Detection of an Orphan X-Ray Flare from a Blazar Candidate EP240709a with the Einstein Probe
Mingjun Liu, Yijia Zhang, Yun Wang, Rui Xue, D. A. H. Buckley, D. A. Howell, Chichuan Jin, Wenxiong Li, I. M. Monageng, Haiwu Pan, Ning-Chen Sun, Samaporn Tinyanont, Lingzhi Wang, Weimin Yuan, Jie An, Moira Andrews, Rungrit Anutarawiramkul, Pathompong Butpan, Yong Chen, Huaqing Cheng, Wei Cui, Cui-Yuan Dai, Lixin Dai, Joseph Farah, Hua Feng, Shao-Yu Fu, Zhen Guo, S. M. Jia, Shuai-Qing Jiang, An Li, Chengkui Li, Dongyue Li, Tzi-Wei Lian, Y. F. Liang, He‐Yang Liu, Xing Liu, Yuan Liu, J. Mao, C. McCully, Megan Newsome, Estefania Padilla Gonzalez, Xin Pan, X. -J. Sun, G. Terreran, Ze-Rui Wang, Qinyu Wu, Hubing Xiao, Dingrong Xiong, D. Xu, Xinpeng Xu, Suijian Xue, Haonan Yang, Jun Yang, Jin Zhang, Wenda Zhang, Wenjie Zhang, Hu Zou
Abstract
Abstract Blazars are often observed to flare across multiple wavelengths. Orphan flares from blazars have only been detected a few times, providing an opportunity to understand the structure of the jet in the accreting system. We report a remarkable orphan X-ray flare from a blazar candidate EP240709a, detected by the Einstein Probe (EP) in July 2024. The multiband spectral properties and variability support EP240709a as a high-energy peaked BL Lacertae-type object. The flux in 0.5–10 keV increased by at least 28 times to the value of low state in 2020, with no significant flaring detected in other bands during the same period. EP240709a exhibits the harder-when-brighter tendency in the X-ray band during the orphan flare, while its infrared-optical spectra are featureless. We employ one-zone and two-zone leptonic synchrotron self-Compton models to perform the spectral energy distribution fitting. Detecting this rare orphan flare shows the potential of EP in discovering peculiar activities from AGN in high-cadence X-ray sky surveys.