TFAW survey - I. Wavelet-based denoising of K2 light curves. Discovery and validation of two new Earth-sized planets in K2 campaign 1

The wavelet-based detrending and denoising method TFAW is applied for the first time to EVEREST 2.0-corrected light curves to further improve the photometric precision of almost all K2 observing campaigns (C1-C8, C12-C18). The performance of both methods is evaluated in terms of 6 h combined differential photometric precision (CDPP), simulated transit detection efficiency, and planet characterization in different SNR regimes. On average, TFAW median 6 h CDPP is ∼30percent better than the one achieved by EVEREST 2.0 for all observing campaigns. Using the TRANSIT LEAST-SQUARES (TLS) algorithm, we show that the transit detection efficiency for simulated Earth-Sun-like systems is ∼8.5× higher for TFAW-corrected light curves than that for EVEREST 2.0 ones. Using the light curves of two confirmed exoplanets, K2-44 b (high SNR) and K2-298 b (low SNR), we show that TFAW yields better Markov chain Monte Carlo posterior distributions, transit parameters compatible with the catalogued ones but with smaller uncertainties, and narrows the credibility intervals. We use the combination of TFAW's improved photometric precision and TLS enhancement of the signal detection efficiency for weak signals to search for new transit candidates in K2 observing campaign 1. We report the discovery of two new K2-C1 Earth-sized planets statistically validated, using the VESPA software: EPIC 201170410.02, with a radius of 1.047+0.276−0.257R⊕ planet orbiting an M-type star, and EPIC 201757695.02, with a radius of 0.908+0.059−0.064R⊕ planet orbiting a K-type star. EPIC 201757695.02 is the 9th smallest planet ever discovered in K2-C1, and the 39th smallest in all K2 campaigns

TFAW: Wavelet-based signal reconstruction to reduce photometric noise in time-domain surveys

Context. There have been many efforts to correct systematic effects in astronomical light curves to improve the detection and characterization of planetary transits and astrophysical variability. Algorithms such as the trend filtering algorithm (TFA) use simultaneously-observed stars to measure and remove systematic effects, and binning is used to reduce high-frequency random noise. Aims: We present TFAW, a wavelet-based modified version of TFA. First, TFAW aims to increase the periodic signal detection and second, to return a detrended and denoised signal without modifying its intrinsic characteristics. Methods: We modified TFA's frequency analysis step adding a stationary wavelet transform filter to perform an initial noise and outlier removal and increase the detection of variable signals. A wavelet-based filter was added to TFA's signal reconstruction to perform an adaptive characterization of the noise- and trend-free signal and the underlying noise contribution at each iteration while preserving astrophysical signals. We carried out tests over simulated sinusoidal and transit-like signals to assess the effectiveness of the method and applied TFAW to real light curves from TFRM. We also studied TFAW's application to simulated multiperiodic signals. Results: TFAW improves the signal detection rate by increasing the signal detection efficiency (SDE) up to a factor ̃2.5× for low S/R light curves. For simulated transits, the transit detection rate improves by a factor ̃2 - 5× in the low-S/R regime compared to TFA. TFAW signal approximation performs up to a factor ̃2× better than bin averaging for planetary transits. The standard deviations of simulated and real TFAW light curves are ̃40% better compared to TFA. TFAW yields better MCMC posterior distributions and returns lower uncertainties, less biased transit parameters and narrower (by approximately ten times) credibility intervals for simulated transits. TFAW is also able to improve the characterization of multiperiodic signals. We present a newly-discovered variable star from TFRM

Robotilter: an automated lens/CCD alignment system for the Evryscope

Camera lenses are increasingly used in wide-field astronomical surveys due to their high performance, wide field-of-view (FOV) unreachable from traditional telescope optics, and modest cost. The machining and assembly tolerances for commercially available optical systems cause a slight misalignment (tilt) between the lens and CCD, resulting in point spread function (PSF) degradation. We have built an automated alignment system (Robotilters) to solve this challenge, optimizing four degrees of freedom¿two tilt axes, a separation axis (the distance between the CCD and lens), and the lens focus (the built-in focus of the lens by turning the lens focusing ring, which moves the optical elements relative to one another) in a compact and low-cost package. The Robotilters remove tilt and optimize focus at the sub-10-μm level, are completely automated, take ≈2 h to run, and remain stable for multiple years once aligned. The Robotilters were built for the Evryscope telescope (a 780-MPix 22-camera array with an 8150-sq. deg FOV and continuous 2-min cadence) designed to detect short-timescale events across extremely large sky areas simultaneously. Variance in quality across the image field, especially the corners and edges compared to the center, is a significant challenge in wide-field astronomical surveys like the Evryscope. The individual star PSFs (which typically extend only a few pixels) are highly susceptible to slight increases in optical aberrations in this situation. The Robotilter solution resulted in a limiting magnitude improvement of 0.5 mag in the center of the image and 1.0 mag in the corners for typical Evryscope cameras, with less distorted and smaller PSFs (half the extent in the corners and edges in many cases). We describe the Robotilter mechanical and software design, camera alignment results, long-term stability, and image improvement. The potential for general use in wide-field astronomical surveys is also explored

