524 research outputs found
USA and RXTE Observations of a Variable Low-Frequency QPO in XTE J1118+480
The USA experiment on ARGOS and RXTE have exensively observed the X-ray
transient XTE J1118+480 during its recent outburst in 2000 April--June. We
present detailed monitoring of the evolution of a low frequency QPO which
drifts from 0.07 Hz to 0.15 Hz during the outburst. We examine possible
correlations of the QPO frequency with the flux and spectral characteristics of
the source, and compare this QPO to low frequency QPOs observed in other black
hole candidates.Comment: Accepted by ApJ Letters, reference added, minor revisions, 6 page
USA Observation of Spectral and Timing Evolution During the 2000 Outburst of XTE J1550-564
We report on timing and spectral observations of the 2000 outburst of XTE
J1550-564 made by the Unconventional Stellar Aspect (USA) Experiment on board
the Advanced Research and Global Observation Satellite (ARGOS). We observe a
low-frequency quasi-periodic oscillation (LFQPO) with a centroid frequency that
tends to increase with increasing flux and a fractional rms amplitude which is
correlated with the hardness ratio. The evolution of the hardness ratio (4--16
keV/1--4 keV) with time and source flux is examined. The hardness-intensity
diagram (HID) shows a cyclical movement in the clockwise direction and possibly
indicates the presence of two independent accretion flows. We observe a
relationship between the USA 4--16 keV count rate and radio observations and
discuss this in the context of previously observed correlations between X-ray,
radio, optical and IR data. We examine our results in the context of models
invoking two accretion flows: a thin disk and a hot sub-Keplerian flow.Comment: 11 pages, 2 figure
The new model of fitting the spectral energy distributions of Mkn 421 and Mkn 501
The spectral energy distribution (SED) of TeV blazars has a double-humped
shape that is usually interpreted as Synchrotron Self Compton (SSC) model. The
one zone SSC model is used broadly but cannot fit the high energy tail of SED
very well. It need bulk Lorentz factor which is conflict with the observation.
Furthermore one zone SSC model can not explain the entire spectrum. In the
paper, we propose a new model that the high energy emission is produced by the
accelerated protons in the blob with a small size and high magnetic field, the
low energy radiation comes from the electrons in the expanded blob. Because the
high and low energy photons are not produced at the same time, the requirement
of large Doppler factor from pair production is relaxed. We present the fitting
results of the SEDs for Mkn 501 during April 1997 and Mkn 421 during March 2001
respectively.Comment: 5 pages, 1 figures, 1table. accepted for publication in Sciences in
China --
Active Galactic Nuclei under the scrutiny of CTA
Active Galactic Nuclei (hereafter AGN) produce powerful outflows which offer
excellent conditions for efficient particle acceleration in internal and
external shocks, turbulence, and magnetic reconnection events. The jets as well
as particle accelerating regions close to the supermassive black holes
(hereafter SMBH) at the intersection of plasma inflows and outflows, can
produce readily detectable very high energy gamma-ray emission. As of now, more
than 45 AGN including 41 blazars and 4 radiogalaxies have been detected by the
present ground-based gamma-ray telescopes, which represents more than one third
of the cosmic sources detected so far in the VHE gamma-ray regime. The future
Cherenkov Telescope Array (CTA) should boost the sample of AGN detected in the
VHE range by about one order of magnitude, shedding new light on AGN population
studies, and AGN classification and unification schemes. CTA will be a unique
tool to scrutinize the extreme high-energy tail of accelerated particles in
SMBH environments, to revisit the central engines and their associated
relativistic jets, and to study the particle acceleration and emission
mechanisms, particularly exploring the missing link between accretion physics,
SMBH magnetospheres and jet formation. Monitoring of distant AGN will be an
extremely rewarding observing program which will inform us about the inner
workings and evolution of AGN. Furthermore these AGN are bright beacons of
gamma-rays which will allow us to constrain the extragalactic infrared and
optical backgrounds as well as the intergalactic magnetic field, and will
enable tests of quantum gravity and other "exotic" phenomena.Comment: 28 pages, 23 figure
The STACEE-32 Ground Based Gamma-ray Detector
We describe the design and performance of the Solar Tower Atmospheric
Cherenkov Effect Experiment detector in its initial configuration (STACEE-32).
STACEE is a new ground-based gamma ray detector using the atmospheric Cherenkov
technique. In STACEE, the heliostats of a solar energy research array are used
to collect and focus the Cherenkov photons produced in gamma-ray induced air
showers. The large Cherenkov photon collection area of STACEE results in a
gamma-ray energy threshold below that of previous detectors.Comment: 45 pages, 25 figures, Accepted for publication in Nuclear Instruments
and Methods
Optical properties of MgH2 measured in situ in a novel gas cell for ellipsometry/spectrophotometry
The dielectric properties of alpha-MgH2 are investigated in the photon energy
range between 1 and 6.5 eV. For this purpose, a novel sample configuration and
experimental setup are developed that allow both optical transmission and
ellipsometric measurements of a transparent thin film in equilibrium with
hydrogen. We show that alpha-MgH2 is a transparent, colour neutral insulator
with a band gap of 5.6 +/- 0.1 eV. It has an intrinsic transparency of about
80% over the whole visible spectrum. The dielectric function found in this work
confirms very recent band structure calculations using the GW approximation by
Alford and Chou [J.A. Alford and M.Y. Chou (unpublished)]. As Pd is used as a
cap layer we report also the optical properties of PdHx thin films.Comment: REVTeX4, 15 pages, 12 figures, 5 table
VERITAS: the Very Energetic Radiation Imaging Telescope Array System
The Very Energetic Radiation Imaging Telescope Array System (VERITAS)
represents an important step forward in the study of extreme astrophysical
processes in the universe. It combines the power of the atmospheric Cherenkov
imaging technique using a large optical reflector with the power of
stereoscopic observatories using arrays of separated telescopes looking at the
same shower. The seven identical telescopes in VERITAS, each of aperture 10 m,
will be deployed in a filled hexagonal pattern of side 80 m; each telescope
will have a camera consisting of 499 pixels with a field of view of 3.5 deg
VERITAS will substantially increase the catalog of very high energy (E >
100GeV) gamma-ray sources and greatly improve measurements of established
sources.Comment: 44 pages, 16 figure
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
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