Search

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.

We present an analysis of eleven years of Strömgren byphotometry of the red semiregular variable star AI Aurigae. An early period determination of 63.9 days is confirmed by the long-term light curve behaviour. The light curve shows semi-regular changes with a mean period of 65 days reaching an amplitude of 0 fraction of minute6 in some cycles. The $b-y$colour changes perfectly parallel the Vlight curve, suggesting radial oscillation to be the main reason for the observed variations. We estimate the main characteristics of the star (mass, radius, effective temperature) that suggest radial pulsation in fundamental or first overtone mode.

Olympus Mons, one of the largest known shield volcanoes in the Solar System, covers an area of >3.2 × 105km2and has a diameter of >600 km, excluding its vast aureole deposits. The structure is five times larger than the largest shield volcano on the Earth. It is situated on the north-west flank of the Tharsis volcanic region, a broad topographic rise on the martian surface. The volcano has three physical subdivisions: the summit caldera, the terraced upper flanks, and the lower flanks, which terminate in a scarp 2–10 km high that nearly surrounds the structure. A large block of images of the Tharsis region, including Olympus Mons, was obtained by the Viking mission1. Here we present a topographic map of Olympus Mons, compiled using various combinations of stereo pairs of these images, together with stereoscopic perspective views generated by image processing techniques.

Light curves in Strömgren b and y for the 1985 season are presented for the uvby standard stars 10 LMi (HD 82635) and 11 LMi (HD 82885). The period of 10 LMi is 40.4 days, with an amplitude of 0.012 magnitude in y. The period of 11 LMi is 18.0 days, with an amplitude of 0.033 magnitude in y. The mechanism of the variability is interpreted as rotational modulation due to active regions on the stars. Observations of 11 LMi as a standard suggest the star was only weakly variable or constant during the interval from 1972 to 1984.

The star BD +59°224 has an interesting history of spectroscopic classification, having been described in the literature variously as a composite G- and F-type star with helium, a K5 star, an O-type star, and an emission-line object similar to a planetary nebula. We present new spectra of this object that clearly show the star to be a binary with a K4.5 Ib primary and a B3 V secondary. We also calculate a reddening and distance [E(B - V) ? 0.75, d ? 2.1 kpc] and point out similarities between this object and the class of ? Aurigae stars.