Gamma-ray quasi-periodicities of blazars. A cautious approach

Covino, S.; Sandrinelli, A.; Treves, A.

doi:10.1093/mnras/sty2720

Cited by 66 publications

(70 citation statements)

References 45 publications

(60 reference statements)

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“…The results are shown in Table. 8 and reveal that in the absence of other physical reasons for restricting the period range the QPV evidence is strongly reduced, with none of the sources reaching 3σ significance. These findings are in line with similar indications in (Covino et al 2019).…”

Section: Global Significancesupporting

confidence: 93%

Evaluating quasi-periodic variations in the γ-ray light curves of Fermi-LAT blazars

Benkhali

Hofmann

Rieger

et al. 2020

A&A

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Context. The detection of periodicities in the light curves of active galactic nuclei (AGN) could have profound consequences for our understanding of the nature and radiation physics of these objects. At high energies (HE; E > 100 MeV) five blazars (PG 1553+113, PKS 2155-304, PKS 0426-380, PKS 0537-441 and PKS 0301-243) have been reported to show year-like quasi-periodic variations (QPVs) with significance > 3σ. As these findings are based on few cycles only, care needs to be taken to properly account for random variations which can produce intervals of seemingly periodic behaviour. Aims. We present results of an updated timing analysis for six blazars (adding PKS 0447-439 to the above), utilizing suitable methods to evaluate their longterm variability properties and to search for QPVs in their light curves. Methods. We generate γ-ray light curves covering almost ten years, study their timing properties and search for QPVs using the Lomb-Scargle Periodogram and the Wavelet Z-transform. Extended Monte Carlo simulations are used to evaluate the statistical significance.Results.(1) Comparing their probability density functions (PDFs), all sources (except PG 1553+113) exhibit a clear deviation from a Gaussian distribution, but are consistent with being log-normal, suggesting that the underlying variability is of a non-linear, multiplicative nature. (2) Apart from PKS 0301-243 the power spectral density (PSD) for all investigated blazars is close to flicker noise (power-law slope −1). (3) Possible QPVs with a local significance > ∼ 3σ are found in all light curves (apart from PKS 0426-380 and PKS 0537-441), with observed periods between (1.7 − 2.8) yr. The evidence is strongly reduced, however, if evaluated in terms of a global significance. Conclusions. Our results advise caution as to the significance of reported year-like HE QPVs in blazars. Somewhat surprisingly, the putative, redhift-corrected periods are all clustering around ∼ 1.6 yr. We speculate on possible implications for QPV generation.

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Section: Global Significancesupporting

confidence: 93%

Evaluating quasi-periodic variations in the γ-ray light curves of Fermi-LAT blazars

Benkhali

Hofmann

Rieger

et al. 2020

A&A

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“…Ackermann et al 2015;Prokhorov & Moraghan 2017;Ait Benkhali et al 2019), there are other studies which demonstrate that there is no solid evidence supporting such hints of year-long periodicities in blazars (e.g. Covino et al 2019). We emphasise that caution is required when claiming quasi-periodicities on timescales of a few years in blazar γ-ray emission.…”

Section: Discussionmentioning

confidence: 73%

A parsec-scale wobbling jet in the high-synchrotron peaked blazar PG 1553+113

Lico

Liu

Giroletti

et al. 2020

A&A

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Context. The detection of quasi-periodic variability in active galactic nuclei (AGNs) in general, and in blazars in particular, is key to our understanding of the origin and nature of these objects as well as their cosmological evolution. PG 1553+113 is the first blazar showing an approximately two-year quasi-periodic pattern in its γ-ray light curve, which is also revealed at optical frequencies.Aims. Such quasi-periodicity might have a geometrical origin, possibly related to the precessing nature of the jet, or could be intrinsic to the source and related to pulsational accretion flow instabilities. In this work we investigate and characterise the high-resolution radio emission properties of PG 1553+113 on parsec scales in order to differentiate between these different physical scenarios. Methods. We monitored the source with the very long baseline array (VLBA) at 15, 24, and 43 GHz during an entire cycle of γ-ray activity in the period 2015-2017, with a cadence of about 2 months, both in total and polarised intensity. We constrained the jet position angle across the different observing epochs by investigating the total intensity ridge lines. Results. We find a core-dominated source with a limb-brightened jet structure extending for ∼ 1.5 mas in the northeast direction whose position angle varies in time in the range ∼ 40 • − 60 • . No clear periodic pattern can be recognized in the VLBA light curves during 2015-2017 or in the 15 GHz Owens Valley Radio Observatory light curve during the period 2008-2018. The core region polarisation percentage varies in the range ∼ 1 − 4%, and the polarisation angle varies from being roughly parallel to roughly transverse to the jet axis. We estimate a rotation measure value in the core region of ∼ −1.0 ± 0.4 × 10 4 rad m −2 . The brightness temperature (T B ) is found to decrease as the frequency increases with an intrinsic value of ∼ 1.5 × 10 10 K and the estimated Doppler factor is ∼ 1.4. Conclusions. Although the jet wobbling motion indicates that geometrical effects can produce an enhanced emission through the Doppler boosting modulation, additional mechanisms are required in order to account for the quasi-periodic variability patterns observed in γ rays. The intrinsic T B value indicates that the total energy in the core region is dominated by the magnetic field.

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“…It is important to note here that in all of these cases the claimed periodic timescales were nearly two years and above (or low temporal frequency), and number of observed periodic cycles were less than 5. More recently, Covino et al (2019) studying several blazars in γ-ray reported no significant periodicities in the light curves.…”

Section: Introductionmentioning

confidence: 97%

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

Bhatta

2019

Monthly Notices of the Royal Astronomical Society

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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.

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Gamma-ray quasi-periodicities of blazars. A cautious approach

Cited by 66 publications

References 45 publications

Evaluating quasi-periodic variations in the γ-ray light curves of Fermi-LAT blazars

Evaluating quasi-periodic variations in the γ-ray light curves of Fermi-LAT blazars

A parsec-scale wobbling jet in the high-synchrotron peaked blazar PG 1553+113

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

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