The EXTraS project: Exploring the X-ray transient and variable sky

¹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, Via A. Corti 12, 20133 Milano, Italy
e-mail: andrea.deluca@inaf.it
² INFN, Sezione di Pavia, via A. Bassi 6, 27100 Pavia, Italy
³ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
⁴ CNR, Istituto di Elettronica e di Ingegneria dell’Informazione e delle Telecomunicazioni, via de Marini 6, 16149 Genova, Italy
⁵ INAF, Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, Italy
⁶ School of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK
⁷ Scuola Universitaria Superiore IUSS Pavia, Piazza della Vittoria 15, 27100 Pavia, Italy
⁸ European Space Astronomy Center (ESA/ESAC), Operations Department, Vilanueva de la Cañada, 28692 Madrid, Spain
⁹ Department of Physics, Stanford University, 382 via Pueblo Mall, Stanford, CA 94305-4013, USA
¹⁰ Dr. Karl Remeis-Observatory and Erlangen Centre for Astroparticle Physics, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
¹¹ INAF, Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
¹² Institut für Astronomie & Astrophysik, Eberhard-Karls Universität Tübingen, Sand 1, 72076 Tübingen, Germany
¹³ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹⁴ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

Received: 29 October 2020
Accepted: 15 March 2021

Abstract

Temporal variability in flux and spectral shape is ubiquitous in the X-ray sky and carries crucial information about the nature and emission physics of the sources. The EPIC instrument on board the XMM-Newton observatory is the most powerful tool for studying variability even in faint sources. Each day, it collects a large amount of information about hundreds of new serendipitous sources, but the resulting huge (and growing) dataset is largely unexplored in the time domain. The project called Exploring the X-ray transient and variable sky (EXTraS) systematically extracted all temporal domain information in the XMM-Newton archive. This included a search and characterisation of variability, both periodic and aperiodic, in hundreds of thousands of sources spanning more than eight orders of magnitude in timescale and six orders of magnitude in flux, and a search for fast transients that were missed by standard image analysis. All results, products, and software tools have been released to the community in a public archive. A science gateway has also been implemented to allow users to run the EXTraS analysis remotely on recent XMM datasets. We give details on the new algorithms that were designed and implemented to perform all steps of EPIC data analysis, including data preparation, source and background modelling, generation of time series and power spectra, and search for and characterisation of different types of variabilities. We describe our results and products and give information about their basic statistical properties and advice on their usage. We also describe available online resources. The EXTraS database of results and its ancillary products is a rich resource for any kind of investigation in almost all fields of astrophysics. Algorithms and lessons learnt from our project are also a very useful reference for any current and future experiment in the time domain.