166 research outputs found
Structure and rheology of the middle Allochthon at 68 ̊N, Scandinavian Caledonides
Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1992.Maps folded in pocket following text.Includes bibliographical references (leaves 41-44).by Eric J. Buchovecky.M.S
A novel biallelic loss-of-function variant in DAND5 causes heterotaxy syndrome
Funding J.A.B. acknowledges funding from iNOVA4Health-UID/Multi/04462/2013, a program financially supported by Fundação para a Ciência e Tecnologia/Ministério da Educação e Ciência through national funds and cofunded by Fundo Europeu de Desenvolvimento Regional (FEDER) under the PT2020 Partnership Agreement.The majority of heterotaxy cases do not obtain a molecular diagnosis, although pathogenic variants in more than 50 genes are known to cause heterotaxy. A heterozygous missense variant in DAND5, a nodal inhibitor, which functions in early development for establishment of right-left patterning, has been implicated in heterotaxy. Recently, the first case was reported of a DAND5 biallelic loss-of-function (LoF) variant in an individual with heterotaxy. Here, we describe a second unrelated individual with heterotaxy syndrome and a homozygous frameshift variant in DAND5 (NM_152654.2:c.197del [p.Leu66ArgfsTer22]). Using an in vitro assay, we demonstrate that the DAND5 c.197del variant is unable to inhibit nodal signaling when compared with the wild-type expression construct. This work strengthens the genetic and functional evidence for biallelic LoF variants in DAND5 causing an autosomal recessive heterotaxy syndrome.publishersversionpublishe
A Luminous and Isolated Gamma-Ray Flare from the Blazar B2 1215+30
B2 1215+30 is a BL-Lac-type blazar that was first detected at TeV energies by the MAGIC atmospheric Cherenkov telescopes and subsequently confirmed by the Very Energetic Radiation Imaging Telescope Array System (VERITAS) observatory with data collected between 2009 and 2012. In 2014 February 08, VERITAS detected a large-amplitude flare from B2 1215+30 during routine monitoring observations of the blazar 1ES 1218+304, located in the same field of view. The TeV flux reached 2.4 times the Crab Nebula flux with a variability timescale of . Multiwavelength observations with Fermi-LAT, Swift, and the Tuorla Observatory revealed a correlated high GeV flux state and no significant optical counterpart to the flare, with a spectral energy distribution where the gamma-ray luminosity exceeds the synchrotron luminosity. When interpreted in the framework of a one-zone leptonic model, the observed emission implies a high degree of beaming, with Doppler factor , and an electron population with spectral index
A Search for Spectral Hysteresis and Energy-dependent Time Lags from X-Ray and TeV Gamma-Ray Observations of Mrk 421
Abstract
Blazars are variable emitters across all wavelengths over a wide range of timescales, from months down to minutes. It is therefore essential to observe blazars simultaneously at different wavelengths, especially in the X-ray and gamma-ray bands, where the broadband spectral energy distributions usually peak. In this work, we report on three target-of-opportunity observations of Mrk 421, one of the brightest TeV blazars, triggered by a strong flaring event at TeV energies in 2014. These observations feature long, continuous, and simultaneous exposures with XMM-Newton (covering the X-ray and optical/ultraviolet bands) and VERITAS (covering the TeV gamma-ray band), along with contemporaneous observations from other gamma-ray facilities (MAGIC and Fermi-Large Area Telescope) and a number of radio and optical facilities. Although neither rapid flares nor significant X-ray/TeV correlation are detected, these observations reveal subtle changes in the X-ray spectrum of the source over the course of a few days. We search the simultaneous X-ray and TeV data for spectral hysteresis patterns and time delays, which could provide insight into the emission mechanisms and the source properties (e.g., the radius of the emitting region, the strength of the magnetic field, and related timescales). The observed broadband spectra are consistent with a one-zone synchrotron self-Compton model. We find that the power spectral density distribution at 4 × 10−4 Hz from the X-ray data can be described by a power-law model with an index value between 1.2 and 1.8, and do not find evidence for a steepening of the power spectral index (often associated with a characteristic length scale) compared to the previously reported values at lower frequencies
Gamma-ray Observations Under Bright Moonlight with VERITAS
Imaging atmospheric Cherenkov telescopes (IACTs) are equipped with sensitive
photomultiplier tube (PMT) cameras. Exposure to high levels of background
illumination degrades the efficiency of and potentially destroys these
photo-detectors over time, so IACTs cannot be operated in the same
configuration in the presence of bright moonlight as under dark skies. Since
September 2012, observations have been carried out with the VERITAS IACTs under
bright moonlight (defined as about three times the night-sky-background (NSB)
of a dark extragalactic field, typically occurring when Moon illumination >
35%) in two observing modes, firstly by reducing the voltage applied to the
PMTs and, secondly, with the addition of ultra-violet (UV) bandpass filters to
the cameras. This has allowed observations at up to about 30 times previous NSB
levels (around 80% Moon illumination), resulting in 30% more observing time
between the two modes over the course of a year. These additional observations
have already allowed for the detection of a flare from the 1ES 1727+502 and for
an observing program targeting a measurement of the cosmic-ray positron
