644 research outputs found
Discovery of VHE gamma-rays from RGB J0152+017
The BL Lac object RGB J0152+017 (z = 0.080) was predicted to be a very
high-energy (VHE; > 100 GeV) gamma-ray source, due to its high X-ray and radio
fluxes.
We report recent observations of this source made in late October and
November 2007 with the H.E.S.S. array consisting of four imaging atmospheric
Cerenkov telescopes. Contemporaneous observations were made in X-rays with the
Swift and RXTE satellites, in the optical band with the ATOM telescope, and in
the radio band with the Nancay Radio Telescope.
As a result, RGB J0152+017 is discovered as a source of VHE gamma-rays by
H.E.S.S. A signal of 173 gamma-ray photons corresponding to a statistical
significance of 6.6 sigmas was found in the data. The energy spectrum of the
source can be described by a powerlaw with a spectral index of = 2.95 +-
0.36stat +- 0.20syst. The integral flux above 300 GeV corresponds to ~2% of the
flux of the Crab nebula. The source spectral energy distribution (SED) can be
described using a two-component (extended jet and blob in jet) non-thermal
synchrotron self-Compton (SSC) leptonic model, plus a thermal host galaxy
component. The parameters that are found are very close to those found for TeV
blazars in similar SSC studies.
The location of its synchrotron peak, as derived from the SED in Swift data,
allows clear classification as a high-frequency-peaked BL Lac (HBL).Comment: 6 pages, 5 figures; accepted to appear in the proceedings of the
Workshop on Blazar Variability across the Electromagnetic Spectrum, 22-25
April 2008, Palaiseau, Franc
Discovery of VHE gamma-rays from Centaurus A
We report the discovery of faint very high energy (VHE, E > 100 GeV)
gamma-ray emission from the radio galaxy Centaurus A in deep observations
performed with the H.E.S.S. experiment. A signal with a statistical
significance of 5.0 sigma is detected from the region including the radio core
and the inner kpc jets. The integral flux above an energy threshold of ~250 GeV
is measured to be 0.8% of the flux of the Crab Nebula and the spectrum can be
described by a power law with a photon index of 2.7 +/- 0.5_stat +/- 0.2_sys.
No significant flux variability is detected in the data set. The discovery of
VHE gamma-ray emission from Centaurus A reveals particle acceleration in the
source to >TeV energies and, together with M 87, establishes radio galaxies as
a class of VHE emitters.Comment: 4 pages, 3 figures, submitted as proceeding to the 44th Recontre de
Moriond "Very High Energy Phenomena in the Universe", contains an updated SED
including the recent Fermi dat
Recommended from our members
The Modular Aerial Sensing System
Satellite remote sensing has enabled remarkable progress in the ocean, earth, atmospheric, and environmental sciences through its ability to provide global coverage with ever-increasing spatial resolution. While exceptions exist for geostationary ocean color satellites, the temporal coverage of low-Earth-orbiting satellites is not optimal for oceanographic processes that evolve over time scales of hours to days. In hydrology, time scales can range from hours for flash floods, to days for snowfall, to months for the snowmelt into river systems. On even smaller scales, remote sensing of the built environment requires a building-resolving resolution of a few meters or better. For this broad range of phenomena, satellite data need to be supplemented with higher-resolution airborne data that are not tied to the strict schedule of a satellite orbit. To address some of these needs, a novel, portable, high-resolution airborne topographic lidar with video, infrared, and hyperspectral imaging systems was integrated. The system is coupled to a highly accurate GPS-aided inertial measurement unit (GPS IMU), permitting airborne measurements of the sea surface displacement, temperature, and kinematics with swath widths of up to 800 m under the aircraft, and horizontal spatial resolution as low as 0.2 m. These data are used to measure ocean waves, currents, Stokes drift, sea surface height (SSH), ocean transport and dispersion, and biological activity. Hydrological and terrestrial applications include measurements of snow cover and the built environment. This paper describes the system, its performance, and present results from recent oceanographic, hydrological, and terrestrial measurements.This research was funded by an NSF MRI (ARRA) grant to WKM, DC, MK, Robert Guza, and Paul Linden at SIO/UCSD for most of the MASS instruments and system development for application to oceanographic, hydrographic and built-environment research. It was also supported by an industry grant to WKM for the October 2011 GoM experiment, and by the Basker Chair Fund for PFL. The collection of other data used here was funded by grants to WKM by ONR (Physical Oceanography)
NectarCAM : a camera for the medium size telescopes of the Cherenkov Telescope Array
NectarCAM is a camera proposed for the medium-sized telescopes of the
Cherenkov Telescope Array (CTA) covering the central energy range of ~100 GeV
to ~30 TeV. It has a modular design and is based on the NECTAr chip, at the
heart of which is a GHz sampling Switched Capacitor Array and a 12-bit Analog
