135 research outputs found
Evidence for proton acceleration up to TeV energies based on VERITAS and Fermi-LAT observations of the Cas A SNR
We present a study of -ray emission from the core-collapse supernova
remnant Cas~A in the energy range from 0.1GeV to 10TeV. We used 65 hours of
VERITAS data to cover 200 GeV - 10 TeV, and 10.8 years of \textit{Fermi}-LAT
data to cover 0.1-500 GeV. The spectral analysis of \textit{Fermi}-LAT data
shows a significant spectral curvature around GeV that is
consistent with the expected spectrum from pion decay. Above this energy, the
joint spectrum from \textit{Fermi}-LAT and VERITAS deviates significantly from
a simple power-law, and is best described by a power-law with spectral index of
with a cut-off energy of TeV. These
results, along with radio, X-ray and -ray data, are interpreted in the
context of leptonic and hadronic models. Assuming a one-zone model, we exclude
a purely leptonic scenario and conclude that proton acceleration up to at least
6 TeV is required to explain the observed -ray spectrum. From modeling
of the entire multi-wavelength spectrum, a minimum magnetic field inside the
remnant of is deduced.Comment: 33 pages, 9 Figures, 6 Table
Measurement of gas-phase OH radical oxidation and film thickness of organic films at the air–water interface using material extracted from urban, remote and wood smoke aerosol
The presence of an organic film on a cloud droplet or aqueous aerosol particle has the potential to alter the chemical, optical and physical properties of the droplet or particle. In the study presented, water insoluble organic materials extracted from urban, remote (Antarctica) and wood burning atmospheric aerosol were found to have stable, compressible, films at the air–water interface that were typically ∼6–18 Å thick. These films are reactive towards gas-phase OH radicals and decay exponentially, with bimolecular rate constants for reaction with gas-phase OH radicals of typically 0.08–1.5 × 10−10 cm3 molecule−1 s−1. These bimolecular rate constants equate to initial OH radical uptake coefficients estimated to be ∼0.6–1 except woodsmoke (∼0.05). The film thickness and the neutron scattering length density of the extracted atmosphere aerosol material (from urban, remote and wood burning) were measured by neutron reflection as they were exposed to OH radicals. For the first time neutron reflection has been demonstrated as an excellent technique for studying the thin films formed at air–water interfaces from materials extracted from atmospheric aerosol samples. Additionally, the kinetics of gas-phase OH radicals with a proxy compound, the lipid 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) was studied displaying significantly different behaviour, thus demonstrating it is not a good proxy for atmospheric materials that may form films at the air–water interface. The atmospheric lifetimes, with respect to OH radical oxidation, of the insoluble organic materials extracted from atmospheric aerosol at the air–water interface were a few hours. Relative to a possible physical atmospheric lifetime of 4 days, the oxidation of these films is important and needs inclusion in atmospheric models. The optical properties of these films were previously reported [Shepherd et al., Atmos. Chem. Phys., 2018, 18, 5235–5252] and there is a significant change in top of the atmosphere albedo for these thin films on core–shell atmospheric aerosol using the film thickness data and confirmation of stable film formation at the air–water interface presented here
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Can houseplants improve indoor air quality by removing CO2 and increasing relative humidity?
High indoor CO2 concentrations and low relative humidity (RH) create an array of well-documented human health issues. Therefore, assessing houseplants’ potential as a low-cost approach to CO2 removal and increasing RH is important.
We investigated how environmental factors such as ’dry’ ( 0.30 m3 m-3) growing substrates, and indoor light levels (‘low’ 10 µmol m-2 s-1, ‘high’ 50 µmol m-2 s-1 and ‘very high’ 300 µmol m-2 s-1), influence the plants’ net CO2 assimilation (‘A’) and water-vapour loss. Seven common houseplant taxa – representing a variety of leaf types, metabolisms and sizes – were studied for their ability to assimilate CO2 across a range of indoor light levels. Additionally, to assess the plants’ potential contribution to RH increase, the plants’ evapo-transpiration (ET) was measured.
