A New Method for Robust High-Precision Time-Series Photometry From Well-Sampled Images: Application to Archival MMT/Megacam Observations of the Open Cluster M37

Abstract

We introduce new methods for robust high-precision photometry from well-sampled images of a non-crowded field with a strongly varying point-spread function. For this work, we used archival imaging data of the open cluster M37 taken by MMT 6.5m telescope. We find that the archival light curves from the original image subtraction procedure exhibit many unusual outliers, and more than 20% of data get rejected by the simple filtering algorithm adopted by early analysis. In order to achieve better photometric precisions and also to utilize all available data, the entire imaging database was re-analyzed with our time-series photometry technique (Multi-aperture Indexing Photometry) and a set of sophisticated calibration procedures. The merit of this approach is as follows: we find an optimal aperture for each star with a maximum signal-to-noise ratio, and also treat peculiar situations where photometry returns misleading information with more optimal photometric index. We also adopt photometric de-trending based on a hierarchical clustering method, which is a very useful tool in removing systematics from light curves. Our method removes systematic variations that are shared by light curves of nearby stars, while true variabilities are preserved. Consequently, our method utilizes nearly 100% of available data and reduce the rms scatter several times smaller than archival light curves for brighter stars. This new data set gives a rare opportunity to explore different types of variability of short (∼minutes) and long (∼1 month) time scales in open cluster stars.

Photometric Study on Stellar Magnetic Activity. I. Flare Variability of Red Dwarf Stars in the Open Cluster M37

“…The archive contains almost 5000 images obtained in r ′ -band over one-month period. We refer the reader to Chang et al (2015a) for a detailed description of our new photometric reduction and light curve production. New light curves allow the analysis of brief transients such as flares.…”

A PSF-based Approach to TESS High quality data Of Stellar clusters (PATHOS) – II. Search for exoplanets in open clusters of the Southern ecliptic hemisphere and their frequency

“…In the last years, many approaches have been developed for the extraction of high precision light curves of stars in crowded regions, based on the use of the Point Spread Functions (PSFs, Nardiello et al 2015a;Libralato et al 2016a) or the Difference Imaging Analysis (Alard & Lupton 1998;Chang et al 2015;Soares-Furtado et al 2017;Bouma et al 2019;Wallace et al 2019;Soares-Furtado et al 2020). In the first paper of this series we presented our project "A PSF-based Approach to TESS High Quality data Of Stellar clusters" (PATHOS, Nardiello et al 2019, hereafter Paper I), aimed at the extraction of high precision light curves for stars in stellar clusters (open and globular clusters) and young associations.…”

New Photometric Pipeline to Explore Temporal and Spatial Variability with KMTNet DEEP-South Observations

“…We utilize the multi-aperture photometry algorithm (Chang et al 2015) in order to: (i) determine the optimal aperture for each object at each epoch, (ii) use an empirical aperture-growth-curve method for flux correction, and (iii) add a new tag that isolates peculiar situations where photometry returns improper measurements. For each frame, we measure photometry in a series of circular apertures (up to 12-pixel radius) without changing the sky annulus, and we use the last aperture (corresponding to 12-pixel radii) as the reference for aperture corrections.…”

“…For each frame, we measure photometry in a series of circular apertures (up to 12-pixel radius) without changing the sky annulus, and we use the last aperture (corresponding to 12-pixel radii) as the reference for aperture corrections. We refer the reader to Chang et al 2015 for details of the photometric performance of the pipeline. The position-dependent PSF variation and pixel-scale variation across fields for given focus result in up to 10% changes in magnitude, depending on the selected aperture size for photometry.…”