to Digital converter. The camera will be equipped with 265 7-photomultiplier
modules, covering a field of view of 8 degrees. Each module includes the
photomultiplier bases, high voltage supply, pre-amplifier, trigger, readout and
Ethernet transceiver. The recorded events last between a few nanoseconds and
tens of nanoseconds. The camera trigger will be flexible so as to minimize the
read-out dead-time of the NECTAr chips. NectarCAM is designed to sustain a data
rate of more than 4 kHz with less than 5\% dead time. The camera concept, the
design and tests of the various subcomponents and results of thermal and
electrical prototypes are presented. The design includes the mechanical
structure, cooling of the electronics, read-out, clock distribution, slow
control, data-acquisition, triggering, monitoring and services.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherlands. All CTA contributions at
arXiv:1508.0589
Nonthermal Emission from Star-Forming Galaxies
The detections of high-energy gamma-ray emission from the nearby starburst
galaxies M82 & NGC253, and other local group galaxies, broaden our knowledge of
star-driven nonthermal processes and phenomena in non-AGN star-forming
galaxies. We review basic aspects of the related processes and their modeling
in starburst galaxies. Since these processes involve both energetic electrons
and protons accelerated by SN shocks, their respective radiative yields can be
used to explore the SN-particle-radiation connection. Specifically, the
relation between SN activity, energetic particles, and their radiative yields,
is assessed through respective measures of the particle energy density in
several star-forming galaxies. The deduced energy densities range from O(0.1)
eV/cm^3 in very quiet environments to O(100) eV/cm^3 in regions with very high
star-formation rates.Comment: 17 pages, 5 figures, to be published in Astrophysics and Space
Science Proceeding
H.E.S.S. observations of gamma-ray bursts in 2003-2007
Very-high-energy (VHE; >~100 GeV) gamma-rays are expected from gamma-ray
bursts (GRBs) in some scenarios. Exploring this photon energy regime is
necessary for understanding the energetics and properties of GRBs. GRBs have
been one of the prime targets for the H.E.S.S. experiment, which makes use of
four Imaging Atmospheric Cherenkov Telescopes (IACTs) to detect VHE gamma-rays.
Dedicated observations of 32 GRB positions were made in the years 2003-2007 and
a search for VHE gamma-ray counterparts of these GRBs was made. Depending on
the visibility and observing conditions, the observations mostly start minutes
to hours after the burst and typically last two hours. Results from
observations of 22 GRB positions are presented and evidence of a VHE signal was
found neither in observations of any individual GRBs, nor from stacking data
from subsets of GRBs with higher expected VHE flux according to a
model-independent ranking scheme. Upper limits for the VHE gamma-ray flux from
the GRB positions were derived. For those GRBs with measured redshifts,
differential upper limits at the energy threshold after correcting for
absorption due to extra-galactic background light are also presented.Comment: 9 pages, 4 tables, 3 figure
Discovery of VHE gamma-rays from the high-frequency-peaked BL Lac object RGB J0152+017
Aims: The BL Lac object RGB J0152+017 (z=0.080) was predicted to be a very
high-energy (VHE; > 100 GeV) gamma-ray source, due to its high X-ray and radio
fluxes. Our aim is to understand the radiative processes by investigating the
observed emission and its production mechanism using the High Energy
Stereoscopic System (H.E.S.S.) experiment. Methods: We report recent
observations of the BL Lac source RGB J0152+017 made in late October and
November 2007 with the H.E.S.S. array consisting of four imaging atmospheric
Cherenkov telescopes. Contemporaneous observations were made in X-rays by the
Swift and RXTE satellites, in the optical band with the ATOM telescope, and in
the radio band with the Nancay Radio Telescope. Results: A signal of 173
gamma-ray photons corresponding to a statistical significance of 6.6 sigma was
found in the data. The energy spectrum of the source can be described by a
powerlaw with a spectral index of 2.95+/-0.36stat+/-0.20syst. The integral flux
above 300 GeV corresponds to ~2% of the flux of the Crab nebula. The source
spectral energy distribution (SED) can be described using a two-component
non-thermal synchrotron self-Compton (SSC) leptonic model, except in the
optical band, which is dominated by a thermal host galaxy component. The
parameters that are found are very close to those found in similar SSC studies
in TeV blazars. Conclusions: RGB J0152+017 is discovered as a source of VHE
gamma-rays by H.E.S.S. The location of its synchrotron peak, as derived from
the SED in Swift data, allows clearly classification it as a
high-frequency-peaked BL Lac (HBL).Comment: Accepted for publication in A&A Letters (5 pages, 4 figures
Detection of variable VHE gamma-ray emission from the extra-galactic gamma-ray binary LMC P3
Context. Recently, the high-energy (HE, 0.1-100 GeV) -ray emission
from the object LMC P3 in the Large Magellanic Cloud (LMC) has been discovered
to be modulated with a 10.3-day period, making it the first extra-galactic
-ray binary.
