Infrared dust echoes from neutron star mergers

A significant fraction of binary neutron star mergers occur in star-forming galaxies where the UV-optical and soft X-ray emission from the relativistic jet may be absorbed by dust and re-emitted at longer wavelengths. We show that, for mergers occurring in gas-rich environment ($n_{\rm H}\gtrsim 0.5\rm \, cm^{-3}$ at a few to tens of pc) and when the viewing angle is less than about 30°, the emission from heated dust should be detectable by James Webb Space Telescope (JWST), with a detection rate of $\sim 1\rm \, yr^{-1}$. The spatial separation between the dust emission and the merger site is a few to 10 milli-arcsecs (for a source distance of 150 Mpc), which may be astrometrically resolved by JWST for sufficiently high signal-noise-ratio detections. Measuring the superluminal apparent speed of the flux centroid directly gives the orbital inclination of the merger, which can be combined with gravitational wave data to measure the Hubble constant. For a line of sight within the jet opening angle, the dust echoes are much brighter and may contaminate the search for kilonova candidates from short gamma-ray bursts, such as the case of GRB 130603B.