Abstracts of Interest
Selected by:
Natasha Atkins
Abstract: 1708.02474
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Title: Did the 2000 November 8 solar flare accelerate protons to >=40 GeV?
(Submitted on 8 Aug 2017)
Abstract: It has been reported that a 5.7sigma directional muon excess coincident with the 2000 July 14 solar flare was registered by the L3 precision muon spectrometer [Ruiguang Wang, Astroparticle Phys., 31(2009) 149]. Using a same analysis method and similar criteria of event selection, we have analyzed the L3 precision muon spectrometer data during November 2000. The result shows that a 4.7sigma muon excess appeared at a time coincident with the solar flare of 8 of November 2000. This muon excess corresponds to above 40 GeV primary protons which came from a sky cell of solid angle 0.048 sr. The probability of being a background fluctuation is estimated to be about 0.1%. It has been convinced that solar protons could be accelerated to tens of GeV in those Class X solar flares which usually arose solar cosmic ray ground level enhancement (GLE) events. However, whether a Class M solar flare like the non-GLE event of 8 November 2000 may also accelerate solar protons to such high energies? It is interesting and noteworthy.
Abstract: 1708.02829
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Title: Very-High-Energy $γ$-Ray Observations of the Blazar 1ES 2344+514 with VERITAS
(Submitted on 9 Aug 2017)
Abstract: We present very-high-energy $\gamma$-ray observations of the BL Lac object 1ES 2344+514 taken by the Very Energetic Radiation Imaging Telescope Array System (VERITAS) between 2007 and 2015. 1ES 2344+514 is detected with a statistical significance above background of $20.8\sigma$ in $47.2$ hours (livetime) of observations, making this the most comprehensive very-high-energy study of 1ES 2344+514 to date. Using these observations the temporal properties of 1ES 2344+514 are studied on short and long times scales. We fit a constant flux model to nightly- and seasonally-binned light curves and apply a fractional variability test, to determine the stability of the source on different timescales. We reject the constant-flux model for the 2007-2008 and 2014-2015 nightly-binned light curves and for the long-term seasonally-binned light curve at the $> 3\sigma$ level. The spectra of the time-averaged emission before and after correction for attenuation by the extragalactic background light are obtained. The observed time-averaged spectrum above 200 GeV is satisfactorily fitted (${\chi^2/NDF = 7.89/6}$) by a power-law function with index $\Gamma = 2.46 \pm 0.06_{stat} \pm 0.20_{sys} $ and extends to at least 8 TeV. The extragalactic-background-light-deabsorbed spectrum is adequately fit (${\chi^2/NDF = 6.73/6}$) by a power-law function with index $\Gamma = 2.15 \pm 0.06_{stat} \pm 0.20_{sys} $ while an F-test indicates that the power-law with exponential cutoff function provides a marginally-better fit ($\chi^2/NDF $ = $2.56 / 5 $) at the 2.1$\sigma$ level. The source location is found to be consistent with the published radio location and its spatial extent is consistent with a point source.
Abstract: 1708.02572
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Title: HAWC Contributions to the 35th International Cosmic Ray Conference (ICRC2017)
(Submitted on 8 Aug 2017)
Abstract: List of proceedings from the HAWC Collaboration presented at the 35th International Cosmic Ray Conference, 12 July - 20 July 2017, Bexco, Busan, Korea.
