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  • Acciari, V. A., et al. (författare)
  • VERITAS Observations of the TeV Binary LS I +61\deg 303 During 2008-2010
  • 2011
  • Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 738:1
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>We present the results of observations of the TeV binary LS I +61° 303 with the VERITAS telescope array between 2008 and 2010, at energies above 300 GeV. In the past, both ground-based gamma-ray telescopes VERITAS and MAGIC have reported detections of TeV emission near the apastron phases of the binary orbit. The observations presented here show no strong evidence for TeV emission during these orbital phases; however, during observations taken in late 2010, significant emission was detected from the source close to the phase of superior conjunction (much closer to periastron passage) at a 5.6 standard deviation (5.6σ) post-trials significance. In total, between 2008 October and 2010 December a total exposure of 64.5 hr was accumulated with VERITAS on LS I +61° 303, resulting in an excess at the 3.3σ significance level for constant emission over the entire integrated data set. The flux upper limits derived for emission during the previously reliably active TeV phases (i.e., close to apastron) are less than 5% of the Crab Nebula flux in the same energy range. This result stands in apparent contrast to previous observations by both MAGIC and VERITAS which detected the source during these phases at 10% of the Crab Nebula flux. During the two year span of observations, a large amount of X-ray data were also accrued on LS I +61° 303 by the<em>Swift</em> X-ray Telescope and the <em>Rossi X-ray Timing Explorer</em> Proportional Counter Array. We find no evidence for a correlation between emission in the X-ray and TeV regimes during 20 directly overlapping observations. We also comment on data obtained contemporaneously by the <em>Fermi</em> Large Area Telescope.</p>
  • Aliu, E., et al. (författare)
  • Multiwavelength Observations of the Previously Unidentified Blazar RX J0648.7+1516
  • 2011
  • Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 742:2
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>We report on the VERITAS discovery of very high energy (VHE) gamma-ray emission above 200 GeV from the high-frequency-peaked BL Lac (HBL) object RX J0648.7+1516 (GB J0648+1516), associated with 1FGL J0648.8+1516. The photon spectrum above 200 GeV is fitted by a power law <em>dN</em>/<em>dE</em> = <em>F</em> <sub>0</sub>(<em>E</em>/<em>E</em> <sub>0</sub>)<sup>–Γ</sup> with a photon index Γ of 4.4 ± 0.8<sub>stat</sub> ± 0.3<sub>syst</sub> and a flux normalization <em>F</em> <sub>0</sub> of (2.3 ± 0.5<sub>stat</sub> ± 1.2<sub>sys</sub>) <strong>×</strong> 10<sup>–11</sup> TeV<sup>–1</sup> cm<sup>–2</sup> s<sup>–1</sup> with <em>E</em> <sub>0</sub> = 300 GeV. No VHE variability is detected during VERITAS observations of RX J0648.7+1516 between 2010 March 4 and April 15. Following the VHE discovery, the optical identification and spectroscopic redshift were obtained using the Shane 3 m Telescope at the Lick Observatory, showing the unidentified object to be a BL Lac type with a redshift of <em>z</em> = 0.179. Broadband multiwavelength observations contemporaneous with the VERITAS exposure period can be used to subclassify the blazar as an HBL object, including data from the MDM observatory, <em>Swift</em>-UVOT, and X-Ray Telescope, and continuous monitoring at photon energies above 1 GeV from the <em>Fermi</em>Large Area Telescope (LAT). We find that in the absence of undetected, high-energy rapid variability, the one-zone synchrotron self-Compton (SSC) model overproduces the high-energy gamma-ray emission measured by the <em>Fermi</em>-LAT over 2.3 years. The spectral energy distribution can be parameterized satisfactorily with an external-Compton or lepto-hadronic model, which have two and six additional free parameters, respectively, compared to the one-zone SSC model.</p>
  • Aliu, E., et al. (författare)
  • VERITAS deep observations of the dwarf spheroidal galaxy Segue 1
  • 2012
  • Ingår i: Physical Review D. - 1550-7998 .- 1550-2368. ; 85:6
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>The VERITAS array of Cherenkov telescopes has carried out a deep observational program on the nearby dwarf spheroidal galaxy Segue 1. We report on the results of nearly 48 hours of good quality selected data, taken between January 2010 and May 2011. No significant γ-ray emission is detected at the nominal position of Segue 1, and upper limits on the integrated flux are derived. According to recent studies, Segue 1 is the most dark matter-dominated dwarf spheroidal galaxy currently known. We derive stringent bounds on various annihilating and decaying dark matter particle models. The upper limits on the velocity-weighted annihilation cross-section are ⟨σv⟩95% CL≲10−23  cm3 s−1, improving our limits from previous observations of dwarf spheroidal galaxies by at least a factor of 2 for dark matter particle masses mχ≳300  GeV. The lower limits on the decay lifetime are at the level of τ95% CL≳1024  s. Finally, we address the interpretation of the cosmic ray lepton anomalies measured by ATIC and PAMELA in terms of dark matter annihilation, and show that the VERITAS observations of Segue 1 disfavor such a scenario.</p>
  • Wang, Li-San, et al. (författare)
  • Rarity of the Alzheimer Disease-Protective APP A673T Variant in the United States.
