A redshifted Fe K$\alpha$ line from the unusual gamma-ray source PMN J1603-4904

TL;DR

This study reports the detection of a redshifted Fe Kα line in the X-ray spectrum of the unusual gamma-ray source PMN J1603-4904, indicating it is a misaligned-jet object possibly a young radio galaxy, challenging previous classifications.

Contribution

First high signal-to-noise X-ray spectrum of PMN J1603-4904 revealing a redshifted Fe Kα line, suggesting a different jet orientation and classification than previously thought.

Findings

Redshifted Fe Kα line at 5.44 keV observed

Redshift of z=0.18 determined for the source

Challenges the BL Lac classification of the source

Abstract

Multiwavelength observations have revealed the highly unusual properties of the gamma-ray source PMN J1603-4904, which are difficult to reconcile with any other well established gamma-ray source class. The object is either a very atypical blazar or compact jet source seen at a larger angle to the line of sight. In order to determine the physical origin of the high-energy emission processes in PMN J1603-4904, we study the X-ray spectrum in detail. We performed quasi-simultaneous X-ray observations with XMM-Newton and Suzaku in 2013 September, resulting in the first high signal-to-noise X-ray spectrum of this source. The 2-10 keV X-ray spectrum can be well described by an absorbed power law with an emission line at 5.44$\pm$0.05 keV (observed frame). Interpreting this feature as a K{\alpha} line from neutral iron, we determine the redshift of PMN J1603-4904 to be z=0.18$\pm$0.01, corresponding to a luminosity distance of 872$\pm$54 Mpc. The detection of a redshifted X-ray emission line further challenges the original BL Lac classification of PMN J1603-4904. This result suggests that the source is observed at a larger angle to the line of sight than expected for blazars, and thus the source would add to the elusive class of gamma-ray loud misaligned-jet objects, possibly a {\gamma}-ray bright young radio galaxy.