848 research outputs found
High zenith angle observations of PKS 2155-304 with the MAGIC telescope
The high frequency peaked BL Lac PKS 2155-304 with a redshift z=0.116 was
discovered 1997 in the VHE range by the University of Durham Mark 6 gamma-ray
telescope in Australia with a flux corresponding to approx. 0.2 times the Crab
Nebula flux. It was later observed and detected with high significance by the
Southern observatories CANGAROO and H.E.S.S. establishing this source as the
best studied Southern TeV blazar. Detection from the Northern hemisphere was
very difficult due to challenging observation conditions under large zenith
angles. In July 2006, the H.E.S.S. collaboration reported an extraordinary
outburst of VHE gamma-emission. During the outburst, the VHE gamma-ray emission
was found to be variable on the time scales of minutes and at a mean flux of
approx. 7 times the flux observed from the Crab Nebula. The MAGIC collaboration
operates a 17m imaging air Cherenkov Telescope at La Palma (Northern
Hemisphere). Follow up observations of the extraordinary outburst have been
triggered in a Target of Opportunity program by an alert from the H.E.S.S.
collaboration. The measured spectrum and light curve are presented.Comment: Contribution to the 31st ICRC, Lodz, Poland, July 200
Gamma-ray signatures of cosmic ray acceleration, propagation, and confinement in the era of CTA
Galactic cosmic rays are commonly believed to be accelerated at supernova
remnants via diffusive shock acceleration. Despite the popularity of this idea,
a conclusive proof for its validity is still missing. Gamma-ray astronomy
provides us with a powerful tool to tackle this problem, because gamma rays are
produced during cosmic ray interactions with the ambient gas. The detection of
gamma rays from several supernova remnants is encouraging, but still does not
constitute a proof of the scenario, the main problem being the difficulty in
disentangling the hadronic and leptonic contributions to the emission. Once
released by their sources, cosmic rays diffuse in the interstellar medium, and
finally escape from the Galaxy. The diffuse gamma-ray emission from the
Galactic disk, as well as the gamma-ray emission detected from a few galaxies
is largely due to the interactions of cosmic rays in the interstellar medium.
On much larger scales, cosmic rays are also expected to permeate the
intracluster medium, since they can be confined and accumulated within clusters
of galaxies for cosmological times. Thus, the detection of gamma rays from
clusters of galaxies, or even upper limits on their emission, will allow us to
constrain the cosmic ray output of the sources they contain, such as normal
galaxies, AGNs, and cosmological shocks. In this paper, we describe the impact
that the Cherenkov Telescope Array, a future ground-based facility for
very-high energy gamma-ray astronomy, is expected to have in this field of
research.Comment: accepted to Astroparticle Physics, special issue on Physics with the
Cherenkov Telescope Arra
The missing GeV {\gamma}-ray binary: Searching for HESS J0632+057 with Fermi-LAT
The very high energy (VHE; >100 GeV) source HESS J0632+057 has been recently
confirmed as a \gamma-ray binary, a subclass of the high mass X-ray binary
(HMXB) population, through the detection of an orbital period of 321 days. We
performed a deep search for the emission of HESS J0632+057 in the GeV energy
range using data from the Fermi Large Area Telescope (LAT). The analysis was
challenging due to the source being located in close proximity to the bright
\gamma-ray pulsar PSR J0633+0632 and lying in a crowded region of the Galactic
plane where there is prominent diffuse emission. We formulated a Bayesian block
algorithm adapted to work with weighted photon counts, in order to define the
off-pulse phases of PSR J0633+0632. A detailed spectral-spatial model of a 5
deg circular region centred on the known location of HESS J0632+057 was
generated to accurately model the LAT data. No significant emission from the
location of HESS J0632+057 was detected in the 0.1-100 GeV energy range
integrating over ~3.5 years of data; with a 95% flux upper limit of F_{0.1-100
GeV} < 3 x 10-8 ph cm-2 s-1. A search for emission over different phases of the
orbit also yielded no significant detection. A search for source emission on
shorter timescales (days--months) did not yield any significant detections. We
also report the results of a search for radio pulsations using the 100-m Green
Bank Telescope (GBT). No periodic signals or individual dispersed bursts of a
likely astronomical origin were detected. We estimated the flux density limit
of < 90/40 \mu Jy at 2/9 GHz. The LAT flux upper limits combined with the
detection of HESS J0632+057 in the 136-400 TeV energy band by the MAGIC
collaboration imply that the VHE spectrum must turn over at energies <136 GeV
placing constraints on any theoretical models invoked to explain the \gamma-ray
emission.Comment: 11 pages, 4 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society (MNRAS) Main Journa
Binaries with the eyes of CTA
The binary systems that have been detected in gamma rays have proven very
useful to study high-energy processes, in particular particle acceleration,
emission and radiation reprocessing, and the dynamics of the underlying
magnetized flows. Binary systems, either detected or potential gamma-ray
emitters, can be grouped in different subclasses depending on the nature of the
binary components or the origin of the particle acceleration: the interaction
of the winds of either a pulsar and a massive star or two massive stars;
accretion onto a compact object and jet formation; and interaction of a
relativistic outflow with the external medium. We evaluate the potentialities
of an instrument like the Cherenkov telescope array (CTA) to study the
non-thermal physics of gamma-ray binaries, which requires the observation of
high-energy phenomena at different time and spatial scales. We analyze the
capability of CTA, under different configurations, to probe the spectral,
temporal and spatial behavior of gamma-ray binaries in the context of the known
or expected physics of these sources. CTA will be able to probe with high
spectral, temporal and spatial resolution the physical processes behind the
gamma-ray emission in binaries, significantly increasing as well the number of
known sources. This will allow the derivation of information on the particle
acceleration and emission sites qualitatively better than what is currently
available.Comment: 23 pages, 13 figures, accepted for publication in Astroparticle
Physics, special issue on Physics with the Cherenkov Telescope Arra
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