The Atacama Cosmology Telescope: Systematic Transient Search of 3-Day Maps

TL;DR

This study conducts a systematic search for millimeter transients in three years of ACT data (2017–2019) using 3-day sky maps across 77–277 GHz. The authors implement a multi-stage pipeline with S/N cuts, cross-array matching, geometric cleaning, mean-flux filtering, forced photometry, and visual inspection to produce 29 transient detections, of which 8 are asteroids, 3 are known, and 14 are new events. Spectral analyses reveal two classes: flat/falling spectra consistent with synchrotron emission from stellar activity and at least one rising-spectrum event indicating thermal processes; most transients associate with rotating variable or cool stars. The work showcases a workable framework for time-domain millimeter astronomy with ACT data and points to future improvements using dedicated transient maps and larger datasets to better characterize event rates for next-generation surveys such as the Simons Observatory and CMB-S4.

Abstract

We conduct a systematic search for transients in three years of data (2017-2019) from the Atacama Cosmology Telescope (ACT). ACT covers 40 percent of the sky at three bands spanning from 77 GHz to 277 GHz. Analysis of 3-day mean-subtracted sky maps, which were match-filtered for point sources, yielded 29 transients detections. Eight of these transients are due to known asteroids, and three others were previously published. Four of these events occur in areas of with poor noise models and thus we cannot be confident they are real transients. We are left with 14 new transient events occurring at 11 unique locations. All of these events are associated with either rotationally variable stars or cool stars. Ten events have flat or falling spectra indicating radiation from synchrotron emission. One event has a rising spectrum indicating a different engine for the flare.

Figures (8)

• Figure 1: Process of the initial detection, with each plot showing a 0.3 deg by 0.3 deg map. The first step is to make a mask (middle) on $S/N$ map (left) selecting pixels that have $S/N >5$. The mask is then applied to the flux map (right), and the candidate position, shown as the red cross mark, is evaluated as the center of mass weighted by the flux values within the selected pixels.
• Figure 2: Left: A histogram of the distance to each detection's nearest neighbor with a binsize of 5 arcminutes. The peak close to zero indicates there are clusters of spurious detections in many of the maps so we cut any candidate with a nearest neighbor of 20 arcminutes or less. Center: A histogram of each candidate's distance from the nearest zero variance contour, defined to be a variance of less than $1.5\times10^{-5} K$, with a bin size of 3 pixels. There is a large peak of candidates near zero variance contours which drops after 3 pixels. We mask out to 5 pixels, cutting $74\%$ of all sources. Right: A histogram of each candidate's distance from the map edge in pixel units with a bin size of 3 pixels. As expected, there is an excess number of candidates near the edge of the map as the map edges are noisy and so appear variable when sampled every three days. At a mask size of 5 pixels, we cut off the peak of candidates near the edge.
• Figure 3: 10$\hbox{$^\prime$}$x10$\hbox{$^\prime$}$ 3-day thumbnail maps for each transient. The upper row is the intensity map with $\pm$5000 $\mu$K color range. The bottom row is the $S/N$ map after applying a matched filter, with $\pm$5 color range. Due to the conjugate gradient iteration used to solve the maximum-likelihood map-making equation only being run for 10 steps, these maps are effectively mildly highpass filtered. The affected scales have negligible weight in the matched filter. Events 2, 9, 10, 13 at the bottom of the table are the four events that are difficult to determine if they are real transients.
• Figure 4: Left: The three transient events consistent with Hygiea observations. Right: The five transient events consistent with Davida observations. All of the plotted positions and observation times are consistent with the asteroids' paths. The position errors are on the order of 0.1 arcminutes.
• Figure 5: Light curves for 18 detected transient events on day time scales from the peak. Each frequency is denoted by a different color. We see that in most cases the peak is correlated with all frequencies.