Multi-wavelength picture of the misaligned BL Lac object 3C 371
Authors: Otero-Santos J. , Raiteri C. M. , Tramacere A. , Escudero Pedrosa J. , Acosta-Pulido J. A. , Carnerero M. I. , Villata M. , Agudo I. , Rahimov I. A. , Andreeva T. S. , Ivanov D. V. , Marchili N. , Righini S. , Giroletti M. , Gurwell M. A. , Savchenko S. S. , Carosati D. , Chen W. P. , Kurtanidze S. O. , Joner M. D. , Semkov E. , Pursimo T. , Benítez E. , Damljanovic G. , Andreuzzi G. , Apolonio G. , Borman G. A. , Bozhilov V. , Galindo-Guil F. J. , Grishina T. S. , Hagen-Thorn V. A. , Hiriart D. , Hsiao H. Y. , Ibryamov S. , Ivanidze R. Z. , Kimeridze G. N. , Kopatskaya E. N. , Kurtanidze O. M. , Larionov V. M. , Larionova E. G. , Larionova L. V. , Minev M. , Morozova D. A. , Nikolashvili M. G. , Ovcharov E. , Sigua L. A. , Stojanovic M. , Troitskiy I. S. , Troitskaya Yu. V. , Tsai A. , Valcheva A. , Vasilyev A. A. , Vince O. , Zaharieva E. , Zhovtan A. V. 
Published: December 2024

Context. The BL Lac object 3C 371 is one of the targets regularly monitored by the Whole-Earth Blazar Telescope (WEBT), a collaboration of observers studying blazar variability on both short and long timescales.

Aims. We aim to evaluate the long-term multi-wavelength (MWL) behaviour of 3C 371, comparing it with results derived from its optical emission in our previous study. For this, we make use of the multi-band campaigns organised by the WEBT collaboration in optical and radio between January 2018 and December 2020, and of public data from Swift and Fermi satellites and the MOJAVE Very Large Interferometry programme.

Methods. We evaluated the variability shown by the source in each band by quantifying the amplitude variability parameter, and also looked for a possible inter-band correlation using the z-discrete correlation function. We also present a deep analysis of the optical-UV, X-ray, and γ-ray spectral variability. With the MOJAVE data, we performed a kinematics analysis, looking for components propagating along the jet and calculating its kinematics parameters. We then used this set of parameters to interpret the source MWL behaviour, modelling its broadband spectral energy distribution (SED) with theoretical blazar emission scenarios.

Results. The MWL variability of the source in the UV, X-ray, and γ-ray bands is comparable to that in optical, especially considering the lower coverage of the first two wavebands. On the other hand, the radio bands show variability of much lower magnitude. Moreover, this MWL emission shows a high degree of correlation, which is compatible with zero lag, again with the exception of the radio emission. The radio VLBI images reveal super-luminal motion of one of the identified components, which we used to set constraints on the jet kinematics and parameters, and to estimate a viewing angle of θ = (9.6 ± 1.6)°, a Doppler factor of δ = 6.0 ± 1.1, and a Lorentz factor of Γ = 6.0 ± 1.8. The polarised radio emission was found to be anti-correlated with the total flux, and to follow the same behaviour as the polarised optical radiation. The optical-UV spectral behaviour shows a mild harder-when-brighter trend on long timescales, and other trends such as redder-when-brighter on shorter timescales. We successfully modelled the broadband emission with a leptonic scenario, where we compared the low and high emission states during the period of complete MWL coverage. The difference between these two states can be ascribed mainly to a hardening of the distribution of particles. The derived features of the source confirm that 3C 371 is a BL Lac whose jet is not well aligned with the line of sight.