61 research outputs found
The imprint of pulsar parameters on the morphology of Pulsar Wind Nebulae
The morphology of young Pulsar Wind Nebulae (PWN) is largely determined by
the properties of the wind injected by the pulsar. We have used a recent
parametrization of the wind obtained from Force Free Electrodynamics
simulations of pulsar magnetospheres to simulate nebulae for different sets of
pulsar parameters. We performed axisymmetric Relativistic Magnetohydrodynamics
simulations to test the morphology dependence of the nebula on the obliquity of
the pulsar and on the magnetization of the pulsar wind. We compare these
simulations to the morphology of the Vela and Crab PWN. We find that the
morphology of Vela can be reproduced qualitatively if the pulsar obliquity
angle is alpha ~45deg and the magnetization of the wind is high (sigma_0 ~
3.0). A morphology similar to the one of the Crab Nebula is only obtained for
low magnetization simulations with alpha >~ 45deg. Interestingly, we find that
Kelvin-Helmholtz instabilities produce small scale turbulences downstream of
the reverse shock of the pulsar wind.Comment: 18 pages, 12 figures, accepted by MNRA
The surprising Crab pulsar and its nebula: A review
The Crab nebula and its pulsar (referred to together as "Crab") have
historically played a central role in astrophysics. True to their legacy,
several unique discoveries have been made recently. The Crab was found to emit
gamma-ray pulsations up to energies of 400 GeV, beyond what was previously
expected from pulsars. Strong gamma-ray flares, of durations of a few days were
discovered from within the nebula, while the source was previously expected to
be stable in flux on these time scales. Here we review these intriguing and
suggestive developments. In this context we give an overview of the
observational properties of the Crab and our current understanding of pulsars
and their nebulae.Comment: Review for Reports on Progress in Physics, 31 pages, 11 figures. For
a video abstract see the journal web page of the article or
http://bcove.me/391e37d
Two exceptional flares of the active galaxy PKS 2155-304 at Very High Energy gamma-rays and their implications on blazar and fundamental physics
In this work an analysis and interpretation of the Very High Energy (VHE, photon energy > 200 GeV) gamma-ray emission measured from the Active Galactic Nuclei (AGN) PKS 2155-304 is presented. The observations were carried out with the H.E.S.S. telescopes between 2005 and 2007. The source underwent two exceptional flares during this period, on 28 and 30 July 2006. The fluxes observed during these nights are among the highest ever observed in VHE astronomy and provided a very rich dataset. During the first flare the source showed a very fast flux variability (~200 s). No time delays could be observed between the emission at different photon energy bands; this lack of dispersion is used to set limits on the energy scale of Lorentz Invariance violations. The second flare was observed simultaneously by the Chandra X-ray satellite, yielding an unprecedented multi-wavelength view on this object. The most striking result of this night is a cubic decay of the gamma-ray flux as a function of the X-ray flux. Such a correlation can not be explained with current blazar models and points towards an X-ray emission from multiple regions within the AGN
An outlier locus relevant in habitat-mediated selection in an alpine plant across independent regional replicates
Habitat types can induce genetic responses in species and may drive adaptive differentiation and evolutionary divergence of populations. In this study, we aimed at detecting loci indicative of adaptation for different habitat types in the alpine plant Arabis alpina. We used a dataset consisting of A. alpina plants collected in scree, nutrient-rich and moist habitat types in two independent regional replicates of the European Alps (the Swiss and French Alps). Genome scans resulting in 825 amplified fragment length polymorphisms (AFLPs) followed by outlier analysis, i.e. looking for excessive differentiation between habitat types, after accounting for heterozygosity and population structure, was used to detect loci under divergent selection for habitat type within and across the alpine regions. The outlier analyses resulted in the detection of a consistent single outlier locus, which showed a higher fragment frequency in moist compared to the other habitat types in both alpine regions. In addition, a posteriori tests for hierarchical population structuring in the dataset did not detect signals confounding selection at this locus (i.e. signals of regional population structure). Thus, we consider this locus indicative of habitat-mediated selection, and we subsequently sequence-characterized and compared it to the Arabidopsis genome. The sequence was found to be a putative homologue to the SIT4 phosphatase-associated family protein. The detection of this locus in two alpine regions and the availability of its genome sequence make this locus a strong candidate worth further exploration in the habitat-mediated selection and genetic adaptation of natural populations in the alpine plant A. alpin
Historical Fermi All-Sky Variability Analysis of Galactic Flares
The Fermi All-sky Variability Analysis (FAVA) provides a photometric
alternative for identifying week-long gamma-ray flares across the entire sky
while being independent of any diffuse Galactic or isotropic emission model. We
reviewed 779 weeks of Fermi-LAT data analyzed by FAVA to estimate the rate and
origin of Galactic gamma-ray flares, and to search for new variable Galactic
gamma-ray transients. We report an estimated yearly rate of ~8.5 Galactic
gamma-ray flares/year with ~1 flare/year coming from unknown sources. Out of
the known gamma-ray sources that are spatially coincident with these detected
flares, we report gamma-ray flares for six of them for the first time. All six
are classified as pulsars, or a source of unknown nature but which positionally
overlaps with known supernova remnants or pulsar wind nebulae. This potentially
means these sites are tentative candidates to be the second known site of a
