Multi-band behaviour of the TeV blazar PG 1553+113 in optical range on diverse time scales

AGARWAL, A.; MIHOV, B.; ANDRUCHOW, I.; CELLONE, SERGIO A.; ANUPAMA, G.C.; AGRAWAL, V.; ZOLA, S.; SLAVCHEVA-MIHOVA, L.; ÖZDÖNMEZ, A.; EGE, E.; RAJ, A.; MAMMANA, L.; ZIBECCHI, L.; FERNÁNDEZ-LAJÚS, E.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2021

0004-6361

The TeV BL Lac object PG 1553+113 is one of the primary candidates for a binary supermassive black hole system.We study the flux and spectral variability of PG 1553+113 on intra-night to long-term time scales using (i) BVRI data collected over 76 nights from January 2016 to August 2019 involving nine optical telescopes and (ii) historical VR data (including ours) obtained for the period 2005--2019.We analyzed the light curves using various statistical tests, fitting and cross-correlation techniques, and methods for periodicity search. We examined the color-magnitude diagrams before and after the corresponding light curves being corrected for the long-term variations.Our intra-night monitoring, supplemented with literature data, result in a low duty cycle of ~(10--17)%. In April 2019, we recorded a flare, which marks the brightest state of pg for the period 2005--2019: R ~ 13.2 mag. This flare is found to show a clockwise spectral hysteresis loop on its VR color-magnitude diagram and a time lag in the sense the V-band leads the $R$ one. We obtain estimates of the radius, the magnetic field strength, and the electron energy that characterize the emission region related to the flare. We find a median period of (2.21 +- 0.04) years using the historical light curves. In addition, we detect a secondary period of about 210 days using the historical light curves corrected for the long-term variations. We briefly discuss the possible origin of this period.