A search for optical laser emission from Proxima Centauri

A search for laser light from Proxima Centauri was performed, including 107 high-resolution, optical spectra obtained between 2004 and 2019 with the HARPS spectrometer. The search for laser light involved rejecting sharp peaks in the spectrum from stellar flares, fluorescent city lights, and elementary particles that directly hit the CCD detector. The search revealed unexpected spectral 'combs' found at equally spaced frequencies, which were not known to the observer nor to the builder of the spectrometer. But they came from stray, optical ghosts of light originating with an interferometric etalon filter and its light source at the telescope. Future observers must be aware of this contamination. The 107 spectra of Proxima Centauri show no evidence of any laser emission nor technological signatures of any type. Of special interest are 29 spectra obtained between March and July 2019 when the candidate technological radio signal, BLC1, was captured by Breakthrough Listen. This search would have revealed laser light from Proxima Centauri pointed toward Earth if the laser had a power at least 20 to 120 kilowatts (depending on wavelength) and was positioned within the 1.3 au field of view, assuming a benchmark laser launcher having a 10-m aperture. Smaller lasers would also have been detected, but would require more power.