1,056 research outputs found
Monte-Carlo simulations of thermal/nonthermal radiation from a neutron-star magnetospheric accretion shell
We discuss the space-and-time-dependent Monte Carlo code we have developed to
simulate the relativistic radiation output from compact astrophysical objects,
coupled to a Fokker-Planck code to determine the self-consistent lepton
populations. We have applied this code to model the emission from a magnetized
neutron star accretion shell near the Alfven radius, reprocessing the radiation
from the neutron sar surface. We explore the parameter space defined by the
accretion rate, stellar surface field and the level of wave turbulence in the
shell. Our results are relevant to the emission from atoll sources, soft-X-ray
transient X-ray binaries containing weakly magnetized neutron stars, and to
recently suggested models of accretion-powered emission from anomalous X-ray
pulsars.Comment: 24 pages, including 7 figures; uses epsf.sty. final version, accepted
for publication in ApJ. Extended introduction and discussio
Analyzing the Multiwavelength Spectrum and Variability of BL Lacertae During the July 1997 Outburst
The multiwavelength spectrum of BL Lacertae during its July 1997 outburst is
analyzed in terms of different variations of the homogeneous leptonic jet model
for the production of high-energy radiation from blazars. We find that a
two-component gamma-ray spectrum, consisting of a synchrotron self-Compton and
an external Compton component, is required in order to yield an acceptable fit
to the broadband spectrum. Our analysis indicates that in BL Lac, unlike other
BL Lac objects, the broad emission line region plays an important role for the
high-energy emission. Several alternative blazar jet models are briefly
discussed. In the appendix, we describe the formalism in which the process of
Comptonization of reprocessed accretion disk photons is treated in the
previously developed blazar jet simulation code which we use.Comment: Now accepted for publication in The Astronomical Journal.
Significantly extended discussion w.r.t. original version. 3 Figures included
using epsf.sty, rotate.st
Lamination And Microstructuring Technology for a Bio-Cell Multiwell array
Microtechnology becomes a versatile tool for biological and biomedical
applications. Microwells have been established long but remained
non-intelligent up to now. Merging new fabrication techniques and handling
concepts with microelectronics enables to realize intelligent microwells
suitable for future improved cancer treatment. The described technology depicts
the basis for the fabrication of a elecronically enhanced microwell. Thin
aluminium sheets are structured by laser micro machining and laminated
successively to obtain registration tolerances of the respective layers of
5..10\^Am. The microwells lasermachined into the laminate are with
50..80\^Am diameter, allowing to hold individual cells within the well.
The individual process steps are described and results on the microstructuring
are given.Comment: Submitted on behalf of EDA Publishing Association
(http://irevues.inist.fr/EDA-Publishing
The prompt X-ray emission of GRB011211: possible evidence of a transient absorption feature
We report on observation results of the prompt X- and gamma-ray emission from
GRB011211. This event was detected with the Gamma-Ray Burst Monitor and one of
the Wide Field Cameras aboard the BeppoSAX satellite. The optical counterpart
to the GRB was soon identified and its redshift determined (z = 2.140), while
with the XMM-Newton satellite, the X-ray afterglow emission was detected.
Evidence of soft X-ray emission lines was reported by Reeves et al. (2002), but
not confirmed by other authors. In investigating the spectral evolution of the
prompt emission we find the possible evidence of a transient absorption feature
at 6.9^{+0.6}_{-0.5} keV during the rise of the primary event. The significance
of the feature is derived with non parametric tests and numerical simulations,
finding a chance probability which ranges from 3x10^{-3} down to 4x10^{-4}. The
feature shows a Gaussian profile and an equivalent width of 1.2^{+0.5}_{-0.6}
keV. We discuss our results and their possible interpretation.Comment: 23 pages, 3 Tables, 6 Figures. Accepted for publication in
Astrophysical Journa
Gamma Rays from Compton Scattering in the Jets of Microquasars: Application to LS 5039
Recent HESS observations show that microquasars in high-mass systems are
sources of VHE gamma-rays. A leptonic jet model for microquasar gamma-ray
emission is developed. Using the head-on approximation for the Compton cross
section and taking into account angular effects from the star's orbital motion,
we derive expressions to calculate the spectrum of gamma rays when nonthermal
jet electrons Compton-scatter photons of the stellar radiation field.
Calculations are presented for power-law distributions of nonthermal electrons
that are assumed to be isotropically distributed in the comoving jet frame, and
applied to -ray observations of LS 5039. We conclude that (1) the TeV
emission measured with HESS cannot result only from Compton-scattered stellar
radiation (CSSR), but could be synchrotron self-Compton (SSC) emission or a
combination of CSSR and SSC; (2) fitting both the HESS data and the EGRET data
associated with LS 5039 requires a very improbable leptonic model with a very
hard electron spectrum. Because the gamma rays would be variable in a leptonic
jet model, the data sets are unlikely to be representative of a simultaneously
measured gamma-ray spectrum. We therefore attribute EGRET gamma rays primarily
to CSSR emission, and HESS gamma rays to SSC emission. Detection of periodic
modulation of the TeV emission from LS 5039 would favor a leptonic SSC or
cascade hadron origin of the emission in the inner jet, whereas stochastic
variability alone would support a more extended leptonic model. The puzzle of
the EGRET gamma rays from LS 5039 will be quickly solved with GLAST. (Abridged)Comment: 17 pages, 11 figures, ApJ, in press, June 1, 2006, corrected eq.
