sci19002 — Announcement

Hubble Catalog of Variables to be released in September

15 May 2019

Antonella Nota (ESA/STScI) & Alceste Bonanos (NOA)

We are only a few months away from unveiling the results of four years of intense effort by astronomers from the National Observatory of Athens, sponsored by ESA and led by Alceste Bonanos. We will present the Hubble Catalog of Variables (HCV): the first full, homogeneous, catalogue of variable sources found in the Hubble Source Catalog (HSC), which is built out of publicly available images obtained with the WFPC2, ACS and WFC3 instruments. Hubble has been observing the sky for over 29 years and has visited some regions of the sky several times over its lifetime. This offers the opportunity to search for variable objects over a range of magnitudes that is difficult to reach with ground-based telescopes. The magnitude depth, along with the superb resolution achieved by Hubble and the long time-baseline of its operation, are the features that make such a variable source catalogue unique. The HSC has recently provided photometric measurements of all sources detected from a homogeneous reduction and analysis of Hubble archival images, thereby enabling such a variability search.

The HCV will be the deepest catalogue of variables available. It will include variable stars in our Galaxy and nearby galaxies, as well as transients and variable active galactic nuclei. In total, the HCV will include 84 428 candidate variable sources (out of 3.7 million HSC sources that were searched for variability) with V ≤ 27 mag; for 11 115 of them the variability is detected in more than one filter. The data points in a light curve range from 5 to over 80, the time baseline ranges from under a day to over 13 years, while ∼ 8% of variables have amplitudes in excess of 1 mag. Variable source identification in a large and inhomogeneous set of photometric measurements has been challenging. Bonanos and her collaborators adopted magnitude-dependent thresholding in median absolute deviation (a robust measure of light curve scatter) combined with sophisticated pre-processing techniques and visual quality control to identify and validate variable sources observed by Hubble five or more times with the same instrument/filter combination. Visual inspection performed on a subset of the candidate variables suggests that at least 80% of the candidate variables that passed an automated quality control are variables or probable variables, rather than spurious detections resulting from blending, residual cosmic rays or calibration errors.

We expect that the catalogue will be an extremely valuable resource for the community. Possible uses will include searches for new variable objects of a particular type for population analysis, detection of unique objects worthy of follow-up studies, identification of sources observed at other wavelengths and photometric characterisation of candidate progenitors of supernovae and other transients in nearby galaxies. The catalogue will be available to the community from the ESA Hubble Science Archive (eHST) at ESAC, and the Mikulski Archive for Space Telescopes (MAST) at STScI.

Stay tuned for the official release!


Antonella Nota
ESA HST Project Scientist, STScI

About the Announcement



Hubble image of variable star RS Puppis
Hubble image of variable star RS Puppis

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