At typical ‘low’ indoor light levels ‘A’ rates were generally low (< 3.9 mg hr-1). Differences between ‘dry’ and ’wet’ plants at typical indoor light levels were negligible in terms of room-level impact. Light compensation points (i.e. light levels at which plants have positive ‘A’) were in the typical indoor light range (1-50 µmol m-2 s-1) only for two studied Spathiphyllum wallisii cultivars and Hedera helix; these plants would thus provide the best CO2 removal indoors. Additionally, increasing indoor light levels to 300 µmol m-2 s-1 would, in most species, significantly increase their potential to assimilate CO2. Species which assimilated the most CO2 also contributed most to increasing RH
Search for Ultraheavy Dark Matter from Observations of Dwarf Spheroidal Galaxies with VERITAS
Dark matter is a key piece of the current cosmological scenario, with weakly
interacting massive particles (WIMPs) a leading dark matter candidate. WIMPs
have not been detected in their conventional parameter space (100 GeV 100 TeV), a mass range accessible with current Imaging
Atmospheric Cherenkov Telescopes. As ultraheavy dark matter (UHDM; 100 TeV) has been suggested as an under-explored alternative to the
WIMP paradigm, we search for an indirect dark matter annihilation signal in a
higher mass range (up to 30 PeV) with the VERITAS gamma-ray observatory. With
216 hours of observations of four dwarf spheroidal galaxies, we perform an
unbinned likelihood analysis. We find no evidence of a -ray signal from
UHDM annihilation above the background fluctuation for any individual dwarf
galaxy nor for a joint-fit analysis, and consequently constrain the
velocity-weighted annihilation cross section of UHDM for dark matter particle
masses between 1 TeV and 30 PeV. We additionally set constraints on the allowed
radius of a composite UHDM particle.Comment: 10 pages, 7 figure
Demonstration of stellar intensity interferometry with the four VERITAS telescopes
High angular resolution observations at optical wavelengths provide valuable
insights in stellar astrophysics, directly measuring fundamental stellar
parameters, and probing stellar atmospheres, circumstellar disks, elongation of
rapidly rotating stars, and pulsations of Cepheid variable stars. The angular
size of most stars are of order one milli-arcsecond or less, and to spatially
resolve stellar disks and features at this scale requires an optical
interferometer using an array of telescopes with baselines on the order of
hundreds of meters. We report on the successful implementation of a stellar
intensity interferometry system developed for the four VERITAS imaging
atmospheric-Cherenkov telescopes. The system was used to measure the angular
diameter of the two sub-mas stars Canis Majoris and Orionis
with a precision better than 5%. The system utilizes an off-line approach where
starlight intensity fluctuations recorded at each telescope are correlated
post-observation. The technique can be readily scaled onto tens to hundreds of
telescopes, providing a capability that has proven technically challenging to
current generation optical amplitude interferometry observatories. This work
demonstrates the feasibility of performing astrophysical measurements with
imaging atmospheric-Cherenkov telescope arrays as intensity interferometers and
the promise for integrating an intensity interferometry system within future
observatories such as the Cherenkov Telescope Array.Comment: Accepted for publication in Nature Astronomy (2020
VERITAS Discovery of VHE Emission from the Radio Galaxy 3C 264: A Multi-Wavelength Study
The radio source 3C 264, hosted by the giant elliptical galaxy NGC 3862, was
observed with VERITAS between February 2017 and May 2019. These deep
observations resulted in the discovery of very-high-energy (VHE; E GeV)
-ray emission from this active galaxy. An analysis of 57 hours of
quality-selected live time yields a detection at the position of the source,
corresponding to a statistical significance of 7.8 standard deviations above
background. The observed VHE flux is variable on monthly time scales, with an
elevated flux seen in 2018 observations. The VHE emission during this elevated
state is well-characterized by a power-law spectrum with a photon index and flux F( GeV) = ( cm s, or approximately 0.7%
of the Crab Nebula flux above the same threshold. 3C 264 () is the
most distant radio galaxy detected at VHE, and the elevated state is thought to
be similar to that of the famously outbursting jet in M 87. Consequently,
extensive contemporaneous multi-wavelength data were acquired in 2018 at the
time of the VHE high state. An analysis of these data, including VLBA, VLA,
HST, Chandra and Swift observations in addition to the VERITAS data, is
presented, along with a discussion of the resulting spectral energy
distribution.Comment: 19 pages, 11 figures, Accepted for publication in Astrophysical
Journa
VERITAS Observations of the Galactic Center Region at Multi-TeV Gamma-Ray Energies
The Galactic Center (GC) region hosts a variety of powerful astronomical
sources and rare astrophysical processes that emit a large flux of non-thermal
radiation. The inner 375 pc x 600 pc region, called the Central Molecular Zone,
is home to the supermassive black hole Sagittarius A*, massive cloud complexes,
