797 research outputs found
Strategy Implementation for the CTA Atmospheric Monitoring Program
The Cherenkov Telescope Array (CTA) is the next generation facility of
Imaging Atmospheric Cherenkov Telescopes. It will reach unprecedented
sensitivity and energy resolution in very-high-energy gamma-ray astronomy. CTA
will detect Cherenkov light emitted within an atmospheric shower of particles
initiated by cosmic-gamma rays or cosmic rays entering the Earth's atmosphere.
From the combination of images the Cherenkov light produces in the telescopes,
one is able to infer the primary particle energy and direction. A correct
energy estimation can be thus performed only if the local atmosphere is well
characterized. The atmosphere not only affects the shower development itself,
but also the Cherenkov photon transmission from the emission point in the
particle shower, at about 10-20 km above the ground, to the detector. Cherenkov
light on the ground is peaked in the UV-blue region, and therefore molecular
and aerosol extinction phenomena are important. The goal of CTA is to control
systematics in energy reconstruction to better than 10%. For this reason, a
careful and continuous monitoring and characterization of the atmosphere is
required. In addition, CTA will be operated as an observatory, with data made
public along with appropriate analysis tools. High-level data quality can only
be ensured if the atmospheric properties are consistently and continuously
taken into account. In this contribution, we concentrate on discussing the
implementation strategy for the various atmospheric monitoring instruments
currently under discussion in CTA. These includes Raman lidars and ceilometers,
stellar photometers and others available both from commercial providers and
public research centres.Comment: (6 pages, 2 figures, Proceedings of the 2nd AtmoHEAD Conference,
Padova, Italy May 19-21, 2014
The Atmospheric Monitoring Strategy for the Cherenkov Telescope Array
The Imaging Atmospheric Cherenkov Technique (IACT) is unusual in astronomy as
the atmosphere actually forms an intrinsic part of the detector system, with
telescopes indirectly detecting very high energy particles by the generation
and transport of Cherenkov photons deep within the atmosphere. This means that
accurate measurement, characterisation and monitoring of the atmosphere is at
the very heart of successfully operating an IACT system. The Cherenkov
Telescope Array (CTA) will be the next generation IACT observatory with an
ambitious aim to improve the sensitivity of an order of magnitude over current
facilities, along with corresponding improvements in angular and energy
resolution and extended energy coverage, through an array of Large (23m),
Medium (12m) and Small (4m) sized telescopes spread over an area of order
~km. Whole sky coverage will be achieved by operating at two sites: one in
the northern hemisphere and one in the southern hemisphere. This proceedings
will cover the characterisation of the candidate sites and the atmospheric
calibration strategy. CTA will utilise a suite of instrumentation and analysis
techniques for atmospheric modelling and monitoring regarding pointing
forecasts, intelligent pointing selection for the observatory operations and
for offline data correction.Comment: 6 pages. To appear in the proceedings of the Adapting to the
Atmosphere conference 201
Wind, jet, hybrid corona and hard X-ray flares: multiwavelength evolution of GRO J1655-40 during the 2005 outburst rise
We have investigated the complex multiwavelength evolution of GRO J1655-40
during the rise of its 2005 outburst. We detected two hard X-ray flares, the
first one during the transition from the soft state to the ultra-soft state,
and the second one in the ultra-soft state. The first X-ray flare coincided
with an optically thin radio flare. We also observed a hint of increased radio
emission during the second X-ray flare. To explain the hard flares without
invoking a secondary emission component, we fit the entire data set with the
eqpair model. This single, hybrid Comptonization model sufficiently fits the
data even during the hard X-ray flares if we allow reflection fractions greater
than unity. In this case, the hard X-ray flares correspond to a Comptonizing
corona dominated by non-thermal electrons. The fits also require absorption
features in the soft and ultra-soft state which are likely due to a wind. In
this work we show that the wind and the optically thin radio flare co-exist.
Finally, we have also investigated the radio to optical spectral energy
distribution, tracking the radio spectral evolution through the quenching of
the compact jet and rise of the optically thin flare, and interpreted all data
using state transition models.Comment: 16 pages, 11 figure
On the coexistence of stellar-mass and intermediate-mass black holes in globular clusters
In this paper, we address the question: What is the probability of
stellar-mass black hole (BH) binaries co-existing in a globular cluster with an
intermediate-mass black hole (IMBH)? Our results suggest that the detection of
one or more BH binaries can strongly constrain the presence of an IMBH in most
Galactic globular clusters. More specifically, the detection of one or more BH
binaries could strongly indicate against the presence of an IMBH more massive
than M in roughly 80\% of the clusters in our
sample. To illustrate this, we use a combination of N-body simulations and
analytic methods to weigh the rate of formation of BH binaries against their
ejection and/or disruption rate via strong gravitational interactions with the
central (most) massive BH.
The eventual fate of a sub-population of stellar-mass BHs (with or without
binary companions) is for all BHs to be ejected from the cluster by the central
IMBH, leaving only the most massive stellar-mass BH behind to form a close
binary with the IMBH. During each phase of evolution, we discuss the rate of
inspiral of the central BH-BH pair as a function of both the properties of the
binary and its host cluster.Comment: 16 pages, 8 figures, 1 table, accepted for publication in MNRA
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