Redshifted Kα Line from the Peculiar Gamma-ray Source PMN J1603–4904

Abstract

The Southern Hemisphere AGN monitoring program TANAMI∗ provides regular VLBI monitoring (at 8 and 22 GHz) and multiwavelength coverage of extragalactic jets south of −30◦ declination. Here we focus on our latest results on the bright, hard-spectrum γ-ray source PMN J1603–4904. Our VLBI observations reveal a symmetric brightness distribution with the brightest, most compact component at the center of the emission region. No significant apparent motion is detected. Long-term monitoring with ATCA in the radio (5-40 GHz) and in the γ-rays by Fermi/LAT shows only mild variability and no major outbursts. Its broadband spectral energy distribution and other multiwavelength properties point to either a very atypical blazar or can be explained as a source seen edge-on, possibly a young radio galaxy. The latter would make PMN J1603– 4904 the first young radio galaxy detected in γ-rays, so additional confirmation is sought. Our recent Suzaku and XMM observations detect a narrow iron line, which results in the first redshift measurement of the system (z = 0.18 ± 0.01). This result suggests that the source is observed at a larger angle to the line of sight than expected for blazars, and allows us to constrain the linear extent of the arcsec-scale structure to be smaller than ∼3 kpc, which in the two-sided jet scenario is in agreement with the small linear scales characteristic of young radio galaxies.