and particle accelerators such as supernova remnants. We present the results of
our improved analysis of the very-high-energy (VHE) gamma-ray emission above 2
TeV from the GC using 125 hours of data taken with the VERITAS
imaging-atmospheric Cherenkov telescope between 2010 and 2018. The central
source VER J1745-290, consistent with the position of Sagittarius A*, is
detected at a significance of 38 standard deviations above the background level
, and we report its spectrum and light curve. Its differential
spectrum is consistent with a power law with exponential cutoff, with a
spectral index of , a flux normalization at 5.3 TeV of
TeV-1 cm-2 s-1, and cutoff energy of
TeV. We also present results on the diffuse emission near
the GC, obtained by combining data from multiple regions along the GC ridge
which yield a cumulative significance of . The diffuse GC ridge
spectrum is best fit by a power law with a hard index of 2.19 0.20,
showing no evidence of a cutoff up to 40 TeV. This strengthens the evidence for
a potential accelerator of PeV cosmic rays being present in the GC. We also
provide spectra of the other sources in our field of view with significant
detections, composite supernova remnant G0.9+0.1 and HESS J1746-285.Comment: 19 pages, 8 figures, Accepted for publication in Astrophysical
Journa
VERITAS and Fermi-LAT constraints on the Gamma-ray Emission from Superluminous Supernovae SN2015bn and SN2017egm
Superluminous supernovae (SLSNe) are a rare class of stellar explosions with
luminosities ~10-100 times greater than ordinary core-collapse supernovae. One
popular model to explain the enhanced optical output of hydrogen-poor (Type I)
SLSNe invokes energy injection from a rapidly spinning magnetar. A prediction
in this case is that high-energy gamma rays, generated in the wind nebula of
the magnetar, could escape through the expanding supernova ejecta at late times
(months or more after optical peak). This paper presents a search for gamma-ray
emission in the broad energy band from 100 MeV to 30 TeV from two Type I SLSNe,
SN2015bn, and SN2017egm, using observations from Fermi-LAT and VERITAS.
Although no gamma-ray emission was detected from either source, the derived
upper limits approach the putative magnetar's spin-down luminosity. Prospects
are explored for detecting very-high-energy (VHE; 100 GeV - 100 TeV) emission
from SLSNe-I with existing and planned facilities such as VERITAS and CTA.Comment: 20 pages, 7 figures, 2 table
An Archival Search for Neutron-Star Mergers in Gravitational Waves and Very-High-Energy Gamma Rays
The recent discovery of electromagnetic signals in coincidence with
neutron-star mergers has solidified the importance of multimessenger campaigns
in studying the most energetic astrophysical events. Pioneering multimessenger
observatories, such as LIGO/Virgo and IceCube, record many candidate signals
below the detection significance threshold. These sub-threshold event
candidates are promising targets for multimessenger studies, as the information
provided by them may, when combined with contemporaneous gamma-ray
observations, lead to significant detections. Here we describe a new method
that uses such candidates to search for transient events using archival
very-high-energy gamma-ray data from imaging atmospheric Cherenkov telescopes
(IACTs). We demonstrate the application of this method to sub-threshold binary
neutron star (BNS) merger candidates identified in Advanced LIGO's first
observing run. We identify eight hours of archival VERITAS observations
coincident with seven BNS merger candidates and search them for TeV emission.
No gamma-ray emission is detected; we calculate upper limits on the integral
flux and compare them to a short gamma-ray burst model. We anticipate this
search method to serve as a starting point for IACT searches with future
LIGO/Virgo data releases as well as in other sub-threshold studies for
multimessenger transients, such as IceCube neutrinos. Furthermore, it can be
deployed immediately with other current-generation IACTs, and has the potential
for real-time use that places minimal burden on experimental operations.
Lastly, this method may serve as a pilot for studies with the Cherenkov
Telescope Array, which has the potential to observe even larger fields of view
in its divergent pointing mode
Measurement of the extragalactic background light spectral energy distribution with VERITAS
The extragalactic background light (EBL), a diffuse photon field in the
optical and infrared range, is a record of radiative processes over the
Universe's history. Spectral measurements of blazars at very high energies
(100 GeV) enable the reconstruction of the spectral energy distribution
(SED) of the EBL, as the blazar spectra are modified by redshift- and
energy-dependent interactions of the gamma-ray photons with the EBL. The
spectra of 14 VERITAS-detected blazars are included in a new measurement of the
EBL SED that is independent of EBL SED models. The resulting SED covers an EBL
wavelength range of 0.56--56 m, and is in good agreement with lower limits
obtained by assuming that the EBL is entirely due to radiation from cataloged
galaxies.Comment: Accepted for publication in The Astrophysical Journa
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