Early soft X-ray to UV emission from double neutron star mergers: implications from the long-term radio and X-ray emissions of GW 170817

Abstract

Recent long-term radio follow-up observations of GW 170817 reveals a simple power-law rising light curve, with a slope of $t^{0.78}$, up to 93 days after the merger. The latest X-ray detection at 109 days is also consistent with such a temporal slope. Such a shallow rise behavior requires a mildly relativistic outflow with a steep velocity gradient profile, so that slower material with larger energy catches up with the decelerating ejecta and re-energizes it. It has been suggested that this mildly relativistic outflow may represent a cocoon of material. We suggest that the velocity gradient profile may form during the stage that the cocoon is breaking out of the merger ejecta, resulted from shock propagation down a density gradient. The cooling of the hot relativistic cocoon material immediately after it breaks out should have produced soft X-ray to UV radiation at tens of seconds to hours after the merger. The soft X-ray emission has a luminosity of $L_{\mathrm{X}}\sim 10^{45}\mathrm{erg}{\mathrm{s}}^{-1}$ over a period of tens of seconds for a merger event like GW 170817. The UV emission shows a rise initially and peaks at about a few hours with a luminosity of $L_{\mathrm{UV}}\sim 10^{42}\mathrm{erg}{\mathrm{s}}^{-1}$. The soft X-ray transients could be detected by future wide-angle X-ray detectors, such as the Chinese mission Einstein Probe. This soft X-ray/UV emission would serve as one of the earliest electromagnetic counterparts of gravitation waves from double neutron star mergers and could provide the earliest localization of the sources.

5 pages, 2 figures, ApJL in press, discussions on the reverse shock emission in the refreshed shock scenario for the long-term radio and X-ray emissions are added

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• The short GRB 170817A: Modelling the off-axis emission and implications on the ejecta magnetization
• Brightening X-ray/Optical/Radio Emission of GW170817/SGRB 170817A: Results of an Electron-Positron Wind from the Central Engine?
• Continued Brightening of the Afterglow of GW170817/GRB 170817A as due to a Delayed Energy Injection
• Polarization of the first-hour macronovae