2,555 research outputs found
Bright X-ray bursts from 1E 1724-3045 in Terzan 2
During about 3 years wide field monitoring of the Galactic Center region by
the WFC telescopes on board the BeppoSAX satellite, a total of 14 type-I X-ray
bursts were detected from the burster 1E 1724-3045 located in the globular
cluster Terzan 2. All the observed events showed evidence for photospheric
radius expansion due to Eddington-limit burst luminosity, thus leading to an
estimate of the source distance (~7.2 kpc). Preliminary results of the analysis
of the bursts are presented.Comment: 5 pages, 2 figures, Proc. 5th Compton Symp., Portsmouth 199
The Accretion Flows and Evolution of Magnetic Cataclysmic Variables
We have used a model of magnetic accretion to investigate the accretion flows
of magnetic cataclysmic variables. Numerical simulations demonstrate that four
types of flow are possible: discs, streams, rings and propellers. The
fundamental observable determining the accretion flow, for a given mass ratio,
is the spin-to-orbital period ratio of the system. If IPs are accreting at
their equilibrium spin rates, then for a mass ratio of 0.5, those with
Pspin/Porb < 0.1 will be disc-like, those with 0.1 < Pspin/Porb < 0.6 will be
stream-like, and those with Pspin/Porb ~ 0.6 will be ring-like. The spin to
orbital period ratio at which the systems transition between these flow types
increases as the mass ratio of the stellar components decreases.
For the first time we present evolutionary tracks of mCVs which allow
investigation of how their accretion flow changes with time. As systems evolve
to shorter orbital periods and smaller mass ratios, in order to maintain spin
equilibrium, their spin-to-orbital period ratio will generally increase. As a
result, the relative occurrence of ring-like flows will increase, and the
occurrence of disc-like flows will decrease, at short orbital periods. The
growing number of systems observed at high spin-to-orbital period ratios with
orbital periods below 2h, and the observational evidence for ring-like
accretion in EX Hya, are fully consistent with this picture.Comment: Accepted for publication in ApJ. 6 figures - included here at low
resolutio
X-ray Spectral Diagnostics of Gamma-Ray Burst Environments
Recently, the detection of discrete features in the X-ray afterglow spectra
of GRB970508 and GRB970828 was reported. The most natural interpretation of
these features is that they are redshifted Fe K emission complexes. The
identification of the line emission mechanism has drastic implications for the
inferred mass of radiating material, end hence the nature of the burst site.
X-ray spectroscopy provides a direct observational constraint on these
properties of gamma-ray bursters. We briefly discuss how these constraints
arise, in the context of an application to the spectrum of GRB970508.Comment: 11 pages, 2 figures, accepted for publication in ApJ Letter
Iron line signatures in X-ray afterglows of GRB by BeppoSAX
We report the possible detection (99.3% of statistical significance) of
redshifted Fe iron line emission in the X-ray afterglow of Gamma-ray burst
GRB970508 observed by BeppoSAX. Its energy is consistent with the redshift of
the putative host galaxy determined from optical spectroscopy. In contrast to
the fairly clean environment expected in the merging of two neutron stars, the
observed line properties would imply that the site of the burst is embedded in
a large mass of material (>0.5 \Ms), consistent with pre-explosion ejecta of
a very massive star. This material could be related with the outburst observed
in the afterglow 1 day after the GRB and with the spectral variations measured
during this phase. We did not find evidence of Fe line in two other GRB with
known redshift (GB971214 and GB980613), but we note that the upper limits are
of the same order of the intensity measured in GB97508 and that none of these
afterglows shows rebursting activity.Comment: 2 pages, 1 figure, proc. of Gamma-Ray Bursts in the Afterglow Era,
Nov. 1998, Rome, F. Frontera & L. Piro ed.s., A&A Suppl. Ser., in pres
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