[Paper Review] Evidence for a decreasing X-ray afterglow emission of GW170817A and GRB 170817A in XMM-Newton

P. D’Avanzo, S. Campana+9 more|arXiv (Cornell University)|Jan 18, 2018

Gamma-ray bursts and supernovae13 references21 citations

TL;DR

This study presents XMM-Newton observations of GW170817A and GRB 170817A at 135 days post-event, revealing a significant decrease in X-ray afterglow flux after a prior steady brightening. The flux decline, confirmed by Hubble optical data and consistent spectral slope, points to geometric or dynamical origins rather than cooling, with both structured jet and isotropic fireball models fitting the data equally well.

ABSTRACT

We report our observation of the short GRB 170817A, associated to the binary neutron star merger event GW 170817, perfomed in the X-ray band with XMM-Newton 135 d after the event (on the 29th December 2017). We find that the X-ray light curve started to decrease with respect to the previous steadily brightening trend. This is also supported by a nearly simultaneous optical Hubble Space Telescope observation. Since the optical-to-X-ray spectral slope did not change with respect to previous observations, we exclude that the decrease in flux is due to the passage of the cooling frequency: its origin must be geometric or dynamical. We interpret all the existing afterglow data with two models: i) a structured jet and ii) a jet-less isotropic fireball with some stratification in its radial velocity structure. Both models fit the data and predict that the radio flux must decrease simultaneously with the optical and the X-ray one, making hard to distinguish between them at the present stage. Polarimetric measures and the rate of short GRB-GW association in future LIGO/Virgo runs will be key to disentangle these two geometrically different scenarios.

Motivation & Objective

• To investigate the evolution of X-ray afterglow emission from GW170817A and GRB 170817A beyond the initial brightening phase.
• To determine whether the observed flux decrease is due to physical processes like cooling or geometric/dynamical effects.
• To compare competing models—structured jet versus isotropic fireball with radial velocity stratification—in explaining the afterglow light curve.
• To assess the implications of the X-ray and optical light curve behavior for understanding the emission geometry of short gamma-ray burst afterglows.

Proposed method

• Performed XMM-Newton X-ray observations of GW170817A on December 29, 2017, 135 days after the event.
• Compared the X-ray light curve with prior observations showing a steady brightening trend to detect a reversal in flux evolution.
• Acquired simultaneous optical data using the Hubble Space Telescope to cross-check flux evolution across bands.
• Analyzed the optical-to-X-ray spectral slope to rule out changes due to cooling frequency passage.
• Fitted the full afterglow light curve with two theoretical models: a structured jet and a stratified, isotropic fireball.
• Evaluated model consistency with multi-wavelength data and discussed observational discriminators such as polarimetry and future GRB-GW detection rates.

Experimental results

Research questions

• RQ1 What caused the X-ray afterglow flux of GW170817A to decrease after a period of steady brightening?
• RQ2 Is the flux decline due to the passage of the cooling frequency or due to geometric or dynamical effects?
• RQ3 Can the observed afterglow light curve be equally well explained by a structured jet model and an isotropic fireball with radial velocity stratification?
• RQ4 What observational diagnostics can distinguish between a structured jet and an isotropic fireball model in future events?
• RQ5 How will the increasing rate of short GRB-GW associations in LIGO/Virgo runs aid in resolving the geometric ambiguity in afterglow models?

Key findings

• The X-ray afterglow of GW170817A showed a clear decrease in flux at 135 days post-event, marking a reversal from the prior steady brightening trend.
• The optical-to-X-ray spectral slope remained constant, indicating that the flux decline was not due to the cooling frequency passing through the X-ray band.
• The observed flux decline is therefore attributed to geometric or dynamical effects rather than spectral evolution.
• Both the structured jet model and the isotropic fireball with radial velocity stratification model fit the observed afterglow light curve equally well.
• The models predict that the radio flux should also decrease in tandem with optical and X-ray flux, making them indistinguishable at this stage.
• Future polarimetric measurements and the increasing number of short GRB-GW associations in LIGO/Virgo observations will be critical to differentiate between the two geometric models.