Temporal and Spectral Variability of OJ 287 before the April--June 2020 Outburst

TL;DR

This study analyzes the temporal and spectral variability of the BL Lac object OJ 287 across optical, UV, and X-ray bands during 2019-2020, revealing moderate variability, soft X-ray dominance, and complex emission mechanisms.

Contribution

It provides a detailed multi-wavelength analysis of OJ 287's variability and spectral properties, highlighting the presence of multiple emission components and their temporal relationships.

Findings

Moderate variability amplitude of 22-31% across wavelengths.

Detection of ~45-day soft lags between optical/UV and soft X-ray emissions.

X-ray spectra dominated by synchrotron emission with a weak softer-when-brighter trend.

Abstract

We present the results of a temporal and spectral study of the BL Lacertae object OJ 287 in optical, UV, and X-ray bands with observations performed by Swift satellite during September 2019--March 2020. In this period, the source showed moderate variability characterized by variability amplitude of $\sim$22--31% in all the wavelengths on a short timescale, except the hard X-ray band which was variable by only $\sim$8%. We observed that the X-ray flux of the source was significantly dominated by the soft photons below $2$ keV. Soft lags of $\sim$45 days were detected between the optical/UV and soft X-ray emissions, while there is no correlation between the hard X-rays and the lower energy bands indicating the presence of two emission components or electron populations. Although two components contribute to the X-ray emission, most of the 0.3--10 keV spectra were well fitted with an absorbed power-law model which outlines the dominance of synchrotron over inverse Compton (IC) mechanism. The X-ray spectra follow a weak "softer when brighter" trend.