Bruce R. Dawson

Southern hemisphere observations of a 1018 eV cosmic ray source near the direction of the Galactic Centre

We report on an analysis of data from the southern hemisphere SUGAR cosmic ray detector. We confirm the existence of an excess of 1018 eV cosmic rays from a direction close to the Galactic Centre, first reported by the AGASA group. We find that the signal is consistent with that from a point source, and we find no evidence for an excess of cosmic rays coming from the direction of the Galactic Centre itself.

The prototype high-resolution Fly's Eye cosmic ray detector

Abstract The High-Resolution Flys Eye (HiRes) is an observatory for the highest energy cosmic rays. It detects the nitrogen fluorescence light induced by the passage of giant cosmic ray extensive air showers through the atmosphere. A two-site prototype of the observatory was operated from September 1994 to November 1996. In this paper we describe the components of that detector, and the procedures used to calibrate the detector and characterise the atmosphere. Data collected by the HiRes prototype are being used for physics studies, including an analysis of the cosmic ray mass composition in the energy range from 10 17 to 10 18 eV .

Extremely high energy cosmic rays

Abstract Cosmic ray experiments at energies above 10 17 eV attempt to measure the energy spectrum, chemical composition, and anisotropy in order to discover where and how such high energy particles are accelerated. Air shower experiments at Sydney (Australia), Haverah Park (UK), Yakutsk (Russia), Akeno (Japan), and Utah (USA) have employed different detection techniques to study these cosmic rays. The methods range from measuring the longitudinal development of the electromagnetic shower in the atmosphere (Flys Eye detector in Utah) to measuring the lateral distribution of muons at ground level (Sydney). Comparison of results requires allowance for the uncertainties and possible biases intrinsic to each method. Although results from the different experiments do not blatantly contradict each other, numerous effects of marginal statistical significance are in dispute. These include the existence of air showers with energies in excess of 10 20 eV and a neutral particle flux from Cygnus X-3. We review the cosmic ray issues and results in this energy region as well as the various detection techniques and analysis procedures. Fundamental questions remain to be answered by more sensitive detectors which are now in development.

3 to 12 millimetre studies of dense gas towards the western rim of supernova remnant RX J1713.7-3946

The young X-ray and gamma-ray-bright supernova remnant RX J1713.7-3946 (SNR G347.3-0.5) is believed to be associated with molecular cores that lie within regions of the most intense TeV emission. Using the Mopra telescope, four of the densest cores were observed using high critical density tracers such as CS(J= 10, J= 21) and its isotopologue counterparts, NH₃(1, 1) and (2, 2) inversion transitions and N₂H⁺(J= 10) emission, confirming the presence of dense gas ≥10⁴cm⁻³ in the region. The mass estimates for Core C range from 40 (from CS) to 80 M⊙ (from NH₃ and N₂H⁺), an order of magnitude smaller than published mass estimates from CO(J= 1-0) observations. We also modelled the energy-dependent diffusion of cosmic ray protons accelerated by RX J1713.7-3946 into Core C, approximating the core with average density and magnetic field values. We find that for considerably suppressed diffusion coefficients (factors χ= 10⁻³ down to 10⁻⁵ the Galactic average), low-energy cosmic rays can be prevented from entering the inner core region. Such an effect could lead to characteristic spectral behaviour in the GeV to TeV gamma-ray and multi-keV X-ray fluxes across the core. These features may be measurable with future gamma-ray and multi-keV telescopes offering arcminute or better angular resolution, and can be a novel way to understand the level of cosmic ray acceleration in RX J1713.7-3946 and the transport properties of cosmic rays in the dense molecular cores.

Simulations of a giant hybrid air shower detector

Abstract We have used Monte Carlo simulations to investigate the capabilities of a giant air shower observatory designed to detect showers initiated by cosmic rays with energies exceeding 10 19 eV. The observatory is to consist of an array of detectors that will characterise the air shower at ground level, and optical detectors to measure the fluorescence light emitted by the shower in the atmosphere. Using these detectors together in a ‘hybrid’ configuration, we find that precise geometrical reconstruction of the shower axis is possible, leading to excellent resolution in energy. The technique is also shown to provide very good reconstruction below 10 19 eV, at energies where the ground array is not fully efficient.

