arXiv subjects

J. Patrick Harding

Publications and source records attributed to J. Patrick Harding.

27 records · Page 2Linked to original sources

arXiv2015

A high-level analysis framework for HAWC

The High Altitude Water Cherenkov (HAWC) Observatory continuously observes gamma-rays between 100 GeV to 100 TeV in an instantaneous field of view of about 2 steradians above the array. The large amount of raw data, the importance of small number statistics, the large dynamic range of gamma-ray signals in time (1 - $10^8$ sec) and angular extent (0.1 - 100 degrees), and the growing need to directly compare results from different observatories pose some special challenges for the analysis of HAWC data. To address these needs, we have designed and implemented a modular analysis framework based on the method of maximum likelihood. The framework facilitates the calculation of a binned Poisson Log-likelihood value for a given physics model (i.e., source model), data set, and detector response. The parameters of the physics model (sky position, spectrum, angular extent, etc.) can be optimized through a likelihood maximization routine to obtain a best match to the data. In a similar way, the parameters of the detector response (absolute pointing, angular resolution, etc.) can be optimized using a well-known source such as the Crab Nebula. The framework was designed concurrently with the Multi-Mission Maximum Likelihood (3ML) architecture, and allows for the definition of a general collection of sources with individually varying spectral and spatial morphologies. Compatibility with the 3ML architecture allows to easily perform powerful joint fits with other observatories. In this contribution, we overview the design and capabilities of the HAWC analysis framework, stressing the overarching design points that have applicability to other astronomical and cosmic-ray observatories.

astro-ph.IM↗

arXiv2015

Dark Matter Annihilation and Decay Searches with the High Altitude Water Cherenkov (HAWC) Observatory

In order to observe annihilation and decay of dark matter, several types of potential sources should be considered. Some sources, such as dwarf galaxies, are expected to have very low astrophysical backgrounds but fairly small dark matter densities. Other sources, like the Galactic center, are expected to have larger densities of dark matter but also have more complicated backgrounds from other astrophysical sources. To search for signatures of dark matter, the large field-of-view of the HAWC detector, covering 2 sr at a time, especially enables searches from sources of dark matter annihilation and decay, which are extended over several degrees on the sky. With a sensitivity over 2/3 of the sky, HAWC has the ability to probe a large fraction of the sky for the signals of TeV-mass dark matter. In particular, HAWC should be the most sensitive experiment to signals coming from dark matter with masses greater than 10-100 TeV. We present the HAWC sensitivity to annihilating and decaying dark matter signals for several likely sources of these signals.

astro-ph.HE↗

arXiv2013

The TeV Cosmic-Ray Anisotropy from Local Dark Matter Annihilation

Several experiments have reported regions in the TeV sky with an excess of cosmic rays. Here we consider the possibility that the excess cosmic rays are coming from dark matter annihilation in nearby subhalos. We provide motivation for dark matter as the source of the excess and show that dark matter annihilations can naturally produce the cosmic ray anisotropy without the need for exotic dark matter models. We show that viable dark matter explanations of the cosmic ray excess are consistent with current measurements of antiprotons, positrons, and gamma-rays. Additionally, we show that the dark matter interpretation of the observed anisotropy predicts detections in several cosmic ray channels by the next generation of experiments.

astro-ph.HE↗

arXiv2012

Models of the Contribution of Blazars to the Anisotropy of the Extragalactic Diffuse Gamma-ray Background

We study the relation between the measured anisotropies in the extragalactic diffuse gamma-ray background (DGRB) and the DGRB spectral intensity, and their potential origin from the unresolved blazar population. Using a physical-evolution model for blazars with a luminosity dependent evolution (LDDE) and an observationally-determined luminosity-dependent blazar spectral energy distribution (SED), we find that blazars can account for the observed anisotropy of the DGRB consistent with their observed source-count distribution, but are in turn constrained in contributing significantly to the observed DGRB intensity. For the best-fit LDDE model accounting for the DGRB anisotropy and source-count distribution, blazars only contribute 5.7^{+2.1}_{-1.0}% (68% CL) of the DGRB intensity above 1 GeV. Requiring a higher fraction of the DGRB intensity contribution by blazars overproduces the DGRB anisotropy, and therefore blazars in the LDDE+SED-sequence model cannot simultaneously account for the DGRB intensity as well as anisotropy. We discuss the limitations of LDDE models. However, these models do not require the many unjustified and observationally-inconsistent simplifying assumptions---including a single power law for all blazar spectra and a simple broken power-law model for their source-count distribution---that are present in much previous work.

astro-ph.HE↗

arXiv2012

Comment on "Joint Anisotropy and Source Count Constraints on the Contribution of Blazars to the Diffuse Gamma-Ray Background"

