Whipple telescope observations of LS I +61 303: 2004–2006

In this paper we present the results of the past two years observations on the galactic microquasar LS I +61 303 with the Whipple 10 m gamma-ray telescope. The recent MAGIC detection of the source between 200 GeV and 4 TeV suggests that the source is periodic with very high energy (VHE) gamma-ray emission linked to its orbital cycle. The entire 50-hour data set obtained with Whipple from 2004 to 2006 was analyzed with no reliable detection resulting. The upper limits obtained in the 2005–2006 season covered several of the same epochs as the MAGIC Telescope detections, albeit with lower sensitivity. Upper limits are placed on emission during the orbital phases of 0→0.1 and 0.8→1, phases which are not included in the MAGIC data set.      

Improvement of gamma-hadron discrimination at TeV energies using a new parameter, image Surface Brightness

The recent advances in TeV γ-ray astronomy using imaging Cerenkov telescopes is mainly due to the good separation between the properties of γ-ray and hadronic showers. A new method for this discrimination is described. The method is mainly based on the change of the image's Size and Surface Brightness (Size/Area) with energy and the differences in these parameters between those initiated by γ-rays and hadrons. The application of the method to the Whipple's 10 m telescope database from the Crab Nebula and Markarian 421 demonstrates its sensitivity to gamma-hadron separation. This study shows the importance of the image Surface Brightness, in addition to the previously used Shape and Orientation parameters.     

Earthquake evidence for compressive stress in the southeast Australian crust

Earthquakes in SE Australia are usually caused by compressive stresses acting in the crust, and are associated with steeply dipping faults. Sometimes the faulting is predominantly strike‐slip, as for the Bowning earthquakes of 1977 and some of the Dalton/Gunning earthquakes; and sometimes it is high‐angle thrust faulting, as for the 1961 Robertson and 1973 Picton earthquakes. No surface expression of the faults associated with any recent earthquakes in SE Australia has been reported.

The directions of the pressure axes, from all the earthquakes for which focal mechanisms have been determined, do not form a consistent pattern. This suggests that the faulting associated with earthquakes in SE Australia is dominated by the geometry of pre‐existing crustal faults or zones of weakness.

In situ stress measurements have not been made near the epicentral areas of the larger recent earthquakes, because of the absence of competent, near‐surface rocks coupled to the crust. However, in the western part of the Lachlan Fold Belt the in situ stress results indicate that the maximum pressure axis is approximately E‐W. The evidence from the focal mechanisms does not preclude the persistence of this stress regime farther to the east, and a regional compressive stress in the crust with an azimuth of about 120° is consistent with most of the earthquake focal mechanisms and the in situ stress measurements throughout SE Australia.     

Stereoscopic observations of gamma rays at the Whipple observatory

Observations of photons at E ≥ 550 GeV from the Crab Nebula are presented and used to assess the potential of multi-telescope systems for γ-ray astronomy.

The Whipple observatory 10 m and 8 m imaging atmospheric erenkov telescopes have been used to provide a stereoscopic view of air showers to make a more complete measurement of air shower parameters. Here we present a measurement of the spread in the arrival direction of primary γ-rays originating from a point source. The data show that the shower arrival direction can be reconstructed with an accuracy of σ = 0.°14.     

Search for shadowing of primary cosmic radiation by the moon at TeV energies

The antimatter research through the earth moon ion spectrometer (ARTEMIS) experiment is described in terms of searching for the moon's shadow in the primary cosmic radiation at TeV energies. Observations were made using the Whipple 10 m Imaging Atmospheric erenkov Telescope, in conjunction with a special optical filter to suppress unwanted moonlight. Monte Carlo simulations were used to predict that the magnitude of shadowing based on a pure proton beam would be 1% of the cosmic ray background. Observations and event classification are discussed, and results of a search for shadowing based on both protons and antiprotons are presented. Atmospheric fluctuations giving rise to non-statistical event rates constituted a limiting factor to the sensitivity of the shadow search as they rivaled or exceeded statistical errors. It is demonstrated that the experiment does not have sufficient sensitivity to moon shadowing which would allow a search for an antimatter component in the primary cosmic ray flux at the percent level. Possible future improvements of the technique are discussed.     

