Blazar OJ 287 After First VHE Activity: Tracking the Re-emergence of the HBL like Component in 2020

TL;DR

This study documents the re-emergence of a high-energy-peaked emission component in OJ 287 after its first VHE activity in 2017, revealing correlated multi-wavelength variability and spectral changes that suggest a connection to long-term outburst cycles.

Contribution

It provides the first detailed multi-wavelength analysis of OJ 287's re-emergence of a HBL-like component post-VHE activity, linking it to long-term optical outbursts.

Findings

Detection of a new HBL-like broadband emission component.

Correlation between optical, UV, and gamma-ray flux variations.

Spectral hardening above 1 GeV during high activity periods.

Abstract

We report the re-emergence of a new broadband emission through a detailed and systematic study of the multi-wavelength spectral and temporal behavior of OJ 287 after its first-ever reported VHE activity in 2017 to date, which includes the second-highest X-ray flux of the source. The source shows high optical to X-ray flux variations, accompanied mainly by strong spectral changes. The optical to X-ray flux variations are correlated and simultaneous except for two durations when they are anti-correlated. The flux variations, however, are anti-correlated with the X-ray spectral state while correlated with optical-UV (ultraviolet). Weekly binned {\it Fermi}-LAT data around the duration of the highest X-ray activity show a few detections with a log-parabola model but none with a power-law; yet the extracted LAT spectral energy distribution (SED) of the high activity duration for both the models is similar and show a hardening above 1 GeV. Further, near-infrared (NIR) data indicate strong spectral change, resembling a thermal component. Overall, the combined optical to gamma-ray broadband spectrum establishes the observed variations to a new high-energy-peaked (HBL) broadband emission component, similar to the one seen during the highest reported X-ray flux state of the source in 2017. The observed activities indicate some peculiar features that seem to be characteristic of this emission component while its appearance, a few years around the claimed \(\sim 12\)-year optical outbursts strongly indicate a connection between the two.