The Deeper, Wider, Faster programme: exploring stellar flare activity with deep, fast cadenced DECam imaging via machine learning

Abstract

We present our 500 pc distance-limited study of stellar flares using the Dark Energy Camera as part of the Deeper, Wider, Faster Program. The data was collected via continuous 20-second cadence g band imaging and we identify 19,914 sources with precise distances from Gaia DR2 within twelve, ∼3 square-degree, fields over a range of Galactic latitudes. An average of ∼74 minutes is spent on each field per visit. All light curves were accessed through a novel unsupervised machine learning technique designed for anomaly detection. We identify 96 flare events occurring across 80 stars, the majority of which are M dwarfs. Integrated flare energies range from ∼1031 – 1037 erg, with a proportional relationship existing between increased flare energy with increased distance from the Galactic plane, representative of stellar age leading to declining yet more energetic flare events. In agreement with previous studies we observe an increase in flaring fraction from M0 – M6 spectral types. Furthermore, we find a decrease in the flaring fraction of stars as vertical distance from the galactic plane is increased, with a steep decline present around ∼100 pc. We find that $\sim 70{{\ \rm per\ cent}}$ of identified flares occur on short timescales of < 8 minutes. Finally we present our associated flare rates, finding a volumetric rate of 2.9 ± 0.3 × 10−6 flares pc−3 hr−1.

Constraints on Stellar Flare Energy Ratios in the NUV and Optical from a Multiwavelength Study of GALEX and Kepler Flare Stars

“…The longest duration Kepler longcadence flare lasts 260 minutes, and the shortest GALEX flare detected spans only 30 s. The short-cadence Kepler flares noted in Namekata et al (2017) and displayed in Figure 15 span the range of durations between these two extremes. We note that a similar plot of flare duration versus energy for the shortduration optical flares in the Webb et al (2021) sample (their Figure 8) shows a similar behavior as the Kepler short-cadence flare characteristics. This evidence supports a continuum of flare durations between a few tens of seconds all the way up to several hours.…”

“…The motivating paper for the current study, Brasseur et al (2019), examined a population of short-duration flares in the NUV using GALEX data at 10 s cadence. Webb et al (2021) used 20 s cadenced optical imaging to study stellar flares within 500 pc across 12 fields of observations with the Dark Energy Camera, and found that the majority of flares occur on timescales less than 8 minutes, with a range of optical flare enhancements ranging from 0.1-1.8 mag. Howard & MacGregor (2021) reported on flare light-curve profiles seen in the TESS bandpass at 20 s cadence, finding that roughly half of the large flares studied exhibited substructure in the rise phase of the flares.…”

“…Optical wavelengths are dominated by emission from the photosphere, with flaring contributions added on top of that. Recently, Webb et al (2021) reported on a 500 pc distance-limited sample of stellar flaring using 20 s cadence imaging at the g band (with 74 minutes on each field) as part of the Deeper, Wider, Faster flare search. This search is the most analogous in terms of cadence and time coverage to the NUV study, which forms the basis of the present project.…”

“…This search is the most analogous in terms of cadence and time coverage to the NUV study, which forms the basis of the present project. Webb et al (2021) found 80 flaring stars out of 19,914 searched, for an incidence of 0.4%, and noted the large preponderance of M dwarfs among their flaring stellar sample. Webb et al's (2021) study demonstrates that fast optical flares do occur.…”

“…Webb et al (2021) found 80 flaring stars out of 19,914 searched, for an incidence of 0.4%, and noted the large preponderance of M dwarfs among their flaring stellar sample. Webb et al's (2021) study demonstrates that fast optical flares do occur. The flaring percentage noted in the NUV is higher than this by about an order of magnitude.…”