The Panchromatic Afterglow of GW170817: The full uniform dataset, modeling, comparison with previous results and implications

TL;DR

This paper compiles and analyzes a comprehensive panchromatic dataset of GW170817's afterglow, refining models, comparing with previous results, and deriving implications for the jet structure, environment, and remnant neutron star.

Contribution

It provides a uniform, reprocessed dataset of GW170817's afterglow and offers refined modeling and analysis, including the impact of proper motion and jet structure on viewing angle estimates.

Findings

Afterglow peaks at 155 days with a power-law rise and decline.

Electron spectral index p=2.168±0.004 is precisely estimated.

Upper limit on host galaxy ISM density is <0.01 cm$^{-3}$.

Abstract

We present the full panchromatic afterglow light curve data of GW170817, including new radio data as well as archival optical and X-ray data, between 0.5 and 940 days post-merger. By compiling all archival data, and reprocessing a subset of it, we have evaluated the impact of differences in data processing or flux determination methods used by different groups, and attempted to mitigate these differences to provide a more uniform dataset. Simple power-law fits to the uniform afterglow light curve indicate a $t^{0.86\pm0.04}$ rise, a $t^{-1.92\pm0.12}$ decline, and a peak occurring at $155\pm4$ days. The afterglow is optically thin throughout its evolution, consistent with a single spectral index ($-0.584\pm0.002$) across all epochs. This gives a precise and updated estimate of the electron power-law index, $p=2.168\pm0.004$. By studying the diffuse X-ray emission from the host galaxy, we place a conservative upper limit on the hot ionized ISM density, $<$0.01 cm$^{-3}$, consistent with previous afterglow studies. Using the late-time afterglow data we rule out any long-lived neutron star remnant having magnetic field strength between 10$^{10.4}$ G and 10$^{16}$ G. Our fits to the afterglow data using an analytical model that includes VLBI proper motion from Mooley et al. (2018), and a structured jet model that ignores the proper motion, indicates that the proper motion measurement needs to be considered while seeking an accurate estimate of the viewing angle.