The optimal on-source region size for detections with counting-type telescopes

Source detection in counting type experiments such as Cherenkov telescopes often involves the application of the classical Eq. 17 from the paper of Li & Ma (1983) to discrete on- and off-source regions. The on- source region is typically a circular area with radius {\theta} in which the signal is expected to appear with the shape of the instrument point spread function (PSF). This paper addresses the question of what is the {\theta} that maximises the probability of detection for a given PSF width and background event density. In the high count number limit and assuming a Gaussian PSF profile, the optimum is found to be at $\zeta_\infty^2 \approx 2.51$ times the squared PSF width $\sigma_\mathrm{PSF39}^2$. While this number is shown to be a good choice in many cases, a dynamic formula for cases of lower count numbers, which favour larger on-source regions, is given. The recipe to get to this parametrisation can also be applied to cases with a non-Gaussian PSF. This result can standardise and simplify analysis procedures, reduce trials and eliminate the need for experience-based ad hoc cut definitions or expensive case-by-case Monte Carlo simulations.Comment: 6 pages, 2 figures. Accepted for publication in Astropart. Phy

The MAGIC telescopes—Status and recent results

The MAGIC telescopes are two Imaging Atmospheric Cherenkov Telescopes located on the Canary island of La Palma. They provide the lowest energy threshold among the existing instruments of the kind, reaching down to 50 GeV in standard trigger mode. This allows us to close the energy gap between satelliteborne and ground-based gamma-ray observations for strong enough sources. During the first five years of monoscopic observations, many interesting results could thus be achieved. With the second MAGIC telescope, which started operation in 2009, the sensitivity could be improved by stereoscopic imaging, and 5 new detections could already be reported in 2010. We present the status of the MAGIC telescopes in 2010 and review the latest results obtained in mono- and stereoscopic mode. This includes, among others, the detection of the head-tail galaxy IC 310, a new multiwavelength study of Mrk 501, an updated lightcurve of the Crab Pulsar

Systematic search for high-energy gamma-ray emission from bow shocks of runaway stars

Context. It has been suggested that the bow shocks of runaway stars are sources of high-energy gamma rays (E > 100 MeV). Theoretical models predicting high-energy gamma-ray emission from these sources were followed by the first detection of non-thermal radio emission from the bow shock of BD+43^\deg 3654 and non-thermal X-ray emission from the bow shock of AE Aurigae. Aims. We perform the first systematic search for MeV and GeV emission from 27 bow shocks of runaway stars using data collected by the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope (Fermi). Methods. We analysed 57 months of Fermi-LAT data at the positions of 27 bow shocks of runaway stars extracted from the Extensive stellar BOw Shock Survey catalogue (E-BOSS). A likelihood analysis was performed to search for gamma-ray emission that is not compatible with diffuse background or emission from neighbouring sources and that could be associated with the bow shocks. Results. None of the bow shock candidates is detected significantly in the Fermi-LAT energy range. We therefore present upper limits on the high-energy emission in the energy range from 100 MeV to 300 GeV for 27 bow shocks of runaway stars in four energy bands. For the three cases where models of the high-energy emission are published we compare our upper limits to the modelled spectra. Our limits exclude the model predictions for Zeta Ophiuchi by a factor $\approx$ 5.Comment: 5 pages, 5 figures, 1 table, accepted by A&

Solar Activation of Molecular Water Oxidation Catalysts Using Semiconductors.

Water splitting has been proposed as a viable route toward the renewable production of hydrogen. However, this approach is limited by the water oxidation half-reaction due to the large energy input needed the reaction is sluggish. Sunlight has been used to create excited state electron/hole pairs that are thermodynamically capable of generating H2 and O2. Molecular (homogeneous) and heterogeneous catalysts have been developed to oxidize water efficiently under illumination. The focus of this dissertation is to anchor molecular catalysts onto heterogeneous light absorbers (semiconductors) to more efficiently oxidize water using sunlight. The most significant discovery is that when a molecular Fe water oxidation catalyst is anchored to WO3, a 60% increase in the rate of oxygen evolution and a 40% increase in selectivity towards water oxidation is achieved. Additionally, when different tsemiconductors are used, the reaction rate is dependent on the energy of the conduction band edge and the band gap. For instance, when the conduction band potential energy is held constant and the band gap is decreased from 2.7 eV (WO3) to 2.1 eV (Fe2O3) the rate enhancement with the Fe catalyst increases from 60% to 273%. In addition to the role of the semiconductor on the photoelectrochemical performance, the molecular species also plays an important role in the photoelectrochemical rate enhancement. The most significant development using various molecular species is that replacing iron with other first-row transition metals (Mn-Zn) results in complexes that are only active toward water oxidation when anchored to WO3. And, the corresponding nickel and copper complexes increase the selectivity towards water oxidation up to 99% compared to bare WO3 (56%). Unfortunately, when the best-performing ruthenium catalyst is anchored to various semiconductors, the rate enhancement for solar water oxidation is negligible compared to the other molecular species used in this work. This work demonstrates for the first time that photoelectrochemically generated minority carriers (holes) from semiconductors can be used directly as oxidants to activate molecular oxidation complexes under solar irradiation. Furthermore, this work is the first to quantify the increase in selectivity towards water oxidation directly for WO3 when modified with molecular species.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111517/1/bklepser_1.pd