The Origin of the Multiwavelength Emission of PKS 0502+049

Abstract

The origin of the multiwavelength emission from PKS 0502+049 neighboring the first cosmic neutrino source TXS 0506+056 is studied using the data observed by Fermi-LAT and Swift UVOT/XRT. This source was in a flaring state in the considered bands before and after the neutrino observations in 2014-2015, characterized by hard emission spectra in the X-ray and $\gamma$-ray bands, $1.5-1.8$ and $\leq2.0$, respectively. During the neutrino observations, the $\gamma$-ray spectrum shows a deviation from a simple power-law shape, indicating a spectral cutoff at $E_c =8.50\pm2.06$ GeV. The spectral energy distributions of PKS 0502+049 are modeled within a one-zone leptonic scenario assuming that high energy $\gamma$-ray emission is produced either by IC scattering of synchrotron or dusty torus photons by the electron population that produce the radio-to-optical emission. Alternatively, the observed $\gamma$-rays are modeled considering inelastic interaction of protons, when the jet interacts with a dense gaseous target. During the neutrino observations, the $\gamma$-ray data are best described when the proton energy distribution is $E_p^{-2.61}$ and if the protons are effectively accelerated up to 10 PeV, the expected neutrino rate is $\sim1.1$ events within 110 days. In principle, if the $\gamma$-ray emission with a hard photon index observed during the flaring periods extends up to TeV, the expected rate can be somewhat higher, but such conditions are hardly possible. Within the hadronic interpretation, the $\gamma$-ray data can be reproduced only when the accretion rate of PKS 0502+049 is in the supper-Eddington regime, as opposed to the leptonic scenario. From the point of view of the necessary energetics as well as considering that the required parameters are physically reasonable, when the neutrinos were observed, the broadband emission from PKS 0502+049 is most likely of a leptonic origin.