A thousand days after the merger: continued X-ray emission from GW170817

TL;DR

This paper reports continued X-ray emission from GW170817 nearly three years post-merger, suggesting ongoing energy injection or afterglow phenomena, with implications for understanding neutron star merger remnants.

Contribution

It provides new long-term X-ray observations of GW170817 and discusses potential models for prolonged emission, advancing knowledge of post-merger evolution.

Findings

X-ray emission detected at 935-942 days post-merger

X-ray flux higher than earlier predictions

Consistent with energy injection or kilonova afterglow models

Abstract

Recent observations with the Chandra X-ray telescope continue to detect X-ray emission from the transient GW170817. In a total exposure of 96.6 ks, performed between March 9 and March 16 2020 (935 d to 942 d after the merger), a total of 8 photons are measured at the source position, corresponding to a significance of about 5 sigma. Radio monitoring with the Australian Telescope Compact Array (ATCA) shows instead that the source has faded below our detection threshold (<33 uJy, 3 sigma). By assuming a constant spectral index beta=0.585, we derive an unabsorbed X-ray flux of approximately 1.4E-15 erg/cm^2/s, higher than earlier predictions, yet still consistent with a simple structured jet model. We discuss possible scenarios that could account for prolonged emission in X-rays. The current dataset appears consistent both with energy injection by a long-lived central engine and with the onset of a kilonova afterglow, arising from the interaction of the sub-relativistic merger ejecta with the surrounding medium. Long-term monitoring of this source will be essential to test these different models.