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Blazars are active galactic nuclei (AGN) with relativistic jets whose non-thermal radiation is extremely variable on various timescales1–3. This variability seems mostly random, although some quasi-periodic oscillations (QPOs), implying systematic processes, have been reported in blazars and other AGN. QPOs with timescales of days or hours are especially rare4in AGN and their nature is highly debated, explained by emitting plasma moving helically inside the jet5, plasma instabilities6,7or orbital motion in an accretion disc7,8. Here we report results of intense optical and γ-ray flux monitoring of BL Lacertae (BL Lac) during a dramatic outburst in 2020 (ref. 9). BL Lac, the prototype of a subclass of blazars10, is powered by a 1.7 × 108MSun(ref. 11) black hole in an elliptical galaxy (distance = 313 megaparsecs (ref. 12)). Our observations show QPOs of optical flux and linear polarization, and γ-ray flux, with cycles as short as approximately 13 h during the highest state of the outburst. The QPO properties match the expectations of current-driven kink instabilities6near a recollimation shock about 5 parsecs (pc) from the black hole in the wake of an apparent superluminal feature moving down the jet. Such a kink is apparent in a microwave Very Long Baseline Array (VLBA) image.

Blazars are active galactic nuclei, which are powerful sources of radiation whose central engine is located in the core of the host galaxy. Blazar emission is dominated by non-thermal radiation from a jet that moves relativistically towards us, and therefore undergoes Doppler beaming. This beaming causes flux enhancement and contraction of the variability timescales, so that most blazars appear as luminous sources characterized by noticeable and fast changes in brightness at all frequencies. The mechanism that produces this unpredictable variability is under debate, but proposed mechanisms include injection, acceleration and cooling of particles, with possible intervention of shock waves or turbulence. Changes in the viewing angle of the observed emitting knots or jet regions have also been suggested as an explanation of flaring events and can also explain specific properties of blazar emission, such as intra-day variability, quasi-periodicity and the delay of radio flux variations relative to optical changes. Such a geometric interpretation, however, is not universally accepted because alternative explanations based on changes in physical conditions—such as the size and speed of the emitting zone, the magnetic field, the number of emitting particles and their energy distribution—can explain snapshots of the spectral behaviour of blazars in many cases. Here we report the results of optical-to-radio-wavelength monitoring of the blazar CTA 102 and show that the observed long-term trends of the flux and spectral variability are best explained by an inhomogeneous, curved jet that undergoes changes in orientation over time. We propose that magnetohydrodynamic instabilities or rotation of the twisted jet cause different jet regions to change their orientation and hence their relative Doppler factors. In particular, the extreme optical outburst of 2016–2017 (brightness increase of six magnitudes) occurred when the corresponding emitting region had a small viewing angle. The agreement between observations and theoretical predictions can be seen as further validation of the relativistic beaming theory.

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We use optical color indices colors from the SDSS database to study the effect of dust in starburst galaxies by measuring the dependence of colors on galaxy inclination. Starburst galaxies with ongoing star formation, are rich with metalsdust and are, therefore, an excellent objects for studying the effect of dust in galaxies. They are selected using the [O III]λ5007Hαvs. [N II]λ6584Hβdiagram, that is, the BPTdiagram. We use Kauffmanns empirical demarcation line in the BPTdiagram to exclude galaxies with active galactic nuclei AGN from the sample because they have different physical and dust properties from normal galaxies. The sample is divided into bins according to galaxy stellar mass and 4000 Å break which is a coarse measure of a galaxy star formation history; SFH and the reddening with inclination is studied as a function of these two physical parameters. Assuming that the dust effect is negligible in the SDSS zband, we derive the attenuation curves for these galaxies. We fit the attenuation curves with a simple power law and use power law index to interpret the relative distribution of dust and stars in the starburst galaxies © 2010 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim