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From Neutrino- to Photon-Cooled in Three Years: Can Fallback Accretion Explain the X-ray Excess in GW170817?
High Energy Astrophysical Phenomena · Submitted 3 Jun 2021
abstract compiled · 2026-09-23 · no language-model review
The authors' main claim The predicted X-ray accretion luminosity at the present epoch is L_X ~ 0.1 Mdot_BH c^2 ~ (2-70)e38 erg/s for beta_FB ~ 1.43-1.66, thus supporting (with caveats) an accretion-powered origin for the X-ray excess in GW170817.
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The abstract, sentence by sentence
An excess in the X-ray emission from the neutron star merger GW170817 above the predicted afterglow was recently detected t~3.4 years post-merger. One possible origin for the excess is accretion onto the newly unshrouded black hole (BH) remnant. While fall-back of the bound dynamical ejecta is insufficient to generate the excess luminosity, L_X ~ 5e38 erg/s, fall-back from the disk wind ejecta-due to their larger mass and lower velocity-remains a viable possibility. We present hydrodynamic alpha-viscosity simulations of the post-merger disk evolution which extend to an unprecedently long timescale t ~ 35 s post-merger, as necessary to capture the end of photodissociation and the asymptotic evolution into the radiatively inefficient regime. Due to inefficient neutrino cooling, the BH accretion rate decays rapidly at late times (Mdot_BH ~ t^(-\beta_BH), where \beta_BH ~ 2.4-2.8), seemingly incompatible with generating the late-time excess. However, the rate at which matter falls back to the inner disk from the equatorial regions (as inferred by the rate matter is unbound in outflows by viscous heating at higher latitudes) decays more gradually, Mdot_fb ~ t^(-\beta_fb) with \beta_fb ~ 1.43 in our alpha ~ 0.03 simulations. By the present epoch, the fall-back rate has become sub-Eddington and the disk can again accrete efficiently, i.e. Mdot_BH ~ Mdot_fb, this time as a result of photon cooling instead of neutrino cooling. The predicted X-ray accretion luminosity at the present epoch is L_X ~ 0.1 Mdot_BH c^2 ~ (2-70)e38 erg/s for beta_FB ~ 1.43-1.66, thus supporting (with caveats) an accretion-powered origin for the X-ray excess in GW170817. The suppressed BH accretion rate prior to the radiatively efficient (sub-Eddington) transition, weeks to months after the merger, is key to avoid overproducing the kilonova luminosity via reprocessing.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | An excess in the X-ray emission from the neutron star merger GW170817 above the predicted afterglow was recently detected t~3.4 years post-merger. | background 0.98 | 0.15 | 0.03 | asserted 0.99 |
| s1 | One possible origin for the excess is accretion onto the newly unshrouded black hole (BH) remnant. | background 0.92 | 0.07 | 0.05 | hedged possibility 1.00 |
| s2 | While fall-back of the bound dynamical ejecta is insufficient to generate the excess luminosity, L_X ~ 5e38 erg/s, fall-back from the disk wind ejecta-due to their larger mass and lower velocity-remains a viable possibility. | background 0.52 | 0.33 | 0.20 | hedged possibility 0.75 |
| s3 | We present hydrodynamic alpha-viscosity simulations of the post-merger disk evolution which extend to an unprecedently long timescale t ~ 35 s post-merger, as necessary to capture the end of photodissociation and the asymptotic evolution into the radiatively inefficient regime. | method 0.85 | 0.95 | 0.04 | asserted 1.00 |
| s4 | Due to inefficient neutrino cooling, the BH accretion rate decays rapidly at late times (Mdot_BH ~ t^(-\beta_BH), where \beta_BH ~ 2.4-2.8), seemingly incompatible with generating the late-time excess. | result 0.88 | 0.66 | 0.04 | asserted 0.97 |
| s5 | However, the rate at which matter falls back to the inner disk from the equatorial regions (as inferred by the rate matter is unbound in outflows by viscous heating at higher latitudes) decays more gradually, Mdot_fb ~ t^(-\beta_fb) with \beta_fb ~ 1.43 in our alpha ~ 0.03 simulations. | result 1.00 | 0.93 | 0.10 | asserted 1.00 |
| s6 | By the present epoch, the fall-back rate has become sub-Eddington and the disk can again accrete efficiently, i.e. Mdot_BH ~ Mdot_fb, this time as a result of photon cooling instead of neutrino cooling. | result 0.77 | 0.79 | 0.22 | asserted 1.00 |
| s7 | The predicted X-ray accretion luminosity at the present epoch is L_X ~ 0.1 Mdot_BH c^2 ~ (2-70)e38 erg/s for beta_FB ~ 1.43-1.66, thus supporting (with caveats) an accretion-powered origin for the X-ray excess in GW170817. | result 0.60 | 0.89 | 0.83 | hedged possibility 0.94 |
| s8 | The suppressed BH accretion rate prior to the radiatively efficient (sub-Eddington) transition, weeks to months after the merger, is key to avoid overproducing the kilonova luminosity via reprocessing. | interpretation 0.93 | 0.60 | 0.09 | asserted 0.98 |
