Tests of Einstein's general theory of relativity have mostly been carried out in weak gravitational fields where the space-time curvature effects are first-order deviations from Newton's theory. Binary pulsars provide a means of probing the strong gravitational field around a neutron star, but strong-field effects may be best tested in systems containing black holes. Here we report such a test in a close binary system of two candidate black holes in the quasar OJ 287. This quasar shows quasi-periodic optical outbursts at 12-year intervals, with two outburst peaks per interval. The latest outburst occurred in September 2007, within a day of the time predicted by the binary black-hole model and general relativity. The observations confirm the binary nature of the system and also provide evidence for the loss of orbital energy in agreement (within 10 per cent) with the emission of gravitational waves from the system. In the absence of gravitational wave emission the outburst would have happened 20 days later.

We present an analysis of high energy (HE; 0.1–300 GeV) γ‐ray observations of 1FGL J1227.9−4852 with the Fermi Gamma‐ray Space Telescope, follow‐up radio observations with the Australia Telescope Compact Array, Giant Metrewave Radio Telescope and Parkes radio telescopes of the same field and follow‐up optical observations with the ESO VLT. We also examine archival XMM–Newton and INTEGRAL X‐ray observations of the region around this source. The γ‐ray spectrum of 1FGL J1227.9−4852 is best fitted with an exponentially cut‐off power law, reminiscent of the population of pulsars observed by Fermi. A previously unknown, compact radio source within the 99.7 per cent error circle of 1FGL J1227.9−4852 is discovered and has a morphology consistent either with an AGN core/jet structure or with two roughly symmetric lobes of a distant radio galaxy. A single bright X‐ray source XSS J12270−4859, a low‐mass X‐ray binary, also lies within the 1FGL J1227.9−4852 error circle and we report the first detection of radio emission from this source. The potential association of 1FGL J1227.9−4852 with each of these counterparts is discussed. Based upon the available data we find the association of the γ‐ray source to the compact double radio source unlikely and suggest that XSS J12270−4859 is a more likely counterpart to the new HE source. We propose that XSS J12270−4859 may be a millisecond binary pulsar and draw comparisons with PSR J1023+0038.

Context. Determining the ages of powerful radio sources is crucial for understanding galaxy evolution, the activity cycle of galactic nuclei, and their influence on the surrounding intergalactic medium. So far, several different methods for estimating the age of classical double radio galaxies have been proposed and widely used in the literature, although each of them faces difficulty due to observational limitations and/or freedom in choosing the underlying model assumptions. Aims. We propose a new approach to determining the ages of FR II type radio sources that, on one hand exploits a dynamical model developed for these objects by Kaiser et al. (1997, MNRAS, 292, 723) and, on the other hand, uses multifrequency radio observations not necessarily restricted to the high-resolution ones.Methods. In particular, we applied the assumed dynamical model to a number of FR II type radio galaxies observed at different radio frequencies and fit -for each frequency separately -the model's free parameters to the quantities of the observed sources. Such a procedure, which in fact enlarged a number of observables, enabled us to determine relatively precise ages and other crucial characteristics (like the jets' kinetic power) for the analyzed sources. Results. The resulting age estimates agree very well with those obtained with the "classical" spectral aging method for objects not older than 10 Myr, for which good-quality spectral data are available. However, this method is also applicable in the case of older sources than this and/or those for which the only available low-resolution radio data do not allow for detailed spectral aging studies. Interestingly, the estimated ages always correspond to the realistic values of the jets' advance velocity of ∼0.01-0.1 c. Conclusions. Our analysis indicates that the main factor precluding precise age determination for FR II type radio galaxies is related to the poorly known shape of the initial electron energy distribution injected by the jet's terminal shocks to the expanding lobes/cocoons. We briefly considered this issue and conclude that the broad-band single power-law form assumed here may be accurate enough for the age estimates, although most likely it does not strictly correspond to some well-defined realistic particle acceleration process. Instead, it should be considered as the simplest model approximation of the initial electron continuum, averaged over a very broad energy range and over the age of the source, with the effective spectral index that may be different for different sources, but within the relatively narrow range of p = 2.0-2.4 suggested by our modeling.

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We present results of multicolour UBVR photometry of the high‐amplitude EC 14026‐type star Balloon 090100001. The data span over a month and consist of more than a hundred hours of observations. Fourier analysis of these data led us to the detection of at least 30 modes of pulsation of which 22 are independent. The frequencies of 13 detected modes group in three narrow ranges, around 2.8, 3.8 and 4.7 mHz, where the radial fundamental mode and the first and second overtones are likely to occur. Surprisingly, we also detect nine independent modes in the low‐frequency domain, between 0.15 and 0.4 mHz. These modes are typical for pulsations found in PG 1716+426‐type stars, discovered recently among cool B‐type subdwarfs. The modes found in these stars are attributed to the high‐order g modes. As both kinds of pulsations are observed in Balloon 090100001, it represents a link between the two classes of pulsating hot subdwarfs. At present, it is probably the most suitable target for testing evolutionary scenarios and internal constitution models of these stars by means of asteroseismology. Three of the modes that we discovered form an equidistant frequency triplet which can be explained by invoking rotational splitting of an ℓ= 1 mode. The splitting amounts to about 1.58 μHz, leading to a rotation period of 7.1 ± 0.1 d.