Swift follow-up of BL Lacertae during a bright state

ATel #14065; Filippo D'Ammando (INAF-IRA Bologna)
on 6 Oct 2020; 16:58 UT
Credential Certification: Filippo D'Ammando (dammando@ira.inaf.it)

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Following the recent strong activity observed from BL Lacertae in optical (ATel #13956, ATel #13958), high-energy gamma-rays (ATel #13933 and #13964), and very-high-energy gamma-rays (ATel #13963 and #14032), Swift target of opportunity observations were performed on October 2, 3, 4, and 5.

Swift/UVOT observations in 2020 October 5 found BL Lacertae about 1 mag brighter in all six filters with respect to the UVOT observations performed on October 4, slightly brighter than the values observed on 2020 August 21 at the Hans-Haffner-Sternwarte (V = 12.545+/-0.003, B = 13.465+/-0.003; ATel #13958), and about 1.5 brighter than the maximum level observed during 2008-2012 by Swift (Raiteri et al. 2013, MNRAS, 436,1530).

The measured magnitudes (Vega system; systematic error included) are:

Band	2020-Oct-02	2020-Oct-03	2020-Oct-04	2020-Oct-05
V	12.72+/-0.04	-	13.23+/-0.07	12.39+/-0.04
B	13.46+/-0.04	13.87+/-0.04	13.98+/-0.07	13.14+/-0.05
U	12.97+/-0.05	13.42+/-0.05	13.53+/-0.07	12.56+/-0.05
W1	13.22+/-0.06	13.72+/-0.06	13.78+/-0.09	12.78+/-0.06
M2	13.94+/-0.11	-	14.68+/-0.15	13.45+/-0.06
W2	13.84+/-0.06	14.33+/-0.06	14.43+/-0.09	13.32+/-0.06

The observed flux densities collected on 2020 October 5 are V = 40.1+/-1.1 mJy, B = 22.4+/-0.6 mJy, U = 13.7+/-0.6 mJy, W1 = 6.9+/-0.4 mJy, M2 = 3.2+/-0.1 mJy, and W2 = 3.5+/-0.2 mJy.

Swift/XRT data were taken during 2020 October 2, 3, 4, and 5 in Photon Counting mode for a net exposure of about 1.0, 1.0, 0.2, and 1.0 ksec, respectively. Taking into consideration the Swift/XRT observations performed during 2005-2020, the source on 2020 October 5 reached the second highest count rate in the 0.3-10 keV energy range (see Swift-XRT Monitoring of Fermi-LAT Sources of Interest ; Stroh & Falcone, 2013, ApJS, 207, 28).

Preliminary analysis has been performed fitting the 0.3-10 keV X-ray spectra 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). Pile-up correction has been applied when needed. The X-ray (0.3-10 keV) flux observed (i.e. not corrected for Galactic extinction) on 2020 October 2, 3, 4, and 5 is (4.8+/-0.2) x10^-11 erg cm^-2 s^-1, (1.2+/-0.1) x10^-11 erg cm^-2 s^-1, (1.0+/-0.2) x10^-11 erg cm^-2 s^-1, and (8.8+/-0.4) x10^-11 erg cm^-2 s^-1, respectively. The X-ray flux level reached on October 5 is higher than previous observations (e.g. Raiteri et al. 2013, MNRAS, 436,1530; Wehrle et al. 2013, ApJ 816, 53), with a significant increase with respect to October 4.

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.