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

Abstract

We present a search for fast optical transients (τ ∼ 0.5 hr − 1 d) using repeated observations of the Pan-STARRS1 Medium-Deep Survey (PS1/MDS) fields. Our search takes advantage of the consecutive g P1 r P1 observations (16.5 min 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 8 transients lack quiescent counterparts, exhibit mild but significant astrometric shifts between the g P1 and r P1 images, colors of (g − r) P1 ≈ 0.5 − 0.8 mag, non-varying light curves, and locations near the ecliptic plane with solar elongations of about 130 deg, 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 FOT (τ ∼ 0.5 hr) 0.12 deg −2 d −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 ∼ day is R FOT 2.4 × 10 −3 deg −2 d −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 nonrelativistic events are limited to M ≈ −10 mag (M ≈ −14 mag) for a timescale of ∼ 0.5 hr (∼ day), while relativistic sources (e.g., gamma-ray bursts, magnetar-powered transients) can reach much larger luminosities. The resulting volumetric rates are 13 Mpc −3 yr −1 (M ≈ −10 mag), 0.05 Mpc −3 yr −1 (M ≈ −14 mag) and 10 −6 Mpc −3 yr −1 (M ≈ −24 mag), significantly above the nova, supernova, and GRB 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 LSST 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.

Rapidly Evolving and Luminous Transients From Pan-Starrs1

“…The transient must be present (>3 σ ) in at least three sequential observations. This criteria selects a unique population of transients than those discussed in Berger et al (2013; which possess timescales shorter than 1 day).…”

iPTF Archival Search for Fast Optical Transients

“…To determine spectral type, we fit the spectra using the PyHammer software package (Kesseli et al 2017). When a spectrum was not available, we used the quiescent Pan-STARRS colors and the relations in West et al (2011) andBerger et al (2013).…”

“…The dominant population of fast optical transients (FOTs) is flares from Galactic low-mass main sequence stars, particularly M dwarfs (e.g., Kulkarni & Rau 2006;Rau et al 2008;Berger et al 2013). These flares are thought to arise from magnetic reconnection events in convective envelopes.…”

An Empirical Limit on the Kilonova Rate from the DLT40 One Day Cadence Supernova Survey

“…Some progress has been made by using theoretical kilonova light curves to calculate the upper limit of their rate in programs like the Dark Energy Survey (Doctor et al 2017). Additionally, there have been several recent attempts to estimate the rate of fast optical transients that evolve on sub-day timescales (τ ∼ 0.5 hours to 1 day ;Berger et al 2013) all the way up to ∼ 10 day timescales .…”