Day 1505

Thursday.

1811.11491
The optical afterglow of GW170817 at one year post-merger
Lamb, et al

Present observations of the optical afterglow of GRB 170817A, made by HST, between Feb and Aug 2018, up to one year after the neutron star merger, GW170817.  The afterglow shows a rapid decline beyond 170 days, and confirms the jet origin for the observed outflow, in contrast to more slowly declining expectations for 'failed-jet' scenarios.  Show here that the broadband (radio, optical, X-ray) afterglow is consistent with a structured outflow where an ultra-relativistic jet, with Lorentz factor Gamma >~100, forms a narrow core (~5 deg) and is surrounded by a wider angular component that extends to ~15 deg, which is itself relativistic (Gamma >~5).  For a 2-component model of this structure, the late-time optical decline, where F~t^-alpha, is alpha=2.2±0.2, and for a Gaussian structure the decline is alpha=2.45±0.22.  Find the Gaussian model to be consistent with both the early ~10 days and late >~290 days data.  The agreement of the optical light curve with the evolution of the broadband spectral energy distribution, and its continued decline, indicates that the optical flux is arising primarily from the afterglow and not any underlying host system.  This provides the deepest limits on any host stellar cluster, with a luminosity <~4000 Sun~M_F606W >~ -4.5.

1811.11584
The effect on cosmological parameter estimation of a parameter dependent covariance matrix
Kodwani, Alonso, Ferreira

Cosmological large-scale structure analyses based on 2pt correlation functions often assume a Gaussian likelihood function with a fixed covariance matrix.  Study the impact on cosmological parameter estimation of ignoring the parameter dependence of this covariance matrix, focusing on the particular case of joint weak-lensing and galaxy clustering analyses.  Using a Fisher matrix formalism (calibrated against exact likelihood evolution in particular simple cases), quantify the effect of using a parameter dependent covariance matrix on both the bias and variance of the parameters.  Confirm that the approximation of a parameter-independent covariance matrix is exceptionally good in all realistic scenarios.  The information content in the covariance matrix (in comparison with the 2 pt function themselves) does not change with the fractional sky coverage.  Therefore the increase in information due to the parameter dependent covariance matrix becomes negligible as the number of modes increases.  Even for surveys covering less than 1% of the sky, this effect only causes a bias of up to O(10%) of the statistical uncertainties, with a misestimation of the parameter uncertainties at the same level or lower.  The effect will only be smaller with future large-area surveys.  Thus for most analyses the effect of a parameter-dependent covariance matrix can be ignored both in terms of the accuracy and precision of the recovered cosmological constraints.