The binary neutron star merger event GW170817 was detected through both electromagnetic radiation and gravitational waves. Its afterglow emission may have been produced by either a narrow relativistic jet or an isotropic outflow. High-spatial-resolution measurements of the source size and displacement can discriminate between these scenarios. We present very-long-baseline interferometry observations, performed 207.4 days after the merger by using a global network of 32 radio telescopes. The apparent source size is constrained to be smaller than 2.5 milli–arc seconds at the 90% confidence level. This excludes the isotropic outflow scenario, which would have produced a larger apparent size, indicating that GW170817 produced a structured relativistic jet. Our rate calculations show that at least 10% of neutron star mergers produce such a jet.

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We present a new sample of compact steep spectrum (CSS) sources with radio luminosity below 1026 W Hz−1 at 1.4 GHz; these are called low‐luminosity compact (LLC) objects. The sources have been selected from the Faint Images of the Radio Sky at Twenty‐cm (FIRST) survey and observed with the multi‐element radio linked interferometer network (MERLIN) at the L and C bands. The main criterion used for selection was the luminosity of the objects, and approximately one‐third of the CSS sources from the new sample have a value of radio luminosity comparable to Fanaroff–Riley type 1 sources (FR Is). About 80 per cent of the sources have been resolved and about 30 per cent have weak extended emission and disturbed structures when compared with the observations of higher‐luminosity CSS sources. We have studied the correlation between radio power and linear size, and the redshift with a larger sample that also included published samples of compact objects and large‐scale FR IIs and FR Is. In the radio power versus linear size diagram, the LLC objects occupy the space below the main evolutionary path of radio objects. We suggest that many of these might be short‐lived objects, and their radio emission may be disrupted several times before they become FR IIs. We conclude that there exists a large population of short‐lived LLC objects unexplored so far, and some of these could be precursors to large‐scale FR Is.

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We study the mid-egress eclipse timing data gathered for the cataclysmic binary HU Aquarii during the years 1993-2014. The (O-C) residuals were previously attributed to a single ∼ 7 Jupiter mass companion in ∼ 5 au orbit or to a stable 2-planet system with an unconstrained outermost orbit. We present 22 new observations gathered between June, 2011 and July, 2014 with four instruments around the world. They reveal a systematic deviation of ∼ 60-120 seconds from the older ephemeris. We re-analyse the whole set of the timing data available. Our results provide an erratum to the previous HU Aqr planetary models, indicating that the hypothesis for a third and fourth body in this system is uncertain. The dynamical stability criterion and a particular geometry of orbits rule out coplanar 2-planet configurations. A putative HU Aqr planetary system may be more complex, e.g., highly non-coplanar. Indeed, we found examples of 3-planet configurations with the middle planet in a retrograde orbit, which are stable for at least 1 Gyr, and consistent with the observations. The (O-C) may be also driven by oscillations of the gravitational quadrupole moment of the secondary, as predicted by the Lanza et al. modification of the Applegate mechanism. Further systematic, long-term monitoring of HU Aqr is required to interpret the (O-C) residuals.