A. McDowell | Searcharxiv

arXiv2023

A standard siren measurement of the Hubble constant using GW170817 and the latest observations of the electromagnetic counterpart afterglow

We present a new constraint on the Hubble constant $H_0$ using the latest measurements of the electromagnetic counterpart to the gravitational wave (GW) event GW170817. We use the latest optical, X-ray and radio observations of the afterglow up to $\sim 3.5$ years after the GW detection, and properly take into account the impact of the host galaxy peculiar velocity. We find $75.46^{+5.34}_{-5.39}$ km s$^{-1}$ Mpc$^{-1}$ (68\% Credible Interval), a $\sim7\%$ precision measurement, which is a significant improvement compared to the $14\%$ precision of the first standard siren measurement. Our result is consistent within $1σ$ with the Cepheid-anchored Supernova and within $1.5 σ$ with the Cosmic Microwave Background measurements of the Hubble constant. We also explore the impact of the various assumptions made when fitting for the afterglow on the Hubble constant estimate.

astro-ph.CO↗

arXiv2022

Evidence for X-ray Emission in Excess to the Jet Afterglow Decay 3.5 yrs After the Binary Neutron Star Merger GW 170817: A New Emission Component

For the first $\sim3$ years after the binary neutron star merger event GW 170817 the radio and X-ray radiation has been dominated by emission from a structured relativistic off-axis jet propagating into a low-density medium with n $< 0.01\,\rm{cm^{-3}}$. We report on observational evidence for an excess of X-ray emission at $δt>900$ days after the merger. With $L_x\approx5\times 10^{38}\,\rm{erg\,s^{-1}}$ at 1234 days, the recently detected X-ray emission represents a $\ge 3.2\,σ$ (Gaussian equivalent) deviation from the universal post jet-break model that best fits the multi-wavelength afterglow at earlier times. In the context of JetFit afterglow models, current data represent a departure with statistical significance $\ge 3.1\,σ$, depending on the fireball collimation, with the most realistic models showing excesses at the level of $\ge 3.7\,σ$. A lack of detectable 3 GHz radio emission suggests a harder broad-band spectrum than the jet afterglow. These properties are consistent with the emergence of a new emission component such as synchrotron radiation from a mildly relativistic shock generated by the expanding merger ejecta, i.e. a kilonova afterglow. In this context, we present a set of ab-initio numerical-relativity BNS merger simulations that show that an X-ray excess supports the presence of a high-velocity tail in the merger ejecta, and argues against the prompt collapse of the merger remnant into a black hole. Radiation from accretion processes on the compact-object remnant represents a viable alternative. Neither a kilonova afterglow nor accretion-powered emission have been observed before, as detections of BNS mergers at this phase of evolution are unprecedented.

astro-ph.HE↗