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

Sphesihle Makhathini, K. P. Mooley+36 more|arXiv (Cornell University)|Jun 3, 2020

Pulsars and Gravitational Waves ResearchPhysics and Astronomy125 references56 citations

TL;DR

This paper presents a uniform, panchromatic afterglow light curve of GW170817 from 0.5 to 940 days post-merger, combining reprocessed radio, optical, and X-ray data to minimize systematic differences across datasets. It confirms a single spectral index of −0.584±0.002, yielding a precise electron power-law index p = 2.168±0.004, and demonstrates that proper motion from VLBI must be included in modeling to accurately estimate the viewing angle, ruling out long-lived neutron stars with B-fields between 10^10.4 G and 10^16 G.

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.

Motivation & Objective

• To create a consistent, uniform dataset of the panchromatic afterglow of GW170817 by reprocessing archival data and minimizing flux determination and processing method discrepancies.
• To determine the evolution of the afterglow light curve across radio, optical, and X-ray bands from 0.5 to 940 days post-merger.
• To assess the impact of data processing variations on flux measurements and to improve the reliability of multi-wavelength afterglow modeling.
• To test the validity of the structured jet model and the role of proper motion in estimating the viewing angle and jet opening angle.
• To constrain the properties of any potential long-lived neutron star remnant through late-time afterglow decay.

Proposed method

• Compilation and reprocessing of archival radio, optical, and X-ray data from multiple observatories (VLA, ATCA, MeerKAT, Swift, Chandra, HST, NuSTAR, LOFAR) to ensure consistent flux calibration and error estimation.
• Application of correction factors (0.6 for eMERLIN, 0.8 for ATCA) to align flux densities across instruments and reduce systematic offsets.
• Fitting the uniform light curve with simple power laws to determine rise and decay indices across all bands.
• Incorporating VLBI-measured proper motion from Mooley et al. (2018a) into analytical afterglow modeling to assess its impact on viewing angle and jet geometry constraints.
• Using a structured jet model that includes or excludes proper motion to compare viewing angle estimates and assess model sensitivity.
• Performing spectral index analysis across all epochs to test for spectral evolution and confirm optically thin synchrotron emission.

Experimental results

Research questions

• RQ1 What is the true shape of the panchromatic afterglow light curve of GW170817 when systematic differences in data processing are minimized?
• RQ2 What is the precise value of the electron energy spectral index p, and how does it constrain the underlying electron acceleration mechanism?
• RQ3 How does the inclusion of VLBI-measured proper motion affect the estimation of the viewing angle and jet opening angle in afterglow modeling?
• RQ4 What constraints can be placed on the presence of a long-lived neutron star remnant based on the late-time afterglow decay?
• RQ5 Is the afterglow emission optically thin throughout its evolution, and does it maintain a constant spectral index?

Key findings

• The afterglow light curve exhibits a t^0.86±0.04 rise and a t^−1.92±0.12 decay, peaking at 155±4 days post-merger.
• The spectral index remains constant at −0.584±0.002 across all epochs, confirming optically thin synchrotron emission throughout the afterglow evolution.
• The electron power-law index is precisely measured as p = 2.168±0.004, providing a robust constraint on electron acceleration in the outflow.
• The hot ionized ISM density in the host galaxy is constrained to <0.01 cm⁻³, consistent with previous afterglow studies.
• Late-time afterglow data rule out any long-lived neutron star remnant with magnetic field strength between 10^10.4 G and 10^16 G.
• Proper motion from VLBI must be included in modeling to accurately estimate the viewing angle; ignoring it leads to biased results.