15 research outputs found
VERITAS Search for VHE Gamma-ray Emission from Dwarf Spheroidal Galaxies
Indirect dark matter searches with ground-based gamma-ray observatories
provide an alternative for identifying the particle nature of dark matter that
is complementary to that of direct search or accelerator production
experiments. We present the results of observations of the dwarf spheroidal
galaxies Draco, Ursa Minor, Bootes 1, and Willman 1 conducted by VERITAS. These
galaxies are nearby dark matter dominated objects located at a typical distance
of several tens of kiloparsecs for which there are good measurements of the
dark matter density profile from stellar velocity measurements. Since the
conventional astrophysical background of very high energy gamma rays from these
objects appears to be negligible, they are good targets to search for the
secondary gamma-ray photons produced by interacting or decaying dark matter
particles. No significant gamma-ray flux above 200 GeV was detected from these
four dwarf galaxies for a typical exposure of ~20 hours. The 95% confidence
upper limits on the integral gamma-ray flux are in the range 0.4-2.2x10^-12
photons cm^-2s^-1. We interpret this limiting flux in the context of pair
annihilation of weakly interacting massive particles and derive constraints on
the thermally averaged product of the total self-annihilation cross section and
the relative velocity of the WIMPs. The limits are obtained under conservative
assumptions regarding the dark matter distribution in dwarf galaxies and are
approximately three orders of magnitude above the generic theoretical
prediction for WIMPs in the minimal supersymmetric standard model framework.
However significant uncertainty exists in the dark matter distribution as well
as the neutralino cross sections which under favorable assumptions could
further lower the limits.Comment: 21 pages, 2 figures, updated to reflect version published in ApJ.
NOTE: M.D. Wood added as autho
Discovery of very high energy gamma rays from PKS 1424+240 and multiwavelength constraints on its redshift
We report the first detection of very-high-energy (VHE) gamma-ray emission
above 140 GeV from PKS 1424+240, a BL Lac object with an unknown redshift. The
photon spectrum above 140 GeV measured by VERITAS is well described by a power
law with a photon index of 3.8 +- 0.5_stat +- 0.3_syst and a flux normalization
at 200 GeV of (5.1 +- 0.9_stat +- 0.5_syst) x 10^{-11} TeV^-1 cm^-2 s^-1, where
stat and syst denote the statistical and systematical uncertainty,
respectively. The VHE flux is steady over the observation period between MJD
54881 and 55003 (2009 February 19 to June 21). Flux variability is also not
observed in contemporaneous high energy observations with the Fermi Large Area
Telescope (LAT). Contemporaneous X-ray and optical data were also obtained from
the Swift XRT and MDM observatory, respectively. The broadband spectral energy
distribution (SED) is well described by a one-zone synchrotron self-Compton
(SSC) model favoring a redshift of less than 0.1. Using the photon index
measured with Fermi in combination with recent extragalactic background light
(EBL) absorption models it can be concluded from the VERITAS data that the
redshift of PKS 1424+240 is less than 0.66.Comment: accepted for publication, Ap
Discovery of VHE -ray emission from the SNR G54.1+0.3
We report the discovery of very high energy gamma-ray emission from the
direction of the SNR G54.1+0.3 using the VERITAS ground-based gamma-ray
observatory. The TeV signal has an overall significance of 6.8 and
appears point-like given the 5 resolution of the instrument. The
integral flux above 1 TeV is 2.5% of the Crab Nebula flux and significant
emission is measured between 250 GeV and 4 TeV, well described by a power-law
energy spectrum dN/dE E with a photon index . We find no evidence of time variability among
observations spanning almost two years. Based on the location, the morphology,
the measured spectrum, the lack of variability and a comparison with similar
systems previously detected in the TeV band, the most likely counterpart of
this new VHE gamma-ray source is the PWN in the SNR G54.1+0.3. The measured
X-ray to VHE gamma-ray luminosity ratio is the lowest among all the nebulae
supposedly driven by young rotation-powered pulsars, which could indicate a
particle-dominated PWN.Comment: 5 pages, 2 figure, Latex, emulateapj style, accepted by the
Astrophysical Journal Letter
The Discovery of gamma-Ray Emission From The Blazar RGB J0710+591
