Blazar S4 0954+65: coincidence of gamma-ray flash and optical emission peak at February, 27th

ATel #14459; V. V. Vlasyuk, O. I. Spiridonova, E. V. Emelianov, A. S. Moskvitin, O. A. Maslennikova (SAO RAS)
on 15 Mar 2021; 17:39 UT
Credential Certification: Alexander Moskvitin (mosk@sao.ru)

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Blazar S4 0954+65 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. Some radio data were collected from archives of Russian radio telescopes RATAN-600 and RT-22.

Since January we have started commissioning observations with second SAO RAS 0.5-m RC500 reflector, manufactured by JSC ASTROSIB (Novosibirsk, Russia)*. In combination with prime-focus corrector and CCD 4Kx4K camera FLI Proline PL16801 telescope realizes 1.5-degree field of view.

We have observed S4 0954+65 every night with appropriate atmospheric conditions in January-March in BVR passbands with both telescopes. Typical accuracy of our data was about 0.01 mag. under good transparency and seeing value (FWHM) better than 3".

Analysis of our data showed that from the beginning of 2021 blazar demonstrates active phase with brightness variations in range between 14.3 and 15.2 mag. in R-band. We can confirm that gamma-ray flaring episode, reported in ATel#14426, is certainly linked with S4 0954+65. After some flux fluctuations in range 14.1 - 14.5 mag., detected by us between February 20th and 26th, brightness of blazar in R-band increased from 14.35 mag. at MJD=59270.965 to 13.88 mag. at MJD=59272.738 (Feb.27, 17.40 UT). Afterwards, brightness of S4 0954+654 reached absolute maximum R_max = 13.81 mag. between 20.0 UT and 21.0 UT and begun slow decrease up to ~ 14.0 mag. (end of observations at 24.0 UT). So, according to our data, optical maximum is very close to the moment of gamma-ray flash event.

The fading of blazar brightness continued within next days till levels between 14.6 mag. and 14.8 mag. in R-band. All our following data didn't reveal its brightness above 14.5 mag. However, blazar is still at high state, which exceeds normal brightness more than 1 magnitude and demonstrates violent intraday variability (about 0.5 mag. within few hours). We are preparing for detailed study of behavior of S4 0954+65 brightness. Following observations are also in our plans.

* Building of complex of 0.5-m optical telescopes RC500 was supported by Russian Scientific Foundation grant No 14-50-0043.