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Detectability of Gravitational-wave counterparts of EP-FXTs observed during the O4b LIGO-Virgo-KAGRA Observing Run

This study systematically investigates the detectability of gravitational-wave counterparts to Einstein Probe Fast X-ray Transients (FXTs) observed during the O4b observing run, finding no significant associations and establishing 90% exclusion distances that disfavor nearby compact binary merger origins for these events.

Original authors: Ansh Chopra, Samuele Ronchini, Biswajit Banerjee, Marica Branchesi, Stefano Ascenzi, Maria Edvige Ravasio, Peter Jonker, Andrew Levan

Published 2026-07-21

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CC BY 4.0

Original authors: Ansh Chopra, Samuele Ronchini, Biswajit Banerjee, Marica Branchesi, Stefano Ascenzi, Maria Edvige Ravasio, Peter Jonker, Andrew Levan

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 universe as a giant, cosmic dance floor where massive objects like black holes and neutron stars spin, spiral, and eventually crash into each other. When these heavyweights collide, they don't just make a sound; they create ripples in the fabric of space and time itself, known as gravitational waves. It's like dropping a giant stone into a pond, sending out waves that travel across the entire universe. For a long time, we could only "hear" these ripples using giant, ultra-sensitive detectors on Earth. But recently, scientists started listening for a different kind of signal: flashes of X-ray light. These are called Fast X-ray Transients (FXTs). They are like sudden, bright sparks in the dark sky that last for seconds or hours. The big mystery is: what causes these sparks? Some scientists think they might be the "afterglow" or the "shout" of those same crashing stars that make the gravitational waves. If we can catch both the ripple (the gravitational wave) and the spark (the X-ray flash) at the same time, it would be like seeing a lightning strike and hearing the thunder simultaneously, proving they come from the same storm. This is the holy grail of "multi-messenger astronomy," helping us understand the violent, hidden lives of stars.

This paper is a massive detective story where the authors tried to link these two clues together. They looked at a specific list of 47 X-ray sparks (FXTs) spotted by a space telescope called the Einstein Probe during a specific time in 2024 and 2025. They then cross-referenced this list with a database of gravitational wave candidates detected by the LIGO, Virgo, and KAGRA observatories during the same period. The goal was to see if any of the X-ray sparks happened at the exact same time and in the exact same spot as a gravitational wave ripple.

The results? The detectives found seven pairs where an X-ray spark and a gravitational wave candidate happened close enough in time to be suspicious. However, when they looked closer, the evidence didn't hold up. The gravitational waves were too faint, and the locations on the sky didn't match up well enough to say they were definitely from the same event. In fact, the number of "suspicious" pairs they found was exactly what you would expect to see just by pure chance, like finding two people wearing the same red shirt in a crowd of thousands. The authors concluded that there is no significant evidence that these specific X-ray sparks were caused by the crashing of neutron stars or black holes that we could detect with our current tools.

So, what does this mean for the origin of these sparks? It doesn't mean they aren't caused by crashing stars; it just means that if they are, the crashes are either too far away or too quiet for our current detectors to hear. To figure this out, the authors calculated a "detectability range." Think of it like a flashlight: they asked, "How far away could a crash be and still be loud enough for us to hear?" They found that for the best-case scenario, our detectors could "hear" a crash up to about 178 million light-years away for two neutron stars, and up to about 349 million light-years away for a neutron star and a black hole. Since most of the X-ray sparks they studied are much farther away than that, it makes sense that we didn't hear them.

The paper suggests that these specific X-ray sparks are disfavored as being caused by nearby crashes of compact stars that our current network of detectors should have been able to catch, though it notes important exceptions. For instance, one of the closest events studied (EP240506a) is at a distance where our detectors might have heard a very massive type of crash, but not the standard ones. The authors are confident in this negative result because they used a rigorous statistical method to prove that the few matches they found were likely just random coincidences. They didn't find a "smoking gun," but they did map out exactly how far our "ears" can hear, which helps scientists know where to look next. As our detectors get more sensitive in the future, we might finally catch that perfect match, but for now, the connection between these X-ray sparks and gravitational waves remains a mystery waiting to be solved.

Technical Summary: Detectability of Gravitational-wave counterparts of EP-FXTs observed during the O4b LIGO-Virgo-KAGRA Observing Run

Problem and Motivation
Fast X-ray transients (FXTs) detected by the Einstein Probe (EP) mission represent a rapidly growing class of extragalactic transients with uncertain physical origins. While some are linked to core-collapse supernovae or tidal disruption events, compact binary coalescence (CBC) systems—specifically binary neutron star (BNS) and neutron star–black hole (NSBH) mergers—have been proposed as progenitors for a subset of these events. If FXTs originate from CBCs, they should be associated with gravitational-wave (GW) emission. However, unlike short Gamma-Ray Bursts (GRBs), many FXTs lack prompt gamma-ray counterparts, and the time delay between the GW merger signal and the onset of X-ray emission is poorly constrained. This uncertainty complicates targeted GW searches, which typically rely on precise trigger times. This work addresses the lack of systematic investigation into GW counterparts for EP-FXTs, specifically those not associated with observed MeV emission, to assess the detectability of associated CBCs and constrain progenitor scenarios.

