Frequency Limits on Naked-Eye Optical Transients Lasting from Minutes to Years

TL;DR

This study uses all-sky monitoring data over three years to establish upper limits on the frequency of bright optical transients visible to the naked eye, spanning durations from minutes to years.

Contribution

It provides the first systematic constraints on the occurrence rates of bright optical transients across a wide range of durations using continuous all-sky observations.

Findings

Fewer than 0.0040 transients per sky at any time for minutes to hours durations.

Less than 160 transients per sky for months to years durations.

Fewer than 50 transients per sky for years to millennia durations.

Abstract

How often do bright optical transients occur on the sky but go unreported? To constrain the bright end of the astronomical transient function, a systematic search for transients that become bright enough to be noticed by the unaided eye was conducted using the all-sky monitors of the Night Sky Live network. Two fisheye continuous cameras (CONCAMs) operating over three years created a data base that was searched for transients that appeared in time-contiguous CCD frames. Although a single candidate transient was found (Nemiroff and Shamir 2006), the lack of more transients is used here to deduce upper limits to the general frequency of bright transients. To be detected, a transient must have increased by over three visual magnitudes to become brighter than visual magnitude 5.5 on the time scale of minutes to years. It is concluded that, on the average, fewer than 0.0040 ($t_{dur} / 60$ seconds) transients with duration $t_{dur}$ between minutes and hours, occur anywhere on the sky at any one time. For transients on the order of months to years, fewer than 160 ($t_{dur} / 1$ year) occur, while for transients on the order of years to millennia, fewer than 50 ($t_{dur}/1$ year)$^2$ occur.