Study on Soft X-ray Prompt Emission from the High-Redshift Gamma-Ray Burst EP240315A

Background

Gamma-Ray Bursts (GRBs) are among the most violent explosive events in the universe, typically lasting from milliseconds to hundreds of seconds. Long Gamma-Ray Bursts (Long GRBs) are believed to originate from the core collapse of massive stars, making them important tools for studying the history of star formation in high-redshift (High-Redshift) galaxies. High-redshift GRBs (redshift z > 4.5) are particularly valuable as they can help us probe the reionization epoch and star formation history of the early universe. However, the detection of high-redshift GRBs is extremely rare, accounting for only about 3% of the GRBs triggered by the Swift satellite, and only about 10% of the GRBs with known redshifts have z > 4.5.

To further study the properties of high-redshift GRBs, especially their radiation behavior in the soft X-ray band, the research team conducted detailed observations of the high-redshift GRB EP240315A using the Wide-Field X-ray Telescope (WXT) onboard the Einstein Probe (EP) satellite. The study aims to reveal the radiation characteristics of high-redshift GRBs in the soft X-ray band and explore their relationship with gamma-ray radiation, thereby providing new insights into the physical processes of the early universe.

Source of the Paper

This research was conducted by multiple research teams from the Chinese Academy of Sciences, with key authors including Gao He, Zhang Chuang, Chen Yizhong, and Wu Xuefeng. The paper was published online on November 21, 2024, in the journal Nature Astronomy, titled “Soft X-ray prompt emission from the high-redshift gamma-ray burst EP240315A.”

Research Process and Results

1. Observations and Data Acquisition

The research team used the WXT onboard the Einstein Probe satellite to observe EP240315A in the 0.5–4 keV band. The GRB was triggered on March 15, 2024, at 20:10:44 (UTC), and its bright peak was later detected through offline analysis by the Burst Alert Telescope (BAT) on the Swift satellite and the Konus-Wind instrument. The optical counterpart was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) approximately 1.1 hours after the X-ray detection, and its redshift of z = 4.859 was confirmed through spectroscopic observations by the Very Large Telescope (VLT), verifying its cosmological origin.

2. Comparison of Soft X-ray and Gamma-ray Radiation

The light curve from WXT showed that the duration of EP240315A in the soft X-ray band was 1,034 ± 81 seconds, significantly longer than its duration in the gamma-ray band (approximately 38–41 seconds). Additionally, WXT triggered the soft X-ray detection 372 seconds before the gamma-ray detection, a much longer lead time compared to GRBs observed by BeppoSAX and HETE-2 (typically tens of seconds). The research team fitted the entire prompt emission spectrum from WXT and found it to be consistent with an absorbed power-law model, with a photon index of α = −1.4.

3. Multi-band Joint Spectral Analysis

The research team performed joint spectral fitting of data from WXT, BAT, and Konus-Wind during the peak period (t0 + 372 s to t0 + 416 s) and found that the broadband spectrum (0.5–1,618 keV) could be well described by a Cutoff Power-Law (CPL) model, with a photon index of −0.97 and a peak energy of 283 keV. This result indicates that the X-ray and gamma-ray emissions of EP240315A share a common origin, both arising from internal dissipation processes within the central engine.

4. Late-time X-ray Observations and Re-brightening Phenomenon

During the periods of 5.7 ks to 7.6 ks and 10.2 ks to 13.4 ks after the prompt emission, WXT detected faint X-ray signals. These late-time X-ray observations showed a simple power-law decline with a slope of −2. Additionally, the Follow-up X-ray Telescope (FXT) onboard EP began observations 42 hours later and conducted 10 observations over approximately 8 days, with the first nine detecting faint X-ray signals. The Chandra X-ray Observatory also conducted two observations at 72 hours and 10.4 days after the burst, with the first observation detecting an X-ray source consistent with FXT’s detection at the same epoch, while the second observation did not detect a significant signal.

5. Multi-band Afterglow Fitting

The research team fitted the multi-band afterglow data of EP240315A and found that the standard GRB afterglow model could well explain the optical, infrared, and radio observations. However, the late-time observations by WXT significantly deviated from the predictions of the afterglow model, suggesting that this radiation more likely originated from late activities of the central engine or a long-lived reverse shock.

Conclusions and Significance

This study is the first to provide a detailed analysis of the soft X-ray radiation characteristics of the high-redshift GRB EP240315A, revealing that its duration in the soft X-ray band is significantly longer than in the gamma-ray band. This finding challenges the traditional central engine activity timescales based on gamma-ray observations, indicating that the soft X-ray band may be more sensitive to detecting faint and soft radiation from the central engine. Additionally, the research team observed a late-time X-ray re-brightening phenomenon, which could be attributed to the reactivation of the central engine or off-axis observations of complex jet structures.

This study not only provides new insights into the physical mechanisms of high-redshift GRBs but also demonstrates the significant potential of the Einstein Probe satellite in detecting high-redshift GRBs. With its large field of view and high sensitivity, EP is expected to discover more high-redshift GRBs in future observations, thereby providing crucial observational data for studying the star formation and reionization history of the early universe.

Research Highlights

1. First Detailed Revelation of Soft X-ray Radiation in High-Redshift GRBs: The research team, for the first time, used the WXT onboard the Einstein Probe satellite to conduct detailed observations of the soft X-ray radiation of the high-redshift GRB EP240315A, revealing that its duration in the soft X-ray band is significantly longer than in the gamma-ray band.
2. Multi-band Joint Spectral Analysis: The research team performed joint spectral fitting of data from WXT, BAT, and Konus-Wind, uncovering the common origin of X-ray and gamma-ray emissions in EP240315A.
3. Discovery of Late-time X-ray Re-brightening: The research team observed a re-brightening phenomenon in the X-ray band during the late stages of the prompt emission, which could be attributed to the reactivation of the central engine or off-axis observations of complex jet structures.

Other Valuable Information

The research team also explored the detection capability of EP240315A at higher redshifts (z = 7.5) and found that similar events could be detected with a high signal-to-noise ratio (SNR ≥ 7) using EP-WXT in the future. This suggests that with further observations by EP, more high-redshift GRBs are likely to be discovered, providing additional observational data for studying the physical processes of the early universe.