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

TL;DR

The STONKS pipeline enables near-real-time detection of long-term X-ray transients by comparing new XMM-Newton observations to archival data, with a variability threshold of $V \ge 5$ to suppress false alerts. Applied to a Galactic plane heritage survey (231 observations; 213 processed), it produced 78 true alerts across 70 sources, including 23 first-time X-ray detections. The study achieves firm classifications for 38 sources (e.g., 7 XRBs, 1 γ-Cas analogue, 1 magnetar candidate) and provisional classifications for 32 more, uncovering a magnetar candidate and several cataclysmic variables at greater distances than typical optical surveys. These results demonstrate STONKS’ potential to reveal highly variable, faint X-ray sources and to deliver valuable alerts to the community, greatly enriching the Galactic X-ray transient census. The work also highlights the role of multi-wavelength associations and spectral analyses in robust source typing, and points to broader implications for stellar evolution populations and accretion physics in compact objects.

Abstract

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. 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. 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. 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. 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.

Figures (15)

• Figure 1: Locations of alerts within Multi-Year Heritage Programme observations in galactic coordinates. Observations are presented as blue circles, with radius 15' as an approximation of the footprint of XMM observations and alert sources are indicated by red diamonds.
• Figure 2: X-ray fluxes of alert sources and apparent optical, NIR, and MIR magnitudes for the associated counterparts of these sources.
• Figure 3: Distributions of X-ray fluxes, distance estimates, and derived luminosities for STONKS alert sources. Panel (a): Comparison of flux distributions for sources with distance estimates (red), and without (blue). Panel (b): Distance estimates for the 35 sources where one is available. Panel (c): Distribution of distances and X-ray luminosities for these 35 sources. In panel (a) we also display the 10 ks sensitivity limits for source detection in some other X-ray transient detectors.
• Figure 4: Comparison of alerts and alert types in this work and as per Q24. The first column represents the alerts per observation, and the other four represent the proportion of alerts in each category. The black solid bar represents the results of Q24, while the red diagonally striped bar represents the total alerts and the blue crossed bar show the true alerts obtained during this work.
• Figure 5: Light curve for source 45, 4XJ1751-2759, during observation 0886121001. The solid line shows the background-subtracted countrate for photons in the energy range 0.2--12.0 keV. Figure adapted from webbe_4xmm_2025.