1,806 research outputs found
Wet deposition at the base of Mt Everest: Seasonal evolution of the chemistry and isotopic composition
The chemistry of wet deposition was investigated during 2012–2014 at the Pyramid International Laboratory in the Upper Khumbu Valley, Nepal, at 5050 m a.s.l., within the Global Atmosphere Watch (GAW) programme. The main hydro-chemical species and stable isotopes of the water molecule were determined for monsoon rain (July–September) and snow samples (October–June). To evaluate the synoptic-scale variability of air masses reaching the measurement site, 5 day back-trajectories were computed for the sampling period. Ion concentrations in precipitation during the monsoon were low suggesting that they represent global regional background concentrations. The associations between ions suggested that the principal sources of chemical species were marine aerosols, rock and soil dust, and fossil fuel combustion. Most chemical species exhibited a pattern during the monsoon, with maxima at the beginning and at the end of the season, partially correlated with the precipitation amount. Snow samples exhibited significantly higher concentrations of chemical species, compared to the monsoon rainfall observations. Particularly during 2013, elevated concentrations of NO3−, SO42− and NH4+ were measured in the first winter snow event, and in May at the end of the pre-monsoon season. The analysis of large-scale circulation and wind regimes as well as atmospheric composition observations in the region indicates the transport of polluted air masses from the Himalayan foothills and Indian sub-continent up to the Himalaya region. During the summer monsoon onset period, the greater values of pollutants can be attributed to air-mass transport from the planetary boundary layer (PBL) of the Indo-Gangetic plains. Isotopic data confirm that during the monsoon period, precipitation occurred from water vapor that originated from the Indian Ocean and the Bay of Bengal; by contrast during the non-monsoon period, an isotopic signature of more continental origin appeared, indicating that the higher recorded NO3− and SO42− concentrations could be ascribed to a change in air circulation patterns. A comparison of recent monsoon deposition chemistry with data from the 1990's shows similar levels of contaminants in the rainfall. However, non-monsoon deposition can be significant, as it largely contributed to the ion wet deposition fluxes for all analyzed species in 2013
The Binary Neutron Star event LIGO/VIRGO GW170817 a hundred and sixty days after merger: synchrotron emission across the electromagnetic spectrum
We report deep Chandra, HST and VLA observations of the binary neutron star
event GW170817 at d after merger. These observations show that GW170817
has been steadily brightening with time and might have now reached its peak,
and constrain the emission process as non-thermal synchrotron emission where
the cooling frequency is above the X-ray band and the synchrotron
frequency is below the radio band. The very simple power-law spectrum
extending for eight orders of magnitude in frequency enables the most precise
measurement of the index of the distribution of non-thermal relativistic
electrons accelerated by a shock launched by a
NS-NS merger to date. We find , which indicates that radiation
from ejecta with dominates the observed emission. While
constraining the nature of the emission process, these observations do
\emph{not} constrain the nature of the relativistic ejecta. We employ
simulations of explosive outflows launched in NS ejecta clouds to show that the
spectral and temporal evolution of the non-thermal emission from GW170817 is
consistent with both emission from radially stratified quasi-spherical ejecta
traveling at mildly relativistic speeds, \emph{and} emission from off-axis
collimated ejecta characterized by a narrow cone of ultra-relativistic material
with slower wings extending to larger angles. In the latter scenario, GW170817
harbored a normal SGRB directed away from our line of sight. Observations at
days are unlikely to settle the debate as in both scenarios the
observed emission is effectively dominated by radiation from mildly
relativistic material.Comment: Updated with the latest VLA and Chandra dat
The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. V. Rising X-ray Emission from an Off-Axis Jet
We report the discovery of rising X-ray emission from the binary neutron star
(BNS) merger event GW170817. This is the first detection of X-ray emission from
a gravitational-wave source. Observations acquired with the Chandra X-ray
Observatory (CXO) at t~2.3 days post merger reveal no significant emission,
with L_x<=3.2x10^38 erg/s (isotropic-equivalent). Continued monitoring revealed
the presence of an X-ray source that brightened with time, reaching L_x\sim
9x10^39 erg/s at ~15.1 days post merger. We interpret these findings in the
context of isotropic and collimated relativistic outflows (both on- and
off-axis). We find that the broad-band X-ray to radio observations are
consistent with emission from a relativistic jet with kinetic energy
E_k~10^49-10^50 erg, viewed off-axis with theta_obs~ 20-40 deg. Our models
favor a circumbinary density n~ 0.0001-0.01 cm-3, depending on the value of the
microphysical parameter epsilon_B=10^{-4}-10^{-2}. A central-engine origin of
the X-ray emission is unlikely. Future X-ray observations at
days, when the target will be observable again with the CXO, will provide
additional constraints to solve the model degeneracies and test our
predictions. Our inferences on theta_obs are testable with gravitational wave
information on GW170817 from Advanced LIGO/Virgo on the binary inclination.Comment: 7 Pages, 4 Figures, ApJL, In Press. Keywords: GW170817, LV
Radio source-component association for the LOFAR Two-metre Sky Survey with region-based convolutional neural networks
Large scale structure and cosmolog
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