Swift detection of record X-ray flare of BL Lacertae

ATel #14069; Filippo D'Ammando (INAF-IRA Bologna)
on 7 Oct 2020; 10:44 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 2020 October 5 (ATel #14065), a new Swift observation of BL Lacertae was performed on October 6 (04:15:06-04:31:00 UTC) for the same Target of Opportunity request.

Swift/XRT data were taken during 2020 October 6 in Windowed Timing mode for a net exposure of about 1.0 ksec. Taking into consideration the Swift/XRT observations performed during 2005-2020, on 2020 October 6 BL Lacertae reached 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 = 3.4 x 10^21 cm^-2 (Raiteri et al. 2010, A&A, 524, 43). The X-ray (0.3-10 keV) flux observed (i.e. not corrected for Galactic extinction) is (3.3+/-0.4) x10^-10 erg cm^-2 s^-1, a factor of 3.5 higher than the flux observed on 2020 October 5, reaching the historical maximum in X-rays.

Swift/UVOT observations performed on 2020 October 6 (04:15:06-04:31:00 UTC) found BL Lacertae at a similar brightness level with respect to the values observed on 2020 October 5. The observed magnitudes (flux densities) are: V = 12.62 +/- 0.04 (32.8 +/- 1.0 mJy), B = 13.30 +/- 0.05 (19.5 +/- 0.6 mJy), U = 12.72 +/- 0.05 (11.8 +/- 0.05 mJy), W1 = 12.90 +/- 0.06 (6.2 +/- 0.3 mJy), M2 = 13.48 +/- 0.06 (3.1 +/- 0.1 mJy), and W2 = 13.41 +/- 0.06 (3.2 +/- 0.2 mJy).

New Swift observations of BL Lacertae are planned on 2020 October 8 and 9. In consideration of the ongoing activity of this source, confirmed also by the Swift observations, we encourage further multiwavelength observations.

We would like to thank the Swift Team for making these observations possible, in particular M. H. Siegel, as the Swift Observatory Duty Scientist.