A. U. Abeysekara

OBSERVATION OF SMALL-SCALE ANISOTROPY IN THE ARRIVAL DIRECTION DISTRIBUTION OF TeV COSMIC RAYS WITH HAWC

The High-Altitude Water Cherenkov (HAWC) Observatory is sensitive to gamma rays and charged cosmic rays at TeV energies. The detector is still under construction, but data acquisition with the partially deployed detector started in 2013. An analysis of the cosmic-ray arrival direction distribution based on 4.9 × 1010 events recorded between 2013 June and 2014 February shows anisotropy at the 10–4 level on angular scales of about 10°. The HAWC cosmic-ray sky map exhibits three regions of significantly enhanced cosmic-ray flux; two of these regions were first reported by the Milagro experiment. A third region coincides with an excess recently reported by the ARGO-YBJ experiment. An angular power spectrum analysis of the sky shows that all terms up to l = 15 contribute significantly to the excesses.

Search for Gamma-Rays from the Unusually Bright GRB 130427A with the HAWC Gamma-Ray Observatory

The first limits on the prompt emission from the long gamma-ray burst (GRB) 130427A in the >100 GeV energy band are reported. GRB 130427A was the most powerful burst ever detected with a redshift z 0.5 and featured the longest lasting emission above 100 MeV. The energy spectrum extends at least up to 95 GeV, clearly in the range observable by the High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory, a new extensive air shower detector currently under construction in central Mexico. The burst occurred under unfavorable observation conditions, low in the sky and when HAWC was running 10% of the final detector. Based on the observed light curve at MeV-GeV energies, eight different time periods have been searched for prompt and delayed emission from this GRB. In all cases, no statistically significant excess of counts has been found and upper limits have been placed. It is shown that a similar GRB close to zenith would be easily detected by the full HAWC detector, which will be completed soon. The detection rate of the full HAWC detector may be as high as one to two GRBs per year. A detection could provide important information regarding the high energy processes at work and the observation of a possible cut-off beyond the Fermi Large Area Telescope energy range could be the signature of gamma-ray absorption, either in the GRB or along the line of sight due to the extragalactic background light.

Search for TeV Gamma-Ray Emission from Point-like Sources in the Inner Galactic Plane with a Partial Configuration of the HAWC Observatory

Author(s): Abeysekara, AU; Alfaro, R; Alvarez, C; Alvarez, JD; Arceo, R; Arteaga-Vela Zquez, JC; Solares, HAA; Barber, AS; Baughman, BM; Bautista-Elivar, N; Reyes, ADB; Belmont, E; Benzvi, SY; Bernal, A; Braun, J; Caballero-Mora, KS; Capistran, T; Carraminana, A; Casanova, S; Castillo, M; Cotti, U; Cotzomi, J; Leon, SCD; Fuente, EDL; Leon, CD; Deyoung, T; Diaz Hernandez, R; Dingus, BL; Duvernois, MA; Ellsworth, RW; Enriquez-Rivera, O; Fiorino, DW; Fraija, N; Garfias, F; Gonzalez, MM; Goodman, JA; Gussert, M; Hampel-Arias, Z; Harding, JP; Hernandez, S; Huntemeyer, P; Hui, CM; Imran, A; Iriarte, A; Karn, P; Kieda, D; Lara, A; Lauer, RJ; Lee, WH; Lennarz, D; Vargas, HL; Linnemann, JT; Longo, M; Raya, GL; Malone, K; Marinelli, A; Marinelli, SS; Martinez, H; Martinez, O; Martinez-Castro, J; Matthews, JA; Miranda-Romagnoli, P; Moreno, E; Mostafa, M; Nellen, L; Newbold, M; Noriega-Papaqui, R; Patricelli, B; Pelayo, R; Perez-Perez, EG; Pretz, J; Ren, Z; Riviere, C; Rosa-Gonzalez, D; Salazar, H; Greus, FS; Sandoval, A; Schneider, M; Sinnis, G; Smith, AJ; Woodle, KS; Springer, RW; Taboada, I; Tibolla, O; Tollefson, K | Abstract:

A SEARCH for BRIEF OPTICAL FLASHES ASSOCIATED with the SETI TARGET KIC 8462852

This research is supported by grants from the U.S. Department of Energy Office of Science, the U.S. National Science Foundation, and the Smithsonian Institution, and by NSERC in Canada.

