http://arxiv.org/abs/1808.06067
In this letter, we report the detection of transient rhythmic oscillations in the $\sim$10-year long Fermi/LAT observations of the famous blazar Mrk 501. Using two widely accepted methods, Lomb-Scargle periodogram and weighted wavelet z-transform, we found a strong signal of quasi-periodic oscillation (QPO) with a periodicity centered around 332 days. In addition, we noted that the $\gamma$-ray flux modulations gradually decayed in strength during the recent years. A large number of Monte Carlo simulations were performed to properly account for the red-noise inherent in the light curve and thereby estimate both the local and global significance of the signal to be above 99\%. In the context where not many $\gamma$-ray QPOs are reported to show more than 5 cycles, this might be one of the few instances where we witness a relatively high frequency, having a period of less than a year, $\gamma$-ray QPO persisting nearly 7 cycles before it weakens. Mrk 501 being one of the best studied sources, the detection, complemented with what has been already known about the source, can lead us to the understanding of extreme conditions prevailing around the supermassive black holes that potentially lead to the launch of the relativistic jets. In addition, the detection might contribute to the broader program of searching for characteristic timescales in the blazar light curves. Furthermore, the core of the distant sources being unresolved by our current telescopes, such timescales provide us with important clues about the processes taking place at the heart of active galactic nuclei. Several possible scenarios that possibly give rise to such a transient $\gamma$-ray QPO are discussed.
G. Bhatta
Tue, 21 Aug 18
7/71
Comments: 5 pages and 4 figures
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