40 +/- 0.05	15.32 +/- 0.05
U	16.28 +/- 0.06	16.02 +/- 0.07	13.93 +/- 0.05	14.54 +/- 0.05	14.44 +/- 0.05
W1	16.35 +/- 0.07	16.21 +/- 0.05	13.87 +/- 0.06	14.48 +/- 0.06	14.38 +/- 0.06
M2	16.50 +/- 0.07	16.29 +/- 0.08	13.87 +/- 0.06	14.50 +/- 0.06	14.41 +/- 0.06
W2	16.64 +/- 0.06	16.51 +/- 0.08	13.98 +/- 0.05	14.63 +/- 0.06	14.51 +/- 0.06

In consideration of the ongoing activity of this 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.