Evryscope and K2 Constraints on TRAPPIST-1 Superflare Occurrence and Planetary Habitability

The nearby ultracool dwarf TRAPPIST-1 possesses several Earth-sized terrestrial planets, three of which have equilibrium temperatures that may support liquid surface water, making it a compelling target for exoplanet characterization. TRAPPIST-1 is an active star with frequent flaring, with implications for the habitability of its planets. Superflares (stellar flares whose energy exceeds 1033 erg) can completely destroy the atmospheres of a cool star's planets, allowing ultraviolet radiation and high-energy particles to bombard their surfaces. However, ultracool dwarfs emit little ultraviolet flux when quiescent, raising the possibility of frequent flares being necessary for prebiotic chemistry that requires ultraviolet light. We combine Evryscope and Kepler observations to characterize the high-energy flare rate of TRAPPIST-1. The Evryscope is an array of 22 small telescopes imaging the entire Southern sky in g' every two minutes. Evryscope observations, spanning 170 nights over 2 yr, complement the 80 day continuous short-cadence K2 observations by sampling TRAPPIST-1's long-term flare activity. We update TRAPPIST-1's superflare rate, finding a cumulative rate of 4.2−0.2+1.9 superflares per year. We calculate the flare rate necessary to deplete ozone in the habitable-zone planets' atmospheres, and find that TRAPPIST-1's flare rate is insufficient to deplete ozone if present on its planets. In addition, we calculate the flare rate needed to provide enough ultraviolet flux to power prebiotic chemistry. We find TRAPPIST-1's flare rate is likely insufficient to catalyze some of the Earthlike chemical pathways thought to lead to ribonucleic acid synthesis, and flux due to flares in the biologically relevant UV-B band is orders of magnitude less for any TRAPPIST-1 planet than has been experienced by Earth at any time in its history

Building the Evryscope: Hardware Design and Performance

The Evryscope is a telescope array designed to open a new parameter space in optical astronomy, detecting short-timescale events across extremely large sky areas simultaneously. The system consists of a 780 MPix 22-camera array with an 8150 sq. deg. field of view, 13″ per pixel sampling, and the ability to detect objects down to {m}g\prime ≃ 16 in each 2-minute dark-sky exposure. The Evryscope, covering 18,400 sq. deg. with hours of high-cadence exposure time each night, is designed to find the rare events that require all-sky monitoring, including transiting exoplanets around exotic stars like white dwarfs and hot subdwarfs, stellar activity of all types within our galaxy, nearby supernovae, and other transient events such as gamma-ray bursts and gravitational-wave electromagnetic counterparts. The system averages 5000 images per night with ∼300,000 sources per image, and to date has taken over 3.0M images, totaling 250 TB of raw data. The resulting light curve database has light curves for 9.3M targets, averaging 32,600 epochs per target through 2018. This paper summarizes the hardware and performance of the Evryscope, including the lessons learned during telescope design, electronics design, a procedure for the precision polar alignment of mounts for Evryscope-like systems, robotic control and operations, and safety and performance-optimization systems. We measure the on-sky performance of the Evryscope, discuss its data analysis pipelines, and present some example variable star and eclipsing binary discoveries from the telescope. We also discuss new discoveries of very rare objects including two hot subdwarf eclipsing binaries with late M-dwarf secondaries (HW Vir systems), two white dwarf/hot subdwarf short-period binaries, and four hot subdwarf reflection binaries. We conclude with the status of our transit surveys, M-dwarf flare survey, and transient detection

EvryFlare. I. Long-term Evryscope Monitoring of Flares from the Cool Stars across Half the Southern Sky

We search for superflares from 4068 cool stars in 2+ yr of Evryscope photometry, focusing on those with high-cadence data from both Evryscope and the Transiting Exoplanet Survey Satellite (TESS). The Evryscope array of small telescopes observed 575 flares from 284 stars, with a median energy of 1034.0 erg. Since 2016, Evryscope has enabled the detection of rare events from all stars observed by TESS through multi-year, high-cadence continuous observing. We report around twice the previous largest number of 1034 erg high-cadence flares from nearby cool stars. We find eight flares with amplitudes of 3+ g' magnitudes, with the largest reaching 5.6 mag and releasing 1036.2 erg. We observe a 1034 erg superflare from TOI-455 (LTT 1445), a mid-M with a rocky planet candidate. We measure the superflare rate per flare-star and quantify the average flaring of active stars as a function of spectral type, including superflare rates, flare frequency distributions, and typical flare amplitudes in g'. We confirm superflare morphology is broadly consistent with magnetic reconnection. We estimate starspot coverage necessary to produce superflares, and hypothesize maximum allowed superflare energies and waiting times between flares corresponding to 100% coverage of the stellar hemisphere. We observe decreased flaring at high Galactic latitudes. We explore the effects of superflares on ozone loss to planetary atmospheres: we observe one superflare with sufficient energy to photodissociate all ozone in an Earth-like atmosphere in one event. We find 17 stars that may deplete an Earth-like atmosphere via repeated flaring. Of the 1822 stars around which TESS may discover temperate rocky planets, we observe 14.6% ± 2% emit large flares