fraction. We provide details of these new observing modes and their performance
relative to the standard VERITAS observations
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
Very-high-energy observations of the binaries V 404 Cyg and 4U 0115+634 during giant X-ray outbursts
Transient X-ray binaries produce major outbursts in which the X-ray flux can
increase over the quiescent level by factors as large as . The low-mass
X-ray binary V 404 Cyg and the high-mass system 4U 0115+634 underwent such
major outbursts in June and October 2015, respectively. We present here
observations at energies above hundreds of GeV with the VERITAS observatory
taken during some of the brightest X-ray activity ever observed from these
systems. No gamma-ray emission has been detected by VERITAS in 2.5 hours of
observations of the microquasar V 404 Cyg from 2015, June 20-21. The upper flux
limits derived from these observations on the gamma-ray flux above 200 GeV of F
cm s correspond to a tiny fraction (about
) of the Eddington luminosity of the system, in stark contrast to that
seen in the X-ray band. No gamma rays have been detected during observations of
4U 0115+634 in the period of major X-ray activity in October 2015. The flux
upper limit derived from our observations is F cm
s for gamma rays above 300 GeV, setting an upper limit on the ratio of
gamma-ray to X-ray luminosity of less than 4%.Comment: Accepted for publication in the Astrophysical Journa
Discovery of very-high-energy emission from RGB J2243+203 and derivation of its redshift upper limit
Very-high-energy (VHE; 100 GeV) gamma-ray emission from the blazar RGB
J2243+203 was discovered with the VERITAS Cherenkov telescope array, during the
period between 21 and 24 December 2014. The VERITAS energy spectrum from this
source can be fit by a power law with a photon index of , and a
flux normalization at 0.15 TeV of . The integrated
\textit{Fermi}-LAT flux from 1 GeV to 100 GeV during the VERITAS detection is
, which is an order of
magnitude larger than the four-year-averaged flux in the same energy range
reported in the 3FGL catalog, (). The detection with VERITAS
triggered observations in the X-ray band with the \textit{Swift}-XRT. However,
due to scheduling constraints \textit{Swift}-XRT observations were performed 67
hours after the VERITAS detection, not simultaneous with the VERITAS
observations. The observed X-ray energy spectrum between 2 keV and 10 keV can
be fitted with a power-law with a spectral index of , and the
integrated photon flux in the same energy band is . EBL model-dependent upper limits
of the blazar redshift have been derived. Depending on the EBL model used, the
upper limit varies in the range from z to z
Measurement of Cosmic-ray Electrons at TeV Energies by VERITAS
Cosmic-ray electrons and positrons (CREs) at GeV-TeV energies are a unique
probe of our local Galactic neighborhood. CREs lose energy rapidly via
synchrotron radiation and inverse-Compton scattering processes while
propagating within the Galaxy and these losses limit their propagation
distance. For electrons with TeV energies, the limit is on the order of a
kiloparsec. Within that distance there are only a few known astrophysical
objects capable of accelerating electrons to such high energies. It is also
possible that the CREs are the products of the annihilation or decay of heavy
dark matter (DM) particles. VERITAS, an array of imaging air Cherenkov
telescopes in southern Arizona, USA, is primarily utilized for gamma-ray
astronomy, but also simultaneously collects CREs during all observations. We
describe our methods of identifying CREs in VERITAS data and present an energy
spectrum, extending from 300 GeV to 5 TeV, obtained from approximately 300
hours of observations. A single power-law fit is ruled out in VERITAS data. We
find that the spectrum of CREs is consistent with a broken power law, with a
break energy at 710 40 140 GeV.Comment: 17 pages, 2 figures, accepted for publication in PR
Gamma-ray observations of Tycho's SNR with VERITAS and Fermi
High-energy gamma-ray emission from supernova remnants (SNRs) has provided a
unique perspective for studies of Galactic cosmic-ray acceleration. Tycho's SNR
is a particularly good target because it is a young, type Ia SNR that is
well-studied over a wide range of energies and located in a relatively clean
environment. Since the detection of gamma-ray emission from Tycho's SNR by
VERITAS and Fermi-LAT, there have been several theoretical models proposed to
explain its broadband emission and high-energy morphology. We report on an
update to the gamma-ray measurements of Tycho's SNR with 147 hours of VERITAS
and 84 months of Fermi-LAT observations, which represents about a factor of two
increase in exposure over previously published data. About half of the VERITAS
data benefited from a camera upgrade, which has made it possible to extend the
TeV measurements toward lower energies. The TeV spectral index measured by
VERITAS is consistent with previous results, but the expanded energy range
softens a straight power-law fit. At energies higher than 400 GeV, the
power-law index is . It
is also softer than the spectral index in the GeV energy range, , measured by this study using
Fermi--LAT data. The centroid position of the gamma-ray emission is coincident
with the center of the remnant, as well as with the centroid measurement of
Fermi--LAT above 1 GeV. The results are consistent with an SNR shell origin of
the emission, as many models assume. The updated spectrum points to a lower
maximum particle energy than has been suggested previously.Comment: Accepted for publication in The Astrophysical Journa
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