We show the conclusions claimed in the manuscript arXiv:1202.5309v1 by Cuoco, Komatsu and Siegal-Gaskins (CKS) are not generally valid. The results in CKS are based on a number of simplifying assumptions regarding the source population below the detection threshold and the threshold flux itself, and do not apply to many physical models of the blazar population. Physical blazar population models that match the measured source counts above the observational threshold can account for 60% of the diffuse gamma-ray background intensity between 1-10 GeV, while the assumptions in CKS limit the intensity to <30%. The shortcomings of the model considered in CKS arise from an over-simplified blazar source model. A number of the simplifying assumptions are unjustified, including: first, the adoption of an assumed power-law source-count distribution, dN/dS, to arbitrary low source fluxes, which is not exhibited in physical models of the blazar population; and, second, the lack of blazar spectral information in calculating the anisotropy of unresolved gamma-ray blazar emission. We also show that the calculation of the unresolved blazars' anisotropy is very sensitive to the spectral distribution of the unresolved blazars through the adopted source resolution threshold value, and must be taken into account in an accurate anisotropy calculation.

astro-ph.CO↗

arXiv2012

Current and Future Constraints on Dark Matter from Prompt and Inverse-Compton Photon Emission in the Isotropic Diffuse Gamma-ray Background

We perform a detailed examination of current constraints on annihilating and decaying dark matter models from both prompt and inverse-Compton emission photons, including both model-dependent and model-independent bounds. We also show that the observed isotropic diffuse gamma-ray background (DGRB), which provides one of the most conservative constraints on models of annihilating weak-scale dark matter particles, may enhance its sensitivity by a factor of ~2 to 3 (95% C.L.) as the Fermi-LAT experiment resolves DGRB contributing blazar sources with five years of observation. For our forecasts, we employ the results of constraints to the luminosity-dependent density evolution plus blazar spectral energy distribution sequence model, which is constrained by the DGRB and blazar source count distribution function.

hep-ph↗

arXiv2012

Constraints on WIMP and Sommerfeld-Enhanced Dark Matter Annihilation from HESS Observations of the Galactic Center

We examine the constraints on models of weakly interacting massive particle (WIMP) dark matter from the recent observations of the Galactic center by the High Energy Spectroscopic System (HESS) telescope. We analyze canonical WIMP annihilation into Standard Model particle final states, including b/b-bar, t/t-bar and W^+ W^-. The constraints on annihilation into b/b-bar is within an order of magnitude of the thermal cross section at ~3 TeV, while the tau^+/tau^- channel is within a factor of ~2 of thermal. We also study constraints on Sommerfeld-enhanced dark matter annihilation models, and find that the gamma-ray observational constraints here rule out all of the parameter space consistent with dark matter annihilation interpretations of PAMELA and the Fermi-LAT e^+/e^- spectrum, in specific classes of models, and strongly constrains these interpretations in other classes. The gamma-ray constraints we find are more constraining on these models, in many cases, than current relic density, cosmic microwave background, halo shape and naturalness constraints.

hep-ph↗

arXiv2011

The Contribution of Blazars to the Extragalactic Diffuse Gamma-ray Background and Their Future Spatial Resolution

We examine the constraints on the luminosity-dependent density evolution model for the evolution of blazars given the observed spectrum of the diffuse gamma-ray background (DGRB), blazar source-count distribution, and the blazar spectral energy distribution sequence model, which relates the observed the blazar spectrum to its luminosity. We show that the DGRB observed by the Large Area Telescope (LAT) aboard the Fermi Gamma Ray Space Telescope can be produced entirely by gamma-ray emission from blazars and nonblazar active galactic nuclei, and that our blazar evolution model is consistent with and constrained by the spectrum of the DGRB and flux source-count distribution function of blazars observed by Fermi-LAT. Our results are consistent with previous work that used EGRET spectral data to forecast the Fermi-LAT DGRB. The model includes only three free parameters, and forecasts that >~ 95% of the flux from blazars will be resolved into point sources by Fermi-LAT with 5 years of observation, with a corresponding reduction of the flux in the DGRB by a factor of ~2 to 3 (95% confidence level), which has implications for the Fermi-LAT's sensitivity to dark matter annihilation photons.

astro-ph.CO↗

arXiv2009

Morphological Tests of the Pulsar and Dark Matter Interpretations of the WMAP Haze

The WMAP haze is an excess in microwave emission coming from the center of the Milky Way galaxy. In the case of synchrotron emission models of the haze, we present tests for the source of radiating high-energy electrons/positrons. We explore several models in the case of a pulsar population or dark matter annihilation as the source. These morphological signatures of these models are small behind the WMAP Galactic mask, but are testable and constrain the source models. We show that detailed measurements of the morphology may distinguish between the pulsar and dark matter interpretations as well as differentiate among different pulsar models and dark matter profile models individually. Specifically, we find that a zero central density Galactic pulsar population model is in tension with the observed WMAP haze. The Planck Observatory's greater sensitivity and expected smaller Galactic mask should potentially provide a robust signature of the WMAP haze as either a pulsar population or the dark matter.

astro-ph.CO↗