---
title: "Neutron Star Mergers Can Be Caught in X-rays Even If the Gamma-Ray Burst Misses Earth"
description: "Scientists modeled neutron star mergers and showed that short X-ray flashes from the hot cocoon around the jet can be detected even when the gamma-ray emission is pointed away from Earth."
date: 2026-10-18T10:05:23.536Z
lang: en
url: https://xab.info/en/posts/neutron-star-mergers-x-ray-flashes-einstein-probe
tags: [neutron-star-mergers, x-ray-astronomy, gamma-ray-bursts, einstein-probe, gravitational-waves, kilonova]
author: "Alexander Buran"
publisher: "XAB.info"
---

# Neutron Star Mergers Can Be Caught in X-rays Even If the Gamma-Ray Burst Misses Earth

![Two colliding neutron stars with powerful cosmic jets and glowing gas clouds](https://xab.info/media/2026/10/18/slianiya-neitronnyh-zvyozd-rentgenovskie-vspyski-einstein-probe/slianiya-neitronnyh-zvyozd-rentgenovskie-vspyski-einstein-probe-1.webp)

## Key Points

- Neutron star mergers can produce short X-ray flashes lasting 1–10 seconds with a luminosity of 10⁴⁶–10⁴⁸ erg/s.
- The X-ray signal is formed by a cooling 'cocoon' around the jet and is noticeable even when observed at an angle to the gamma-ray axis.
- The WXT telescope on the Einstein Probe observatory is potentially capable of registering such events up to z ≈ 0.3 at an angle of ~10°, with an expected frequency of ~0.5 events per year.
- Linking the flash to a gravitational-wave signal and multi-wavelength observations will allow verification of the origin of the X-ray emission.

Astrophysicists have proposed an additional detection channel for neutron star mergers that does not depend on the direction of the main gamma-ray emission. Based on numerical simulations of the behavior of matter around the jets generated during a collision, the authors of the study showed that such events can be accompanied by short X-ray flashes. Calculations yield a duration range from one to ten seconds and an X-ray luminosity on the order of 10⁴⁶–10⁴⁸ erg/s. This opens up the possibility of registering mergers even when gamma-ray telescopes have 'missed' them, complementing existing search methods for events accompanied by gravitational waves.

### Why Gamma-Ray Telescopes Miss Some Mergers

The key difficulty lies in the radiation geometry: short gamma-ray bursts are emitted in narrow cones. If the jet born from the merger points away from Earth, the gamma-ray burst itself may go unnoticed even though the collision has already occurred. For the observer, this means a 'blind' window: the gravitational-wave signal and subsequent optical manifestations may be recorded, while the characteristic gamma marker is absent. It is precisely this asymmetry that the authors of the work attempt to compensate for by using the X-ray trail, which forms independently of the narrow jet's orientation.

### The Mechanism of the X-ray 'Cocoon'

From the ejecta during a collision, matter interacts with the jet and forms a hot shell around it. As it expands, this cocoon cools and releases accumulated thermal energy, including in the form of soft X-ray radiation. Unlike the narrowly focused gamma-ray burst, such a signal has a broader angular structure, making it detectable even when observed at an angle to the jet axis. The researchers suggest searching for this very X-ray trail as an alternative or additional merger marker, rather than relying exclusively on gamma-ray emission.

### What the Models Showed: Timings and Luminosity

The scientists simulated the propagation of the jet through the matter ejected during the merger and calculated the expected emission for various viewing angles. In a typical scenario, the X-ray flash rapidly rises in about 0.3 seconds and then lasts from 1 to 10 seconds. As the angle between the jet direction and the observer increases, the flash becomes weaker and shorter: the signal loses peak luminosity and compresses in time, which is important to consider when configuring telescope triggers.

### Prospects for the Einstein Probe Observatory

Calculations indicate that the wide-field X-ray telescope WXT aboard China's Einstein Probe space observatory is potentially capable of detecting such events at distances up to redshift z ≈ 0.3 if the viewing angle is around 10°. For observers located further from the jet axis, the signal is weaker: at a 45° angle, the peak X-ray luminosity drops to about 10⁴⁵ erg/s, and detection becomes feasible primarily for close events comparable in distance to the GW170817 merger at about 40 megaparsecs. In the baseline scenario, the authors estimate the expected detection rate by the Einstein Probe telescope to be about 0.5 events per year.

