The FSRQ B2 1308+326: the activity phase at epoch of high-energy flare in 2024 February

ATel #16515; V. V. Vlasyuk, O. I. Spiridonova, O. A. Maslennikova, A. S. Moskvitin (Special Astrophysical Observatory of the Rus. Ac. Sci.)
on 8 Mar 2024; 13:47 UT
Credential Certification: Alexander Moskvitin (mosk@sao.ru)

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The flat spectrum radio quasar B2 1308+326 is demonstrating its activity phase in optical range about the epoch of powerful flare in gamma-rays, as it was reported by Fermi LAT Collaboration (ATel #16497). The daily averaged gamma-ray flux (E>100MeV) of (3.1+/-0.4) X 10^-6 photons cm^-2 s^-1 on February 27, --- the highest LAT daily flux ever observed for this source. Afterwards, activity of B2 1308+326 was confirmed by data from Swift-XRT data (ATel #16504).

This object is observing by SAO RAS team on a regular basis in search of possible correlation between its optical and radio activity events. Till end of 2020 our photometrical studies were performed mainly using by 1-m SAO RAS reflector. Since January 2021 we have started observations with SAO RAS 0.5-m reflector (hereafter --- AS-500/2), manufactured by JSC ASTROSIB (Novosibirsk, Russia). In combination with prime-focus corrector and CCD camera FLI Proline PL16801 this telescope realized 1.5-degree field of view.

Since 2023 July the regular observing with AS-500/2 were continued in Cassegrain focus using by a back-illuminated electron-multiplied CCD camera Andor IXonEM 897. This camera realized 7' field of view with the 0."82/pixel data sampling (1 pixel = 16 microns). The system readout noise is about 6e- with the gain equals to 3e-/ADU.

Analysis of our data showed that within first decades of 2024 January the blazar demonstrated brightness variations in range between 15.0 mag and 15.8 mag in R-band. Afterwards, it's brightness increased to to 14.49 mag at MJD=60333.1 (Jan.24, 01:30UT). After some flux fluctuations in range 14.3--14.7 mag, detected by us between January 23th and February 12th, brightness of B2 1308+326 reached our local maximum mR = 13.95 mag at MJD=60365.97 (Feb.25, 23:20UT). Unfortunately, due to close full Moon and poor weather conditions the following photometrical estimates were obtained only by 6 days --- at MJD=60371.91 (Mar.02, 21:45UT). They gave some fainter value: mR = 14.18 mag. So we cannot suggest definitely about brightness of the blazar within this week. The current behavior of the B2 1308+326 flux can be presented as slow fading from mR = 14.2 to 14.4 mag and downwards.

Typical accuracy of our data was about 0.01 mag under good transparency and seeing value (FWHM) between 1.5" and 2.5". The estimates of brightness from the both (1-m and 0.5-m) telescopes agree well within their instrumental errors.

As before, for 1-m data we used comparison star F, assuming it's R=14.00 mag. Data from 0.5-m telescope were calibrated using by star C from Smith et al (1985, AJ, 90, 1184).

More accurate reduction is necessary. We strongly encourage further multi-wavelength observations.