STONKS first results: Long-term transients in the XMM-Newton Galactic plane survey

¹ Institute for Research in Astrophysics and Planetology (IRAP), CNRS, Toulouse 31400, 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 INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
⁴ Max-Planck Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
⁵ Como Lake Center for Astrophysics (CLAP), DiSAT, Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
⁶ Inter-University Centre for Astronomy and Astrophysics (IUCAA), Ganeshkhind, Pune 411007, India
⁷ National Science Foundation Fellow, Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
⁸ Department of Astronomy, University of Illinois, 1002 W. Green St., Urbana, IL 61801, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 21 October 2025
Accepted: 1 December 2025

Abstract

Context. The study of astronomical transients at high energies provides insights into some of the most extreme physical events in the universe; however, carrying out their detection and fast follow-up studies are limited by instrumental constraints. Search for Transient Object in New observations using Known Sources (STONKS) is a near-real-time transient detection system for XMM-Newton offering the capability to detect transients in XMM-Newton observations at fainter fluxes than can be achieved with wide survey instruments.

Aims. We present the transients detected with the STONKS pipeline found in an XMM-Newton multi-year heritage survey of the Galactic plane to identify and classify highly variable X-ray sources that have recently been reported in this region.

Methods. We examined the alerts created by the STONKS pipeline from over 200 XMM observations of the Galactic plane, screening for instrumental effects. The 78 alerts associated with 70 real astrophysical sources were then subjected to further temporal and spectral analysis.

Results. From the 70 sources we identified, we were able to classify 32 with a high degree of confidence, including 7 X-ray binaries, 1 γ-Cas analogue, and 1 magnetar candidate. Of the 70 sources, 23 were detected for the first time in X-rays.

Conclusions. This systematic analysis of publicly available data has shown the value and potential of STONKS in the application to XMM-Newton observations. It will enable the community to detect transient and highly variable sources at fainter fluxes than with any other X-ray transient detection systems.