The high-frequency-peaked BL Lacertae object RGB J0710+591 was observed in
the very high-energy (VHE; E > 100 GeV) wave band by the VERITAS array of
atmospheric Cherenkov telescopes. The observations, taken between 2008 December
and 2009 March and totaling 22.1 hr, yield the discovery of VHE gamma rays from
the source. RGB J0710+591 is detected at a statistical significance of 5.5
standard deviations (5.5{\sigma}) above the background, corresponding to an
integral flux of (3.9 +/- 0.8) x 10-12 cm-2 s-1 (3% of the Crab Nebula's flux)
above 300 GeV. The observed spectrum can be fit by a power law from 0.31 to 4.6
TeV with a photon spectral index of 2.69 +/- 0.26stat +/- 0.20sys. These data
are complemented by contemporaneous multiwavelength data from the Fermi Large
Area Telescope, the Swift X-ray Telescope, the Swift Ultra-Violet and Optical
Telescope, and the Michigan-Dartmouth-MIT observatory. Modeling the broadband
spectral energy distribution (SED) with an equilibrium synchrotron self-Compton
model yields a good statistical fit to the data. The addition of an
external-Compton component to the model does not improve the fit nor brings the
system closer to equipartition. The combined Fermi and VERITAS data constrain
the properties of the high-energy emission component of the source over 4
orders of magnitude and give measurements of the rising and falling sections of
the SED
Sex difference and intra-operative tidal volume: Insights from the LAS VEGAS study
BACKGROUND: One key element of lung-protective ventilation is the use of a low tidal volume (VT). A sex difference in use of low tidal volume ventilation (LTVV) has been described in critically ill ICU patients.OBJECTIVES: The aim of this study was to determine whether a sex difference in use of LTVV also exists in operating room patients, and if present what factors drive this difference.DESIGN, PATIENTS AND SETTING: This is a posthoc analysis of LAS VEGAS, a 1-week worldwide observational study in adults requiring intra-operative ventilation during general anaesthesia for surgery in 146 hospitals in 29 countries.MAIN OUTCOME MEASURES: Women and men were compared with respect to use of LTVV, defined as VT of 8 ml kg-1 or less predicted bodyweight (PBW). A VT was deemed 'default' if the set VT was a round number. A mediation analysis assessed which factors may explain the sex difference in use of LTVV during intra-operative ventilation.RESULTS: This analysis includes 9864 patients, of whom 5425 (55%) were women. A default VT was often set, both in women and men; mode VT was 500 ml. Median [IQR] VT was higher in women than in men (8.6 [7.7 to 9.6] vs. 7.6 [6.8 to 8.4] ml kg-1 PBW, P < 0.001). Compared with men, women were twice as likely not to receive LTVV [68.8 vs. 36.0%; relative risk ratio 2.1 (95% CI 1.9 to 2.1), P < 0.001]. In the mediation analysis, patients' height and actual body weight (ABW) explained 81 and 18% of the sex difference in use of LTVV, respectively; it was not explained by the use of a default VT.CONCLUSION: In this worldwide cohort of patients receiving intra-operative ventilation during general anaesthesia for surgery, women received a higher VT than men during intra-operative ventilation. The risk for a female not to receive LTVV during surgery was double that of males. Height and ABW were the two mediators of the sex difference in use of LTVV.TRIAL REGISTRATION: The study was registered at Clinicaltrials.gov, NCT01601223
Discovery of tev gamma-ray emission fromtycho’s supernova remnant
We report the discovery of TeV gamma-ray emission from the Type Ia supernova remnant (SNR) G120.1+1.4, known as Tycho's SNR. Observations performed in the period 2008-2010 with the VERITAS ground-based gamma-ray observatory reveal weak emission coming from the direction of the remnant, compatible with a point source located at 00(h)25(m)27(s).0, +64 degrees 10'50 '' (J2000). The TeV photon spectrum measured by VERITAS can be described with a power law dN/dE = C(E/3.42 TeV)(-Gamma) with Gamma = 1.95 +/- 0.51(stat) +/- 0.30(sys) and C = (1.55 +/- 0.43(stat) +/- 0.47(sys)) x 10(-14) cm(-2) s(-1) TeV-1. The integral flux above 1 TeV corresponds to similar to 0.9% of the steady Crab Nebula emission above the same energy, making it one of the weakest sources yet detected in TeV gamma rays. We present both leptonic and hadronic models that can describe the data. The lowest magnetic field allowed in these models is similar to 80 mu G, which may be interpreted as evidence for magnetic field amplification
Discovery of TeV Gamma Ray Emission from Tycho's Supernova Remnant.