The Study of TeV Variability and the Duty Cycle of Mrk 421 from 3 Yr of Observations with the Milagro Observatory

TeV-flaring activity with timescales as short as tens of minutes and an orphan TeV flare have been observed from the blazar Markarian 421 (Mrk 421). The TeV emission from Mrk 421 is believed to be produced by leptonic synchrotron self-Compton (SSC) emission. In this scenario, correlations between the X-ray and the TeV fluxes are expected, TeV orphan flares are hardly explained, and the activity (measured as duty cycle) of the source at TeV energies is expected to be equal to or less than that observed in X-rays if only SSC is considered. To estimate the TeV duty cycle of Mrk 421 and to establish limits on its variability at different timescales, we continuously observed Mrk 421 with the Milagro observatory. Mrk 421 was detected by Milagro with a statistical significance of 7.1 standard deviations between 2005 September 21 and 2008 March 15. The observed spectrum is consistent with previous observations by VERITAS. We estimate the duty cycle of Mrk 421 for energies above 1 TeV for different hypotheses of the baseline flux and for different flare selections and we compared our results with the X-ray duty cycle estimated by Resconi et al. The robustness of the results is discussed.

CONSTRAINTS ON THE EMISSION MODEL OF THE 'NAKED-EYE BURST' GRB 080319B

On 2008 March 19, one of the brightest gamma-ray bursts (GRBs) ever recorded was detected by several groundand space-based instruments spanning the electromagnetic spectrum from radio to gamma rays. With a peak visual magnitude of 5.3, GRB 080319B was dubbed the “naked-eye” GRB, as an observer under dark skies could have seen the burst without the aid of an instrument. Presented here are results from observations of the prompt phase of GRB 080319B taken with the Milagro TeV observatory. The burst was observed at an elevation angle of 47 ◦ . Analysis of the data is performed using both the standard air shower method and the scaler or single-particle technique, which results in a sensitive energy range that extends from ∼ 5G eV to>20 TeV. These observations provide the only direct constraints on the properties of the high-energy gamma-ray emission from GRB 080319B at these energies. No evidence for emission is found in the Milagro data, and upper limits on the gamma-ray flux above 10 GeV are derived. The limits on emission between ∼25 and 200 GeV are incompatible with the synchrotron self-Compton model of gamma-ray production and disfavor a corresponding range (2 eV–16 eV) of assumed synchrotron peak energies. This indicates that the optical photons and soft (∼650 keV) gamma rays may not be produced by the same electron population.

Experimental Constraints on ?-Ray Pulsar Gap Models and the Pulsar GeV to Pulsar Wind Nebula TeV Connection

The pulsar emission mechanism in the gamma ray energy band is poorly understood. Currently, there are several models under discussion in the pulsar community. These models can be constrained by studying the collective properties of a sample of pulsars, which became possible with the large sample of gamma ray pulsars discovered by the Fermi Large Area Telescope. In this paper we develop a new experimental multi-wavelength technique to determine the beaming factor dependance on spin-down luminosity of a set of GeV pulsars. This technique requires three input parameters: pulsar spin-down luminosity, pulsar phase-averaged GeV flux, and TeV or X-ray flux from the associated pulsar wind nebula (PWN). The analysis presented in this paper uses the PWN TeV flux measurements to study the correlation between and . The measured correlation has some features that favor the Outer Gap model over the Polar Cap, Slot Gap, and One Pole Caustic models for pulsar emission in the energy range of 0.1?100 GeV, but one must keep in mind that these simulated models failed to explain many of the most important pulsar population characteristics. A tight correlation between the pulsar GeV emission and PWN TeV emission was also observed, which suggests the possibility of a linear relationship between the two emission mechanisms. In this paper we also discuss a possible mechanism to explain this correlation.

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 ...

VERITAS and multiwavelength observations of the BL Lacertae object 1ES 1741+196

We present results from multiwavelength observations of the BL Lacertae object 1ES 1741+196, including results in the very-high-energy

Milagro observations of potential TeV emitters

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