507 research outputs found
Anisotropic charge transport in non-polar GaN QW: polarization induced charge and interface roughness scattering
Charge transport in GaN quantum well (QW) devices grown in non-polar
direction has been theoretically investigated . Emergence of anisotropic line
charge scattering mechanism originating as a result of anisotropic rough
surface morphology in conjunction with in-plane built-in polarization has been
proposed. It has shown that in-plane growth anisotropy leads to large
anisotropic carrier transport at low temperatures. At high temperatures, this
anisotropy in charge transport is partially washed out by strong isotropic
optical phonon scattering in GaN QW.Comment: 4 pages, 4 figure
Radio Spectra of Giant Radio Galaxies from RATAN-600 Data
Measurements of the flux densities of the extended components of seven giant
radio galaxies obtained using the RATAN-600 radio telescope at wavelengths of
6.25 and 13 cm ar e presented. The spectra of components of these radio
galaxies are constructed using these new RA TAN-600 data together with data
from the WENSS, NVSS, and GB6 surveys. The spectral indices in the stu died
frequency range are calculated, and the need for detailed estimates of the
integrated contributi on of such objects to the background emission is
demonstrated.Comment: 7 pages, 2 figures, 5 table
Sqrt{shat}_{min} resurrected
We discuss the use of the variable sqrt{shat}_{min}, which has been proposed
in order to measure the hard scale of a multi parton final state event using
inclusive quantities only, on a SUSY data sample for a 14 TeV LHC. In its
original version, where this variable was proposed on calorimeter level, the
direct correlation to the hard scattering scale does not survive when effects
from soft physics are taken into account. We here show that when using
reconstructed objects instead of calorimeter energy and momenta as input, we
manage to actually recover this correlation for the parameter point considered
here. We furthermore discuss the effect of including W + jets and t tbar+jets
background in our analysis and the use of sqrt{shat}_{min} for the suppression
of SM induced background in new physics searches.Comment: 23 pages, 9 figures; v2: 1 figure, several subsections and references
as well as new author affiliation added. Corresponds to published versio
A multifrequency study of giant radio sources-II. Spectral ageing analysis of the lobes of selected sources
Multifrequency observations with the GMRT and the VLA are used to determine
the spectral breaks in consecutive strips along the lobes of a sample of
selected giant radio sources (GRSs) in order to estimate their spectral ages.
The maximum spectral ages estimated for the detected radio emission in the
lobes of our sources range from 6 to 36 Myr with a median value of
20 Myr using the classical equipartition fields. Using the magnetic field
estimates from the Beck & Krause formalism the spectral ages range from 5
to 38 Myr with a median value of 22 Myr. These ages are significantly
older than smaller sources. In all but one source (J1313+6937) the spectral age
gradually increases with distance from the hotspot regions, confirming that
acceleration of the particles mainly occurs in the hotspots. Most of the GRSs
do not exhibit zero spectral ages in the hotspots, as is the case in earlier
studies of smaller sources. This is likely to be largely due to contamination
by more extended emission due to relatively modest resolutions. The injection
spectral indices range from 0.55 to 0.88 with a median value of
0.6. We discuss these values in the light of theoretical expectations,
and show that the injection spectral index appears to be correlated with
luminosity and/or redshift as well as with linear size.Comment: 12 Pages, 13 Figures, 9 Tables, Accepted for publication in MNRA
Restrictions on parameters of power-law magnetic field decay for accreting isolated neutron stars
In this short note we discuss the influence of power-law magnetic field decay
on the evolution of old accreting isolated neutron stars. We show, that,
contrary to exponential field decay (Popov & Prokhorov 2000), no additional
restrictions can be made for the parameters of power-law decay from the
statistics of isolated neutron star candidates in ROSAT observations.
