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

Holocene Precipitation Variability, Prehistoric Agriculture, and Natural and Human-Set Fires in Costa Rica

This dissertation presents the results of compound-specific stable hydrogen isotope analysis of n-alkanes from terrestrial leaf waxes preserved in sediment cores from three lakes in Costa Rica to reconstruct variations in paleohydrology during the Holocene. Results were compared with pollen and charcoal data from the same cores to examine relationships between paleohydrology, vegetation change, prehistoric agriculture, and fire, and with archaeological evidence in the watersheds of two lakes to better understand prehistoric human-environment interactions. Lago de las Morrenas 1 (9.4925 °N, 83.4848 °W, 3480 m) is in the Chirripó páramo of Costa Rica, which was never permanently occupied by prehistoric people. The analyses demonstrate 10,000 years of millennial-scale variations in hydroclimate at Morrenas 1, which was dry during the Early Holocene, mesic during the Middle Holocene, and dry over the Late Holocene. The Morrenas sediments record local manifestations of the 8200 BP event, the 5200 BP event, the Terminal Classic Drought (TCD), and the Little Ice Age (LIA). Laguna Bonillita (9.9921 °N, 83.6114 °W, 450 m), in the Caribbean lowlands of central Costa Rica, has a 2700-year history of continuous maize agriculture. The alkane data show variations in paleohydrology over the Late Holocene and local manifestations of the TCD and the LIA that match patterns throughout the circum-Caribbean. The Bonillita watershed was intensively farmed across the entire history of the lake. Changes in prehistoric culture and maize farming are temporally linked to climate change at Bonillita. The data indicate that maize agriculture benefitted from episodes of drier climate in this lowland rainforest environment. Laguna Santa Elena (8.9290 °N, 82.9257 °W, 1055 m), in the Diquís archaeological subregion of southern Pacific Costa Rica, has a 2000-year history of maize agriculture. The analyses document variations in Late Holocene paleohydrology, including local manifestations of the TCD and the LIA, that had important consequences for prehistoric people. Santa Elena may have experienced a decrease in rainfall during the TCD, but unlike the Caribbean side of the Isthmus, the amplitude of this drought event does not appear abnormal on centennial to millennial timescales. Two population collapses inferred to take place during dry periods instead happened during wet intervals at Santa Elena