5,861 research outputs found
Systematic and quantitative approach for the identification of high energy gamma-ray source populations
A large fraction of the detections to be made by the Gamma-ray Large Area
Space Telescope (GLAST) will initially be unidentified. We argue that
traditional methodological approaches to identify individuals and/or
populations of -ray sources will encounter procedural limitations.
These limitations will hamper our ability to classify source populations lying
in the anticipated dataset with the required degree of confidence, particularly
those for which no member has yet been convincingly detected in the predecessor
experiment EGRET. Here we suggest a new paradigm for achieving the
classification of -ray source populations based on the implementation
of an a priori protocol to search for theoretically-motivated candidate
sources. In order to protect the discovery potential of the sample, it is
essential that such paradigm will be defined before the data is unblinded. Key
to the new procedure is a statistical assessment by which the discovery of a
new population can be claimed. Although we explicitly refer here to the case of
GLAST, the scheme we present may be adapted to other experiments confronted
with a similar problematic.Comment: In press in The Astrophysical Journal Letters. Accepted on July 12,
200
The new surprising behaviour of the two "prototype" blazars PKS 2155-304 and 3C 279
Recent VHE observations have unveiled a surprising behaviour in two
well-known blazars at opposite sides of the blazar sequence. PKS 2155-304 have
shown for the first time in an HBL a large Compton dominance, high gamma-ray
luminosities and a cubic relation between X-ray and VHE fluxes. 3C 279 is the
first FSRQ detected at VHE. The high luminosity required to overcome the
significant absorption caused by the BLR emission cannot be easily reconciled
with the historical and quasi-simultaneous SED properties. Both cases shed a
new light on the structure and ambient fields of blazars. Contrary to previous
claims, it is also shown that 3C 279 --as any FSRQ-- cannot in general provide
robust constraints on the EBL.Comment: Proceedings of "4th Heidelberg International Symposium on High Energy
Gamma-Ray Astronomy 2008" (Gamma 2008), July 7-11, 2008. Slightly refined
text with updated reference
EGRET upper limits and stacking searches of gamma-ray observations of luminous and ultra-luminous infrared galaxies
We present a stacking analysis of EGRET -ray observations at the
positions of luminous and ultraluminous infrared galaxies. The latter were
selected from the recently presented HCN survey, which is thought to contain
the most active star forming regions of the universe. Different sorting
criteria are used and, whereas no positive collective detection of -ray
emission from these objects we determined both collective and individual upper
limits. The upper most excess we find appears in the case of ULIRGs ordered by
redshift, at a value of 1.8.Comment: Accepted for publication in the Astrophysical Journa
The GeV-TeV Connection in Galactic gamma-ray sources
Recent observations with atmospheric Cherenkov telescope systems such as
H.E.S.S. and MAGIC have revealed a large number of new sources of
very-high-energy (VHE) gamma-rays from 100 GeV - 100 TeV, mostly concentrated
along the Galactic plane. At lower energies (100 MeV - 10 GeV) the
satellite-based instrument EGRET revealed a population of sources clustering
along the Galactic Plane. Given their adjacent energy bands a systematic
correlation study between the two source catalogues seems appropriate. Here,
the populations of Galactic sources in both energy domains are characterised on
observational as well as on phenomenological grounds. Surprisingly few common
sources are found in terms of positional coincidence and spectral consistency.
These common sources and their potential counterparts and emission mechanisms
will be discussed in detail. In cases of detection only in one energy band, for
the first time consistent upper limits in the other energy band have been
derived. The EGRET upper limits are rather unconstraining due to the
sensitivity mismatch to current VHE instruments. The VHE upper limits put
strong constraints on simple power-law extrapolation of several of the EGRET
spectra and thus strongly suggest cutoffs in the unexplored energy range from
10 GeV - 100 GeV. Physical reasons for the existence of cutoffs and for
differences in the source population at GeV and TeV energies will be discussed.
