Optical Flashes from Beam-Driven Light Sails with the Roman, Rubin and Euclid Observatories

Abstract — v1

The primary challenge of rocket propulsion is the burden of accelerating the spacecraft’s own fuel. Light sails leave the propellant at home, with the achievable speed set by the sail area, the thermal tolerance of its material, and the power of the driving array. In Guillochon & Loeb (2015) we showed that leakage from a microwave array driving such a sail between habitable worlds produces Jansky-level radio transients lasting tens of seconds at $100$ pc. We take that leak to optical and near-infrared wavelengths, where Fresnel matching would shrink the aperture to $\sim\!20$–$100$ m at $1\,\mu{\rm m}$, but the intensity on that aperture is $2\times10^{8}\,{\rm W\,m^{-2}}$, five orders above published directed-energy loadings. Spreading $1.5$ TW at $\sim20\,{\rm kW\,m^{-2}}$ is a $\sim10$ km phased array, near the emitters’ thermal limit; holding the published $10^{3}\,{\rm W\,m^{-2}}$ loading takes a $\sim40$ km array. A tenth-wave RMS piston residual on $40$ m tiles leaks $0.054$ of the power ($80$ GW) into a halo. The typical Galactic detection is a single $\sim30$ s peak at $m_{\rm AB}\simeq19.4$ in F146 at $8$ kpc; a pair $87$ s apart, about one in four, is the confirmation test. Roman’s bulge survey reaches $109$ kpc at $8\sigma$ on a faint host, so the flash is visible anywhere in the Galaxy if the beam points at us. One launch per star per $6$ h gives $N_{\rm det}=0.061\,f_{\rm b}$. For these beamers, $N_{\rm det}=1$ requires $\Gamma_{6{\rm h}}\simeq16$ (launches per star per $6$ h), and a background-free null search limits $\Gamma_{6{\rm h}}\lesssim49$, three times that rate. In practice the threshold is set by PSF-coincident stellar and instrumental transients. If intensity-limited optical beamers are commonly employed in our Galaxy, this activity could be revealed by Roman, Rubin and Euclid at no additional observing cost.