We report the discovery of TeV gamma-ray emission from the Type Ia supernova
remnant (SNR) G120.1+1.4, known as Tycho's supernova remnant. Observations
performed in the period 2008-2010 with the VERITAS ground-based gamma-ray
observatory reveal weak emission coming from the direction of the remnant,
compatible with a point source located at (J2000). The TeV photon
spectrum measured by VERITAS can be described with a power-law with and
cmsTeV. The integral flux above 1 TeV corresponds to percent of the steady Crab Nebula emission above the same energy, making
it one of the weakest sources yet detected in TeV gamma rays. We present both
leptonic and hadronic models which can describe the data. The lowest magnetic
field allowed in these models is G, which may be interpreted as
evidence for magnetic field amplification.Comment: 6 pages, 3 figures. Accepted in ApJ
VERITAS 2008-2009 monitoring of the variable gamma-ray source M87
M87 is a nearby radio galaxy that is detected at energies ranging from radio
to VHE gamma-rays. Its proximity and its jet, misaligned from our
line-of-sight, enable detailed morphological studies and extensive modeling at
radio, optical, and X-ray energies. Flaring activity was observed at all
energies, and multi-wavelength correlations would help clarify the origin of
the VHE emission. In this paper, we describe a detailed temporal and spectral
analysis of the VERITAS VHE gamma-ray observations of M87 in 2008 and 2009. In
the 2008 observing season, VERITAS detected an excess with a statistical
significance of 7.2 sigma from M87 during a joint multi-wavelength monitoring
campaign conducted by three major VHE experiments along with the Chandra X-ray
Observatory. In February 2008, VERITAS observed a VHE flare from M87 occurring
over a 4-day timespan. The peak nightly flux above 250GeV was 7.7% of the Crab
Nebula flux. M87 was marginally detected before this 4-day flare period, and
was not detected afterwards. Spectral analysis of the VERITAS observations
showed no significant change in the photon index between the flare and
pre-flare states. Shortly after the VHE flare seen by VERITAS, the Chandra
X-ray Observatory detected the flux from the core of M87 at a historical
maximum, while the flux from the nearby knot HST-1 remained quiescent. Acciari
et al. (2009) presented the 2008 contemporaneous VHE gamma-ray, Chandra X-ray,
and VLBA radio observations which suggest the core as the most likely source of
VHE emission, in contrast to the 2005 VHE flare that was simultaneous with an
X-ray flare in the HST-1 knot. In 2009, VERITAS continued its monitoring of M87
and marginally detected a 4.2 sigma excess corresponding to a flux of ~1% of
the Crab Nebula. No VHE flaring activity was observed in 2009.Comment: 17 pages, 5 figures, accepted for publication in Ap
TeV and Multi Wavelength Observations of Mrk 421 IN 2006 - 2008
We report on TeV gamma-ray observations of the blazar Mrk 421 (redshift of 0.031) with the VERITAS observatory and the Whipple 10m Cherenkov telescope. The excellent sensitivity of VERITAS allowed us to sample the TeV gamma-ray fluxes and energy spectra with unprecedented accuracy where Mrk 421 was detected in each of the pointings. A total of 47.3 hrs of VERITAS and 96 hrs of Whipple 10m data were acquired between January 2006 and June 2008. We present the results of a study of the TeV gamma-ray energy spectra as a function of time, and for different flux levels. On May 2nd and 3rd, 2008, bright TeV gamma-ray flares were detected with fluxes reaching the level of 10 Crab. The TeV gamma-ray data were complemented with radio, optical, and Xray observations, with flux variability found in all bands except for the radio waveband. The combination of the RXTE and Swift X-ray data reveal spectral hardening with increasing flux levels, often correlated with an increase of the source activity in TeV gamma-rays. Contemporaneous spectral energy distributions were generated for 18 nights, each of which are reasonably described by a one-zone SSC model.Fil: Acciari, V. A.. Harvard-Smithsonian Center for Astrophysics; Estados UnidosFil: Aliu, E.. Columbia University; Estados UnidosFil: Arlen, T.. University of California; Estados UnidosFil: Aune, T.. University of California; Estados UnidosFil: Beilicke, M.. University of Washington; Estados UnidosFil: Benbow, W.. Harvard-Smithsonian Center for Astrophysics; Estados UnidosFil: Boltuch, D.. University of Delaware; Estados UnidosFil: Bradbury, S. M.. University of Leeds; Reino UnidoFil: Buckley, J. H.. Washington University; Estados UnidosFil: Bugaev, V.. Washington University; Estados UnidosFil: Byrum, K.. Argonne National Laboratory; Estados UnidosFil: Cannon, A.. University College Dublin; IrlandaFil: Cesarini, A.. National University of Ireland Galway; IrlandaFil: Ciupik, L.. Adler Planetarium and Astronomy Museum,; Estados UnidosFil: Cui, W.. Purdue University; Estados UnidosFil: Dickherber, R.. University of Washington; Estados UnidosFil: Duke, C.. Grinnell College; Estados UnidosFil: Falcone, A.. University of Pennsylvania; Estados UnidosFil: Finley, J. P.. Purdue University; Estados UnidosFil: Finnegan, G.. University of Utah; Estados UnidosFil: Fortson, L.. Adler Planetarium and Astronomy Museum; Estados UnidosFil: Furniss, A.. University of California; Estados UnidosFil: Galante, N.. Harvard-Smithsonian Center for Astrophysics; Estados UnidosFil: Gall, D.. University of Iowa; Estados UnidosFil: Gillanders, G. H.. National University of Ireland Galway; IrlandaFil: Godambe, S.. University of Utah; Estados UnidosFil: Grube, J.. Adler Planetarium and Astronomy Museum; Estados UnidosFil: Guenette, R.. McGill University; CanadáFil: Pichel, Ana Carolina. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Rovero, Adrian Carlos. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: The Veritas Collaboration