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Journal Article

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


Nedora,  V.
Computational Relativistic Astrophysics, AEI-Golm, MPI for Gravitational Physics, Max Planck Society;
Multi-messenger Astrophysics of Compact Binaries;

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Hajela, A., Margutti, R., Bright, J. S., Alexander, K. D., Metzger, B. D., Nedora, V., et al. (2022). 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. The Astrophysical Journal Letters, 927(1): L17. doi:10.3847/2041-8213/ac504a.

Cite as: https://hdl.handle.net/21.11116/0000-000A-310B-3
For the first $\sim3$ years after the binary neutron star merger event GW
170817 the radio and X-ray radiation has been dominated by emission from a
structured relativistic off-axis jet propagating into a low-density medium with
n $< 0.01\,\rm{cm^{-3}}$. We report on observational evidence for an excess of
X-ray emission at $\delta t>900$ days after the merger. With $L_x\approx5\times
10^{38}\,\rm{erg\,s^{-1}}$ at 1234 days, the recently detected X-ray emission
represents a $\ge 3.2\,\sigma$ (Gaussian equivalent) deviation from the
universal post jet-break model that best fits the multi-wavelength afterglow at
earlier times. In the context of JetFit afterglow models, current data
represent a departure with statistical significance $\ge 3.1\,\sigma$,
depending on the fireball collimation, with the most realistic models showing
excesses at the level of $\ge 3.7\,\sigma$. A lack of detectable 3 GHz radio
emission suggests a harder broad-band spectrum than the jet afterglow. These
properties are consistent with the emergence of a new emission component such
as synchrotron radiation from a mildly relativistic shock generated by the
expanding merger ejecta, i.e. a kilonova afterglow. In this context, we present
a set of ab-initio numerical-relativity BNS merger simulations that show that
an X-ray excess supports the presence of a high-velocity tail in the merger
ejecta, and argues against the prompt collapse of the merger remnant into a
black hole. Radiation from accretion processes on the compact-object remnant
represents a viable alternative. Neither a kilonova afterglow nor
accretion-powered emission have been observed before, as detections of BNS
mergers at this phase of evolution are unprecedented.