  • 2015
  • Ingår i: JAMA neurology. - 2168-6157. ; 72:2
  • Tidskriftsartikel (refereegranskat)abstract
    • Recently, a rare variant in the amyloid precursor protein gene (APP) was described in a population from Iceland. This variant, in which alanine is replaced by threonine at position 673 (A673T), appears to protect against late-onset Alzheimer disease (AD). We evaluated the frequency of this variant in AD cases and cognitively normal controls to determine whether this variant will significantly contribute to risk assessment in individuals in the United States.
  • Acciari, V. A., et al. (författare)
  • Gamma-Ray Observations of the Be/Pulsar Binary 1A 0535+262 During a Giant X-Ray Outburst
  • 2011
  • Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 733:2
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>Giant X-ray outbursts, with luminosities of about 10<sup>37</sup> erg s<sup>–1</sup>, are observed roughly every five years from the nearby Be/pulsar binary 1A 0535+262. In this article, we present observations of the source with VERITAS at very high energies (VHEs;<em>E</em> &gt;100 GeV) triggered by the X-ray outburst in 2009 December. The observations started shortly after the onset of the outburst and provided comprehensive coverage of the episode, as well as the 111 day binary orbit. No VHE emission is evident at any time. We also examined data from the contemporaneous observations of 1A 0535+262 with the <em>Fermi</em>/Large Area Telescope at high-energy photons (<em>E</em> &gt; 0.1 GeV) and failed to detect the source at GeV energies. The X-ray continua measured with the<em>Swift</em>/X-Ray Telescope and the <em>RXTE</em>/PCA can be well described by the combination of blackbody and Comptonized emission from thermal electrons. Therefore, the gamma-ray and X-ray observations suggest the absence of a significant population of non-thermal particles in the system. This distinguishes 1A 0535+262 from those Be X-ray binaries (such as PSR B1259-63 and LS I +61°303) that have been detected at GeV-TeV energies. We discuss the implications of the results on theoretical models.</p>
  • Aliu, E., et al. (författare)
  • VERITAS Observations of the Unusual Extragalactic Transient Swift J164449.3+573451
  • 2011
  • Ingår i: Astrophysical Journal Letters. - 2041-8205 .- 2041-8213. ; 738:2
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>We report on very high energy (&gt;100 GeV) gamma-ray observations of Swift J164449.3+573451, an unusual transient object first detected by the <em>Swift</em> Observatory and later detected by multiple radio, optical, and X-ray observatories. A total exposure of 28 hr was obtained on Swift J164449.3+573451 with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) during 2011 March 28-April 15. We do not detect the source and place a differential upper limit on the emission at 500 GeV during these observations of 1.4 <strong>×</strong> 10<sup>–12</sup> erg cm<sup>–2</sup> s<sup>–1</sup> (99% confidence level). We also present time-resolved upper limits and use a flux limit averaged over the X-ray flaring period to constrain various emission scenarios that can accommodate both the radio-through-X-ray emission detected from the source and the lack of detection by VERITAS.</p>
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