8,979 research outputs found
A stacking method to study the gamma-ray emission of source samples based on the co-adding of Fermi LAT count maps
We present a stacking method that makes use of co-added maps of gamma-ray
counts produced from data taken with the Fermi Large Area Telescope. Sources
with low integrated gamma-ray fluxes that are not detected individually may
become detectable when their corresponding count maps are added. The combined
data set is analyzed with a maximum likelihood method taking into account the
contribution from point-like and diffuse background sources. For both simulated
and real data, detection significance and integrated gamma-ray flux are
investigated for different numbers of stacked sources using the public Fermi
Science Tools for analysis and data preparation. The co-adding is done such
that potential source signals add constructively, in contrast to the signals
from background sources, which allows the stacked data to be described with
simply structured models. We show, for different scenarios, that the stacking
method can be used to increase the cumulative significance of a sample of
sources and to characterize the corresponding gamma-ray emission. The method
can, for instance, help to search for gamma-ray emission from galaxy clusters.Comment: accepted for publication in Astronomy & Astrophysics, 10 pages, 12
figure
A planetary eclipse map of CoRoT-2a. Comprehensive lightcurve modeling combining rotational-modulation and transits
We analyze the surface structure of the planet host star CoRoT-2a using a
consistent model for both the `global' (i.e., rotationally modulated)
lightcurve and the transit lightcurves, using data provided by the CoRoT
mission. Selecting a time interval covering two stellar rotations and six
transits of the planetary companion CoRoT-2b, we adopt a `strip' model of the
surface to reproduce the photometric modulation inside and outside the transits
simultaneously. Our reconstructions show that it is possible to achieve
appropriate fits for the entire sub-interval using a low-resolution surface
model with 36 strips. The surface reconstructions indicate that the brightness
on the eclipsed section of the stellar surface is (6 +/- 1) % lower than the
average brightness of the remaining surface. This result suggests a
concentration of stellar activity in a band around the stellar equator similar
to the behavior observed on the Sun.Comment: accepted by A&A on 12/09/200
Planetary eclipse mapping of CoRoT-2a. Evolution, differential rotation, and spot migration
The lightcurve of CoRoT-2 shows substantial rotational modulation and
deformations of the planet's transit profiles caused by starspots. We
consistently model the entire lightcurve, including both rotational modulation
and transits, stretching over approximately 30 stellar rotations and 79
transits. The spot distribution and its evolution on the noneclipsed and
eclipsed surface sections are presented and analyzed, making use of the high
resolution achievable under the transit path.
We measure the average surface brightness on the eclipsed section to be
(5\pm1) % lower than on the noneclipsed section. Adopting a solar spot
contrast, the spot coverage on the entire surface reaches up to 19 % and a
maximum of almost 40 % on the eclipsed section. Features under the transit
path, i.e. close to the equator, rotate with a period close to 4.55 days.
Significantly higher rotation periods are found for features on the noneclipsed
section indicating a differential rotation of . Spotted
and unspotted regions in both surface sections concentrate on preferred
longitudes separated by roughly 180 deg.Comment: Paper accepted by A&A 17/02/2010. For a better resolution paper
please visit my homepage:
http://www.hs.uni-hamburg.de/EN/Ins/Per/Huber/index.htm
Real-time observation of interfering crystal electrons in high-harmonic generation
Accelerating and colliding particles has been a key strategy to explore the
texture of matter. Strong lightwaves can control and recollide electronic
wavepackets, generating high-harmonic (HH) radiation which encodes the
structure and dynamics of atoms and molecules and lays the foundations of
attosecond science. The recent discovery of HH generation in bulk solids
combines the idea of ultrafast acceleration with complex condensed matter
systems and sparks hope for compact solid-state attosecond sources and
electronics at optical frequencies. Yet the underlying quantum motion has not
been observable in real time. Here, we study HH generation in a bulk solid
directly in the time-domain, revealing a new quality of strong-field
excitations in the crystal. Unlike established atomic sources, our solid emits
HH radiation as a sequence of subcycle bursts which coincide temporally with
the field crests of one polarity of the driving terahertz waveform. We show
that these features hallmark a novel non-perturbative quantum interference
involving electrons from multiple valence bands. The results identify key
mechanisms for future solid-state attosecond sources and next-generation
lightwave electronics. The new quantum interference justifies the hope for
all-optical bandstructure reconstruction and lays the foundation for possible
quantum logic operations at optical clock rates
An ab initio and dynamics study of the photodissociation of nitric acid HNO3
We investigated the photodissociation of HNO3 within the first (300 nm) and the third (200 nm) absorption band. The relevant S1 and S3 potential energy surfaces were calculated by taking into account the N-O single bond and N=O “double” bond distances. The striking feature of the dynamical analysis is a bifurcation of the wave packet on the S3 surface which explains the branching into the two
reaction pathways with the products OH+NO2 and O+HONO found in experiments. Dissociation on the S1 surface is predicted to proceed along a single channel leading to OH+NO2, both in their electronic ground states.
Corresponding author
- …