8,008 research outputs found
Self-sustained magnetoelectric oscillations in magnetic resonant tunneling structures
The dynamic interplay of transport, electrostatic, and magnetic effects in
the resonant tunneling through ferromagnetic quantum wells is theoretically
investigated. It is shown that the carrier-mediated magnetic order in the
ferromagnetic region not only induces, but also takes part in intrinsic,
robust, and sustainable high-frequency current oscillations over a large window
of nominally steady bias voltages. This phenomenon could spawn a new class of
quantum electronic devices based on ferromagnetic semiconductors.Comment: 5 pages, 4 figure
ROSAT PSPC detection of soft X-ray absorption in GB 1428+4217: The most distant matter yet probed with X-ray spectroscopy
We report on a ROSAT PSPC observation of the highly-luminous z = 4.72
radio-loud quasar GB 1428+4217 obtained between 1998 December 11 and 17, the
final days of the ROSAT satellite. The low-energy sensitivity of the PSPC
detector was employed to constrain the intrinsic X-ray absorption of the
currently most distant X-ray detected object. Here we present the detection of
significant soft X-ray absorption towards GB 1428+4217, making the absorbing
material the most distant matter yet probed with X-ray spectroscopy. X-ray
variability by 25+-8 per cent is detected on a timescale of 6500 s in the rest
frame. The X-ray variation requires an unusually high radiative efficiency of
at least 4.2, further supporting the blazar nature of the source.Comment: 6 pages incl. 6 figures, accepted for publication in Monthly Notice
Ion-supported tori: a thermal bremsstrahlung model for the X-ray Background
We discuss the possibility that a significant contribution of the hard X-ray
Background is the integrated emission from a population of galaxies undergoing
advection-dominated accretion in their nuclei. Owing to poor coupling between
ions and electrons and to efficient radiative cooling of the electrons, the
accreting plasma is two-temperature, with the ions being generally much hotter
than the electrons and forming an ion-supported torus. We show that the
electron te mperature then saturates at approximately 100keV independent of
model parameters. At this temperature the hard X-ray emission is dominated by
bremsstrahlung radiation. We find that this physical model gives an excellent
fit to the spectrum of the XRB in the 3-60 keV range, provided that there is
some evolution associated with the spectral emissivity which must peak at a
redshift of about 2. We estimate that such galaxies contribute only to a small
fraction of the local X-ray volume emissivity. The model implies a higher mean
black hole mass than is obtained from the evolution of quasars alone.Comment: 7 pages, 7 ps figures, uses mn.sty (included). Submitted for
publication to MNRA
Does size matter? Statistical limits of paleomagnetic field reconstruction from small rock specimens
Some remarks about intrinsic parity in Ryder's derivation of the Dirac equation
This work is a comment on Ryder's derivation of the Dirac equation, with
emphasis on the physical contents of this equation: the notion of particles and
antiparticles according to the Stueckelberg-Feynman interpretation, the
opposite intrinsic parity between particles and antiparticles, and the spin.Comment: 4 pages, Revte
Magnetic flares in accretion disc coronae and the Spectral States of black hole candidates: the case of GX 339-4
We present a model for the different X-ray spectral states displayed by
Galactic Black Hole Candidates (GBHC). We discuss the physical and spectral
implications for a magnetically structured corona in which magnetic flares
result from reconnection of flux tubes rising from the accretion disk by the
magnetic buoyancy instability. Using observations of one of the best studied
examples, GX339-4, we identify the geometry and the physical conditions
characterizing each of these states. We find that, in the Soft state, flaring
occurs at small scale heights above the accretion disk. The soft thermal-like
spectrum is the result of heating and consequent re-radiation of the hard
X-rays produced by such flares. The hard tail is produced by Comptonization of
the soft field radiation. Conversely, the hard state is the result of flares
triggered high above the underlying accretion disk which produce X-rays via
Comptonization of either internal synchrotron radiation or soft disk photons.
The spectral characteristics of the different states are naturally accounted
for by the choice of geometry: when flares are triggered high above the disk
the system is photon-starved, hence the hard Comptonized spectrum of the hard
state. Intense flaring close to the disk greatly enhances the soft-photon field
with the result that the spectrum softens. We interpret the two states as being
related to two different phases of magnetic energy dissipation. In the Soft
state, Parker instability in the disk favours the emergence of large numbers of
relatively low magnetic field flux tubes. In the hard state, only intense
magnetic fields become buoyant. The model can also qualitatively account for
the observed short timescale variability and the characteristics of the X-ray
reflected component of the hard state.Comment: submitted to MNRAS, Feb. 1998, 10 pages, 3 figures in MNRAS LaTex
styl
Magnetic flares and the optical variability of the X-ray transient XTE J1118+480
The simultaneous presence of a strong quasi periodic oscillation of period of
about 10 seconds in the optical and X-ray lightcurves of the X-ray transient
XTE J1118+480 suggests that a significant fraction of the optical flux
originates from the inner part of the accretion flow, where most of the X-rays
are produced. We present a model of magnetic flares in an accretion disc corona
where thermal cyclo-synchrotron emission contributes significantly to the
optical emission, while the X-rays are produced by inverse Compton scattering
of the soft photons produced by dissipation in the underlying disc and by the
synchrotron process itself. Given the observational constraints, we estimate
the values for the coronal temperature, optical depth and magnetic field
intensity, as well as the accretion rate for the source. Within our model we
predict a correlation between optical and hard X-ray variability and an
anticorrelation between optical and soft X-rays. We also expect optical
variability on flaring timescales (about tens of milliseconds), with a power
density spectrum similar to the one observed in the X-ray band. Finally we use
both the available optical/EUV/X-ray spectral energy distribution and the low
frequency time variability to discuss limits on the inner radius of the
optically thick disc.Comment: 5 pages, included 1 figure. One reference corrected. Submitted to
MNRA
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