Methodology
The authors conducted a systematic search for GW counterparts to 47 EP-FXTs detected during the second half of the fourth observing run (O4b) of the LIGO-Virgo-KAGRA (LVK) network (April 2024 – January 2025).

Sample Selection: The sample includes 47 EP-FXTs, 11 of which have measured redshifts. Events associated with known GRBs or flaring stars were excluded to avoid overlap with dedicated LVK GRB searches.
Temporal Search Window: A conservative search window was defined for each FXT as $[t_{FXT} - 1000\,\text{s}, t_{FXT} + T_{100}]$ , where $t_{FXT}$ is the EP trigger time and $T_{100}$ is the transient duration. The 1000-second pre-trigger window accounts for potential delays in X-ray emission relative to the merger (e.g., off-axis jet emission, magnetar spin-down, or shock breakout), while the post-trigger window extends to the end of the transient.
GW Candidate Matching: The authors cross-matched the FXT sample against the 5th Gravitational-Wave Transient Catalog (GWTC-5), focusing on CBC candidates identified by PyCBC, GstLAL, and MBTA pipelines. Coherent WaveBurst (cWB) candidates were excluded as they target unmodeled bursts rather than CBCs.
Significance Assessment: Seven temporally coincident GW candidates were identified. To evaluate their significance, the authors employed a joint ranking statistic ( $\Lambda$ $Λ$ ) that combines:
- Temporal coincidence.
- Sky localization overlap ( $I_\Omega$ ) between the GW probability map and the FXT position.
- The GW candidate's Bayes factor ($BCI$) for a coherent signal.
- An empirical background distribution was generated using 1000 random GW candidates and 1000 random sky positions to estimate p-values and joint false alarm rates (FAR).
Exclusion Distance Analysis: In the absence of a significant detection, the authors utilized the Targeted Detectability Range (TDR) framework. This method injects CBC waveforms (BNS and NSBH) into the detector noise at the specific sky position and time of each FXT to calculate the 90% exclusion distance ( $D_{90}$ ). This accounts for the specific detector network configuration (H1, L1, V1 availability) and antenna response at the source location, assuming an inclination prior of $0^\circ < \iota < 45^\circ$ .

Key Results

No Significant Associations: The analysis found no statistically significant GW counterparts for any of the 47 FXTs. The seven temporally coincident candidates (e.g., EP241125a with S241125fj) were consistent with random chance. The joint p-values and FARs indicated no evidence for a physical association; the distribution of p-values was consistent with a uniform random distribution.
Temporal Coincidence Statistics: The expected number of random temporal overlaps was $\lambda \approx 4.53$ . The observation of 7 coincidences yielded a Poisson probability of $p \approx 0.172$ , confirming that temporal overlap alone is insufficient to claim a physical link.
Exclusion Distances: The TDR analysis established that the LVK network during O4b could exclude a nearby CBC origin for these events only within specific distance limits.
- For BNS systems ( $1.4+1.4\,M_\odot$ ), the typical median exclusion distance was $\sim 178$ Mpc.
- For NSBH systems ( $10+1.4\,M_\odot$ ), the typical median exclusion distance was $\sim 349$ Mpc.
- These distances vary significantly based on the number of operating interferometers and the source's sky position relative to the detector antenna pattern.
Redshift Constraints: For the 11 sources with measured redshifts, the inferred luminosity distances generally exceeded the calculated exclusion distances. The closest source, EP240506a ( $z \approx 0.12$ , $D_L \approx 560$ Mpc), lies beyond the exclusion range for most BNS and NSBH configurations, except for a specific massive NSBH scenario ( $20+2\,M_\odot$ ) where the exclusion distance ( $D_{90} \approx 597$ Mpc) approaches the source distance.

Significance and Claims
The paper claims to present the first systematic investigation of GW counterparts to EP-FXTs. Its primary significance lies in establishing a methodology for connecting wide-field X-ray transients with GW observations in the absence of prompt gamma-ray triggers.

The authors conclude that while no significant GW–FXT association was found, the results provide 90% exclusion-distance constraints that disfavor a nearby compact-binary merger origin for the majority of the sample. The study demonstrates that current GW sensitivity is insufficient to detect CBCs associated with FXTs at the typical distances of the observed population (often $>500$ Mpc), unless the events are exceptionally massive or nearby.

The work emphasizes that the lack of detection does not rule out a CBC origin for the broader FXT population but rather constrains the parameter space to sources beyond the current detection horizon. The authors posit that longer joint observation periods between the LVK and Einstein Probe, coupled with improved GW detector sensitivity in future runs (IR1 and beyond), will be necessary to identify genuine associations and place tighter constraints on the progenitor scenarios of these transients. The TDR framework is highlighted as a valuable, low-latency tool for post-event diagnostics and for guiding follow-up strategies by quantifying the sensitivity of the GW network to specific transient locations.

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