INTEGRAL and Swift/XRT observations of the source PKS 0208-512

TL;DR

This study presents combined INTEGRAL and Swift/XRT observations of the blazar PKS 0208-512 during a gamma-ray flare decay, revealing X-ray variability and potential MeV excess emission, suggesting a jet with electron-positron plasma.

Contribution

First combined high-energy observations of PKS 0208-512 during a gamma-ray flare decay, proposing a jet composition with electron-positron plasma based on detected excess emission.

Findings

X-ray spectrum with photon index ~1.64

30% X-ray luminosity variation over a month

Possible 3.2 sigma excess in 0.5-1 MeV band

Abstract

The active galaxy PKS 0208-512, detected at lower energies by COMPTEL, has been claimed to be a MeV blazar from EGRET. We report on the most recent INTEGRAL observations of the blazar PKS 0208-512, which are supplemented by Swift ToO observations. The high energy X-ray and gamma-ray emission of PKS 0208-512 during August - December 2008 has been studied using 682 ks of INTEGRAL guest observer time and ~ 56 ks of Swift/XRT observations. These data were collected during the decay of a gamma-ray flare observed by Fermi/LAT. At X-ray energies (0.2 - 10 keV) PKS 0208-512 is significantly detected by Swift/XRT, showing a power-law spectrum with a photon index of ~ 1.64. Its X-ray luminosity varied by roughly 30% during one month. At hard X-/soft gamma-ray energies PKS 0208-512 shows a marginally significant (~ 3.2 sigma) emission in the 0.5-1 MeV band when combining all INTEGRAL/SPI data. Non-detections at energies below and above this band by INTEGRAL/SPI may indicate intrinsic excess emission. If this possible excess is produced by the blazar, one possible explanation could be that its jet consists of an abundant electron-positron plasma, which may lead to the emission of an annihilation radiation feature. Assuming this scenario, we estimate physical parameters of the jet of PKS 0208-512.