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

This paper presents the first results of the STONKS pipeline, a near-real-time transient detection system for XMM-Newton, which successfully identified and classified 70 astrophysical sources—including 32 high-confidence transients and 23 X-ray detections for the first time—from over 200 Galactic plane observations, demonstrating its superior capability to detect faint, highly variable X-ray sources compared to other systems.

Original authors: Robbie Webbe, E. Quintin, N. A. Webb, Gabriele Ponti, Tong Bao, Chandreyee Maitra, Shifra Mandel, Samaresh Mondal

Published 2026-01-28

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CC BY 4.0

Original authors: Robbie Webbe, E. Quintin, N. A. Webb, Gabriele Ponti, Tong Bao, Chandreyee Maitra, Shifra Mandel, Samaresh Mondal

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the night sky as a giant, bustling city. Most of the time, the "stars" (the buildings) stay the same. But sometimes, a building suddenly lights up with a blinding flash, or a dark alley suddenly glows with a new neon sign. These are astronomical transients—cosmic events that change brightness over time. Finding them is like trying to spot a single flickering streetlight in a city that never sleeps, but most telescopes are either too wide-angle to see the dim ones or too slow to catch the quick flashes.

This paper introduces a new tool called STONKS (Search for Transient Object in New observations using Known Sources). Think of STONKS as a super-smart security guard for the XMM-Newton telescope, a powerful "eye" in space that looks at the Milky Way (our galaxy's disk).

Here is how the paper explains their work, broken down simply:

1. The Problem: The "Needle in a Haystack"

The Milky Way is crowded with stars. Most telescopes designed to find new, flashing lights (transients) have to look at huge areas of the sky quickly. Because they move so fast, they can only see the very brightest "streetlights." They miss the fainter, dimmer ones that are actually very interesting.

2. The Solution: STONKS

STONKS is a computer program that acts like a comparative detective.

How it works: Every time the XMM-Newton telescope takes a new picture of the galaxy, STONKS compares it to a massive library of old pictures taken by XMM-Newton and other telescopes over the years.
The Analogy: Imagine you have a photo album of your neighborhood from the last 20 years. If you take a new photo today and the computer instantly spots that a house you've never seen before has suddenly appeared, or an old house has suddenly doubled in brightness, STONKS raises an alarm.
The Advantage: Because XMM-Newton takes very long, detailed "portraits" of specific spots (unlike the wide-angle "panoramas" of other telescopes), STONKS can spot changes in objects that are much fainter than anything else can see.

3. The Investigation: Cleaning the Clutter

The researchers ran STONKS on over 200 observations of the galactic plane. The computer generated 142 "alarms." However, space is messy. Sometimes, bright stars just outside the camera's view create fake "ghost rings" that look like new sources, or the heavy dust in our galaxy tricks the computer into thinking a dim object is bright.

The team acted like art restorers, manually checking each alarm to remove the "ghosts" and false alarms.

Result: They filtered out 64 fake alerts.
The Real Deal: They were left with 78 true alerts linked to 70 real cosmic objects.

4. The Discoveries: What Did They Find?

Out of these 70 objects, the team successfully identified what 32 of them were. It was a mix of familiar and strange cosmic characters:

The "New Neighbors" (23 sources): 23 of these objects had never been seen in X-rays before. They were completely new discoveries.
The "Cosmic Flares" (Stars): Many were regular stars that suddenly flared up, like a cosmic version of a solar flare.
The "Cosmic Vampires" (Cataclysmic Variables): They found new candidates for these systems, where a white dwarf star is greedily eating a companion star. Some of these were found much farther away than usual, suggesting our "population census" of these stars was missing many distant members.
The "Magnetar Candidate": They caught a rare, highly magnetic neutron star (a magnetar) right as it was waking up from a nap and bursting into a bright flare. This is like catching a sleeping dragon just as it opens its eyes and breathes fire.
The "Gamma-Cas Analogue": They found a star behaving like a famous, mysterious type of star known for having a disk of gas swirling around it, and they even spotted it pulsing (beating like a heart) in a way that helps scientists understand its structure.
The "Black Hole/Neutron Star" Candidate: They found a system likely powered by a black hole or neutron star that was hidden by thick dust, only visible because it suddenly got brighter.

5. Why This Matters

The paper concludes that STONKS is a game-changer. It allows astronomers to see fainter and more variable objects than any other current system.

The Takeaway: By using this "comparative detective" method on existing telescope data, the team proved they can find cosmic "ghosts" and "flashes" that were previously invisible. This helps scientists build a more complete picture of the extreme physics happening in our galaxy, from stars exploding to black holes feasting.

In short, STONKS didn't just find a few new stars; it showed us that there is a whole hidden layer of the universe's "light show" that we were previously too blind to see.

Technical Summary: STONKS First Results: Long-term Transients in the XMM-Newton Galactic Plane Survey

Problem and Context
The detection of high-energy astronomical transients offers critical insights into extreme physical events, yet current capabilities are often limited by instrumental constraints. Wide-field survey missions (e.g., Swift, SVOM, Einstein Probe) maximize detection likelihood through large fields of view but typically operate with high-flux detection thresholds. Conversely, pointed observations by XMM-Newton offer significantly deeper sensitivity (down to $\sim 3.1 \times 10^{-16}$ erg cm $^{-2}$ s $^{-1}$ in 0.5–2.0 keV for $\sim 100$ ks exposures) but lack a systematic, near-real-time mechanism to identify long-term variability against the background of archival data. This gap limits the ability to detect faint, highly variable sources, particularly in the crowded and highly absorbed Galactic plane.

