2,573 research outputs found
Glycosylation of Stannyl Ceramides Promoted by Modified Montmorillonite in Supercritical Carbon Dioxide
The direct glycosylation of ceramides in supercritical carbon dioxide (scCO₂) successfully proceeded to produce β-glycolipids in high yield and with full stereoselectivity. The reaction is promoted by montmorillonite modified with a superacid (CF₃SO₃H). The value of this protocol was demonstrated in the efficient synthesis of isoglobotrihexosylceramide (iGB3)
Starspots on the fastest rotators in the Beta Pic moving group
Aims: We carried out high-resolution spectroscopy and BV(I)_C photometric
monitoring of the two fastest late-type rotators in the nearby Beta Pictoris
moving group, HD199143 (F7V) and CD-641208 (K7V). The motivation for this work
is to investigate the rotation periods and photospheric spot patterns of these
very young stars, with a longer term view to probing the evolution of rotation
and magnetic activity during the early phases of main-sequence evolution. We
also aim to derive information on key physical parameters, such as rotational
velocity and rotation period. Methods: We applied maximum entropy (ME) and
Tikhonov regularizing (TR) criteria to derive the surface spot map
distributions of the optical modulation observed in HD199143 (F7 V) and
CD-641208 (K7V). We also used cross-correlation techniques to determine stellar
parameters such as radial velocities and rotational velocities. Lomb-Scargle
periodograms were used to obtain the rotational periods from differential
magnitude time series. Results: We find periods and inclinations of 0.356 days
and 21.5deg for HD199143, and 0.355 days and 50.1deg for CD-641208. The spot
maps of HD199143 obtained from the ME and TR methods are very similar, although
the latter gives a smoother distribution of the filling factor. Maps obtained
at two different epochs three weeks apart show a remarkable increase in spot
coverage amounting to ~7% of the surface of the photosphere over a time period
of only ~20 days. The spot maps of CD-641208 from the two methods show good
longitudinal agreement, whereas the latitude range of the spots is extended to
cover the whole visible hemisphere in the TR map. The distributions obtained
from the first light curve of HD199143 show the presence of an extended and
asymmetric active longitude with the maximum filling factor at longitude
~325degree.Comment: Accepted by A&A. 13 pages, 13 figures (4 online included), 5 Table
Observation of the Crab Nebula with the HAWC Gamma-Ray Observatory
The Crab Nebula is the brightest TeV gamma-ray source in the sky and has been
used for the past 25 years as a reference source in TeV astronomy, for
calibration and verification of new TeV instruments. The High Altitude Water
Cherenkov Observatory (HAWC), completed in early 2015, has been used to observe
the Crab Nebula at high significance across nearly the full spectrum of
energies to which HAWC is sensitive. HAWC is unique for its wide field-of-view,
nearly 2 sr at any instant, and its high-energy reach, up to 100 TeV. HAWC's
sensitivity improves with the gamma-ray energy. Above 1 TeV the
sensitivity is driven by the best background rejection and angular resolution
ever achieved for a wide-field ground array.
We present a time-integrated analysis of the Crab using 507 live days of HAWC
data from 2014 November to 2016 June. The spectrum of the Crab is fit to a
function of the form . The data is well-fit with values of
, , and
log when
is fixed at 7 TeV and the fit applies between 1 and 37 TeV. Study of the
systematic errors in this HAWC measurement is discussed and estimated to be
50\% in the photon flux between 1 and 37 TeV.