Cosmic Neutrinos and the Energy Budget of Galactic and Extragalactic Cosmic Rays
Although kilometer-scale neutrino detectors such as IceCube are discovery
instruments, their conceptual design is very much anchored to the observational
fact that Nature produces protons and photons with energies in excess of
10^{20} eV and 10^{13} eV, respectively. The puzzle of where and how Nature
accelerates the highest energy cosmic particles is unresolved almost a century
after their discovery. We will discuss how the cosmic ray connection sets the
scale of the anticipated cosmic neutrino fluxes. In this context, we discuss
the first results of the completed AMANDA detector and the science reach of its
extension, IceCube.Comment: 13 pages, Latex2e, 3 postscript figures included. Talk presented at
the International Workshop on Energy Budget in the High Energy Universe,
Kashiwa, Japan, February 200
Multi-band optical-NIR variability of blazars on diverse timescales
To search for optical variability on a wide range of timescales, we have
carried out photometric monitoring of two flat spectrum radio quasars, 3C 454.3
and 3C 279, plus one BL Lac, S5 0716+714, all of which have been exhibiting
remarkably high activity and pronounced variability at all wavelengths. CCD
magnitudes in B, V, R and I pass-bands were determined for 7000 new
optical observations from 114 nights made during 2011 - 2014, with an average
length of 4 h each, at seven optical telescopes: four in Bulgaria, one
in Greece, and two in India. We measured multiband optical flux and colour
variations on diverse timescales. Discrete correlation functions were computed
among B, V, R, and I observations, to search for any time delays. We found weak
correlations in some cases with no significant time lags. The structure
function method was used to estimate any characteristic time-scales of
variability. We also investigated the spectral energy distribution of the three
blazars using B, V, R, I, J and K pass-band data. We found that the sources
almost always follows a bluer-when-brighter trend. We discuss possible physical
causes of the observed spectral variability.Comment: Accepted for publication in MNRAS, 16 pages, 11 figures, 5 tables,
plus supplementary material containing additional figures and tables (please
contact authors for it
Nature of Intra-night Optical Variability of BL Lacertae
We present the results of extensive multi-band intra-night optical monitoring
of BL Lacertae during 2010--2012. BL Lacertae was very active in this period
and showed intense variability in almost all wavelengths. We extensively
observed it for a total for 38 nights; on 26 of them observations were done
quasi-simultaneously in B, V, R and I bands (totaling 113 light curves), with
an average sampling interval of around 8 minutes. BL Lacertae showed
significant variations on hour-like timescales in a total of 19 nights in
different optical bands. We did not find any evidence for periodicities or
characteristic variability time-scales in the light curves.
The intranight variability amplitude is generally greater at higher
frequencies and decreases as the source flux increases.
We found spectral variations in BL Lacertae in the sense that the optical
spectrum becomes flatter as the flux increases but in several flaring states
deviates from the linear trend suggesting different jet components contributing
to the emission at different times.Comment: 12 Pages, 5 figures, 3 Tables, Accepted for Publication in MNRA
Modeling the Multiwavelength Spectra and Variability of BL Lacertae in 2000
BL Lacertae was the target of an extensive multiwavelength monitoring
campaign in the second half of 2000. In this paper, we are using leptonic and
hadronic jet models to fit the observed broadband spectra and spectral
variability patterns. We start out with global spectral models. Subsequently,
we investigate various flaring scenarios for comparison with the observed
short-term variability. For our leptonic jet model, we find that the short-term
variability, in particular the optical and X-ray spectral variability, can be
best represented with a flaring scenario dominated by a spectral-index change
of the spectrum of ultrarelativistic electrons injected into the jet. Based on
this result, a detailed model simulation, reproducing the observed optical and
X-ray spectral variability and broadband SED of BL Lacertae simultaneously, is
presented. Our leptonic modeling results are compared to fits using the
hadronic synchrotron-proton blazar (SPB) model. That model can reproduce the
observed SEDs of BL Lacertae in a scenario with muon-synchrotron dominated
high-energy emission. It requires a significantly higher magnetic field than
the leptonic model (~ 40 G vs. ~ 2 G in the leptonic model) and a lower Doppler
factor associated with the bulk motion of the emission region (D ~ 8 vs. D ~ 18
in the leptonic model). The hadronic model predicts a significantly larger >
100 GeV flux than the leptonic models, well within the anticipated capabilities
of VERITAS and MAGIC.Comment: Accepted for publication in ApJ. Uses AASTEX LaTeX macros.41 pages,
including 12 figure
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