| 1. |
- Archambault, S., et al.
(författare)
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Deep Broadband Observations of the Distant Gamma-Ray Blazar PKS 1424+240
- 2014
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Ingår i: Astrophysical Journal Letters. - 2041-8205 .- 2041-8213. ; 785:1
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Tidskriftsartikel (refereegranskat)abstract
- We present deep VERITAS observations of the blazar PKS 1424+240, along with contemporaneous Fermi Large Area Telescope, Swift X-ray Telescope, and Swift UV Optical Telescope data between 2009 February 19 and 2013 June 8. This blazar resides at a redshift of z ≥ 0.6035, displaying a significantly attenuated gamma-ray flux above 100 GeV due to photon absorption via pair-production with the extragalactic background light. We present more than 100 hr of VERITAS observations over three years, a multiwavelength light curve, and the contemporaneous spectral energy distributions. The source shows a higher flux of (2.1 ± 0.3) × 10–7 photons m–2 s–1 above 120 GeV in 2009 and 2011 as compared to the flux measured in 2013, corresponding to (1.02 ± 0.08) × 10–7 photons m–2 s–1 above 120 GeV. The measured differential very high energy (VHE; E ≥ 100 GeV) spectral indices are Γ = 3.8 ± 0.3, 4.3 ± 0.6 and 4.5 ± 0.2 in 2009, 2011, and 2013, respectively. No significant spectral change across the observation epochs is detected. We find no evidence for variability at gamma-ray opacities of greater than τ = 2, where it is postulated that any variability would be small and occur on timescales longer than a year if hadronic cosmic-ray interactions with extragalactic photon fields provide a secondary VHE photon flux. The data cannot rule out such variability due to low statistics.
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| 2. |
- Archambault, S., et al.
(författare)
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Discovery of a New TeV Gamma-Ray Source : VER J0521+211
- 2013
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 776:2
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Tidskriftsartikel (refereegranskat)abstract
- We report the detection of a new TeV gamma-ray source, VER J0521+211, based on observations made with the VERITAS imaging atmospheric Cherenkov Telescope Array. These observations were motivated by the discovery of a cluster of >30 GeV photons in the first year of Fermi Large Area Telescope observations. VER J0521+211 is relatively bright at TeV energies, with a mean photon flux of (1.93 ± 0.13stat ± 0.78sys) × 10–11 cm–2 s–1 above 0.2 TeV during the period of the VERITAS observations. The source is strongly variable on a daily timescale across all wavebands, from optical to TeV, with a peak flux corresponding to ~0.3 times the steady Crab Nebula flux at TeV energies. Follow-up observations in the optical and X-ray bands classify the newly discovered TeV source as a BL Lac-type blazar with uncertain redshift, although recent measurements suggest z = 0.108. VER J0521+211 exhibits all the defining properties of blazars in radio, optical, X-ray, and gamma-ray wavelengths.
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| 3. |
- Aliu, E., et al.
(författare)
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Investigating the TeV Morphology of MGRO J1908+06 with VERITAS
- 2014
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 787:2
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Tidskriftsartikel (refereegranskat)abstract
- We report on deep observations of the extended TeV gamma-ray source MGRO J1908+06 made with the VERITAS very high energy gamma-ray observatory. Previously, the TeV emission has been attributed to the pulsar wind nebula (PWN) of theFermi-LAT pulsar PSR J1907+0602. We detect MGRO J1908+06 at a significance level of 14 standard deviations (14σ) and measure a photon index of 2.20 ± 0.10stat ± 0.20sys. The TeV emission is extended, covering the region near PSR J1907+0602 and also extending toward SNR G40.5-0.5. When fitted with a two-dimensional Gaussian, the intrinsic extension has a standard deviation of σsrc = 044 ± 002. In contrast to other TeV PWNe of similar age in which the TeV spectrum softens with distance from the pulsar, the TeV spectrum measured near the pulsar location is consistent with that measured at a position near the rim of G40.5-0.5, 033 away.
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| 4. |
- Santo, C.E., et al.
(författare)
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Optimization of coagulation-flocculation and flotation parameters for the treatment of a petroleum refinery effluent from a Portuguese Plant
- 2012
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Ingår i: Chemical Engineering Journal. - : Elsevier. - 1385-8947 .- 1873-3212. ; 183, s. 117-123
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Tidskriftsartikel (refereegranskat)abstract
- Petroleum refineries generate significant amounts of wastewater which have to be treated and processed before their discharge into water streams. The refinery wastewater treatment plants (WWTPPs) employ physico-chemical processes to achieve effluents of satisfactory oil content to be further treated by biological processes. In the present study, coagulation-flocculation and flotation processes are optimised to reduce the concentrations of organic matter, oil and grease and other contaminants in order to obtain an effluent with suitable characteristics ready to be treated by an aerobic biological process. PAX-18 (17% Al2O3), aluminium sulphate (Al-2(SO4)(3)) and ferric sulphate (Fe-2(SO4)(3)) are chosen for affecting coagulation-flocculation as a primary treatment. NALCO 71408 is employed as flocculant. Treatment efficiency is evaluated in terms of chemical oxygen demand (COD), total organic carbon (TOC) and turbidity measurements. The experiments are conducted both in discontinuous and continuous mode to assess the feasibility of the process. Flotation is investigated as a post-treatment process for the removal of emulsified hydrocarbons and satisfactory results are obtained.
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| 5. |
- Santo, C.E., et al.
(författare)
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Sulphide removal from petroleum refinery wastewaters by catalytic oxidation
- 2012
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Ingår i: Desalination and Water Treatment. - : Informa UK Limited. - 1944-3994 .- 1944-3986. ; 46:1-3, s. 256-263
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Tidskriftsartikel (refereegranskat)abstract
- The catalytic oxidation of sulphides present in oil refinery wastewaters was investigated in the present study. The wastewaters were obtained from the wastewater treatment plant of the oil refinery in Matosinhos (Portugal), Galp Energia. Air, NaOCl and H2O2 were chosen as oxidants and Fe3+ and Mn2+ as the two catalysts to assess the effective combination of catalyst-oxidant for sulphide removal after oil separation in parallel plate interceptors. Primarily, air (oxygen) was used as the oxidant and the efficiency of two catalysts (Fe3+ and Mn2+) for sulphide removal was evaluated. Experimental data suggested that Fe3+ catalysed sulphide removal in the presence of oxygen was more effective than Mn2+ catalysed reaction. In a subsequent study, oxygen was replaced by NaOCl and H2O2, and the potential of various catalyst-oxidant combinations, NaOCl + [Fe3+], NaOCl + [Mn2+], H2O2 + [Fe3+], H2O2 + [Mn2+], was assessed. The NaOCl + [Fe3+] combination achieved the maximum sulphide oxidation. Pseudo-first-order and pseudo-second-order reaction models were fitted to the kinetic experimental data. The influence of temperature on the kinetic rate was also investigated.
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