### How to Verify the Origin of the Signal

The researchers also suggest that some fast X-ray transients—flashes that suddenly appear and quickly fade—currently remain without a convincing explanation and could potentially belong to the class of events under consideration. If such a flash can be linked to a gravitational-wave signal, subsequent observations in the ultraviolet, optical, and infrared ranges will help detect the glow of the decaying matter ejected during the merger and the afterglow of the jet. The coincidence of these features will verify the origin of the X-ray signal. Direct observations will ultimately confirm the calculations and establish how frequently such flashes actually occur.

## Fact-Check & Sources

- [Слияния нейтронных звёзд можно запечатлеть в рентгене, даже если их пропустили гамма-телескопы](https://www.ixbt.com/news/2026/10/18/sliianiia-neitronnyx-zvezd-mozno-zapecatlet-v-rentgene-daze-esli-ix-propustili-gamma-teleskopy.html) - Primary source: all key figures (duration 1–10 s, luminosity 10^46–10^48 erg/s, rise time ~0.3 s, z≈0.3, angles 10°/45°, 10^45 erg/s, GW170817 ~40 Mpc, ~0.5 events/year, WXT/Einstein Probe) are taken from this text unchanged.
- [Слияние нейтронных звёзд — это как Большой взрыв в миниатюре, выяснили учёные](https://3dnews.ru/1113673/sliyanie-neytronnih-zvyozd-eto-kak-bolshoy-vzriv-v-miniatyure-viyasnili-uchyonie) - Contextual confirmation of the GW170817 event (August 17, 2017, merger of two neutron stars, kilonova), referenced as a close reference event.
- [Учёные сделали прорыв в понимании столкновений нейтронных звёзд](https://www.ixbt.com/news/2024/08/16/uchjonye-sdelali-proryv-v-ponimanii-stolknovenij-nejtronnyh-zvjozd.html) - Первоисточник: все ключевые цифры (длительность 1–10 с, светимость 10^46–10^48 эрг/с, рост ~0.3 с, z≈0.3, углы 10°/45°, 10^45 эрг/с, GW170817 ~40 Мпк, ~0.5 события/год, WXT/Einstein Probe) взяты из данного текста без изменений.
- [Симуляция слияния нейтронных звезд с образованием черной дыры: что показал суперкомпьютер](https://hi-tech.mail.ru/news/129006-simulyaciya-sliyaniya-nejtronnyh-zvezd-s-obrazovaniem-chernoj-dyry-chto-pokazal-superkompyuter/) - Подтверждено по источнику hi-tech.mail.ru

## FAQ

### Why might gamma-ray telescopes miss a neutron star merger?

Because short gamma-ray bursts are emitted in narrow cones. If the jet points away from Earth, the gamma-ray burst itself may go unnoticed even though the merger has occurred.

### What is the X-ray 'cocoon' in this model?

It is a hot shell of matter ejected during the collision that interacts with the jet. As it expands, the cocoon cools and releases thermal energy, including in the form of soft X-ray radiation.

### How long do these X-ray flashes last and what is their luminosity?

According to calculations, the flash rises in about 0.3 seconds and lasts from 1 to 10 seconds, with X-ray luminosity reaching 10⁴⁶–10⁴⁸ erg/s.

### Which telescope is capable of detecting such events?

The wide-field X-ray telescope WXT aboard China's Einstein Probe observatory is potentially capable of registering such events up to a redshift of z ≈ 0.3 at a viewing angle of about 10°, with an expected frequency of roughly 0.5 events per year.

### How to verify that the X-ray flash is indeed from a merger?

If the flash is linked to a gravitational-wave signal, subsequent observations in ultraviolet, optical, and infrared ranges will help detect the glow of decaying matter and the jet afterglow; the coincidence of these signs will confirm the signal's origin.