Design and Operation of the ATLAS Transient Science Server

Abstract

The Asteroid Terrestrial impact Last Alert System (ATLAS) system consists of two 0.5 m Schmidt telescopes with cameras covering 29 square degrees at plate scale of 1.86 arcsec per pixel. Working in tandem, the telescopes routinely survey the whole sky visible from Hawaii (above δ > − 50 ° ) every two nights, exposing four times per night, typically reaching o < 19 magnitude per exposure when the moon is illuminated and c < 19.5 magnitude per exposure in dark skies. Construction is underway of two further units to be sited in Chile and South Africa which will result in an all-sky daily cadence from 2021. Initially designed for detecting potentially hazardous near earth objects, the ATLAS data enable a range of astrophysical time domain science. To extract transients from the data stream requires a computing system to process the data, assimilate detections in time and space and associate them with known astrophysical sources. Here we describe the hardware and software infrastructure to produce a stream of clean, real, astrophysical transients in real time. This involves machine learning and boosted decision tree algorithms to identify extragalactic and Galactic transients. Typically we detect 10–15 supernova candidates per night which we immediately announce publicly. The ATLAS discoveries not only enable rapid follow-up of interesting sources but will provide complete statistical samples within the local volume of 100 Mpc. A simple comparison of the detected supernova rate within 100 Mpc, with no corrections for completeness, is already significantly higher (factor 1.5 to 2) than the current accepted rates.

PS15cey and PS17cke: prospective candidates from the Pan-STARRS Search for kilonovae

“…We also use ATLAS to search for transients within a fixed local volume (Smith et al 2020) to shallower depths than Pan-STARRS but with significantly higher cadence. ATLAS is a twin 0.5-m telescope system with units based on Haleakala and Mauna Loa, Hawaii (Tonry et al 2018).…”

“…The survey images are processed as described by Tonry et al (2018) and are photometrically and astrometrically calibrated immediately. Each image has a deep reference sky image subtracted and transient sources detected as described in Smith et al (2020). We make use of ATLAS data to supplement these two Pan-STARRS discoveries.…”

“…We have also forced photometry in all difference images made by the ATLAS survey (Tonry et al 2018) to check for any outbursting signature at this sky position. As described by Smith et al (2020), ATLAS typically observes with a sequence of 4 × 30 s exposures every two nights, with gaps due to weather, with difference images of detections of positive sources being carried out automatically. We forced photometric PSF measurements at the position of PS17cke and found no significant flux in the history of ATLAS imaging.…”

“…As discussed in previous sections, we have forced photometry throughout the history of Pan-STARRS observations between MJD 57805 and MJD 58879 and find no significant flux excess above 21st magnitude. For more complete temporal coverage and a search for activity at the position of PS17cke, we have analysed all ATLAS data at this position (Tonry et al 2018;Smith et al 2020). We have generated forced photometry at the location of PS17cke from the ATLAS images and manually inspected all exposures for the duration of its survey.…”

“…The kilonovae candidates presented in this paper represent examples of the kind of objects we are searching for in the Local Universe independent of GW triggers within the PSSK (Smartt et al 2019) and the ATLAS volume limited survey within 100 Mpc (Smith et al 2020). This may be a fruitful search for degenerate mergers in the gap between the LIGO-Virgo Consortium observing runs of O3 and O4 and may also discover kilonovae beyond the horizon distances of the GW detectors or in periods when they are not locked and observing.…”