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Over the past decade, long-duration gamma-ray bursts (GRBs)--including the subclass of X-ray flashes (XRFs)--have been revealed to be a rare variety of type Ibc supernova. Although all these events result from the death of massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those of ordinary type Ibc supernovae by many orders of magnitude. The essential physical process that causes a dying star to produce a GRB or XRF, and not just a supernova, is still unknown. Here we report radio and X-ray observations of XRF 060218 (associated with supernova SN 2006aj), the second-nearest GRB identified until now. We show that this event is a hundred times less energetic but ten times more common than cosmological GRBs. Moreover, it is distinguished from ordinary type Ibc supernovae by the presence of 10(48) erg coupled to mildly relativistic ejecta, along with a central engine (an accretion-fed, rapidly rotating compact source) that produces X-rays for weeks after the explosion. This suggests that the production of relativistic ejecta is the key physical distinction between GRBs or XRFs and ordinary supernovae, while the nature of the central engine (black hole or magnetar) may distinguish typical bursts from low-luminosity, spherical events like XRF 060218.

Past studies have suggested that long-duration gamma-ray bursts have a 'standard' energy of E(gamma) approximately 10(51) erg in the ultra-relativistic ejecta, after correcting for asymmetries in the explosion ('jets'). But a group of sub-energetic bursts, including the peculiar GRB980425 associated with the supernova SN1998bw (E(gamma) approximately 10(48) erg), has recently been identified. Here we report radio observations of GRB030329 that allow us to undertake calorimetry of the explosion. Our data require a two-component explosion: a narrow (5 degrees opening angle) ultra-relativistic component responsible for the gamma-rays and early afterglow, and a wide, mildly relativistic component that produces the radio and optical afterglow more than 1.5 days after the explosion. The total energy release, which is dominated by the wide component, is similar to that of other gamma-ray bursts, but the contribution of the gamma-rays is energetically minor. Given the firm link of GRB030329 with SN2003dh, our result indicates a common origin for cosmic explosions in which, for reasons not yet understood, the energy in the highest-velocity ejecta is extremely variable.

The galactic superluminal source GRS 1915+105 was observed byBeppoSAX at several occasions from October 1996 to April 2000. In April 1999 the 2–10 keV light curve showed a long series of quasi regular pulses with a recurrence time of about 50 s. Some preliminary results of a spectral analysis are presented.

Gamma-ray bursts (GRBs) are thought to arise when an extremely relativistic outflow of particles from a massive explosion (the nature of which is still unclear) interacts with material surrounding the site of the explosion. Observations of the evolving changes in emission at many wavelengths allow us to investigate the origin of the photons, and so potentially determine the nature of the explosion. Here we report the results of γ-ray, optical, infrared, submillimetre, millimetre and radio observations of the burst GRB990123 and its afterglow. Our interpretation of the data indicates that the initial and afterglow emissions are associated with three distinct regions in the fireball. The peak flux of the afterglow, one day after the burst, has a lower frequency than observed for other bursts; this explains the short-lived radio emission. We suggest that the differences between bursts reflect variations in the magnetic-field strength in the afterglow-emitting regions.

THE Sunyaev–Zel'dovich effect1 is a distortion imposed on the 2.7-K cosmic microwave background radiation when the microwave photons are scattered by the hot gas in galaxy clusters. At radio wavelengths it appears as a decrement (of about 0.5 mK) in the brightness temperature of the background radiation. Measurements of this effect can provide information about the physics of galaxy clusters, and can also potentially be used to determine the Hubble constant, by constraining the size of the cluster along the line of sight. Successful detections have been made with singledish radio telescopes2,3, from which one-dimensional profiles of the temperature decrement are constructed. But these observations are susceptible to several systematic errors, such as corruption of the measured signal by radio sources located close to the line of sight. To circumvent these problems, we have obtained a two-dimensional image, using an interferometric (rather than single-dish) approach, of structure in the microwave background in the vicinity of the galaxy cluster Abell 2218. Our measurements, combined with X-ray observations of the scattering gas in the same cluster, support a low value of the Hubble constant.

The radio source 0957 + 561 has at least four components. Two coincide with the optical QSOs, which is in accordance with the hypothesis that the QSOs are images of a single object due to a gravitational lens. There are details of spectra and structure which are more difficult to reconcile with the hypothesis.

Sagittarius A, the intense radio source at the centre of our Galaxy, has no counterpart in the Andromeda Nebula although the structure of the nucleus is otherwise similar. The bright supernova of 1885 in Andromeda produces far less radio emission than would be expected from a comparison with remnants of galactic supernovae.

THE radio outbursts1 of Cygnus X-3 have been observed at several frequencies since September 4, 1972. We have made measurements of the flux density variations at 5.0, 2.7 and 1.4 GHz and the position and an upper limit to the angular size at 5 GHz; in addition the HI absorption spectrum has been determined and upper limits have been placed on the flux densities at 150 and 81.5 MHz. The instruments used were the Five-Kilometre (5 GHz), One-Mile (2.7 and 5 GHz) and Half-Mile (1.4 GHz) telescopes, the 4C antenna (150 MHz) and the 4½ acre array (81.5 MHz).

ON the basis of some new observations made at a frequency of 2,700 MHz, Shimmins, Bolton and Wall1 have derived radio source counts having a slope of −1.4. They claim that these differ significantly from the Cambridge results and cast doubt on the validity of the interpretation of the counts in terms of evolutionary cosmological models. We wish to point out that these doubts are unfounded, and that there is little discrepancy between the Australian observations and the Cambridge source-counts which have been made at 178 and 408 MHz.