42,189 research outputs found
Optical and X-ray Properties of CAL 83: I. Quasi-periodic Optical and Supersoft Variability
We have studied the long-term (~ years) temporal variability of the prototype
supersoft X-ray source (SSS) CAL 83 in the LMC, using data from the MACHO and
OGLE projects. The CAL 83 light curve exhibits dramatic brightness changes of
~1 mag on timescales of ~450 days, and spends typically ~200 days in the
optical low state. Combined with archival XMM-Newton X-ray observations these
represent the most extensive X-ray/optical study to date of this system, and
reveal in much greater detail that the X-ray light curve is anti-correlated
with the optical behaviour. This is remarkably similar to the behaviour of the
"transient" SSS, RX J0513.9-6951, where the SSS outbursts recur on a timescale
of ~168 days, and also anti-correlate with the optical flux. We performed
simple blackbody fits to both high and low state X-ray spectra, and find that
the blackbody temperature and luminosity decrease when the optical counterpart
brightens. We interpret these long-term variations in terms of the limit-cycle
model of Hachisu & Kato (2003a), which provides further support for these
systems containing massive (~1.3 Msun) white dwarfs. In addition, we have
refined their orbital periods in the MACHO and OGLE-III light curves to values
of 1.047529(1) days and 0.762956(5) days for CAL 83 and RX J0513.9-6951,
respectively.Comment: 9 pages, 8 figures, 3 tables, accepted for publication in MNRA
Spectroscopic Identification of the Infrared Counterpart to GX5-1
Using CGS4 on UKIRT, we have obtained a 1.95-2.45 micron infrared spectrum of
the primary candidate counterpart to the bright Z LMXB GX5-1. IR photometry by
Naylor, Charles, & Longmore (1992) and the astrometry of Jonker et al. (2000)
had previously identified this star as the most likely counterpart to GX5-1.
The spectrum presented here clearly shows Brackett gamma and He lines in
emission, for the first time confirming the identity of the counterpart.
Similar to our previous spectroscopy of the Z source LMXBs Sco X-1 and Sco X-2
(Bandyopadhyay et al. 1999), the K-band spectrum of GX5-1 shows emission lines
only. We briefly discuss the implications of this spectrum for the nature of
the Z sources.Comment: accepted for publication as a Letter in MNRA
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Integer polyhedra for program analysis
Polyhedra are widely used in model checking and abstract interpretation. Polyhedral analysis is effective when the relationships between variables are linear, but suffers from imprecision when it is necessary to take into account the integrality of the represented space. Imprecision also arises when non-linear constraints occur. Moreover, in terms of tractability, even a space defined by linear constraints can become unmanageable owing to the excessive number of inequalities. Thus it is useful to identify those inequalities whose omission has least impact on the represented space. This paper shows how these issues can be addressed in a novel way by growing the integer hull of the space and approximating the number of integral points within a bounded polyhedron
Simultaneous X-ray and optical observations of the flaring X-ray source, Aquila A-1
During the summer of 1978 the recurrent transient X-ray source, Aquila X-1, underwent its first major outburst in two years. The results of extensive observations at X-ray and optical wavelengths throughout this event, which lasted for approximately two months are presented. The peak X-ray luminosity was approximately 1.3 times that of the Crab and exhibited spectral dependent flickering on timescales approximately 5 minutes. The observations are interpreted in terms of a standard accretion disk model withparticular emphasis on the similarities to Sco X-1 and other dward X-ray systems, although the transient nature of the system remains unexplained. It was found that Aquila X-1 can be described adequately by the semi-detached Roche lobe model and yields a mass ratio of less than or approximate to 3.5
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