Abstract: 1708.02094
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Title: On the compatibility of the IceCube results with an universal neutrino spectrum
(Submitted on 7 Aug 2017)
Abstract: There is mounting evidence that the IceCube findings cannot be described simply invoking a single power-law spectrum for cosmic neutrinos. We discuss which are the minimal modifications of the spectrum that are required by the existing observations and we obtain a universal cosmic neutrino spectrum, i.e. valid for all neutrino flavors. Our approach to such task can be outlined in three points: 1) we rely on the throughgoing muon analysis above 200 TeV and on the high-energy starting events (HESE) analysis below this energy, requiring the continuity of the spectrum; 2) we assume that cosmic neutrinos are subject to three-flavor neutrino oscillations in vacuum; 3) we make no assumption on the astrophysical mechanism of production, except for no tau neutrino component at the source. We test our model using the information provided by HESE shower-like events and by the lack of double pulses and resonant events. We find that a two power-law spectrum is compatible with all observations. The model agrees with the standard picture of pion decay as a source of neutrinos, and indicates a slight preference for a pgamma mechanism of production. We discuss the tension between the HESE and the throughgoing muons datasets around few tens TeV, focussing on the angular distributions of the spectra. The expected number of smoking-gun signatures of nu-tau induced events (referred to as double pulses) is quantified: in the baseline model we predict 0.65 double pulse events in 5.7 years. Uncertainties in the predictions are quantified.
Abstract: 1708.02025
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Title: WebPlotDigitizer, a polyvalent and free software to extract spectra from old astronomical publications: application to ultraviolet spectropolarimetry
(Submitted on 7 Aug 2017)
Abstract: In this contribution, we present WebPlotDigitizer, a polyvalent and free software developed to facilitate easy and accurate data extraction from a variety of plot types. We describe the numerous features of this numerical tool and present its relevance when applied to astrophysical archival research. We exploit WebPlotDigitizer to extract ultraviolet spectropolarimetric spectra from old publications that used the Hubble Space Telescope, Lick Observatory 3m Shane telescope and Astro-2 mission to observe the Seyfert-2 AGN NGC 1068. By doing so, we compile all the existing ultraviolet polarimetric data on NGC 1068 to prepare the ground for further investigations with the future high-resolution spectropolarimeter POLLUX on-board of the proposed Large UV/Optical/Infrared Surveyor (LUVOIR) NASA mission.
Abstract: 1708.01540
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Title: On the detectability of BL Lac objects by IceCube
(Submitted on 4 Aug 2017)
Abstract: Since 2010 IceCube observed around 50 high-energy neutrino events of cosmic origin above 60 TeV, but the astrophysical sources of these events are still unknown. We recently proposed high-energy emitting BL Lac (HBL) objects as candidate emitters of high-energy neutrinos. Assuming a direct proportionality between high-energy gamma-ray and very-high energy neutrino fluxes, we calculated the expected neutrino event number in a year for IceCube and the presently under construction Km3NeT. To give a value of the significance of a detection we considered also the background for the single sources. To this aim we derived the through-going muon rate, generated by muon neutrino including the effect of Earth absorption, the density of the Earth and the cross section $\nu N$. Applying this calculation both to HBL sources and the atmospherical neutrino background, we can calculate the expected significance of the detection by IceCube, showing that our scenario is compatible with a no detection of HBL.
Abstract: 1708.01446
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Full Text: [ PostScript, PDF]
Title: Summary of the 12th IACHEC Meeting
(Submitted on 4 Aug 2017)
Abstract: We summarize the outcome of the 12th meeting of the International Astronomical Consortium for High Energy Calibration (IACHEC), held at the UCLA conference center in Lake Arrowhead (California) in March 2017. 56 scientists directly involved in the calibration of operational and future high-energy missions gathered during 3.5 days to discuss the status of the X-ray payload inter-calibration, as well as possible ways to improve it. The "Thermal Supernovas Remnant" (SNR) Working Group presented a recently published paper on 1E0102.2-7219 as a calibration standard in the 0.5-1.0 keV band. A new method to measure the high-energy spectrum of the Crab Nebula and pulsar with NuSTAR without using its optics may yield a new absolute flux standard in the 3-7 keV band. A new ACIS contamination model - released with CALDB version 4.7.3 - leads to a significant improvement in modeling the spectral, spatial, and temporal properties of the contaminant. The first calibration results of the scientific payload on board Hitomi confirm the excellent performance of the instruments before the spacecraft operation problems leading to its loss. Finally, the meeting discussed extensively a novel statistic approach to formally identify in which direction the effective areas of different instruments would need to be changed to bring them into concordance. This method could inform future further calibration efforts.