We also briefly discuss the fate of old magnetars with and without field
decay, and describe parameters of old accreting magnetars.Comment: 8 pages including 3 PostScript figure
The stransverse mass, MT2, in special cases
This document describes some special cases in which the stransverse mass,
MT2, may be calculated by non-iterative algorithms. The most notable special
case is that in which the visible particles and the hypothesised invisible
particles are massless -- a situation relevant to its current usage in the
Large Hadron Collider as a discovery variable, and a situation for which no
analytic answer was previously known. We also derive an expression for MT2 in
another set of new (though arguably less interesting) special cases in which
the missing transverse momentum must point parallel or anti parallel to the
visible momentum sum. In addition, we find new derivations for already known
MT2 solutions in a manner that maintains manifest contralinear boost invariance
throughout, providing new insights into old results. Along the way, we stumble
across some unexpected results and make conjectures relating to geometric forms
of M_eff and H_T and their relationship to MT2.Comment: 11 pages, no figures. v2 corrects minor typos. v3 corrects an
incorrect statement in footnote 8 and inserts a missing term in eq (3.9). v4
and v5 correct minor typos spotted by reader
On the Origin of X-ray Emission From Millisecond Pulsars in 47 Tuc
The observed spectra and X-ray luminosities of millisecond pulsars in 47 Tuc
can be interpreted in the context of theoretical models based on strong, small
scale multipole fields on the neutron star surface. For multipole fields that
are relatively strong as compared to the large scale dipole field, the emitted
X-rays are thermal and likely result from polar cap heating associated with the
return current from the polar gap. On the other hand, for weak multipole
fields, the emission is nonthermal and results from synchrotron radiation of
pairs created by curvature radiation. The X-ray luminosity, , is
related to the spin down power, , expressed in the form with and for strong and weak
multipole fields respectively. If the polar cap size is of the order of the
length scale of the multipole field, and , the polar cap
temperature is .
A comparison of the X-ray properties of millisecond pulsars in globular
clusters and in the Galactic field suggests that the emergence of relatively
strong small scale multipole fields from the neutron star interior may be
correlated with the age and evolutionary history of the underlying neutron
star.Comment: 25 pages, 2 figures, accepted for publication in Ap
General Relativistic Electromagnetic Fields of a Slowly Rotating Magnetized Neutron Star. I. Formulation of the equations
We present analytic solutions of Maxwell equations in the internal and
external background spacetime of a slowly rotating magnetized neutron star. The
star is considered isolated and in vacuum, with a dipolar magnetic field not
aligned with the axis of rotation. With respect to a flat spacetime solution,
general relativity introduces corrections related both to the monopolar and the
dipolar parts of the gravitational field. In particular, we show that in the
case of infinite electrical conductivity general relativistic corrections due
to the dragging of reference frames are present, but only in the expression for
the electric field. In the case of finite electrical conductivity, however,
corrections due both to the spacetime curvature and to the dragging of
reference frames are shown to be present in the induction equation. These
corrections could be relevant for the evolution of the magnetic fields of
pulsars and magnetars. The solutions found, while obtained through some
simplifying assumption, reflect a rather general physical configuration and
could therefore be used in a variety of astrophysical situations.Comment: A few typos corrected; matches the versions in MNRA
Age-related changes in matching novel objects across viewpoints
Peer reviewedPreprin
Neutron Star Physics in the Square Kilometre Array Era: An Indian Perspective
It is an exceptionally opportune time for astrophysics when a number of next-generation mega-instruments are poised to observe the Universe across the entire electromagnetic spectrum with unprecedented data quality. The Square Kilometre Array (SKA) is undoubtedly one of the major components of this scenario. In particular, the SKA is expected to discover tens of thousands of new neutron stars giving a major fillip to a wide range of scientific investigations. India has a sizeable community of scientists working on different aspects of neutron star physics with immediate access to both the uGMRT (an SKA pathfinder) and the recently launched X-ray observatory Astrosat. The current interests of the community largely centre around studies of (a) the generation of neutron stars and the SNe connection, (b) the neutron star population and evolutionary pathways, (c) the evolution of neutron stars in binaries and the magnetic fields, (d) the neutron star equation of state, (e) the radio pulsar emission mechanism, and (f) the radio pulsars as probes of gravitational physics. Most of these studies are the main goals of the SKA first phase, which is likely to be operational in the next four years. This article summarizes the science goals of the Indian neutron star community in the SKA era, with significant focus on coordinated efforts among the SKA and other existing/upcoming instruments
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