The SDSS image survey worked in drift scan mode, opening its shutter for extended periods and imaging a continuous strip of the sky. Objects with large variabilities within 1 minutes show extreme colors. In this project, we search for fast optical transients at the time scale of about one minute in SDSS photometry objects.
One class of event with fast and large variability is white dwarfs eclipsing by a companion (such as M dwarf, brown dwarf, white dwarf, or planets). Especially, earth-like planets in the habitable zone of a white dwarf have transit time of about 2 minutes. We combine the Gaia, PS1, and SDSS data and search for eclipsing events for a large sample of white dwarfs. We find a few fast eclipsing events. This can be used to constrain the frequency of earth-like planets around white dwarfs, or the frequency of accretion compact binaries.
Among the few fast eclipling events around white dwarf, we followed up two objects in more detail.
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SDSS J040749.30-000716.6, The eclipse duration is about 1 minute. The object is more than 1.5 magnitudes fainter during eclipse. The spectra (in spectra folder) shows strong double-peaked HeI absorption lines, indicating it is an AM CVn system. Our imaging follow-up shows it has an orbital period of 35 minutes. This object is among the very few eclipsing AM CVn systems in which the mass and radius of the binary can be accurately measured (e.g. Gaia14aae, Green et al. 2018).
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SDSS J021737.54+024850.0, The eclipse duration is about 5 minutes. The object is more than 2 magnitudes fainter during eclipse. The orbital period of the binary is between 1.4 to 48 hours, and probably a few hours based on Poisson probability .. The spectra shows broad H Balmer absorption lines and double-peaked H Balmer emission lines on top of the absorption lines. It is likely a system in which the white dwarf is accreting from a late-type M-dwarf or brown dwarf.
Another class of event with fast and large variability is the optical afterglow of Gama-ray bursts. We didn't find any candidate events.