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

Abstract

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.

Multi-band behaviour of the TeV blazar PG 1553+113 in optical range on diverse timescales

“…As long as the flux variations of the corrected LCs are achromatic, we should rely on geometric causes of this periodicity, for example, the emitting region follows a spiral path within the jet or the jet itself has a helical geometry. The jet wobbling in PG 1553+113, recently found by Lico et al (2020) could contribute to some extent to the periodic STV. It is worth noting that Bhatta et al (2016) found a secondary period of ∼400 days for the blazar OJ 287 in addition to the main one of ∼12 years.…”

“…Nilsson et al (2018) did not find the claimed year-long period in their Rband LC. In addition, Lico et al (2020) found no signs for clear periodic variability at the radio frequencies.…”

“…The first set is based on the multi-wavelength SED modellingδ varies from 20 (Tavecchio et al 2010) to 40 (Aleksić et al 2015). The second, single-value set is based on the Very Long Baseline Array observationsδ is equal to 1.4 (Lico et al 2020). This discrepancy is typical for TeV HBLs and reflects the so called 'Doppler crisis' (Tavecchio 2006).…”

Quasi-Periodic Oscillations in the long term radio light curves of the blazar AO 0235+164

“…Such structures can be inferred from the wiggling of knots observed in VLBI jets that can be resolved transversely at different epochs (e.g. Piner et al, 2010;Lico et al, 2020). They can be understood in terms of shocks propagating outward through the jets and interacting with a preexisting helical structure, presumably related to the toroidal component of the jet magnetic field, even in an otherwise cylindrical or conical jet.…”

The application of ridgelines in extended radio source cross-identification

“…Blundell & Bowler 2004). Total intensity ridgelines based on the method of Pushkarev et al (2017), where the integrated intensity is equal on both sides of the ridgeline, have been used by multiple authors to investigate properties of the ridgelines themselves, such as their spectral index and the electric vector position angle (see Li et al (2018); Pushkarev et al (2019); Kravchenko et al (2020); Lico et al (2020)).…”