Exploring the Origin of Multiwavelength Activities of High-redshift Flat-spectrum Radio Quasar PKS 1502+106 during 2014–2018

The origin of the multiband activities (outbursts/flares) of blazars is still a heavily debated topic. Shock and magnetic reconnection have long been considered as possible triggers of the multiband activities. In this paper, we present an exploration of the origin of multiband activities for a high-redshift (z = 1.8385) flat-spectrum radio quasar PKS. 1502+106. Utilizing multiband data from radio to gamma-ray and optical polarization observations, we investigate two dramatic activities in detail: a gamma-ray-dominated outburst in 2015 and an optical-dominated outburst in 2017. Our main results are as follows. (I) A fast gamma-ray flare with a flux-doubling timescale as short as 1 hr in 2015 is discovered. Based on the variability timescale, the physical parameters of the flaring region (e.g., minimum Doppler factor, emission region size, etc.) are constrained. At the peak of the flare, the gamma-ray spectrum hardens to Gamma(gamma) = 1.82 +/- 0.04 and exhibits an obvious curvature/break characteristic that is caused by the typical "cooling break." Modeling of multiband spectral energy distributions reveal a very hard electronic energy spectrum with the electronic spectral index of 1.07 +/- 0.53. This result suggests that this fast gamma-ray flare may be triggered by magnetic reconnection. (II) During the outburst in 2017, the degree of optical polarization and optical fluxes showed a very tight correlation. By analyzing the Stokes parameters of polarization observations, our results show that this outburst could be triggered by a transverse shock with a compression ratio of eta > 2.2, and the magnetic field intensity of the shock emission region is about 0.032. G.