Session E01: Astrophysics II

Recent cosmology results from the Dark Energy Survey

Over the next decade, large galaxy surveys will map billions of galaxies and probe cosmic structure formation with high statistical precision. This talk will focus on the opportunities and challenges of cosmological analyses in the presence of complex systematic effects, using recent results from the Dark Energy Survey as pathfinder examples. In particular, I will describe different cosmological probes measured from DES data and summarize recent analyses combining galaxy clustering, weak lensing, cluster clustering and cluster abundances, as well as constraints on baryons and galaxy biasing from small scales.   

Geometric modeling for the Event Horizon Telescope: Investigating the width of the ring feature in M87*

The 2017 Event Horizon Telescope (EHT) observations of the core of the galaxy M87 are the first electromagnetic observations probing event horizon scales of a black hole. The data strongly favor an observational appearance dominated by a ring of approximately 40 micro-arcseconds in diameter. However, many interesting questions remain about the appearance of the source. In particular, the thickness of the ring is much less certain. I will argue that some of the geometric modeling results are in tension with theoretical expectations - the observed ring is too narrow - and explore whether this tension can be resolved by alternative data analysis methods. First, I will report on our independent verification of a subset of the EHT collaboration’s geometric modeling results, using a new code built from scratch. Second, I will discuss some subtleties in the choice of likelihood function used in model-fitting, and test the sensitivity of the results on the choice of method. We find that the choice of likelihood function does in fact bias the results for ring width, however, which likelihood approximation is the best choice for this dataset remains uncertain.   

SPECTROSCOPIC ANALYSIS OF HYDRODYNAMICAL MODELS OF TYPE IA SUPERNOVAE

Type Ia supernovae (SNe Ia) are the result of thermonuclear explosions of white dwarfs (WD) in binary systems. Due to their consistent luminosity, SNe Ia are important  standardizable candles for cosmology, and have helped to reveal the presence of dark energy and obtain precise values of the Hubble constant. Further study of SNe Ia could also help understand the exact nature of their stellar progenitors, which remains in question. Recently-developed computational techniques demonstrate the possibility for advanced hydrodynamical modeling of SNe Ia, which can help identify their features through simulation. We present a pipeline of multiphysics and multiscale simulation codes, which are capable of modeling the initial few seconds of the hydrodynamics of the explosion (FLASH), through the subsequent detailed nucleosynthesis (TORCH), and radiative transfer weeks later (SuperNu). A final synthetic spectral classification (SNID) can then be used to enable direct comparison between the model predictions and observed SNe Ia.   

Searching for the Diffuse Gamma-Ray Background with HAWC

The high-energy Diffuse Gamma-Ray Background (DGRB) is dominated by an isotropic extragalactic emission of gamma rays uncorrelated with any known sources, and potentially dark matter annihilation or decay emissions in galactic structures. While well-characterized at MeV-GeV energies, the DGRB has never been observed at energies above 1 TeV. The High Altitude Water Cherenkov (HAWC) observatory, located in central Mexico at 4100 m above sea level, detects TeV-energy gamma rays and cosmic rays continuously with a wide field-of-view. With its high-energy reach and its large area coverage, HAWC is well-suited to notably improve searches for the DGRB. Using 535 days of HAWC observations and with strict cuts on gamma/hadron separation parameters to better isolate gamma-ray signal from cosmic-ray contamination, we will constrain the DGRB above 10 TeV, as well as discuss prospective implications for multi-messenger and dark matter studies.   

Investigating Cosmic Ray Propagation

Understanding the propagation of charged cosmic rays through our galaxy is crucial to understand a range of astrophysical phenomena. Unfortunately, modeling this propagation is extremely non-trivial, with many uncertain parameters affecting the predicted fluxes of these particles. I will demonstrate a tool that I've developed which utilizes DRAGON2, a cosmic ray propagation program, and scans parameter space to find values of these propagation parameters that describe our galaxy. The output of the program is compared to the proton and antiproton flux data taken by AMS-02, a particle detector located on the ISS. This leads to a best-fit point that, when input into DRAGON2, gives spectra that closely match the AMS-02 data.   

A blinded search for orbital periodicity in VHE gamma-ray emission in High Mass X-ray Binary LS I $+$61 303.

LS I $+$61 303, made up of a Be star and a compact object,~ is one of only 11 VHE gamma-ray binaries observed in our universe thus far. The nature of the compact object in LS I $+$61 303 is still debated. As a part of an ongoing study of LS I $+$61 303, I used ten years of data from the Very Energetic Radiation Imaging Telescope Array System (VERITAS) and existing data from the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescope to search for the orbital period of the binary system. I performed a Pearson Correlation Coefficient (PCC) analysis of 200 phase-folded light-curves for orbital periods ranging from 15.0 days to 35.0 days. A maximum value of PCC$=$0.70 is at an orbital period of 26.5 days, which is consistent with the original observed period of 26.496 days found in the radio lightcurve data.