1,068 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 XMM-Newton Slew view of IGRJ17361-4441: a transient in the globular cluster NGC 6388
IGRJ17361-4441 is a hard transient recently observed by the INTEGRAL
satellite. The source, close to the center of gravity of the globular cluster
NGC 6388, quickly became the target of follow-up observations conducted by the
Chandra, Swift/XRT and RXTE observatories. Here, we concentrate in particular
on a set of observations conducted by the XMM-Newton satellite during two
slews, in order to get the spectral information of the source and search for
spectral variations. The spectral parameters determined by the recent
XMM-Newton slew observations were compared to the previously known results. The
maximum unabsorbed -ray flux in the 0.5-10 keV band as detected by the
XMM-Newton slew observations is erg cm
s, i.e. consistent with that observed by the Swift/XRT satellite 15 days
earlier. The spectrum seems to be marginally consistent () with that derived from the previous high energy observation.Comment: Accepted for publication on New Astronomy, 2012. A sentence about the
globular cluster 47 Tuc was partially rewritten to avoid confusio
Calcium-Rich Gap Transients: Tidal Detonations of White Dwarfs?
We hypothesize that at least some of the recently discovered class of
calcium-rich gap transients are tidal detonation events of white dwarfs (WDs)
by black holes (BHs) or possibly neutron stars. We show that the properties of
the calcium-rich gap transients agree well with the predictions of the tidal
detonation model. Under the predictions of this model, we use a follow-up X-ray
observation of one of these transients, SN 2012hn, to place weak upper limits
on the detonator mass of this system that include all intermediate-mass BHs
(IMBHs). As these transients are preferentially in the stellar haloes of
galaxies, we discuss the possibility that these transients are tidal
detonations of WDs caused by random flyby encounters with IMBHs in dwarf
galaxies or globular clusters. This possibility has been already suggested in
the literature but without connection to the calcium-rich gap transients. In
order for the random flyby cross-section to be high enough, these events would
have to be occurring inside these dense stellar associations. However, there is
a lack of evidence for IMBHs in these systems, and recent observations have
ruled out all but the very faintest dwarf galaxies and globular clusters for a
few of these transients. Another possibility is that these are tidal
detonations caused by three-body interactions, where a WD is perturbed toward
the detonator in isolated multiple star systems. We highlight a number of ways
this could occur, even in lower-mass systems with stellar-mass BHs or neutron
stars. Finally, we outline several new observational tests of this scenario,
which are feasible with current instrumentation.Comment: 10 pages, 1 figure, accepted for publication in MNRA
Four new black hole candidates identified in M31 globular clusters with Chandra and XMM-Newton
We have identified four new black hole candidates in M31 globular clusters
using 123 Chandra, and 4 XMM-Newton observations of the M31 central region. The
X-ray source associated with Bo 163 (XB163) is a recurrent transient, with the
highest luminosity ~1.4E+38 erg/s, considerably brighter than any outbursts
from the neutron star transients Aql X-1 or 4U 1608-452; the outburst
apparently started ~45 days earlier than the observed peak, hence the
luminosity could have been considerably higher. We identified XB082, XB153 and
XB185 as BHCs by observing "low state" emission spectra at luminosities that
exceed the threshold for neutron star binaries. The probability that these are
neutron star systems with anisotropic emission beamed toward us is <4E-4, and
their variability suggests emission from a single source. We therefore conclude
that these systems likely contain black holes rather than neutron stars. We
have now identified 4 persistently bright BHCs in the region; the probability
that these are all background AGN is <1E-20. According to theory, the donors
could be tidally captured main sequence stars, or white dwarves in
ultra-compact binaries. We find that GCs that are particularly massive (XB082)
or metal rich (XB144) can host bright X-ray sources in addition to those that
are both (XB163). Our method may reveal BHCs in other bright X-ray sources.Comment: Accepted for publication in ApJ. 17 pages, 5 figure
Tools and Procedures for the CTA Array Calibration
The Cherenkov Telescope Array (CTA) is an international initiative to build
the next generation ground-based very-high-energy gamma-ray observatory. Full
sky coverage will be assured by two arrays, one located on each of the northern
and southern hemispheres. Three different sizes of telescopes will cover a wide
energy range from tens of GeV up to hundreds of TeV. These telescopes, of which
prototypes are currently under construction or completion, will have different
mirror sizes and fields-of-view designed to access different energy regimes.
Additionally, there will be groups of telescopes with different optics system,
camera and electronics design. Given this diversity of instruments, an overall
coherent calibration of the full array is a challenging task. Moreover, the CTA
requirements on calibration accuracy are much more stringent than those
achieved with current Imaging Atmospheric Cherenkov Telescopes, like for
instance: the systematic errors in the energy scale must not exceed 10%.In this
contribution we present both the methods that, applied directly to the acquired
observational CTA data, will ensure that the calibration is correctly performed
to the stringent required precision, and the calibration equipment that,
external to the telescopes, is currently under development and testing.
Moreover, some notes about the operative procedure to be followed with both
methods and instruments, will be described. The methods applied to the
observational CTA data include the analysis of muon ring images, of carefully
selected cosmic-ray air shower images, of the reconstructed electron spectrum
and that of known gamma-ray sources and the possible use of stereo techniques
hardware-independent. These methods will be complemented with the use of
calibrated light sources located on ground or on board unmanned aerial
vehicles.Comment: All CTA contributions at arXiv:1709.0348
From Supermassive Black Holes to Dwarf Elliptical Nuclei: a Mass Continuum
Considerable evidence suggests that supermassive black holes reside at the
centers of massive galactic bulges. At a lower galactic mass range, many dwarf
galaxies contain extremely compact nuclei that structurally resemble massive
globular clusters. We show that both these types of central massive objects
(CMO's) define a single unbroken relation between CMO mass and the luminosity
of their host galaxy spheroid. Equivalently, M_CMO is directly proportional to
the host spheroid mass over 4 orders of magnitude. We note that this result has
been simultaneously and independently identified by Cote et al. (2006), see
also Ferrarese et al. (2006). We therefore suggest that the dE,N nuclei may be
the low-mass analogs of supermassive black holes, and that these two types of
CMO's may have both developed starting from similar initial formation
processes. The overlap mass interval between the two types of CMO's is small,
and suggests that for M_CMO > 10^7 M_sun, the formation of a black hole was
strongly favored, perhaps because the initial gas infall to the center was too
rapid and violent for star formation to occur efficiently.Comment: 4 pages, 2 figures, submitted to ApJ
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