A search for fast optical transients in the Pan-STARRS1 medium-deep survey: M-dwarf flares, asteroids, limits on extragalactic rates, and implications for LSST
- Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
- Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)
- Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544 (United States)
- Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States)
- Department of Physics, Durham University, South Road, Durham DH1 3LE (United Kingdom)
- Department of Physics, Harvard University, Cambridge, MA 02138 (United States)
We present a search for fast optical transients (τ ∼ 0.5 hr-1 day) using repeated observations of the Pan-STARRS1 Medium-Deep Survey (PS1/MDS) fields. Our search takes advantage of the consecutive g {sub P1} r {sub P1} observations (16.5 minutes in each filter), by requiring detections in both bands, with non-detections on preceding and subsequent nights. We identify 19 transients brighter than 22.5 AB mag (S/N ≳ 10). Of these, 11 events exhibit quiescent counterparts in the deep PS1/MDS templates that we identify as M4-M9 dwarfs at d ≈ 0.2-1.2 kpc. The remaining eight transients lack quiescent counterparts, exhibit mild but significant astrometric shifts between the g {sub P1} and r {sub P1} images, colors of (g – r){sub P1} ≈ 0.5-0.8 mag, non-varying light curves, and locations near the ecliptic plane with solar elongations of about 130°, which are all indicative of main-belt asteroids near the stationary point of their orbits. With identifications for all 19 transients, we place an upper limit of R {sub FOT}(τ ∼ 0.5 hr) ≲ 0.12 deg{sup –2} day{sup –1} (95% confidence level) on the sky-projected rate of extragalactic fast transients at ≲ 22.5 mag, a factor of 30-50 times lower than previous limits; the limit for a timescale of ∼1 day is R {sub FOT} ≲ 2.4 × 10{sup –3} deg{sup –2} day{sup –1}. To convert these sky-projected rates to volumetric rates, we explore the expected peak luminosities of fast optical transients powered by various mechanisms, and find that non-relativistic events are limited to M ≈ –10 to ≈ – 14 mag for a timescale of ∼0.5 hr to ∼1 day, while relativistic sources (e.g., gamma-ray bursts, magnetar-powered transients) can reach much larger luminosities. The resulting volumetric rates are ≲ 13 Mpc{sup –3} yr{sup –1} (M ≈ –10 mag), ≲ 0.05 Mpc{sup –3} yr{sup –1} (M ≈ –14 mag), and ≲ 10{sup –6} Mpc{sup –3} yr{sup –1} (M ≈ –24 mag), significantly above the nova, supernova, and gamma-ray burst rates, respectively, indicating that much larger surveys are required to provide meaningful constraints. Motivated by the results of our search, we discuss strategies for identifying fast optical transients in the Large Synoptic Survey Telescope main survey, and reach the optimistic conclusion that the veil of foreground contaminants can be lifted with the survey data, without the need for expensive follow-up observations.
- OSTI ID:
- 22348555
- Journal Information:
- Astrophysical Journal, Vol. 779, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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