The Hubble Catalog of Variables (HCV)

TL;DR

The Hubble Catalog of Variables (HCV) identifies approximately 52,000 candidate variable sources from the Hubble Source Catalog, using a robust variability detection method, revealing diverse variable objects including stars and active galactic nuclei.

Contribution

This work introduces a new variability detection approach applied to the Hubble Source Catalog, resulting in a large catalog of candidate variables with validation through visual inspection.

Findings

About 52,000 candidate variables identified.

70% of multi-filter candidates are confirmed as true variables.

Candidates span magnitudes 15-27, including stars and AGN.

Abstract

The Hubble Source Catalog (HSC) combines lists of sources detected on images obtained with the WFPC2, ACS and WFC3 instruments aboard the Hubble Space Telescope (HST) available in the Hubble Legacy Archive. The catalog contains time-domain information with about two million of its sources detected with the same instrument and filter in at least five HST visits. The Hubble Catalog of Variables (HCV) project aims to identify HSC sources showing significant brightness variations. A magnitude-dependent threshold in the median absolute deviation of photometric measurements (an outlier-resistant measure of lightcurve scatter) is adopted as the variability-detection statistic. It is supplemented with a cut in $\chi_{\rm red}^2$ that removes sources with large photometric errors. A pre-processing procedure involving bad image identification, outlier rejection and computation of local magnitude zero-point corrections is applied to HSC lightcurves before computing the variability detection statistic. About 52000 HSC sources are identified as candidate variables, among which 7800 show variability in more than one filter. Visual inspection suggests that $\sim 70\%$ of the candidates detected in multiple filters are true variables while the remaining $\sim 30\%$ are sources with aperture photometry corrupted by blending, imaging artifacts or image processing anomalies. The candidate variables have AB magnitudes in the range 15-27$^{m}$ with the median 22$^{m}$. Among them are the stars in our own and nearby galaxies as well as active galactic nuclei.