EvryFlare. II. Rotation Periods of the Cool Flare Stars in TESS across Half the Southern Sky

We measure rotation periods and sinusoidal amplitudes in Evryscope light curves for 122 two-minute K5–M4 TESS targets selected for strong flaring. The Evryscope array of telescopes has observed all bright nearby stars in the south, producing 2-minute cadence light curves since 2016. Long-term, high-cadence observations of rotating flare stars probe the complex relationship between stellar rotation, starspots, and superflares. We detect periods from 0.3487 to 104 days and observe amplitudes from 0.008 to 0.216 g' mag. We find that the Evryscope amplitudes are larger than those in TESS with the effect correlated to stellar mass (p-value = 0.01). We compute the Rossby number (Ro) and find that our sample selected for flaring has twice as many intermediate rotators (0.04 Ro Ro Ro > 0.44) rotators; this may be astrophysical or a result of period detection sensitivity. We discover 30 fast, 59 intermediate, and 33 slow rotators. We measure a median starspot coverage of 13% of the stellar hemisphere and constrain the minimum magnetic field strength consistent with our flare energies and spot coverage to be 500 G, with later-type stars exhibiting lower values than earlier-type stars. We observe a possible change in superflare rates at intermediate periods. However, we do not conclusively confirm the increased activity of intermediate rotators seen in previous studies. We split all rotators at Ro ∼ 0.2 into bins of PRot PRot > 10 days to confirm that short-period rotators exhibit higher superflare rates, larger flare energies, and higher starspot coverage than do long-period rotators, at p-values of 3.2 × 10−5, 1.0 × 10−5, and 0.01, respectively

Variables in the Southern Polar Region Evryscope 2016 Data Set

The regions around the celestial poles offer the ability to find and characterize long-term variables from ground-based observatories. We used multi-year Evryscope data to search for high-amplitude (≈5% or greater) variable objects among 160,000 bright stars (mv σ limiting magnitude of g = 16 in dark time. In this study, covering all stars 9 M⊙) secondaries with K-dwarf primaries, strong candidates for precision mass–radius measurements

TFAW survey II: six newly validated planets and 13 planet candidates from K2

Searching for Earth-sized planets in data from Kepler's extended mission (K2) is a niche that still remains to be fully exploited. The TFAW survey is an ongoing project that aims to re-analyse all light curves in K2 C1-C8 and C12-C18 campaigns with a wavelet-based detrending and denoising method, and the period search algorithm TLS to search for new transit candidates not detected in previous works. We have analysed a first subset of 24 candidate planetary systems around relatively faint host stars (10.9 < Kp < 15.4) to allow for follow-up speckle imaging observations. Using vespa and TRICERATOPS, we statistically validate six candidates orbiting four unique host stars by obtaining false-positive probabilities smaller than 1 per cent with both methods. We also present 13 vetted planet candidates that might benefit from other, more precise follow-up observations. All of these planets are sub-Neptune-sized with two validated planets and three candidates with sub-Earth sizes, and have orbital periods between 0.81 and 23.98 d. Some interesting systems include two ultra-short-period planets, three multiplanetary systems, three sub-Neptunes that appear to be within the small planet Radius Gap, and two validated and one candidate sub-Earths (EPIC 210706310.01, K2-411 b, and K2-413 b) orbiting metal-poor stars

EvryFlare. III. Temperature Evolution and Habitability Impacts of Dozens of Superflares Observed Simultaneously by Evryscope and TESS

Superflares may provide the dominant source of biologically relevant UV radiation to rocky habitable-zone M-dwarf planets (M-Earths), altering planetary atmospheres and conditions for surface life. The combined line and continuum flare emission has usually been approximated by a 9000 K blackbody. If superflares are hotter, then the UV emission may be 10 times higher than predicted from the optical. However, it is unknown for how long M-dwarf superflares reach temperatures above 9000 K. Only a handful of M-dwarf superflares have been recorded with multiwavelength high-cadence observations. We double the total number of events in the literature using simultaneous Evryscope and Transiting Exoplanet Survey Satellite observations to provide the first systematic exploration of the temperature evolution of M-dwarf superflares. We also increase the number of superflaring M dwarfs with published time-resolved blackbody evolution by ∼10×. We measure temperatures at 2 minutes cadence for 42 superflares from 27 K5–M5 dwarfs. We find superflare peak temperatures (defined as the mean of temperatures corresponding to flare FWHM) increase with flare energy and impulse. We find the amount of time flares emit at temperatures above 14,000 K depends on energy. We discover that 43% of the flares emit above 14,000 K, 23% emit above 20,000 K and 5% emit above 30,000 K. The largest and hottest flare briefly reached 42,000 K. Some do not reach 14,000 K. During superflares, we estimate M-Earths orbiting <200 Myr stars typically receive a top-of-atmosphere UV-C flux of ∼120 W m−2 and up to 103 W m−2, 100–1000 times the time-averaged X-ray and UV flux from Proxima Cen