9,343 research outputs found
Modelling the variability of 1ES1218+30.4
The blazar 1ES1218+30.4 has been previously detected by the VERITAS and MAGIC
telescopes in the very high energies. The new detection of VERITAS from
December 2008 to April 2009 proves that 1ES1218+30.4 is not static, but shows
short-time variability. We show that the time variability may be explained in
the context of a self-consistent synchrotron-self Compton model, while the long
time observation do not necessarily require a time-resolved treatment. The
kinetic equations for electrons and photons in a plasma blob are solved
numerically including Fermi acceleration for electrons as well as synchrotron
radiation and Compton scattering. The light curve observed by VERITAS can be
reproduced in our model by assuming a changing level of electron injection
compared to the constant state of 1ES1218+30.4. The multiwavelength behaviour
during an outburst becomes comprehensible by the model. The long time
measurements of VERITAS are still explainable via a constant emission in the
SSC context, but the short outbursts each require a time-resolved treatment.Comment: 4 pages, 3 figures; accepted in A&A; AA/2010/1429
The post-outburst photometric behaviour of V838 Mon
The unusual eruptive variable discovered in Monoceros in 2002 January
underwent dramatic photometric and spectroscopic changes in the months prior to
its 2002 June-August conjunction with the Sun. Optical and infrared (IR)
photometry obtained at the South African Astronomical Observatory (SAAO)
between 2002 January and June (JD 2452280-440) is presented here in an analysis
of the star's post-outburst behaviour. The light curve indicated 3 eruptions
took place in 2002 January, February and March. SAAO echelle spectra obtained
in the week prior to the March maximum indicated the ejection of a new shell of
material. JHKL photometry obtained during 2002 April showed the development of
an IR excess due to the formation of a dust shell. The shell appears to be
largely responsible for the rapid fade in the optical flux during 2002
April-May (Delta V > 6 mag within 3 weeks). Blueing of the optical colours
during the decline is likely due either to the revealing of an emission line
region surrounding V838 Mon, or the unveiling of the progenitor or a
spatially-close early-type star.Comment: 7 pages, 7 figures - accepted for MNRA
SSC radiation in BL Lac sources, the end of the tether
The synchrotron-self Compton (SSC) radiation process is widely held to
provide a close representation of the double peaked spectral energy
distributions from BL Lac Objects (BL Lacs), which are marked by non-thermal
beamed radiations, highly variable on timescales of days or less. Their
outbursts in the gamma ray relative to the optical/X rays might be surmised to
be enhanced in BL Lacs as these photons are upscattered via the inverse Compton
(IC) process. From the observed correlations among the spectral parameters
during optical/X-ray variations we aim at predicting corresponding correlations
in the gamma-ray band, and the actual relations between the gamma-ray and the
X-ray variability consistent with the SSC emission process. We start from the
homogeneous single-zone SSC source model, with log-parabolic energies
distributions of emitting electron as required by the X-ray data of many
sources. We find relations among spectral parameters of the IC radiation in
both the Thomson (for Low energy BL Lacs) and the Klein-Nishina regimes (mainly
for High energy BL Lacs) and we compute how variability is driven by a smooth
increase of key source parameters, primarily the root mean square electron
energy. The single component SSC source model in the Thomson regime turns out
to be adequate for many LBL sources. However, the simple model meets its limits
with the fast/strong flares recently reported for a few sources in the TeV
range; these require sudden accelerations of emitting electrons in a second
source component.Comment: 12 pages, 2 tables, 8 figure
Broad-band nonthermal emission from molecular clouds illuminated by cosmic rays from nearby supernova remnants
Molecular clouds are expected to emit non-thermal radiation due to cosmic ray
interactions in the dense magnetized gas. Such emission is amplified if a cloud
is located close to an accelerator of cosmic rays and if energetic particles
can leave the accelerator site and diffusively reach the cloud. We consider
here the situation in which a molecular cloud is located in the proximity of a
supernova remnant which is efficiently accelerating cosmic rays and gradually
releasing them in the interstellar medium. We calculate the multiwavelength
spectrum from radio to gamma rays which is emerging from the cloud as the
result of cosmic ray interactions. The total energy output is dominated by the
gamma ray emission, which can exceed the emission in other bands by an order of
magnitude or more. This suggests that some of the unidentified TeV sources
detected so far, with no obvious or very weak counterparts in other
wavelengths, might be in fact associated with clouds illuminated by cosmic rays
coming from a nearby source. Moreover, under certain conditions, the gamma ray
spectrum exhibit a concave shape, being steep at low energies and hard at high
energies. This fact might have important implications for the studies of the
spectral compatibility of GeV and TeV gamma ray sources.Comment: 13 pages, 6 figures, submitted to MNRA
Supersonic water masers in 30 Doradus
We report on extremely high velocity molecular gas, up to -80 km/s relative
to the ambient medium, in the giant star-formation complex 30 Doradus in the
Large Magellanic Cloud (LMC), as observed in new 22 GHz H2O maser emission
spectra obtained with the Mopra radio telescope. The masers may trace the
velocities of protostars, and the observed morphology and kinematics indicate
that current star formation occurs near the interfaces of colliding
stellar-wind blown bubbles. The large space velocities of the protostars and
associated gas could result in efficient mixing of the LMC. A similar mechanism
in the Milky Way could seed the galactic halo with relatively young stars and
gas.Comment: 11 pages plus 1 PS and 1 EPS figure, uses AASTeX preprint style;
accepted for publication in Astrophysical Journal Letter
High-energy gamma-ray emission from the inner jet of LS I+61 303: the hadronic contribution revisited
LS I+61 303 has been detected by the Cherenkov telescope MAGIC at very high
energies, presenting a variable flux along the orbital motion with a maximum
clearly separated from the periastron passage. In the light of the new
observational constraints, we revisit the discussion of the production of
high-energy gamma rays from particle interactions in the inner jet of this
system. The hadronic contribution could represent a major fraction of the TeV
emission detected from this source. The spectral energy distribution resulting
from p-p interactions is recalculated. Opacity effects introduced by the photon
fields of the primary star and the stellar decretion disk are shown to be
essential in shaping the high-energy gamma-ray light curve at energies close to
200 GeV. We also present results of Monte Carlo simulations of the
electromagnetic cascades developed very close to the periastron passage. We
conclude that a hadronic microquasar model for the gamma-ray emission in LS I
+61 303 can reproduce the main features of its observed high-energy gamma-ray
flux.Comment: 6 pages. Sligth improvements made. Accepted version by Astrophysics
and Space Scienc
Homeomorphic Embedding for Online Termination of Symbolic Methods
Well-quasi orders in general, and homeomorphic embedding in particular, have gained popularity to ensure the termination of techniques for program analysis, specialisation, transformation, and verification. In this paper we survey and discuss this use of homeomorphic embedding and clarify the advantages of such an approach over one using well-founded orders. We also discuss various extensions of the homeomorphic embedding relation. We conclude with a study of homeomorphic embedding in the context of metaprogramming, presenting some new (positive and negative) results and open problems
Using the fluorescence decay of 2-aminopurine to investigate conformational change in the recognition sequence of the EcoRV DNA-(adenine-N6)-methyltransferase on enzyme binding
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