The Growing Population of Radio Galaxies in the Gamma-Ray Sky of INTEGRAL and Fermi – ASI

New radio surveys, such as the VLA Sky Survey (VLASS) and Rapid ASKAP Continuum Survey (RACS), conducted with advanced radio telescopes, are providing new opportunities for studying the gamma-ray sky. Radio galaxies, known for their impressive relativistic jets visible in the radio band that extend beyond their host galaxies, are among the least represented extragalactic sources in the high-energy catalog by Fermi/LAT. Detecting these radio galaxies requires both high sensitivity and resolution.

Thanks to the recently released VLASS and RACS data, scientists have discovered that an increasing number of extragalactic sources observed by Fermi/LAT are associated with radio galaxies. In a recent study led by the INTEGRAL group at INAF-IAPS in Rome, researchers focused on one of these objects (IGR J18249-3243), which was previously detected in the hard X-ray band by the IBIS instrument onboard INTEGRAL during its initial survey year. This source belongs to the Fanaroff-Riley II class and exhibits intense radio-emitting lobes at the far end of its jets, extending tens of kiloparsecs from the central nucleus. The study of its emissions across various bands, including radio, X-ray, hard X-ray, and gamma-ray, revealed peculiar properties. Notably, the flux at Fermi/LAT energies (GeV) was found to be ten times stronger than expected. By comparing these observations with a theoretical emission model, researchers concluded that the radio lobes likely contribute significantly to the gamma-ray emissions.

This finding not only confirms recent theoretical studies on high-energy emissions in radio galaxies closer to us than IGR J18249-3243 but also provides a new perspective on gamma-ray emissions in higher redshift sources. Previously, these emissions were predominantly attributed to strong blazars, where the jet is primarily responsible for the emission. With the upcoming operation of the Square Kilometer Array (SKA) in the next decade, it is anticipated that an increasing number of objects like IGR J18249-3243 will be discovered, further populating the gamma-ray sky.

Image Caption: The VLASS survey image of the radio galaxy IGR J18249-3243. The colors represent the intensity of the radio flux at 3 GHz, while the cross indicates the core position measured by Swift/XRT, with its corresponding uncertainty depicted as a circle.