Oral Presentation

A month-long QPO in gamma-ray emission from a blazar

Author(s): Zhongxiang Wang

Presenter: Zhongxiang Wang (Shanghai Astronomical Observatory)

Quasi-periodic oscillations (QPOs) are one rare while intriguing phenomenon seen in AGN. Previously, optical and X-ray QPOs, as well as year-long gamma-ray QPOs, have been found, with short intra-day oscillations possibly reflecting the innermost stable orbit of a supermassive black hole (SMBH) and long yearly oscillations being interpreted as evidence of the binary nature of the central SMBH system. Here we report the discovery of the first clear case of a month-long QPO in gamma-ray emission from a blazar. Since 2016 October, the blazar, PKS 2247-131, has undergone a gamma-ray outburst. From our analysis of the Fermi data, we find a 5-sigma QPO signal in the gamma-ray emission after the initial flux peak of the outburst. We show that the QPO is explained by considering a helical structure in the jet. The viewing angle to the dominant emission region in the jet undergoes periodic changes, producing the observed flux modulation through changes in the relativistic Doppler boosting effect. The case of PKS 2247-131 may appear to be the first time that a jet has been found to exhibit a helical structure through clear flux modulation of its high-energy flux.