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  Fallback Accretion Model for the Years-to-Decades X-ray Counterpart to GW170817

Ishizaki, W., Ioka, K., & Kiuchi, K. (in preparation). Fallback Accretion Model for the Years-to-Decades X-ray Counterpart to GW170817.

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2104.04433.pdf (Preprint), 322KB
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 Creators:
Ishizaki, Wataru, Author
Ioka, Kunihito, Author
Kiuchi, Kenta1, Author              
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1Computational Relativistic Astrophysics, AEI-Golm, MPI for Gravitational Physics, Max Planck Society, ou_2541714              

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Free keywords: Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE, Astrophysics, Solar and Stellar Astrophysics, astro-ph.SR
 Abstract: A new component was reported in the X-ray counterpart to the binary neutron-star merger and gravitational wave event GW170817, exceeding the afterglow emission from an off-axis structured jet. The afterglow emission from the kilonova/macronova ejecta may explain the X-ray excess but exceeds the radio observations if the spectrum is the same. We propose a fallback accretion model that a part of ejecta from the neutron star merger falls back and forms a disk around the central compact object. In the super-Eddington accretion phase, the X-ray luminosity stays near the Eddington limit of a few solar masses and the radio is weak, as observed. This will be followed by a power law decay with index $-5/3$. The duration of the constant luminosity phase conveys the initial fallback timescale $t_0$ in the past. The current multi-year duration requires $t_0 > 3$--$30$ sec, suggesting that the disk wind rather than the dynamical ejecta falls back after the jet launch. Future observations in the next decades will probe the timescale of $t_0 \sim 10$--$10^4$ sec, around the time of extended emission in short gamma-ray bursts. The fallback accretion has not been halted by the {\it r}-process heating, implying that fission is weak on the year scale. We predict that the X-ray counterpart will disappear in a few decades due to the {\it r}-process halting.

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 Dates: 2021-04-09
 Publication Status: Not specified
 Pages: 10 pages, 2 figures
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 Table of Contents: -
 Rev. Type: -
 Identifiers: arXiv: 2104.04433
 Degree: -

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