84,374 research outputs found

### Prospects for Detection of Synchrotron Emission from Secondary Electrons and Positrons in Starless Cores: Application to G0.216+0.016

We investigate the diffusion of cosmic rays into molecular cloud complexes.
Using the cosmic-ray diffusion formalism of Protheroe, et al. (2008), we
examine how cosmic rays diffuse into clouds exhibiting different density
structures, including a smoothed step-function, as well as Gaussian and
inverse-$r$ density distributions, which are well known to trace the structure
of star-forming regions. These density distributions were modelled as an
approximation to the Galactic centre cloud G0.216+0.016, a recently-discovered
massive dust clump that exhibits limited signs of massive star formation and
thus may be the best region in the Galaxy to observe synchrotron emission from
secondary electrons and positrons. Examination of the resulting synchrotron
emission, produced by the interaction of cosmic ray protons interacting with
ambient molecular matter producing secondary electrons and positrons reveals
that, due to projection effects, limb-brightened morphology results in all
cases. However, we find that the Gaussian and inverse-$r$ density distributions
show much broader flux density distributions than step-function distributions.
Significantly, some of the compact (compared to the $2.2''$ resolution, 5.3 GHz
JVLA observations) sources show non-thermal emission, which may potentially be
explained by the density structure and the lack of diffusion of cosmic rays
into the cloud. We find that we can match the 5.3 and 20 GHz flux densities of
the non-thermal source JVLA~1 and 6 from Rodr\'{\i}guez & Zapata (2014) with a
local cosmic ray flux density, a diffusion coefficient suppression factor of
$\chi=0.1-0.01$ for a coefficient of $3\times10^{27}$ cm$^2$ s$^{-1}$, and a
magnetic field strength of 470 $\mu$G.Comment: 4 pages, 2 figures, accepted for publication in the Astrophysical
Journal Letter

### Pulsar state switching, timing noise and free precession

Recent radio pulsar observations have shown that a number of pulsars display
interesting long term periodicities in their spin-down rates. At least some of
these pulsars also undergo sharp changes in pulse profile. This has been
convincingly attributed to the stars abruptly switching between two different
magnetospheric states. The sharpness of these transitions has been taken as
evidence against free precession as the mechanism behind the long term
variations. We argue that such a conclusion is premature. By performing a
simple best-fit analysis to the data, we show that the relationship between the
observed spin and modulation periods is of approximately the correct form to be
accounted for by the free precession of a population of neutron stars with
strained crusts, the level of strain being similar in all of the stars, and
consistent with the star retaining a memory of a former faster rotation rate.
We also provide an argument as to why abrupt magnetospheric changes can occur
in precessing stars, and how such changes would serve to magnify the effect of
precession in the timing data, making the observation of the precession more
likely in those stars where such switching occurs. We describe how future
observations could further test the precession hypothesis advanced here.Comment: Additional reference inserted; to appear in MNRA

### Parameter choices and ranges for continuous gravitational wave searches for steadily spinning neutron stars

We consider the issue of selecting parameters and their associated ranges for
carrying out searches for continuous gravitational waves from steadily rotating
neutron stars. We consider three different cases (i) the "classic" case of a
star spinning about a principal axis; (ii) a biaxial star, not spinning about a
principal axis; (iii) a triaxial star spinning steady, but not about a
principal axis (as described in Jones, MNRAS vol 402, 2503 (2010)). The first
of these emits only at one frequency; the other two at a pair of harmonically
related frequencies. We show that in all three cases, when written in terms of
the original "source parameters", there exist a number of discrete
degeneracies, with different parameter values giving rise to the same
gravitational wave signal. We show how these can be removed by suitably
restricting the source parameter ranges. In the case of the model as written
down by Jones, there is also a continuous degeneracy. We show how to remove
this through a suitable rewriting in terms of "waveform parameters", chosen so
as to make the specialisations to the other stellar models particularly simple.
We briefly consider the (non-trivial) relation between the assignment of prior
probabilities on one set of parameters verses the other. The results of this
paper will be of use when designing strategies for carrying out searches for
such multi-harmonic gravitational wave signals, and when performing parameter
estimation in the event of a detection.Comment: Updated to match version accepted by MNRAS: One new equation
(equation 82)); typo (sign-error) corrected in equation (88); one more
paragraph inserted into Summary and Discussion sectio

