The EXOD search for faint transients in XMM-Newton observations

Part II

¹ Institute for Research in Astrophysics and Planetology (IRAP), CNRS, 31400 Toulouse, France
² European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
³ Astronomical Institute, Academy of Sciences, Boční II 1401, 14131 Prague, Czech Republic
⁴ Jodrell Bank Centre for Astrophysics, University of Manchester, Oxford Road, Manchester M13 9PL, UK
⁵ Telespazio France, Space Systems and Operations (SSO), 31110 Toulouse, France
⁶ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁷ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France

^* Corresponding author.

Received: 31 December 2024
Accepted: 12 March 2025

Abstract

Context. The XMM-Newton observatory has accumulated a vast archive of over 17 000 X-ray observations over the last 25 years. However, the standard data processing pipelines may fail to detect certain types of transient X-ray sources, due to their short-lived or dim nature. Identifying these transient sources is important for understanding the full range of temporal X-ray behaviour, as well as understanding the types of sources that could be routinely detected by future missions such as Athena.

Aims. The aim of this work is to reprocess XMM-Newton archival observations using newly developed dedicated software in order to identify neglected and missed transient X-ray sources that were not detected by the existing pipeline.

Methods. We used a new approach that builds upon previous methodologies, by transforming event lists into data cubes, which are then searched for transient variability in short time windows. Our method enhances the detection capabilities in the Poisson regime by accounting for the statistical properties of sparse count rates, and allowing the search for transients in previously discarded periods of high background activity.

Results. Our reprocessing efforts identified 32 247 variable sources at the three-sigma level and 4083 sources at the five-sigma level in 12 926 XMM archival observations. We highlight four noteworthy sources: a candidate quasi-periodic eruption (QPE), a new magnetar candidate, a previously undetected Galactic hard X-ray burst, and a possible X-ray counterpart to a Galactic radio pulsar.

Conclusions. Our method demonstrates a new, fast, and effective way to process event list data from XMM-Newton, which is efficient in finding rapid outburst-like or eclipsing behaviour. This technique can be adapted for use with future telescopes, such as Athena, and can be generalised to other photon counting instruments operating in the low-count Poisson regime.