ABSTRACT

OJ 287 is a well-studied binary black hole (BBH) system, which occasionally exhibits bright X-ray and optical flares. Here we present a detailed spectral study of its second brightest X-ray flare observed during 2019–2020 using archival Swift and Nuclear Spectroscopic Telescope Array (NuSTAR) observations along with Target of Opportunity observations from AstroSat. The entire flaring period is divided into three states, defined as low, intermediate and high states. The variation of the hardness ratio with 0.3–10.0 keV integrated flux suggests a ‘softer-when-brighter’ behaviour, as also previously reported based on flux-index variations. Simultaneous high-state X-ray spectra obtained using Swift, NuSTAR and AstroSat are very steep with a power-law index >2. A significant spectral change is observed in the spectrum obtained with the Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) onboard AstroSat, which is consistent with the spectrum from NuSTARand from the X-ray Telescope (XRT) onboard Swift. Together, optical–ultraviolet and X-ray spectra during the high-flux state suggest the emergence of a new high BL Lacertae component. We have modelled the synchrotron peak with the publicly available code called gamera for low-, intermediate- and high-flux states. Our modelling suggests the need for a high magnetic field to explain the high state under leptonic scenarios. An increase in the magnetic field value inside the jet could be linked to an increase in the accretion rate, as expected in the BH–disc impact scenario. The colour–magnitude diagram reveals a ‘bluer-when-brighter’ spectral energy distribution chromatism during the flaring period. Different chromatism or no chromatism on various occasions suggests a complex origin of optical emission, which is believed to be produced by disc impact or through synchrotron emission in the jet.

中文翻译：

---

使用 AstroSat、Swift 和 NuSTAR 对 OJ 287 2019-2020 年耀斑的综合研究

摘要

OJ 287 是一个经过充分研究的双黑洞 (BBH) 系统，它偶尔会出现明亮的 X 射线和光学耀斑。在这里，我们使用档案Swift和核光谱望远镜阵列( NuSTAR ) 观测以及来自AstroSat的机会目标观测，对其在 2019-2020 年期间观测到的第二亮 X 射线耀斑进行了详细的光谱研究。整个燃烧期分为三个状态，定义为低、中、高状态。硬度比随 0.3-10.0 keV 积分通量的变化表明“更亮时更软”的行为，正如先前基于通量指数变化所报告的那样。使用Swift、NuSTAR和AstroSat非常陡峭，幂律指数 >2。甲显著光谱变化是与软X射线望远镜（SXT）和大面积的X射线正比计数器（LAXPC）板上获得的光谱中观察到AstroSat，这与从光谱一致NuSTAR并从X射线望远镜（ XRT) 板载Swift。总之，高通量状态下的光学-紫外线和 X 射线光谱表明出现了一种新的高 BL Lacertae 成分。我们使用名为gamera的公开代码对同步加速器峰值进行了建模用于低、中和高通量状态。我们的模型表明需要高磁场来解释轻子情景下的高状态。正如在 BH 圆盘撞击情景中所预期的那样，射流内部磁场值的增加可能与吸积率的增加有关。颜色-幅度图揭示了在燃烧期间“更亮时更蓝”的光谱能量分布色差。不同场合的不同色差或无色差表明光发射的复杂起源，据信这是由圆盘撞击或通过喷流中的同步加速器发射产生的。