A search for transient, monochromatic light from the Galactic plane

The Galactic plane was searched for transient, monochromatic light at optical and near-IR wavelengths to detect pulses shorter than 1 s. An objective-prism Schmidt telescope of 0.28-m aperture and a CMOS camera were used to observe 973 square deg, with 8864 exposures of 1-s each, within a strip 2.1 deg wide along the Galactic plane, from Galactic longitude -4 deg to +248 deg. All exposures were analysed for transient, monochromatic sources using a 'difference image' algorithm that yielded 11 candidate sources. All 11 sources were found to be associated with either astrophysical emission-line objects or aircraft with subsecond blinking lights. Our survey 'rediscovered' many Wolf-Rayet stars, M dwarf flare stars, and planetary nebulae. It also identified an aircraft, of unknown type, that apparently had a nearly monochromatic lamp and a xenon lamp. This survey would have revealed optical and near-IR pulses having a power of similar to 180 GW (wavelength dependent) if emitted by a 10-m aperture laser located 1 kiloparsec away. These non-detections of laser pulses from the Galactic plane, including a 10-deg region towards the Galactic centre, add to the non-detections from more than 5000 nearby stars. Indeed, all-sky surveys for emission-line objects (e.g. ionized gas, supernovae remnants, and active galactic nuclei) would have revealed lasers of a wide range of average brightness, wavelength, and cadence. The absence of beacons reveals more of a SETI desert, notably at the intensely surveyed optical and radio wavelengths.