Methodology
This study applies the Search for Transient Object in New observations using Known Sources (STONKS) pipeline to the XMM-Newton Multi-Year Heritage Programme (MYHP) survey of the Galactic plane (conducted between March 2021 and October 2024). The methodology involves:

Pipeline Application: The STONKS pipeline (Quintin et al. 2024) cross-matches new XMM-Newton source lists against a multi-mission catalogue (including XMM, Swift, Chandra, ROSAT, and eROSITA). It calculates a variability metric ( $V$ ) by comparing new flux measurements ( $F_{new,low}$ ) against archival upper limits or past fluxes ( $F_{past,up}$ ), triggering alerts for $V \ge 5$ .
Data Processing: The analysis covered 231 observations. Source lists were processed, and alerts were screened to remove instrumental artefacts, such as spurious sources caused by bright sources outside the field of view or spectral assumption errors due to high neutral hydrogen column densities ( $N_H$ ) in the Galactic plane.
Classification: The remaining alerts were subjected to temporal and spectral analysis. This included:
- Examining light curves and spectra using XMMSAS.
- Cross-matching with SIMBAD, Gaia DR3, 2MASS, and AllWISE to identify optical, near-infrared (NIR), and mid-infrared (MIR) counterparts.
- Utilizing machine learning classifications from CLAXBOI and MUWCLASS.
- Fitting X-ray spectra with absorbed blackbody, power-law, plasma, and thermal bremsstrahlung models.
- Searching for periodicities using epoch folding and $Z^2_n$ tests.

Key Results
From the 231 observations, the pipeline initially generated 142 alerts. After screening for observational artefacts, 78 true alerts associated with 70 unique astrophysical sources were identified.

Classification Success: Of the 70 sources, 32 were classified with high confidence, and an additional 32 received provisional classifications. The firm classifications included:
- 21 Stars (including 1 RS CVn).
- 7 X-ray Binaries (XRBs), including 1 High-Mass XRB (HMXB).
- 4 Cataclysmic Variables (CVs).
- 4 Young Stellar Objects (YSOs).
- 1 $\gamma$ -Cas analogue.
- 1 Magnetar candidate.
Novelty: 23 sources were detected in X-rays for the first time.
Flux Sensitivity: The alert sources spanned a flux range of over three orders of magnitude ( $5.1 \times 10^{-15}$ to $9.7 \times 10^{-12}$ erg s $^{-1}$ cm $^{-2}$ ). The median flux ( $2.44 \times 10^{-13}$ erg s $^{-1}$ cm $^{-2}$ ) is below the detection limits of other transient-focused observatories like MAXI.
Specific Discoveries:
- Magnetar Candidate (4XMM J175136.8-275858): Detected in a high-flux state with a flux increase of nearly two orders of magnitude. Spectral analysis showed a blackbody temperature $kT > 10$ keV (unconstrained fit $\sim 200$ keV), consistent with magnetar outbursts.
- $\gamma$ -Cas Analogue (HD 162718): Identified in a high-flux state with evolving pulsations ( $P \sim 2.1$ ks) and spectral changes indicating increased high-energy emission.
- New CV Candidates: Three sources (4XMM J173828.0-292842, 4XMM J173409.6-322946, and 2CXJ1800-2403) were identified as CV candidates based on spectral fits (thermal bremsstrahlung) and flux ratios. Two exhibited potential periodicities ( $\sim 13$ ks and $\sim 8.2$ ks).
- New XRB Candidate: Source 4XMM J175033.4-264858 showed a hard power-law spectrum, no optical counterpart, and high absorption, suggesting a low/hard state XRB.
Population Characteristics: The study found that CVs identified by STONKS tend to be located at greater distances ( $d \approx 1.43$ kpc average for those with estimates) compared to optically selected samples, suggesting STONKS can detect more distant, highly absorbed populations.

Significance and Claims
The paper asserts that this systematic analysis demonstrates the value of the STONKS pipeline in applying XMM-Newton observations to transient detection. Key claims include:

Enhanced Sensitivity: STONKS enables the community to detect transient and highly variable sources at fluxes fainter than achievable with any other X-ray transient detection system, specifically leveraging the deep exposures of pointed XMM-Newton observations.
Systematic Utility: The pipeline successfully identified highly variable sources, including new candidates for magnetars, $\gamma$ -Cas analogues, CVs, and XRBs, which might otherwise remain undetected or unclassified in standard survey data.
Operational Integration: As of the AO24 proposal cycle, STONKS is being applied to all XMM-Newton observations, with alerts shared with Principal Investigators and the community (with approval), facilitating prompt follow-up for long-duration transients (e.g., magnetar outbursts, CV outbursts) that are too faint for wide-field monitors but too long-lived to be missed by pointed observations.

The authors conclude that while some sources remain unclassified or require future follow-up to confirm provisional classifications, the pipeline has proven effective in identifying a population of highly variable X-ray sources in the Galactic plane that extends the known population of compact objects and variable stars to greater distances and fainter fluxes.

1. The Problem: The "Needle in a Haystack"

2. The Solution: STONKS

3. The Investigation: Cleaning the Clutter

4. The Discoveries: What Did They Find?

5. Why This Matters

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