Confirmation of the Crab flux serves to establish the HAWC instrument's
sensitivity for surveys of the sky. The HAWC survey will exceed sensitivity of
current-generation observatories and open a new view of 2/3 of the sky above 10
TeV.Comment: Submitted 2017/01/06 to the Astrophysical Journa
Daily monitoring of TeV gamma-ray emission from Mrk 421, Mrk 501, and the Crab Nebula with HAWC
We present results from daily monitoring of gamma rays in the energy range
to TeV with the first 17 months of data from the High
Altitude Water Cherenkov (HAWC) Observatory. Its wide field of view of 2
steradians and duty cycle of % are unique features compared to other TeV
observatories that allow us to observe every source that transits over HAWC for
up to hours each sidereal day. This regular sampling yields
unprecedented light curves from unbiased measurements that are independent of
seasons or weather conditions. For the Crab Nebula as a reference source we
find no variability in the TeV band. Our main focus is the study of the TeV
blazars Markarian (Mrk) 421 and Mrk 501. A spectral fit for Mrk 421 yields a
power law index and
an exponential cut-off
TeV. For Mrk 501, we find an index and exponential cut-off TeV. The light curves for both sources show clear
variability and a Bayesian analysis is applied to identify changes between flux
states. The highest per-transit fluxes observed from Mrk 421 exceed the Crab
Nebula flux by a factor of approximately five. For Mrk 501, several transits
show fluxes in excess of three times the Crab Nebula flux. In a comparison to
lower energy gamma-ray and X-ray monitoring data with comparable sampling we
cannot identify clear counterparts for the most significant flaring features
observed by HAWC.Comment: 18 pages, 10 figures, accepted for publication in The Astrophysical
Journa
All-particle cosmic ray energy spectrum measured by the HAWC experiment from 10 to 500 TeV
We report on the measurement of the all-particle cosmic ray energy spectrum
with the High Altitude Water Cherenkov (HAWC) Observatory in the energy range
10 to 500 TeV. HAWC is a ground based air-shower array deployed on the slopes
of Volcan Sierra Negra in the state of Puebla, Mexico, and is sensitive to
gamma rays and cosmic rays at TeV energies. The data used in this work were
taken from 234 days between June 2016 to February 2017. The primary cosmic-ray
energy is determined with a maximum likelihood approach using the particle
density as a function of distance to the shower core. Introducing quality cuts
to isolate events with shower cores landing on the array, the reconstructed
energy distribution is unfolded iteratively. The measured all-particle spectrum
is consistent with a broken power law with an index of prior to
a break at ) TeV, followed by an index of . The
spectrum also respresents a single measurement that spans the energy range
between direct detection and ground based experiments. As a verification of the
detector response, the energy scale and angular resolution are validated by
observation of the cosmic ray Moon shadow's dependence on energy.Comment: 16 pages, 11 figures, 4 tables, submission to Physical Review
Extended gamma-ray sources around pulsars constrain the origin of the positron flux at Earth
The unexpectedly high flux of cosmic ray positrons detected at Earth may
originate from nearby astrophysical sources, dark matter, or unknown processes
of cosmic-ray secondary production. We report the detection, using the
HighAltitude Water Cherenkov Observatory (HAWC), of extended tera-electron volt
gamma-ray emission coincident with the locations of two nearby middle-aged
pulsars (Geminga and PSR B0656+14). The HAWC observations demonstrate that
these pulsars are indeed local sources of accelerated leptons, but the measured
tera-electron volt emission profile constrains the diffusion of particles away
from these sources to be much slower than previously assumed. We demonstrate
that the leptons emitted by these objects are therefore unlikely to be the
origin of the excess positrons, which may have a more exotic origin.Comment: 16 pages (including supplementary material), 5 figure
Constraining the Ratio in TeV Cosmic Rays with Observations of the Moon Shadow by HAWC
An indirect measurement of the antiproton flux in cosmic rays is possible as
the particles undergo deflection by the geomagnetic field. This effect can be
measured by studying the deficit in the flux, or shadow, created by the Moon as
it absorbs cosmic rays that are headed towards the Earth. The shadow is
displaced from the actual position of the Moon due to geomagnetic deflection,
which is a function of the energy and charge of the cosmic rays. The
displacement provides a natural tool for momentum/charge discrimination that
can be used to study the composition of cosmic rays. Using 33 months of data
comprising more than 80 billion cosmic rays measured by the High Altitude Water
Cherenkov (HAWC) observatory, we have analyzed the Moon shadow to search for
TeV antiprotons in cosmic rays. We present our first upper limits on the
fraction, which in the absence of any direct measurements, provide
the tightest available constraints of on the antiproton fraction for
energies between 1 and 10 TeV.Comment: 10 pages, 5 figures. Accepted by Physical Review
Search for very-high-energy emission from Gamma-ray Bursts using the first 18 months of data from the HAWC Gamma-ray Observatory
The High Altitude Water Cherenkov (HAWC) Gamma-ray Observatory is an
extensive air shower detector operating in central Mexico, which has recently
completed its first two years of full operations. If for a burst like GRB
130427A at a redshift of 0.34 and a high-energy component following a power law
with index -1.66, the high-energy component is extended to higher energies with
no cut-off other than from extragalactic background light attenuation, HAWC
would observe gamma rays with a peak energy of 300 GeV. This paper
reports the results of HAWC observations of 64 gamma-ray bursts (GRBs) detected
by and , including three GRBs that were also
detected by the Large Area Telescope (-LAT). An ON/OFF analysis
method is employed, searching on the time scale given by the observed light
curve at keV-MeV energies and also on extended time scales. For all GRBs and
time scales, no statistically significant excess of counts is found and upper
limits on the number of gamma rays and the gamma-ray flux are calculated. GRB
170206A, the third brightest short GRB detected by the Gamma-ray Burst Monitor
on board the satellite (-GBM) and also
detected by the LAT, occurred very close to zenith. The LAT measurements can
neither exclude the presence of a synchrotron self-Compton (SSC) component nor
constrain its spectrum. Instead, the HAWC upper limits constrain the expected
cut-off in an additional high-energy component to be less than
for reasonable assumptions about the energetics and redshift of the burst.Comment: 19 pages, 6 figures, published in Ap
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