Abstract: 1708.01153
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Full Text: [ PostScript, PDF]
Title: Intrinsic spectra of H.E.S.S. blazars : what would we see without EBL absorption
(Submitted on 3 Aug 2017)
Abstract: The vast majority of extragalactic sources detected in the very high energy (E> 100 GeV) domaine are active galactic nuclei (AGN) located at cosmological distances. During their travel towards Earth, the emitted gamma-rays suffer from absorption by the extragalactic background light (EBL). The density of the EBL is not very well constrained by direct or indirect measurement which leads to uncertainties on the intrinsic spectrum of the sources. High-quality AGN spectra obtained with the High Energy Stereoscopic System (H.E.S.S.) have been used to perform a model-independent measurement of the EBL spectral energy distribution. While the precision of this measurement remains limited, it reflects the sensitivity of H.E.S.S. alone to the EBL and offers the possibility to access the intrinsic spectra of AGNs in a consistent manner, taking into account the derived uncertainties on the EBL spectral energy distribution.
In this contribution, we study the intrinsic spectra as measured by H.E.S.S. and by the Fermi Large Area Telescope (LAT) of several blazars of the H.E.S.S. sky. This provides the opportunity to have new insight into the emission processes at play in the jets of AGN. The data presented consist of monitoring data of quiescent state of blazars and also bright blazar flares, such as PKS~2155-304, Mrk~421, recorded by H.E.S.S.
Abstract: 1708.01088
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Full Text: [ PostScript, PDF]
Title: GRB Observations with H.E.S.S. II
(Submitted on 3 Aug 2017 (v1), last revised 7 Aug 2017 (this version, v2))
Abstract: The High Energy Stereoscopic System (H.E.S.S.) has been searching for counterparts of Gamma Ray Bursts (GRBs) for many years. In 2012 the system was upgraded with a fifth $28$ m diameter telescope (CT5) which is equipped with faster motors for rapid repointing, marking the start of the second phase of H.E.S.S. operation (H.E.S.S. II). CT5s large light collection area of $600\,{\rm m}^{2}$ improves the sensitivity to low-energy gamma-rays and even extends the energy range below $100$ GeV. The search for counterparts continues now in the energy range of tens of GeV to tens of TeV. A detection in this energy range would open a new window to the part of the spectrum of these highly energetic explosions which Fermi-LAT has only successfully detected in a reduced subset of events, with rather limited statistics. In the past years, H.E.S.S. has performed followup observations based on GRB detections by Swift-BAT and Fermi-GBM/-LAT. This Target of Opportunity observation program was carried out with a generalised Target of Opportunity Alert system. This contribution will highlight key features of the Target of Opportunity Alert system, present follow-up statistics of GRBs as well as detailed results of promising follow-up observations.
Abstract: 1708.01083
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Full Text: [ PostScript, PDF]
Title: Target of Opportunity Observations of Blazars with H.E.S.S
(Submitted on 3 Aug 2017)
Abstract: The very-high-energy (VHE, E>100 GeV) extragalactic sky is dominated by blazars, a class of active galactic nuclei which show rapid variability at all wavelengths. Target of Opportunity (ToO) observations triggered by flaring activity detected at longer wavelengths are, thus, an important part of the blazar observing strategy of H.E.S.S., an array of five imaging atmospheric Cherenkov telescopes sensitive to VHE photons. In this contribution we detail the H.E.S.S. extragalactic ToO program, describing the specific procedures currently in place to follow up on multi-wavelength alerts. The program is illustrated by discussing a few recent noteworthy targets observed with the H.E.S.S. phase II array over the last two years of blazar ToO observations.
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