Aims. This work aims at the detection of very-high-energy (VHE, >100 GeV)
-ray emission and the search for modulation of the VHE signal with the
orbital period of the binary system.
Methods. LMC P3 has been observed with the High Energy Stereoscopic System
(H.E.S.S.); the acceptance-corrected exposure time is 100 h. The data set has
been folded with the known orbital period of the system in order to test for
variability of the emission. Energy spectra are obtained for the orbit-averaged
data set, and for the orbital phase bin around the VHE maximum.
Results. VHE -ray emission is detected with a statistical
significance of 6.4 . The data clearly show variability which is
phase-locked to the orbital period of the system. Periodicity cannot be deduced
from the H.E.S.S. data set alone. The orbit-averaged luminosity in the
TeV energy range is erg/s. A luminosity of erg/s is reached during 20% of the orbit. HE and VHE
-ray emissions are anti-correlated. LMC P3 is the most luminous
-ray binary known so far.Comment: 5 pages, 3 figures, 1 table, accepted for publication in A&
Characterizing the gamma-ray long-term variability of PKS 2155-304 with H.E.S.S. and Fermi-LAT
Studying the temporal variability of BL Lac objects at the highest energies
provides unique insights into the extreme physical processes occurring in
relativistic jets and in the vicinity of super-massive black holes. To this
end, the long-term variability of the BL Lac object PKS 2155-304 is analyzed in
the high (HE, 100 MeV 200 GeV)
gamma-ray domain. Over the course of ~9 yr of H.E.S.S observations the VHE
light curve in the quiescent state is consistent with a log-normal behavior.
The VHE variability in this state is well described by flicker noise
(power-spectral-density index {\ss}_VHE = 1.10 +0.10 -0.13) on time scales
larger than one day. An analysis of 5.5 yr of HE Fermi LAT data gives
consistent results ({\ss}_HE = 1.20 +0.21 -0.23, on time scales larger than 10
days) compatible with the VHE findings. The HE and VHE power spectral densities
show a scale invariance across the probed time ranges. A direct linear
correlation between the VHE and HE fluxes could neither be excluded nor firmly
established. These long-term-variability properties are discussed and compared
to the red noise behavior ({\ss} ~ 2) seen on shorter time scales during
VHE-flaring states. The difference in power spectral noise behavior at VHE
energies during quiescent and flaring states provides evidence that these
states are influenced by different physical processes, while the compatibility
of the HE and VHE long-term results is suggestive of a common physical link as
it might be introduced by an underlying jet-disk connection.Comment: 11 pages, 16 figure
Long-term (trophic) purinergic signalling: purinoceptors control cell proliferation, differentiation and death
The purinergic signalling system, which uses purines and pyrimidines as chemical transmitters, and purinoceptors as effectors, is deeply rooted in evolution and development and is a pivotal factor in cell communication. The ATP and its derivatives function as a 'danger signal' in the most primitive forms of life. Purinoceptors are extraordinarily widely distributed in all cell types and tissues and they are involved in the regulation of an even more extraordinary number of biological processes. In addition to fast purinergic signalling in neurotransmission, neuromodulation and secretion, there is long-term (trophic) purinergic signalling involving cell proliferation, differentiation, motility and death in the development and regeneration of most systems of the body. In this article, we focus on the latter in the immune/defence system, in stratified epithelia in visceral organs and skin, embryological development, bone formation and resorption, as well as in cancer. Cell Death and Disease (2010) 1, e9; doi:10.1038/cddis.2009.11; published online 14 January 201
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