We present a multiwavelength study of the flat-spectrum radio quasar CTA 102 during 2013–2017 .
We use radio-to-optical data obtained by the Whole Earth Blazar Telescope , 15 GHz data from the Owens Valley Radio Observatory , 91 and 103 GHz data from the Atacama Large Millimeter Array , near-infrared data from the Rapid Eye Monitor telescope , as well as data from the Swift ( optical-UV and X-rays ) and Fermi ( \gamma rays ) satellites to study flux and spectral variability and the correlation between flux changes at different wavelengths .
Unprecedented \gamma -ray flaring activity was observed during 2016 November–2017 February , with four major outbursts .
A peak flux of ( 2158 \pm 63 ) \times 10 ^ { -8 } ph cm ^ { -2 } s ^ { -1 } , corresponding to a luminosity of ( 2.2 \pm 0.1 ) \times 10 ^ { 50 } erg s ^ { -1 } , was reached on 2016 December 28 .
These four \gamma -ray outbursts have corresponding events in the near-infrared , optical and UV bands , with the peaks observed at the same time .
A general agreement between X-ray and \gamma -ray activity is found .
The \gamma -ray flux variations show a general , strong correlation with the optical ones with no time lag between the two bands and a comparable variability amplitude .
This \gamma -ray/optical relationship is in agreement with the geometrical model that has successfully explained the low-energy flux and spectral behaviour , suggesting that the long-term flux variations are mainly due to changes in the Doppler factor produced by variations of the viewing angle of the emitting regions .
The difference in behaviour between radio and higher-energy emission would be ascribed to different viewing angles of the jet regions producing their emission .