Pulsars, transients and relativistic astrophysics

Abstract:

We report the results of a long-term campaign of radio, soft- and hard- X-ray observations of the galactic black hole candidate GX 339-4. In the Low-Hard X-ray state the system displays a strong 3-way linear correlation between soft-and hard-X-rays and radio emission, implying a coupling between the Comptonising corona and a radio-emitting compact jet. In this state the radio emission is linearly polarised at a level of around 2%, with an almost constant polarisation angle, indicative of a favored axis in this system probably related to the compact jet and/or black hole spin axis. In the Off X-ray state the radio emission declines with the X-ray emission to below detectable levels, suggesting that it is simply a lower-luminosity version of the Low-Hard state. In the High-Soft state both the hard-X-ray and radio emission are suppressed. We also note that the transitions from the Low-Hard state to the High-Soft state (and the reverse) are possibly associated with discrete ejection(s) of expanding relativistic plasma.

Abstract:

We have analyzed in detail a set of Rossi X-ray Timing Explorer (RXTE) observations of the galactic microquasar GRS 1915+105 corresponding to times when quasi-periodic oscillations in the infrared have been reported. From time-resolved spectral analysis, we have estimated the mass accretion rate through the (variable) inner edge of the accretion disk. We compare this accretion rate to an estimate of the mass/energy outflow rate in the jet. We discuss the possible implications of these results in terms of disk-instability and jet ejection, and in particular note an apparent anti-correlation between the accretion and ejection rates, implying that the gas expelled in the jet must leave the accretion disk before reaching its innermost radius.

Abstract:

In this paper we present high spatial resolution radio images of the puzzling binary system β Lyrae obtained with MERLIN at 5 GHz. We find a nebula surrounding the binary with a brightness temperature of (11000 ± 700) K approximately 40 AU across. This definitively confirms the thermal origin of the radio emission, which is consistent with emission from the wind of the B6-8 II component (mass loss of order of 10-7 M⊙yr-1), ionized by the radiation field of the hotter companion. This nebula, surrounding the binary, is the proof that β Lyrae evolved in a non-conservative way, i. e. not all the mass lost by the primary is accretted by the secondary, and present measurements indicate that almost 0.015 M⊙ had been lost from the system since the onset of the Roche lobe overflow phase. Moreover, the nebula is aligned with the jet-like structures inferred from recent optical measurements, indicating a possible connection among them.

Abstract:

We have previously published intermediate to high resolution spectroscopic observations of approximately 80 early B-type main-sequence stars situated in 19 Galactic open clusters/associations with Galactocentric distances distributed over 6 ≤ Rg ≤ 18 kpc. This current study collates and re-analyses these equivalent-width datasets using LTE and non-LTE model atmosphere techniques, in order to determine the stellar atmospheric parameters and abundance estimates for C, N, O, Mg, Al and Si. The latter should be representative of the present-day Galactic interstellar medium. Our extensive observational dataset permits the identification of sub-samples of stars with similar atmospheric parameters and of homogeneous subsets of lines. As such, this investigation represents the most extensive and systematic study of its kind to date. We conclude that the distribution of light elements (C, O, Mg & Si) in the Galactic disk can be represented by a linear, radial gradient of -0.07 ± 0.01 dex kpc-1. Our results for nitrogen and oxygen viz. (-0.09 ± 0.01 dex kpc-1 and -0.067 ± 0.008 dex kpc-1) are in excellent agreement with that found from the study of H II regions. We have also examined our datasets for evidence of an abrupt discontinuity in the metallicity of the Galactic disk near a Galactocentric distance of 10 kpc (see Twarog et al. 1997). However, there is no evidence to suggest that our data would be better fitted with a two-zone model. Moreover, we observe a N/O gradient of -0.04 ± 0.02 dex kpc-1 which is consistent with that found for other spiral galaxies (Vila-Costas & Edmunds 1993).