338 research outputs found
Hard X-ray emitting black hole fed by accretion of low angular momentum matter
Observed spectra of Active Galactic Nuclei (AGN) and luminous X-ray binaries
in our Galaxy suggest that both hot (~10^9 K) and cold (~10^6 K) plasma
components exist close to the central accreting black hole. Hard X-ray
component of the spectra is usually explained by Compton upscattering of
optical/UV photons from optically thick cold plasma by hot electrons.
Observations also indicate that some of these objects are quite efficient in
converting gravitational energy of accretion matter into radiation. Existing
theoretical models have difficulties in explaining the two plasma components
and high intensity of hard X-rays. Most of the models assume that the hot
component emerges from the cold one due to some kind of instability, but no one
offers a satisfactory physical explanation for this. Here we propose a solution
to these difficulties that reverses what was imagined previously: in our model
the hot component forms first and afterward it cools down to form the cold
component. In our model, accretion flow has initially a small angular momentum,
and thus it has a quasi-spherical geometry at large radii. Close to the black
hole, the accreting matter is heated up in shocks that form due to the action
of the centrifugal force. The hot post-shock matter is very efficiently cooled
down by Comptonization of low energy photons and condensates into a thin and
cold accretion disk. The thin disk emits the low energy photons which cool the
hot component.Comment: 15 pages, 2 figures, submitted to ApJ Let
Growth rate of the initial magnetic energy in isotropic velocity field with non-Gaussian distribution
We propose a method for including the effects of non-Gaussian velocity field
distribution in the estimation of growth rate of the magnetic energy in a
random flow with finite memory time. The method allows a reduction to the
Gaussian case that was investigates earlier. For illustration we consider the
multivariate Laplace distribution and compare it against the Gaussian one
Evidence for "Propeller" Effects In X-ray Pulsars GX 1+4 And GROJ1744-28
We present observational evidence for "propeller" effects in two X-ray
pulsars, GX 1+4 and GROJ1744-28. Both sources were monitored regularly by the
Rossi X-ray Timing Explorer (RXTE) throughout a decaying period in the X-ray
brightness. Quite remarkably, strong X-ray pulsation became unmeasurable when
total X-ray flux had dropped below a certain threshold. Such a phenomenon is a
clear indication of the propeller effects which take place when pulsar
magnetosphere grows beyond the co-rotation radius as a result of the decrease
in mass accretion rate and centrifugal force prevents accreting matter from
reaching the magnetic poles. The entire process should simply reverse as the
accretion rate increases. Indeed, steady X-ray pulsation was reestablished as
the sources emerged from the non-pulsating faint state. These data allow us to
directly derive the surface polar magnetic field strength for both pulsars:
3.1E+13 G for GX 1+4 and 2.4E+11 G for GROJ1744-28. The results are likely to
be accurate to within a factor of 2, with the total uncertainty dominated by
the uncertainty in estimating the distances to the sources. Possible mechanisms
for the persistent emission observed in the faint state are discussed in light
of the extreme magnetic properties of the sources.Comment: 12 pages including 3 ps figures. To appear in ApJ Letters Vol. 48
Equation of state of superfluid neutron matter and the calculation of pairing gap
We present a Quantum Monte Carlo study of the zero temperature equation of
state of neutron matter and the computation of the pairing gap in the
low-density regime with fm. The system is described by a
non-relativistic nuclear Hamiltonian including both two-- and three--nucleon
interactions of the Argonne and Urbana type. This model interaction provides
very accurate results in the calculation of the binding energy of light nuclei.
A suppression of the gap with respect to the pure BCS theory is found, but
sensibly weaker than in other works that attempt to include polarization
effects in an approximate way
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