8,970 research outputs found
Gamma-Ray Burst Spectral Features: Interpretation as X-ray Emission From A Photoionized Plasma
Numerous reports have been made of features, either in emission or
absorption, in the 10 - 1000 keV spectra of some gamma-ray bursts. Originally
interpreted in the context of Galactic neutron star models as cyclotron line
emission and annihilation features, the recent demonstration that
the majority of GRBs lie at cosmological distances make these explanations
unlikely. In this letter, we adopt a relativistic fireball model for
cosmological GRBs in which dense, metal rich blobs or filaments of plasma are
entrained in the relativistic outflow. In the context of this model, we
investigate the conditions under which broadband features, similar to those
detected, can be observed. We find a limited region of parameter space capable
of reproducing the observed GRB spectra. Finally, we discuss possible
constraints further high-energy spectral observations could place on fireball
model parameters.Comment: Accepted for publication in Astrophysical Journal Letters Four pages,
2 figure
Magnetic properties and complex magnetic phase diagram in non centrosymmetric EuRhGe and EuIrGe single crystals
We report the magnetic properties of two Eu based compounds, single
crystalline EuIrGe and EuRhGe, inferred from magnetisation, electrical
transport, heat capacity and Eu M\"{o}ssbauer spectroscopy. These
previously known compounds crystallise in the non-centrosymmetric, tetragonal,
, BaNiSn-type structure. Single crystals of EuIrGe and EuRhGe
were grown using high temperature solution growth method using In as flux.
EuIrGe exhibits two magnetic orderings at = 12.4 K, and
= 7.3 K. On the other hand EuRhGe presents a single magnetic
transition with a = 12 K. Eu M\"{o}ssbauer spectra present
evidence for a cascade of transitions from paramagnetic to incommensurate
amplitude modulated followed by an equal moment antiferromagnetic phase at
lower temperatures in EuIrGe, the transitions having a substantial first
order character. On the other hand the Eu M\"{o}ssbauer spectra at 4.2
and 9 K in EuRhGe present evidence of a single magnetic transition. In both
compounds a superzone gap is observed for the current density
[001], which enhances with transverse magnetic field. The magnetisation
measured up to 14 T shows the occurrence of field induced transitions, which
are well documented in the magnetotransport data as well. The magnetic phase
diagram constructed from these data is complex, revealing the presence of many
phases in the phase space
Anisotropic magnetic and superconducting properties of pure and Co-doped CaFeAs single crystals
We report anisotropic dc magnetic susceptibility , electrical
resistivity , and heat capacity measurements on the single
crystals of CaFeCoAs for = 0 and 0.06. Large sized single
crystals were grown by the high temperature solution method with Sn as the
solvent. For the pure compound with = 0, a high temperature transition at
170 K is observed which is attributed to a combined spin density wave (SDW)
ordering and a structural phase transition. On the other hand, for the Co-doped
samples for = 0.06, the SDW transition is suppressed while
superconductivity is observed at 17 K. The superconducting transition
has been confirmed from the magnetization and electrical resistivity studies.
The Fe M\"ossbauer spectrum in CaFeAs indicates that the SDW
ordering is incommensurate. In the Co-doped sample, a prominent paramagnetic
line at 4.2 K is observed indicating a weakening of the SDW state.Comment: 4 pages 5 figures. Submitted to Phys. Rev.
Distribution of the second virial coefficients of globular proteins
George and Wilson [Acta. Cryst. D 50, 361 (1994)] looked at the distribution
of values of the second virial coefficient of globular proteins, under the
conditions at which they crystallise. They found the values to lie within a
fairly narrow range. We have defined a simple model of a generic globular
protein. We then generate a set of proteins by picking values for the
parameters of the model from a probability distribution. At fixed solubility,
this set of proteins is found to have values of the second virial coefficient
that fall within a fairly narrow range. The shape of the probability
distribution of the second virial coefficient is Gaussian because the second
virial coefficient is a sum of contributions from different patches on the
protein surface.Comment: 5 pages, including 3 figure
Variability Profiles of Millisecond X-Ray Pulsars: Results of Pseudo-Newtonian 3D MHD Simulations
We model the variability profiles of millisecond period X-ray pulsars. We
performed three-dimensional magnetohydrodynamic simulations of disk accretion
to millisecond period neutron stars with a misaligned magnetic dipole moment,
using the pseudo-Newtonian Paczynski-Wiita potential to model general
relativistic effects. We found that the shapes of the resulting funnel streams
of accreting matter and the hot spots on the surface of the star are quite
similar to those for more slowly rotating stars obtained from earlier
simulations using the Newtonian potential. The funnel streams and hot spots
rotate approximately with the same angular velocity as the star. The spots are
bow-shaped (bar-shaped) for small (large) misalignment angles. We found that
the matter falling on the star has a higher Mach number when we use the
Paczynski-Wiita potential than in the Newtonian case.
Having obtained the surface distribution of the emitted flux, we calculated
the variability curves of the star, taking into account general relativistic,
Doppler and light-travel-time effects. We found that general relativistic
effects decrease the pulse fraction (flatten the light curve), while Doppler
and light-travel-time effects increase it and distort the light curve. We also
found that the light curves from our hot spots are reproduced reasonably well
by spots with a gaussian flux distribution centered at the magnetic poles. We
also calculated the observed image of the star in a few cases, and saw that for
certain orientations, both the antipodal hot spots are simultaneously visible,
as noted by earlier authors.Comment: 9 pages, 10 figures, accepted for publication in ApJ; corrected some
typo
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