Interstellar gas towards CTB 37A and the TeV gamma-ray source HESS J1714−385

Observations of dense molecular gas towards the supernova remnants CTB 37A (G348.5+0.1) and G348.5-0.0 were carried out using the Mopra and Nanten2 radio telescopes. We present CO(2-1) and CS(1-0) emission maps of a region encompassing the CTB 37A TeV gamma-ray emission, HESS J1714-385, revealing regions of dense gas within associated molecular clouds. Some gas displays good overlap with gamma-ray emission, consistent with hadronic gamma-ray emission scenarios. Masses of gas towards the HESS J1714-385 TeV gamma-ray emission region were estimated, and were of the order of 10^3-10^4 solar masses. In the case of a purely hadronic origin for the gamma-ray emission, the cosmic ray flux enhancement is ~80-1100 times the local solar value. This enhancement factor and other considerations allow a discussion of the age of CTB 37A, which is consistent with ~10^4 yr.

Evidence for neutral emission above 10 exp 17 eV from LMC X - 4

We have searched the data set of the SUGAR cosmic ray detector for indications of emission above 10 17 eV from two binary X-ray sources, Vela X-ray sources, Vela X-1 and LMC X-4. There is no indication of a signal from Vela X-1 and an upper limit on the flux is set at 1.5×10 −16 cm −2 s −1 above 2×10 17 eV (95% C.L.). However, we find evidence for neutral emission from LMC X-4 with a confidence of 99.6%, having observed a 1.4 day modulation in the signal consistent with the orbital period of the binary system

Dense gas towards the RX J1713.7-3946 supernova remnant

We present results from a Mopra 7 mm-wavelength survey that targeted the dense gas-tracing CS(1-0) transition towards the young γ -ray-bright supernova remnant, RX J1713.7−3946 (SNR G 347.3−0.5). In a hadronic γ -ray emission scenario, where cosmic ray (CR) protons interact with gas to produce the observed γ -ray emission, the mass of potential CR target material is an important factor. We summarise newly discovered dense gas components, towards Cores G and L, and Clumps N1, N2, N3, and T1, which have masses of 1 – 10 4 M. We argue that these components are not likely to contribute significantly to γ -ray emission in a hadronic γ -ray emission scenario. This would be the case if RX J1713.7−3946 were at either the currently favoured distance of 1 kpc or an alternate distance (as suggested in some previous studies) of 6 kpc. This survey also targeted the shock-tracing SiO molecule. Although no SiO emission corresponding to the RX J1713.7−3946 shock was observed, vibrationally excited SiO(1-0) maser emission was discovered towards what may be an evolved star. Observations taken 1 yr apart confirmed a transient nature, since the intensity, line-width, and central velocity of SiO(J = 1-0,v = 1,2) emission varied significantly.

Steerable laser system for UV atmospheric monitoring at the High-Resolution Fly's Eye

Monitoring the aerosol component of the lowest 10 km of the atmosphere at UV wavelengths (300 - 400 nm) is an important part of the High Resolution Flys Eye astrophysics experiment. Our method of atmospheric monitoring uses a frequency tripled YAG laser and a steering system that can point the beam anywhere in the sky. The same detector that measures scintillation light from high energy cosmic rays also measures light scattered from this laser system over a range of laser energies, geometries, and polarizations. This paper describes the technique, the laser system, and some recent measurements.

Two-point Angular Autocorrelation Function and the Origin of the Highest-energy Cosmic Rays

Construction of the Pierre Auger Observatory for the study of the highest-energy cosmic rays is about to begin. Prior to the availability of data from that experiment, decisions should be made on techniques for the analysis of the directional properties of those data. We examine here one possible analysis tool, the two-point angular autocorrelation function. As a concrete example, data from the SUGAR array are examined in this way. Possible clustering of the data is observed, and the identification of such clustering with candidate astronomical objects in a purpose-developed catalogue is investigated.