The low-frequency radio emission in blazar PKS2155-304

TL;DR

This study presents detailed low-frequency radio observations of blazar PKS 2155-304, revealing jet morphology, variability, and diffuse emission, providing insights into jet emission mechanisms and emphasizing the importance of low-frequency radio interferometry.

Contribution

First detailed low-frequency radio imaging and monitoring of PKS 2155-304, revealing jet structure, variability, and diffuse emission, advancing understanding of blazar jet emission mechanisms.

Findings

Detected extended kpc-scale jet and lobe morphology.

Observed high variability in jet emission during outburst.

Identified diffuse emission with steep spectral index at low frequencies.

Abstract

We report radio imaging and monitoring observations in the frequency range 0.235 - 2.7 GHz during the flaring mode of PKS 2155-304, one of the brightest BL Lac objects. The high sensitivity GMRT observations not only reveal extended kpc-scale jet and FRI type lobe morphology in this erstwhile `extended-core' blazar but also delineate the morphological details, thanks to its arcsec scale resolution. The radio light curve during the end phase of the outburst measured in 2008 shows high variability (8.5%) in the jet emission in the GHz range, compared to the lower core variability (3.2%) seen at the lowest frequencies. The excess of flux density with a very steep spectral index in the MHz range supports the presence of extra diffuse emission at low frequencies. The analysis of multi wavelength (radio/ optical/ gamma-ray) light curves at different radio frequencies confirms the variability of the core region and agrees with the scenario of high energy emission in gamma-rays due to inverse Compton emission from a collimated relativistic plasma jet followed by synchrotron emission in radio. Clearly, these results give an interesting insight of the jet emission mechanisms in blazars and highlight the importance of studying such objects with low frequency radio interferometers like LOFAR and the SKA and its precursor instruments.