Evidence for X-Ray Emission in Excess to the Jet-afterglow Decay 3.5 yr after the Binary Neutron Star Merger GW 170817: A New Emission Component

Abstract

For the first ∼3 yrs 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 cm−3. We report on observational evidence for an excess of X-ray emission at δt > 900 days after the merger. With L x ≈ 5 × 1038 erg s−1 at 1234 days, the recently detected X-ray emission represents a ≥3.2σ (Gaussian equivalent) deviation from the universal post-jet-break model that best fits the multiwavelength afterglow at earlier times. In the context of JetFit afterglow models, current data represent a departure with statistical significance ≥3.1σ, depending on the fireball collimation, with the most realistic models showing excesses at the level of ≥3.7σ. A lack of detectable 3 GHz radio emission suggests a harder broadband 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 binary neutron star (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.

Comprehensive study of mass ejection and nucleosynthesis in binary neutron star mergers leaving short-lived massive neutron stars

“…1 shows the cumulative kinetic energy histogram of βΓ. The spectrum shape is in broad agreement with the previously reported results (Hotokezaka et al 2016(Hotokezaka et al , 2018Hajela et al 2022) apart from the extension to a higher βΓ side than previously. The dependence of Y e and velocity distributions on the grid resolution is discussed in Appendix C.…”

“…Although we here evaluate the mass of the postmerger ejecta assuming that all the internal energy will be converted to the kinetic energy (i.e., Bernoulli's criterion), a fraction of matter can turn around before this conversion completes and then fall back onto the black hole-disk system (e.g., Ishizaki et al 2021b). This fall-back matter may energize late-time electromagnetic emission such as that observed in the afterglow of GRB 170817A (Ishizaki et al 2021a;Hajela et al 2022). To clarify the uncertainty in the effect of fall back, we need a computation for the very long-term evolution of the outflowed matter (e.g., Kawaguchi et al 2021 andRosswog et al 2014).…”

Thinking Outside the Box: Numerical Relativity with Particles

“…While the high-velocity ejecta are still not well-resolved, we note that our simulations here have an order of magnitude more particles than the approximate GR simulations in which these fast ejecta were originally identified [113]. This high-velocity component could have important observational consequences: it may produce an early blue/UV transient on a time scale of several minutes to an hour preceding the main kilonova event [113], and at late times, it may be responsible for synchrotron emission [114][115][116]. Another interesting result in a multi-messenger context is that the ejecta distribution only shows moderate deviations from spherical symmetry, so that the resulting electromagnetic emission could be reasonably modeled with simple approaches.…”

Electromagnetic Counterparts of Binary-neutron-star Mergers Leading to a Strongly Magnetized Long-lived Remnant Neutron Star

“…In addition to the jet afterglow, a long-term synchrotron flare is expected to arise from the merger ejecta on timescales of ∼1-10 4 yr (Nakar & Piran 2011;Hotokezaka & Piran 2015;Hotokezaka et al 2018;Margalit & Piran 2020). Interestingly, the X-ray flux observed around 3.5 yr after GW170817 exceeds the expected flux of the jet afterglow, suggesting that the ejecta afterglow starts to dominate over the jet component (Hajela et al 2022;Troja et al 2022). However, the origin of the X-ray excess is sill under debate because the radio flux is still consistent with the prediction of the jet afterglow (Balasubramanian et al 2021).…”