Tracking OP 313 across wavelengths reveals its blazar behavior

An international team of astronomers has analyzed the data from long-term multiwavelength monitoring of a distant blazar known as OP 313. Results of the new study, published January 18 on the arXiv preprint server, shed more light on the peculiar behavior of this object.

Compact quasi-stellar objects

Blazars are very compact quasi-stellar objects (quasars) associated with supermassive black holes (SMBHs) at the centers of active, giant elliptical galaxies. They are the most luminous and extreme subclass of active galactic nuclei (AGNs). The characteristic features of blazars are highly collimated relativistic jets oriented very close to our line of sight.

Based on their optical emission properties, astronomers generally divide blazars into two classes: flat-spectrum radio quasars (FSRQs) that feature prominent and broad optical emission lines, and BL Lacertae objects (BL Lacs), which do not.

A distant blazar awakes

OP 313, also known as B2 1308+326, is a blazar of the FSRQ class at a redshift of approximately 0.997. It was initially identified in 1959 as a variable star and is so far the most distant quasar detected at very high energy (VHE) gamma rays.

On November 22, 2023, after over a decade of quiescence, OP 313 entered a phase of its highest activity in high-energy gamma rays, which lasted till March 2024. During this phase, the blazar experienced two major gamma-ray flares.

A group of astronomers led by Chiara Bartolini of the University of Trento in Italy recently decided to investigate the long-term multiwavelength observational data of OP 313, focusing on the quiescent phase, hoping to understand the origin of the blazar’s recent flaring activity.

«Given that OP 313 exhibited persistent high-state activity since 2022, following more than a decade of quiescence, we analyzed 15 years of data from radio to high-energy gamma rays to unveil the mechanisms responsible for the observed flares,» the researchers explained.

Strong variability, eight flares and a new jet

The study conducted by Bartolini’s team detected strong variability of OP 313 from the optical band to gamma rays, with an increasing flux starting from 2022. The optical, ultraviolet, X-ray, and gamma-ray light curves appear to be well correlated. However, a different behavior has been identified in the radio band, with the flux being high in 2008 and a new increasing trend in 2019. This occurred without a clear correlation with the other bands.

Examining the gamma-ray band, the astronomers identified eight flaring periods and detected a new, well-defined jet component, which was first spotted on April 13, 2021. Afterward, they investigated the interplay between particle acceleration and radiative cooling by means of a hysteresis pattern—a loop-like structure that provides insight into the particle acceleration and cooling mechanisms occurring within the blazar’s jet. Their findings revealed hints of this pattern for three flaring periods.

According to the study, the recent activity of OP 313 is triggered by new jet components emerging from the blazar’s core and interacting with a standing shock in the blazar’s jet. The results indicate that the gamma-ray emitting region is located far from the central engine and the dusty torus acts as the dominant source of seed photons.

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