The First VHE Activity of OJ 287 and the Extragalactic Background Light

TL;DR

This paper reports the first detection of very-high-energy emission from OJ 287 during an intense X-ray flare, and uses this to constrain the extragalactic background light spectrum through spectral analysis.

Contribution

It presents the first VHE detection of OJ 287 and systematically constrains the EBL spectrum using simultaneous X-ray and VHE data with Bayesian methods.

Findings

VHE emission detected at 100--560 GeV from OJ 287

EBL spectrum consistent with a flat power-law shape

Curved EBL models fit data slightly better but are statistically comparable

Abstract

The BL Lacertae (BL Lac) object OJ 287 underwent an intense X-ray activity phase, exhibiting its brightest recorded X-ray flare in 2016-2017, characterized by much softer X-ray spectra and, concurrently, its first-ever recorded very-high-energy (VHE) emission (100--560 GeV), reported by the VERITAS observatory. Broadband spectral energy distribution reveals a new jet emission component similar to high-synchrotron-peaked BL Lac objects, thereby implying the soft X-ray spectrum for the synchrotron emission. Using the advantage of simultaneous X-ray and VHE spectral information, as well as the source being a low-synchrotron-peaked BL Lac object, we systematically explored the extragalactic background light (EBL) spectrum by demanding that the VHE spectrum cannot be harder than the X-ray spectrum. We used three different phenomenological forms of the EBL spectral shape (power-law, parabola, and polynomial) motivated by current constraints on the EBL with the Bayesian Monte Carlo approach to infer the credible EBL range. Our study favors an almost flat power-law spectral shape and is consistent with previous studies. The other spectral forms capable of capturing curvature though result in a better statistics value; the improvement is statistically insignificant given the additional parameters.