Detection of an Orphan X-ray Flare from a Blazar Candidate EP240709a with Einstein Probe

TL;DR

This paper reports the detection of a rare orphan X-ray flare from a blazar candidate using Einstein Probe, providing insights into jet structure and demonstrating the telescope's capability in discovering peculiar AGN activities.

Contribution

The study presents the first detection of an orphan X-ray flare from a blazar candidate with detailed spectral analysis and modeling, highlighting Einstein Probe's potential in high-cadence sky surveys.

Findings

Flux increased by at least 28 times in 0.5-10 keV band.

Detected harder-when-brighter X-ray behavior during the flare.

Infrared-optical spectra remained featureless during the event.

Abstract

Blazars are often observed to flare across multiple wavelengths. Orphan flares from blazars have been only 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 Einstein Probe (EP) in July 2024. The multi-band spectral properties and variability support EP240709a as a high-energy peaked BL Lacertae-type object. The flux in 0.5-10 keV increases by at least 28 times to the value of low state in 2020, with non-detection of remarkable flaring 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.