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Original abstract
An excess in the X-ray emission from the neutron star merger GW170817 above the predicted afterglow was recently detected t~3.4 years post-merger. One possible origin for the excess is accretion onto the newly unshrouded black hole (BH) remnant. While fall-back of the bound dynamical ejecta is insufficient to generate the excess luminosity, L_X ~ 5e38 erg/s, fall-back from the disk wind ejecta-due to their larger mass and lower velocity-remains a viable possibility. We present hydrodynamic alpha-viscosity simulations of the post-merger disk evolution which extend to an unprecedently long timescale t ~ 35 s post-merger, as necessary to capture the end of photodissociation and the asymptotic evolution into the radiatively inefficient regime. Due to inefficient neutrino cooling, the BH accretion rate decays rapidly at late times (Mdot_BH ~ t^(-\beta_BH), where \beta_BH ~ 2.4-2.8), seemingly incompatible with generating the late-time excess. However, the rate at which matter falls back to the inner disk from the equatorial regions (as inferred by the rate matter is unbound in outflows by viscous heating at higher latitudes) decays more gradually, Mdot_fb ~ t^(-\beta_fb) with \beta_fb ~ 1.43 in our alpha ~ 0.03 simulations. By the present epoch, the fall-back rate has become sub-Eddington and the disk can again accrete efficiently, i.e. Mdot_BH ~ Mdot_fb, this time as a result of photon cooling instead of neutrino cooling. The predicted X-ray accretion luminosity at the present epoch is L_X ~ 0.1 Mdot_BH c^2 ~ (2-70)e38 erg/s for beta_FB ~ 1.43-1.66, thus supporting (with caveats) an accretion-powered origin for the X-ray excess in GW170817. The suppressed BH accretion rate prior to the radiatively efficient (sub-Eddington) transition, weeks to months after the merger, is key to avoid overproducing the kilonova luminosity via reprocessing.
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References
How the literature uses it
4 arXiv papers cite this in 4 sentences
Papers citing it
Cites it as background 4 papers
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Computational efforts focus on predicting the ejecta mass and composition, the kilonova signal, neutrino luminosities, the gravitational-wave signature, and the nature of the remnant [ref] .
Neutrino transport and flavor instabilities in a post-merger disk · Introduction · 0.92
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Long-term viscous hydrodynamic models of the disk outflow have been carried out for a decade now, and have led to most of our current understanding of the disk ejecta [ref] .
Viscous hydrodynamic evolution of neutron star merger accretion disks: a code comparison · Introduction · 0.81
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By comparing the observational data with theoretical models of BNS mergers, we have managed to draw an overview about the merger process [ref] .
Self-consistent picture of the mass ejection from a one second-long binary neutron star merger leaving a short-lived remnant in general-relativistic neutrino-radiation magnetohydrodynamic simulation · Introduction · 0.86
What it cites · 16 sentences without a confident label
Compares with 3 papers
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Interpreted in this way, we see that the black hole accretion rate at $t ≈3.4$ years may indeed be sufficient to explain the X-ray excess from GW170817, which we show for comparison with a blue square in Fig. [ref] [ref] .
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 · Results · 0.90
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Interpreted in this way, we see that the black hole accretion rate at $t ≈3.4$ years may indeed be sufficient to explain the X-ray excess from GW170817, which we show for comparison with a blue square in Fig. [ref] [ref] .
Accurate flux calibration of GW170817: is the X-ray counterpart on the rise? · Results · 0.90
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This contrasts with alternative kilonova afterglow model, for which $L_X$ is predicted to decay slower, or even rise, in time (e.g., [ref] ), and should eventually be accompanied by radio emission in excess of the ordinary afterglow contribution.
Dynamical ejecta synchrotron emission as possible contributor to the changing behaviour of GRB170817A · Conclusions · 0.99
Cites as background 1 paper
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The neutron star merger GW170817 [ref] was accompanied by radiation covering the electromagnetic spectrum, from radio to gamma-rays [ref] .
First Multimessenger Observations of a Neutron Star Merger · Introduction · 0.97
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