Finally, predictions will be derived for common GeV - TeV sources for the
upcoming GLAST mission bridging for the first time the energy gap between
current GeV and TeV instruments.Comment: (1) Kavli Institute for Particle Astrophysics and Cosmology (KIPAC),
Stanford, USA (2) Stanford University, W.W. Hansen Experimental Physics Lab
(HEPL) and KIPAC, Stanford, USA (3) ICREA & Institut de Ciencies de l'Espai
(IEEC-CSIC) Campus UAB, Fac. de Ciencies, Barcelona, Spain. (4) School of
Physics and Astronomy, University of Leeds, UK. Paper Submitted to Ap
Orthographically Mediated Inhibition Effects: Evidence of Activational Feedback During Visual Word Recognition
According to the multistage activation model of visual word recognition (Besner & Smith, 1992a, 1992b; Borowsky & Besner, 1993), during visual word recognition, activation can spread from semantic to orthographic representations via a feedback mechanism. Two experiments were conducted in order to test directly whether or not such feedback occurs, and if so, under what conditions. In order to directly measure feedback, a mediated priming paradigm was utilized. In this paradigm, participants named aloud targets that were preceded either by a semantically related prime (e.g., dog - cat! or by a prime that is related to the target via a mediating word (e.g., dog - (cat) - vat). In this case the mediating word cat is semantically related to the prime, and is both orthographically and phonologically related to the target. Direct evidence of activational feedback was obtained in the form of mediated inhibition effects which were found in the presence of semantic priming effects. These mediated inhibition effects are consistent with activational feedback, thus, they support the multistage activation model of visual word recognition and not the activation-verification model (Paap, Newsome, McDonald, & Schvaneveldt, 1982; Paap, McDonald, Schvaneveldt & Noel, 1987)
On hadronic beam models for quasars and microquasars
Most of the hadronic jet models for quasars (QSOs) and microquasars (MQs)
found in literature represent beams of particles (e.g. protons). These
particles interact with the matter in the stellar wind of the companion star in
the system or with crossing clouds, generating gamma-rays via proton-proton
processes. Our aim is to derive the particle distribution in the jet as seen by
the observer, so that proper computation of the -ray and neutrino
yields can be done. We use relativistic invariants to obtain the transformed
expressions in the case of a power-law and power-law with a cutoff particle
distribution in the beam. We compare with previous expressions used earlier in
the literature. We show that formerly used expressions for the particle
distributions in the beam as seen by the observer are in error, differences
being strongly dependent on the viewing angle. For example, for
( is the Lorentz factor of the blob) and angles larger than , the earlier-used calculation entails an over-prediction (order of
magnitude or more) of the proton spectra for , whereas it always
over-predicts (two orders of magnitude) the proton spectrum at lower energies,
disregarding the viewing angle. All the results for photon and neutrino fluxes
in hadronic models in beams that have made use of the earlier calculation are
affected. Given that correct gamma-ray fluxes will be in almost any case
significantly diminished in comparison with published results, and that the
time of observations in Cherenkov facilities grows with the square of the
flux-reduction factor in a statistically limited result, the possibility of
observing hadronic beams is undermined.Comment: Accepted for publication in A&A Letter
Electron-beam-induced shift in the apparent position of a pinned vortex in a thin superconducting film
When an electron beam strikes a superconducting thin film near a pinned
vortex, it locally increases the temperature-dependent London penetration depth
and perturbs the circulating supercurrent, thereby distorting the vortex's
magnetic field toward the heated spot. This phenomenon has been used to
visualize vortices pinned in SQUIDs using low-temperature scanning electron
microscopy. In this paper I develop a quantitative theory to calculate the
displacement of the vortex-generated magnetic-flux distribution as a function
of the distance of the beam spot from the vortex core. The results are
calculated using four different models for the spatial distribution of the
thermal power deposited by the electron beam.Comment: 9 pages, 6 figures, resubmitted to PRB with referee-suggested
revisions, includes new paragraph on numerical evaluatio
The H.E.S.S. multi-messenger program
Based on fundamental particle physics processes like the production and
subsequent decay of pions in interactions of high-energy particles, close
connections exist between the acceleration sites of high-energy cosmic rays and
the emission of high-energy gamma rays and high-energy neutrinos. In most cases
these connections provide both spatial and temporal correlations of the
different emitted particles. The combination of the complementary information
provided by these messengers allows to lift ambiguities in the interpretation
of the data and enables novel and highly sensitive analyses. In this
contribution the H.E.S.S. multi-messenger program is introduced and described.
The current core of this newly installed program is the combination of
high-energy neutrinos and high-energy gamma rays. The search for gamma-ray
emission following gravitational wave triggers is also discussed. Furthermore,
the existing program for following triggers in the electromagnetic regime was
extended by the search for gamma-ray emission from Fast Radio Bursts (FRBs). An
overview over current and planned analyses is given and recent results are
presented.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherland
- …