155 research outputs found
IonâIon Interactions and Conduction Mechanism of Highly Conductive Fluorohydrogenate Ionic Liquids
Ionâion interactions in highly conductive fluorohydrogenate ionic liquids (ILs) are discussed in this study. Low-temperature crystal structures of DMIm(FH)âF and DMIm(FH)âF (DMIm = 1, 3-dimethylimidazolium) are determined by single-crystal X-ray diffraction to obtain the location of each ion in the crystal lattice. Interaction energies between the imidazolium cation and fluorohydrogenate anions are evaluated with the aid of quantum mechanical calculations where the configuration of the ions is taken from the crystal structures of DMIm(FH)âF and DMIm(FH)âF as well as the previously determined EMImFHF (EMIm = 1-ethyl-3-methylimidazolium). The calculation suggests that the interaction energies are mainly dominated by electrostatic interactions as in the cases of other imidazolium salts, and the low viscosity and high conductivity of fluorohydrogenate ILs are derived from their dynamic properties. The HF unit exchanging between fluorohydrogenate anions weakens the cationâanion interactions and produces smaller anionic diffusion species
Potential of EBL and cosmology studies with the Cherenkov Telescope Array
Very high energy (VHE, E >100 GeV) gamma-rays are absorbed via interaction
with low-energy photons from the extragalactic background light (EBL) if the
involved photon energies are above the threshold for electron-positron pair
creation. The VHE gamma-ray absorption, which is energy dependent and increases
strongly with redshift, distorts the VHE spectra observed from distant objects.
The observed energy spectra of the AGNs carry, therefore, an imprint of the
EBL. The detection of VHE gamma-ray spectra of distant sources (z = 0.11 -
0.54) by current generation Imaging Atmospheric Cherenkov Telescopes (IACTs)
enabled to set strong upper limits on the EBL density, using certain basic
assumptions about blazar physics. In this paper it is studied how the improved
sensitivity of the Cherenkov Telescope Array (CTA) and its enlarged energy
coverage will enlarge our knowledge about the EBL and its sources. CTA will
deliver a large sample of AGN at different redshifts with detailed measured
spectra. In addition, it will provide the exciting opportunity to use gamma ray
bursts (GRBs) as probes for the EBL density at high redshifts.Comment: 12 pages, 9 figures, to appear in Astroparticle Physics. arXiv admin
note: text overlap with arXiv:1005.119
Unraveling the Nature of Unidentified High Galactic Latitude Fermi/LAT Gamma-ray Sources with Suzaku
We report on the results of deep X-ray follow-up observations of four
unidentified Fermi/LAT gamma-ray sources at high Galactic latitudes using
Suzaku. The studied objects were detected with high significance during the
first 3 months of Fermi/LAT operation, and subsequently better localized in the
Fermi/LAT 1 year catalog (1FGL). Possible associations with pulsars and active
galaxies have subsequently been discussed, and our observations provide an
important contribution to this debate. In particular, an X-ray point source was
found within the 95% confidence error circle of 1FGL J1231.1-1410. X-ray
spectrum is well-fitted by a blackbody with an additional power-law. This
supports the recently claimed identification of this source with a millisecond
pulsar (MSP) PSR J1231-1411. Concerning 1FGL J1311.7-3429, two X-ray sources
were found within the LAT error circle. Even though the X-ray spectral and
variability properties were accessed, their nature and relationship with the
gamma-ray source remain uncertain. We found several weak X-ray sources in the
field of 1FGL J1333.2+5056, one coinciding with CLASS J1333+5057. We argue the
available data are consistent with the association between these two objects.
Finally, we have detected an X-ray source in the vicinity of 1FGL J2017.3+0603.
This object was recently suggested to be associated with a newly discovered MSP
PSR J2017+0603, because of the spatial-coincidence and the gamma-ray pulse
detection. We have only detected the X-ray counterpart of the CLASS J2017+0603,
while we determined an X-ray flux upper limit at the pulsar position. All in
all, our studies indicate while a significant fraction of unidentified high
Galactic latitude gamma-ray sources is related to the pulsar and blazar
phenomena, associations with other classes of astrophysical objects are still
valid options.Comment: Accepted for publication in the Ap
Development of the photomultiplier tube readout system for the first Large-Sized Telescope of the Cherenkov Telescope Array
The Cherenkov Telescope Array (CTA) is the next generation ground-based very
high energy gamma-ray observatory. The Large-Sized Telescope (LST) of CTA
targets 20 GeV -- 1 TeV gamma rays and has 1855 photomultiplier tubes (PMTs)
installed in the focal plane camera. With the 23 m mirror dish, the night sky
background (NSB) rate amounts to several hundreds MHz per pixel. In order to
record clean images of gamma-ray showers with minimal NSB contamination, a fast
sampling of the signal waveform is required so that the signal integration time
can be as short as the Cherenkov light flash duration (a few ns). We have
developed a readout board which samples waveforms of seven PMTs per board at a
GHz rate. Since a GHz FADC has a high power consumption, leading to large heat
dissipation, we adopted the analog memory ASIC "DRS4". The sampler has 1024
capacitors per channel and can sample the waveform at a GHz rate. Four channels
of a chip are cascaded to obtain deeper sampling depth with 4096 capacitors.
After a trigger is generated in a mezzanine on the board, the waveform stored
in the capacitor array is subsequently digitized with a low speed (33 MHz) ADC
and transferred via the FPGA-based Gigabit Ethernet to a data acquisition
system. Both a low power consumption (2.64 W per channel) and high speed
sampling with a bandwidth of 300 MHz have been achieved. In addition, in
order to increase the dynamic range of the readout we adopted a two gain system
achieving from 0.2 up to 2000 photoelectrons in total. We finalized the board
design for the first LST and proceeded to mass production. Performance of
produced boards are being checked with a series of quality control (QC) tests.
We report the readout board specifications and QC results.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherlands. All CTA contributions at
arXiv:1508.0589
- âŚ