Pulsars, transients and relativistic astrophysics

Abstract:

We present JHK photometry and spectroscopy of RX J0019.8+2156. The spectrum appears to be dominated by the accretion disc to at least 2.4 μm, over any other source of emission. We find Paschen, Brackett and He II lines strongly in emission, but no He I. There are satellite lines approximately 850km s-1 either side of the strongest, unblended hydrogen lines. These satellite lines may be the spectral signature of jets from the accretion disc.

Abstract:

As part of a programme to investigate spatial variations in the Galactic chemical composition, we have been searching for normal B-type stars and A-type supergiants near the Galactic center. During this search we have found eleven peculiar stars, and in some cases performed detailed abundance analyses of them which suggest that they may be at a post-AGB evolutionary stage. The A-type post-AGB candidates show [Fe/H]=-1.0 to -2.0, and [O/Fe]∼+1.4, typical of the post-AGB abundance patterns discussed in the literature. One star, LS 3591 (=SAO 243756), has also been examined recently by Oudmaijer (1996); its spectrum appears to be changing very rapidly, which may indicate erratic mass loss or the incipient formation of a planetary nebula. A B-type post-AGB candidate, LS 4950, has a similar spectrum to a well studied post-AGB star, LSIV -12 111. However, an examination of the line strengths and elemental abundances of LS 4950 show that it is peculiar for both a Population II, post-AGB, B-type star and for a normal, Population I, B-type supergiant. Two other B-type stars, LS 4825 and LS 5112, are either post-AGB stars near the Galactic center or normal B-type supergiants lying well beyond the Galactic center. In addition, several Be-type stars have been newly (or more clearly) identified from our spectra.

Abstract:

Two rival hypotheses have been proposed for the origin of the compact radio flux observed in radio-quiet quasars (RQQs). It has been suggested that the radio emission in these objects, typically some two or three orders of magnitude less powerful than in radio-loud quasars (RLQs), represents either emission from a circumnuclear starburst or is produced by radio jets with bulk kinetic powers 10^3 times lower than those of RLQs with similar luminosity ratios in other wavebands. We describe the results of high resolution (parsec-scale) radio-imaging observations of a sample of 12 RQQs using the Very Long Baseline Array (VLBA). We find strong evidence for jet-producing central engines in 8 members of our sample.