Swift follow-up observations of BL Lacertae during a new flaring activity

ATel #14350; Filippo D'Ammando (INAF-IRA Bologna)
on 26 Jan 2021; 17:38 UT
Credential Certification: Filippo D'Ammando (dammando@ira.inaf.it)

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After the recent optical and gamma-ray flaring activity of the blazar BL Lacertae (ATel #14318, #14328, #14330, #14334), significant variability in brightness has been observed in X-rays by NICER during 2021 January 18-22 (ATel #14342). Following this activity, Swift target of opportunity observations were performed on 2021 January 25.

Swift-XRT data were taken in Photon Counting mode for a net exposure of about 1.0 ksec. Preliminary analysis has been performed fitting the 0.3-10 keV X-ray spectra with an absorbed power-law with a photon index of 2.25 +/- 0.11 and the Galactic absorption corresponding to a hydrogen column density of n_H = 2.7 x 10^21 cm^-2, in agreement with the value obtained by the analysis of the joint NICER and NuSTAR spectrum obtained on 2020 October 11-12 (ATel #14096). The X-ray (0.3-10 keV) flux observed (i.e. not corrected for Galactic extinction) is (1.4+/-0.1) x10^-10 erg cm^-2 s^-1, a factor of 2.5 lower than the historical maximum observed in X-rays on 2020 October 6 (ATel #14069) but higher than all other fluxes observed for this source so far (ATel #14065).

Swift-UVOT observations performed on 2021 January 25 found BL Lacertae at a similar brightness level with respect to the values observed on 2020 October 5 and 6 in the optical filters, and approximately 0.4-0.5 magnitude less bright in the UV filters. The observed magnitudes are V = 12.39 +/- 0.04, B = 13.19 +/- 0.05, U = 12.67 +/- 0.05, W1 = 13.26 +/- 0.06, M2 = 13.91 +/- 0.06, and W2 = 13.91 +/- 0.06.

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

We would like to thank the Swift Team for making these observations possible, in particular K. L. Page, as the Swift Observatory Duty Scientist.