### Three evolutionary paths for magnetar oscillations

Quasi-periodic oscillations have been seen in the light curves following
several magnetar giant flares. These oscillations are of great interest as they
probably provide our first ever view of the normal modes of oscillation of
neutron stars. The state-of-the-art lies in the study of the oscillations of
elastic-magnetic stellar models, mainly with a view to relating the observed
frequencies to the structure and composition of the star itself. We advance
this programme by considering several new physical mechanisms that are likely
to be important for magnetar oscillations. These relate to the
superfluid/superconducting nature of the stellar interior, and the damping of
the modes, both through internal dissipation mechanisms and the launching of
waves into the magnetosphere. We make simple order-of-magnitude estimates to
show that both the frequencies and the damping time of magnetar oscillations
can evolve in time, identifying three distinct `pathways' that can be followed,
depending upon the initial magnitude of the mode excitation. These results are
interesting as they show that the information buried in magnetar QPOs may be
even richer than previously thought, and motivate more careful examination of
magnetar light curves, to search for signatures of the different types of
evolution that we have identified.Comment: To appear in MNRAS. This version reflects changes made in response to
referee's comments, mainly extra discussion in Section 2.

### Ultra-violet Finiteness in Noncommutative Supersymmetric Theories

We consider the ultra-violet divergence structure of general noncommutative
supersymmetric $U(N_c)$ gauge theories, and seek theories which are all-orders
finite.Comment: 11 pages, Tex, one figure. Uses harvmac (big) and eps

### Fayet-Iliopoulos D-terms and anomaly mediated supersymmetry breaking

We show that in a minimal extension of the MSSM by means of an extra U(1)
gauge group, the negative mass-squared problem characteristic of the Anomaly
Mediated Supersymmetry Breaking scenario is naturally solved by means of
Fayet-Iliopoulos D-terms. We derive a set of sum rules for the sparticle masses
which are consequences of the resulting framework.Comment: 11 pages, including 1 figure. Plain TeX. Uses Harvmac and epsf. Final
version to appear in Phys. Lett. B; some minor improvement

### The Exact Tachyon Beta-Function for the Wess-Zumino-Witten Model

We derive an exact expression for the tachyon $\beta$-function for the
Wess-Zumino-Witten model. We check our result up to three loops by calculating
the three-loop tachyon $\beta$-function for a general non-linear $\sigma$-model
with torsion, and then specialising to the case of the WZW model.Comment: 13pp, uses harvmac (with figures appended as a a standard PostScript
file), LTH 31

### Quasi-infra-red fixed points and renormalisation group invariant trajectories for non-holomorphic soft supersymmetry breaking

In the MSSM the quasi-infra-red fixed point for the top-quark Yukawa coupling
gives rise to specific predictions for the soft-breaking parameters. We discuss
the extent to which these predictions are modified by the introduction of
additional ``non-holomorphic'' soft-breaking terms. We also show that in a
specific class of theories there exists an RG-invariant trajectory for the
``non-holomorphic'' terms, which can be understood using a holomorphic spurion
term.Comment: 24 pages, TeX, two figures. Uses Harvmac (big) and epsf. Minor errors
corrected, and the RG trajectory explained in terms of a holomorphic spurion
ter

### The Gaugino \beta-Function

We present an elegant exact formula for the gaugino $\beta$-function in a
softly-broken supersymmetric gauge theory, of the form $\beta_M = {\cal
O}(\beta_g/g)$, where $\beta_g$ is the gauge $\beta$ function and ${\cal O}$ is
a simple differential operator acting on the gauge coupling $g$ and the Yukawa
coupling.Comment: 11 pages, tex, no figures. Uses harvmac. Minor error in derivation of
Eq. (14) correcte

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