Improving Sensitivity to Weak Pulsations with Photon Probability Weighting

All gamma-ray telescopes suffer from source confusion due to their inability to focus incident high-energy radiation, and the resulting background contamination can obscure the periodic emission from faint pulsars. In the context of the Fermi Large Area Telescope, we outline enhanced statistical tests for pulsation in which each photon is weighted by its probability to have originated from the candidate pulsar. The probabilities are calculated using the instrument response function and a full spectral model, enabling powerful background rejection. With Monte Carlo methods, we demonstrate that the new tests increase the sensitivity to pulsars by more than 50% under a wide range of conditions. This improvement may appreciably increase the completeness of the sample of radio-loud gamma-ray pulsars. Finally, we derive the asymptotic null distribution for the H-test, expanding its domain of validity to arbitrarily complex light curves.Comment: 10 pages, 11 figures, published by ApJ; v2 fixes an error in Eq.

Single Pulse Variability in Gamma-ray Pulsars

The Fermi Large Area Telescope receives $\ll$1 photon per rotation from any $\gamma$-ray pulsar. However, out of the billions of monitored rotations of the bright pulsars Vela (PSR~J0835$-$4510) and Geminga (PSR~J0633$+$1746), a few thousand have $\geq$2 pulsed photons. These rare pairs encode information about the variability of pulse amplitude and shape. We have cataloged such pairs and find the observed number to be in good agreement with simple Poisson statistics, limiting any amplitude variations to $<$19% (Vela) and $<$22% (Geminga) at 2$\sigma$ confidence. Using an array of basis functions to model pulse shape variability, the observed pulse phase distribution of the pairs limits the scale of pulse shape variations of Vela to $<$13% while for Geminga we find a hint of $\sim$20% single-pulse shape variability most associated with the pulse peaks. If variations last longer than a single rotation, more pairs can be collected, and we have calculated upper limits on amplitude and shape variations for assumed coherence times up to 100 rotations, finding limits of $\sim$1% (amplitude) and $\sim$3% (shape) for both pulsars. Because a large volume of the pulsar magnetosphere contributes to $\gamma$-ray pulse production, we conclude that the magnetospheres of these two energetic pulsars are stable over one rotation and very stable on longer time scales. All other $\gamma$-ray pulsars are too faint for similar analyses. These results provide useful constraints on rapidly-improving simulations of pulsar magnetospheres, which have revealed a variety of large-scale instabilities in the thin equatorial current sheets where the bulk of GeV $\gamma$-ray emission is thought to originate.Comment: 12 pages, accepted in Ap

Stable Isotope Analysis of Lake Sediments from Laguna Santa Elena and Laguna Azul, Costa Rica

Lake sediments are increasingly important archives of human-environment interactions and paleoclimate in the neotropics. In Costa Rica, Anchukaitis and Horn (Palaeogeography, Palaeoclimatology, Palaeoecology 221: 35–54, 2005) established a land-use history for Laguna Santa Elena (8.9306 N, 82.9275 W, 1055 m elevation), a small lake in the Diquís archaeological region, based on pollen and charcoal analyses of a 7-meter sediment core. I carried out stable carbon and nitrogen isotope and loss-on-ignition analyses at higher resolution to extend the existing 2000-year record. The new geochemical data parallel major trends in botanical proxies but also reveal aspects of human and environmental dynamics not apparent in the prior analysis. Inferred changes in land use in the watershed are consistent with archaeological evidence. Geochemical trends strongly suggest a population collapse at the site around the time of the Terminal Classic Drought of the Mayan region. The generally close correspondence between microfossil assemblages and geochemistry in the Santa Elena core demonstrates the usefulness of stable isotope analysis as a first line of investigation in paleoenvironmental research. Sediment samples for carbon isotope analysis need to be acidified to remove carbonates that can affect isotope measurements, and debate exists over whether nitrogen isotope analysis can use these acidified samples or require non-acidified samples. My thesis research tested the effects of pre-analysis acidification of sediment and soil samples from Laguna Santa Elena and a second lake in Costa Rica, Laguna Azul (9.9558 N, 83.6519 W, 630 m elevation) in the Central Highlands-Atlantic Watershed archaeological region. Results show that acidification may cause statistically significant differences in nitrogen isotope values. These differences appear to be random and unpredictable, and can manifest as either positive or negative shifts that have the potential to alter or even reverse relative trends in nitrogen isotope signals in lake sediment profiles. More tests are needed, but the results of this analysis suggest that researchers should avoid dual-mode analysis, in which data for both stable carbon and nitrogen isotopes are obtained from a single acidified sample, and should continue analyzing an additional non-acidified sample to obtain nitrogen isotope values