Blazar Mrk 501 shows rhythmic oscillations in its γ-ray emission

Abstract

Quasi-periodic oscillations (QPO) originating from innermost regions of blazars can provide unique perspective of some of the burning issues in blazar studies including disk-jet connection, launch of relativistic jets from the central engine, and other extreme conditions near the fast rotating supermassive black holes. However, a number of hurdles associated with searching QPOs in blazars e.g., red-noise dominance, modest significance of the detection and periodic modulation lasting for only a couple of cycles, make it difficult to estimate the true significance of the detection. In this work, we report a ∼ 330-day QPO in the Fermi/LAT observations of the blazar Mrk 501 spanning nearly a decade. To establish consistency of the result, we adopted multiple approaches to the time series analysis and employed four widely known methods. Among these, Lomb-Scargle periodogram and weighted wavelet z-transform represent frequency domain based methods whereas epoch folding and z-transformed discrete auto-correlation function are time-domain based analysis. Power spectrum response method was followed to properly account for the red-noise, largely inherent in blazar light curves. Both local and global significance of the signal were found to be above 99% over possible spurious detection. In the context where not many γ-ray QPOs have been reported to last more than 5 cycles, this might be one of the few instances where we witness a sub-year timescale γ-ray QPO persisting nearly 7 cycles. A number of possible scenarios linked with binary supermassive black hole, relativistic jets, and accretion disks can be invoked to explain the transient QPO.

FACT - Probing the Periodicity of Mrk 421 and Mrk 501

“…Both blazars are known to exhibit extreme variability, which could be caused by a number of processes. Recently, hints of a quasi periodic oscillation (QPO) of Mrk 501 with a period of about 322 days have been found with Fermi-LAT at GeV energies featuring global and local significances of the signal above 99% [4]. Observing such QPOs also in the TeV-range for Mrk 421 or Mrk 501 would lead to a better understanding of the variability of AGN.…”

Study of Periodic Signals from Blazars

“…As a result, the hunt for periodic signals in blazar light curves has become an active field of research: blazar OJ 287 is famous for its ∼12-year periodicity detected in its historical optical light curve [6,7] and another famous source PG 1553 + 113 is found to show 2.18-year periodic modulations in its γ-ray observations from the Fermi/LAT, which was first reported in [8]. In addition, several authors have reported the existence of QPOs of various characteristic timescales in numerous blazar multi-frequency light curves including the four cases of QPO detection by our research group see [4,[9][10][11] and the references therein.…”

“…The analyses revealed an underlying sub-year timescale (332 d) γ-ray QPO that persisted nearly 7 cycles. Applying PSRESP analysis the significance of the detection was found to be above 99% against possible spurious detection for details see [11]. • J1043+2408: A ∼ 560-d QPO was detected in the radio (15 GHz) observations of the BL Lac source J1043+2408.…”

Summary of the 36 th ICRC Gamma-ray Indirect sessions

“…Curiously, the FACT monitoring of another bright blazar Mrk 501 has revealed a hint of quasi-periodic behaviour with a period of 332-days [5]. Even while no statistical significance of the hint, or possible systematics effects have been evaluated yet, it is interesting to note that a similar period has been obtained in the analysis of Fermi-LAT data of the same source [58].…”