variable gamma-ray pulsar wind nebula (PWN), after the famous Crab Nebula's
PWN. Additionally, we identify 9 unassociated flares that are unlikely to have
originated from known gamma-ray sources.Comment: Submitted to ApJ Feb. 28 2024. Minor edits in Section 4 and 5 for
clarity. 26 pages, 6 figures, 5 Table
Search for the imprint of axion-like particles in the highest-energy photons of hard -ray blazars
Axion-like particles (ALPs), predicted in theories beyond the Standard Model,
can have observational effects on the transparency of the Universe to
rays in the presence of magnetic fields. In this work, we search for effects
compatible with the existence of ALPs with 80 months of data from the Large Area Telescope, by comparing the distributions of observed
highest energy photons from sources beyond redshifts of z 0.1 with
theoretical predictions in the presence of ALPs. We find no evidence for an
increased -ray transparency due to ALPs and therefore we set limits on
the ALPs parameters assuming a value of the intergalactic magnetic field
strength of 1 nG. Photon-ALP couplings above GeV are excluded
for ALP masses neV. These limits exclude a region of the
parameter space not covered by other -ray telescopes and are compatible
with constraints imposed by other experiments.Comment: 23 pages, 8 figures, 2 table
Deep Chandra Observations of the Pulsar Wind Nebula Created by PSR B0355+54
We report on Chandra X-ray Observatory (CXO) observations of the pulsar wind
nebula (PWN) associated with PSR B0355+54 (eight observations with a 395 ks
total exposure, performed over an 8 month period). We investigated the spatial
and spectral properties of the emission coincident with the pulsar, compact
nebula (CN), and extended tail. We find that the CN morphology can be
interpreted in a way that suggests a small angle between the pulsar spin axis
and our line-of-sight, as inferred from the radio data. On larger scales,
emission from the 7' (2 pc) tail is clearly seen. We also found hints of two
faint extensions nearly orthogonal to the direction of the pulsar's proper
motion. The spectrum extracted at the pulsar position can be described with an
absorbed power-law + blackbody model. The nonthermal component can be
attributed to magnetospheric emission, while the thermal component can be
attributed to emission from either a hot spot (e.g., a polar cap) or the entire
neutron star surface. Surprisingly, the spectrum of the tail shows only a
slight hint of cooling with increasing distance from the pulsar. This implies
either a low magnetic field with fast flow speed, or particle re-acceleration
within the tail. We estimate physical properties of the PWN and compare the
morphologies of the CN and the extended tail with those of other bow shock PWNe
observed with long CXO exposures.Comment: 11 pages, 8 figure
A population of gamma-ray emitting globular clusters seen with the Fermi Large Area Telescope
Globular clusters with their large populations of millisecond pulsars (MSPs)
are believed to be potential emitters of high-energy gamma-ray emission. Our
goal is to constrain the millisecond pulsar populations in globular clusters
from analysis of gamma-ray observations. We use 546 days of continuous
sky-survey observations obtained with the Large Area Telescope aboard the Fermi
Gamma-ray Space Telescope to study the gamma-ray emission towards 13 globular
clusters. Steady point-like high-energy gamma-ray emission has been
significantly detected towards 8 globular clusters. Five of them (47 Tucanae,
Omega Cen, NGC 6388, Terzan 5, and M 28) show hard spectral power indices and clear evidence for an exponential cut-off in the range
1.0-2.6 GeV, which is the characteristic signature of magnetospheric emission
from MSPs. Three of them (M 62, NGC 6440 and NGC 6652) also show hard spectral
indices , however the presence of an exponential cut-off
can not be unambiguously established. Three of them (Omega Cen, NGC 6388, NGC
6652) have no known radio or X-ray MSPs yet still exhibit MSP spectral
properties. From the observed gamma-ray luminosities, we estimate the total
number of MSPs that is expected to be present in these globular clusters. We
show that our estimates of the MSP population correlate with the stellar
encounter rate and we estimate 2600-4700 MSPs in Galactic globular clusters,
commensurate with previous estimates. The observation of high-energy gamma-ray
emission from a globular cluster thus provides a reliable independent method to
assess their millisecond pulsar populations that can be used to make
constraints on the original neutron star X-ray binary population, essential for
understanding the importance of binary systems in slowing the inevitable core
collapse of globular clusters.Comment: Accepted for publication in A&A. Corresponding authors: J.
Kn\"odlseder, N. Webb, B. Pancraz
Fermi Large Area Telescope Constraints on the Gamma-ray Opacity of the Universe
The Extragalactic Background Light (EBL) includes photons with wavelengths
from ultraviolet to infrared, which are effective at attenuating gamma rays
with energy above ~10 GeV during propagation from sources at cosmological
distances. This results in a redshift- and energy-dependent attenuation of the
gamma-ray flux of extragalactic sources such as blazars and Gamma-Ray Bursts
(GRBs). The Large Area Telescope onboard Fermi detects a sample of gamma-ray
blazars with redshift up to z~3, and GRBs with redshift up to z~4.3. Using
photons above 10 GeV collected by Fermi over more than one year of observations
for these sources, we investigate the effect of gamma-ray flux attenuation by
the EBL. We place upper limits on the gamma-ray opacity of the Universe at
various energies and redshifts, and compare this with predictions from
well-known EBL models. We find that an EBL intensity in the optical-ultraviolet
wavelengths as great as predicted by the "baseline" model of Stecker et al.
(2006) can be ruled out with high confidence.Comment: 42 pages, 12 figures, accepted version (24 Aug.2010) for publication
in ApJ; Contact authors: A. Bouvier, A. Chen, S. Raino, S. Razzaque, A.
Reimer, L.C. Reye
- …