Swift XRT/UVOT follow-up of the recent optical activity of blazar BL Lacertae

ATel #14774; Raj Prince (CFT), Rukaiya Khatoon (Tezpur Univ), Aditi Agarwal (RRI)
on 11 Jul 2021; 16:40 UT
Credential Certification: Aditi Agarwal (aditiagarwal.phy@gmail.com)

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A continuous optical flaring activity has been recorded recently for the blazar BL Lacertae (ICRS coords. (ep=J2000): RA 22 02 43.291 Dec +42 16 39.978), and reported by Atels 14751 & 14773. We proposed follow-up Swift-ToO observations for this object. The observations conducted on July 8 and July 11, 2021 suggest that the source is in the elevated state in X-ray, optical and UV bands.

Here, we report the preliminary analysis results using the Swift-XRT and UVOT observations. The extracted Swift-XRT spectrum in the 0.3-10 keV range can be fitted by an absorbed power-law model, with the absorption fixed at the Galactic value of N_H = 0.18 x 10^22 cm^-2 (Kalberla et al. 2005, A&A, 440, 775). The X-ray flux measured on July 8, 2021 is (2.07+/-0.19) x 10^-11 erg cm^-2 s^-1 with power-law spectral index 1.74+/-0.41. On July 11, 2021, the flux increased by a factor of ~3 and reached to (6.56+/-0.20) x 10^-11 erg cm^-2 s^-1 and the spectrum became softer with photon spectral index 2.22+/-0.11.

On July 8, 2021, the UVOT observations were performed with four filters, U, B, W1, and W2. The magnitude obtained in AB system are following-

U = 13.91 +/- 0.03

B = 13.26 +/- 0.03

W1 = 14.59 +/- 0.03

W2 = 15.42 +/- 0.03

The source observed in even brighter state on July 11 2021 with following magnitudes,

U = 13.35 +/- 0.03

B = 12.78 +/- 0.03

V = 12.17 +/- 0.03

W1 = 13.98+/- 0.03

M2 = 14.83+/-0.03

W2 = 15.75 +/- 0.03

Our proposed ToO monitoring will continue to monitor this source till July 26, 2021. We encourage further multi-wavelength observations of this object. We would like to thank the Swift Team for the extremely rapid and efficient scheduling of the ToO observations.