Swift detection of record X-ray flare of the AGN PKS 0521-36

ATel #15041; F. D'Ammando (INAF-IRA Bologna)
on 16 Nov 2021; 14:36 UT
Credential Certification: Filippo D'Ammando (dammando@ira.inaf.it)

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After the increase of activity observed in optical, UV, and X-rays by Swift on 2021 November 10 (ATel #15029), a new Swift observation of the AGN PKS 0521-36 was performed on November 15.

Swift/XRT data were taken in Photon Counting mode for a total exposure of 2.5 ksec. Taking into consideration the Swift/XRT observations performed during 2005-2021, PKS 0521-36 reached on 2021 November 15 the highest count rate in the 0.3-10 keV energy range observed so far (see Swift-XRT Monitoring of Fermi-LAT Sources of Interest; Stroh & Falcone, 2013, ApJS, 207, 28).

Preliminary analysis has been performed fitting the X-ray spectrum in the 0.3-10 keV energy range with an absorbed power-law with the Galactic absorption corresponding to a hydrogen column density of n_H = 3.58 x10^20 cm^-2 (Kalberla et al. 2005, A&A, 440, 775) and a photon index of 1.40 +/- 0.07. The corresponding 2-10 keV flux is (3.53 +/- 0.17) x10^-11 erg cm^-2 s^-1, showing an increasing trend of the flux with respect to Swift/XRT observations performed on 2021 November 10 and a flux higher than the peak value observed in 2010 June, at the time of a previous gamma-ray flaring activity (see D'Ammando et al. 2015, MNRAS, 450, 3975).

Simultaneous Swift/UVOT observations were performed on 2021 November 15. The measured magnitudes are V = 14.56 +/- 0.04, B = 15.20 +/- 0.05, U = 14.60 +/- 0.05, W1 = 14.76 +/- 0.06, M2 = 14.86 +/- 0.06, and W2 = 14.97 +/- 0.06, showing an increase of activity also in optical and UV bands with respect to Swift/UVOT observations performed on 2021 November 10 (ATel #15029).

In consideration of the ongoing activity of this source shown by the Swift observations, we strongly encourage further multi-wavelength observations.

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