Hydrologic response to valley‐scale structure in alpine headwaters

Few systematic studies of valley‐scale geomorphic drivers of streamflow regimes in complex alpine headwaters have compared response between catchments. As a result, little guidance is available for regional‐scale hydrological research and monitoring efforts that include assessments of ecosystem function. Physical parameters such as slope, elevation range, drainage area and bedrock geology are often used to stratify differences in streamflow response between sampling sites within an ecoregion. However, these metrics do not take into account geomorphic controls on streamflow specific to glaciated mountain headwaters. The coarse‐grained nature of depositional features in alpine catchments suggests that these landforms have little water storage capacity because hillslope runoff moves rapidly just beneath the rock mantle before emerging in fluvial networks. However, recent studies show that a range of depositional features, including talus slopes, protalus ramparts and ‘rock‐ice’ features may have more storage capacity than previously thought. To better evaluate potential differences in streamflow response among basins with extensive coarse depositional features and those without, we examined the relationships between streamflow discharge, stable isotopes, water temperature and the amplitude of the diurnal signal at five basin outlets. We also quantified the percentages of colluvial channel length measured along the stepped longitudinal profile. Colluvial channels, characterized by the presence of surficial, coarse‐grained depositional features, presented sediment‐rich, transport‐limited morphologies that appeared to have a cumulative effect on the timing and volume of flow downstream. Measurements taken from colluvial channels flowing through depositional landforms showed median recession constants (K_r) of 0.9–0.95, δ¹⁸O values of ≥?14.5 and summer diurnal amplitudes ≤0.8 as compared with more typical surface water recession constant values of 0.7, δ¹⁸O ≤ ?13.5 and diurnal amplitudes >2.0. Our results demonstrated strong associations between the percentage of colluvial channel length within a catchment and moderated streamflow regimes, water temperatures, diurnal signals and depleted δ¹⁸O related to groundwater influx. Copyright © 2014 John Wiley & Sons, Ltd.     

A Survey of Unidentified Egret Sources at TeV Energies

The Whipple Observatory 10 m γ-ray telescope has been used to survey the error boxes of 24 EGRET unidentified sources in an attempt to find counterparts at energies of 350 GeV and above. In no case is a statistically significant signal found in the EGRET error box which implies that, at least for this sample, the γ-ray spectra of these sources steepen between 100 MeV and 350 GeV.     

A high resolution imaging detector for TeV gamma-ray astronomy

Details are presented of an atmospheric Cherenkov telescope for use in very high energy gamma-ray astronomy which consists of a cluster of 109 close-packed photomultiplier tubes at the focus of a 10 meter optical reflector. The images of the Cherenkov flashes generated both by gamma-ray and charged cosmic-ray events are digitized and recorded. Subsequent off-line analysis of the images improves the significance of the signal to noise ratio by a factor of 10 compared with non-imaging techniques.     

A New TeV Source Confirmed in Whipple Archival Data: TeV J2032+41

A re-analysis of data near Cygnus X-3 in 1989–1990 using the Whipple Observatory atmospheric Cherenkov imaging telescope confirms the existence of the TeV J2032 + 4130 source first reported at a conference by the Crimean Astrophysical Observatory and confirmed independently by the HEGRA Collaboration in a referred publication. The significance of the Whipple observations at the a priori HEGRA position is 3.3σ. The peak signal was found at RA = 20 h 32 m, Dec = +41^∘ 33′. This is 0.6^∘ north of Cygnus X-3. The flux level (12% of the level of the Crab Nebula) is intermediate between that reported by the Crimean (100%) and HEGRA (3%) groups.