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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 withn< 0.01 cm−3. We report on observational evidence for an excess of X-ray emission atδt> 900 days after the merger. WithLx≈ 5 × 1038erg s−1at 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 ofJetFitafterglow 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.
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Synchrotron afterglow model for AT 2022cmc: jetted tidal disruption event or engine-powered supernova?
ABSTRACT AT 2022cmc is a luminous optical transient (νLν ≳ 1045 erg s−1) accompanied by decaying non-thermal X-rays (peak duration tX ≲ days and isotropic energy EX,iso ≳ 1053 erg) and a long-lived radio/mm synchrotron afterglow, which has been interpreted as a jetted tidal disruption event (TDE). Both an equipartition analysis and a detailed afterglow model reveal the radio/mm emitting plasma to be expanding mildly relativistically (Lorentz factor $$\Gamma \gtrsim \, \mathrm{ few}$$ ) with an opening angle θj ≃ 0.1 and roughly fixed energy Ej,iso ≳ few × 1053 erg into an external medium of density profile n ∝ R−k with k ≃ 1.5–2, broadly similar to that of the first jetted TDE candidate Swift J1644+57 and consistent with Bondi accretion at a rate of ∼$$10^{-3}\,\dot{M}_{\rm Edd}$$ on to a 106 M⊙ black hole before the outburst. The rapidly decaying optical emission over the first days is consistent with fast-cooling synchrotron radiation from the same forward shock as the radio/mm emission, while the bluer slowly decaying phase to follow likely represents a separate thermal emission component. Emission from the reverse shock may have peaked during the first days, but its non-detection in the optical band places an upper bound Γj ≲ 100 on the Lorentz factor of the unshocked jet. Although a TDE origin for AT 2022cmc is indeed supported by some observations, the vast difference between the short-lived jet activity phase tX ≲ days and the months-long thermal optical emission also challenges this scenario. A stellar core-collapse event giving birth to a magnetar or black hole engine of peak duration ∼1 d offers an alternative model also consistent with the circumburst environment, if interpreted as a massive star wind.
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- Award ID(s):
- 2009255
- PAR ID:
- 10412594
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 522
- Issue:
- 3
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 4028-4037
- Size(s):
- p. 4028-4037
- Sponsoring Org:
- National Science Foundation
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