SVOM Detects One of the Most Distant Bursts in the Universe and Observes the Farthest Supernova Ever Detected

On March 14, 2025, the Franco-Chinese satellite SVOM (Space-based multi-band astronomical Variable Objects Monitor) detected an exceptional gamma-ray burst, named GRB 250314A, originating from the farthest reaches of the Universe. As soon as the alert was triggered by the ECLAIRs and GRM instruments, the satellite reoriented itself to position its X-ray (MXT) and visible (VT) narrow-field instruments toward the source. Thanks to joint observations from multiple observatories and satellites, including the James Webb Space Telescope (JWST), this burst was confirmed as one of the most distant ever detected.
GRB 250314A is a long gamma-ray burst, resulting from the explosion of a star when the Universe was only 730 million years old. Its light traveled for nearly 13 billion years before being detected by our instruments. 110 days after SVOM’s discovery, JWST targeted the host galaxy of this burst. Preliminary photometric analyses suggest that it may be associated with a supernova—the violent gravitational collapse of a massive star at the end of its life—closely resembling local supernovae of the same type. This result could indicate a surprising continuity in the explosion processes of massive stars (>20 solar masses) from the early Universe to the present day.

A Flawless Sequence

At 12:56 UTC, the ECLAIRs alert instrument detected a signal from the depths of the Universe and immediately triggered an alert, starting the countdown. The procedure was instantly activated: the satellite reoriented to observe the relevant region of the sky with its MXT and VT narrow-field instruments.

ECLAIRs and GRM counters onboard the satellite during the detection of gamma-ray burst GRB 250314A

13:07 UTC: The Chinese GRM instrument confirmed the detection.

13:23 UTC: The team decided to issue a circular to alert the entire scientific community.

The “Gamma-ray Coordinates Network” (GCN) advocates sent out GCN Circular 39719, allowing observatories and satellites to target the region of the sky potentially affected by the burst. 19 circulars were issued regarding GRB 250314A (source : https://gcn.nasa.gov/circulars?view=index&query=250314A )

The Scientific Community on Alert

Thanks to the alert transmitted by the GRB advocates, other observatories joined the effort to detect X-ray, optical, and infrared counterparts.

The Neil Gehrels Swift space observatory and the Einstein Probe satellite pointed their instruments toward the source. An X-ray counterpart was localized by Swift’s X-ray detector. The Nordic Optical Telescope (NOT) discovered the near-infrared counterpart within the error box provided by Swift. Observations from the Einstein Probe confirmed the transient nature of the X-ray counterpart, a key indicator of a gamma-ray burst afterglow.

However, SVOM’s MXT and VT instruments did not detect a counterpart, which could indicate that the event was either extremely distant or heavily absorbed by surrounding gas and dust.

17 hours after the alert, using the precise position provided by the NOT, the Very Large Telescope (VLT), equipped with its X-shooter spectrograph, began observations. The optical/infrared spectrum revealed a redshift (z) of approximately 7.3, a measurement later supported by photometric observations from the Gran Telescopio Canarias. GRB 250314A ultimately proved to be the 5th most distant GRB ever identified. The previous high-redshift GRB (z > 7) had been identified about 12 years earlier!

110 days later, the JWST observed the burst’s location, and its NIRCam instrument detected an infrared emission. In the JWST images, the emission seen in the reddest filters is interpreted as a combination of the host galaxy and the emergence of the associated supernova. An additional observation, already scheduled in 9 months, should confirm the supernova’s disappearance.

Distribution of gamma-ray bursts as a function of their distance from the cosmic microwave background (CMB), from the first light of the Universe (emitted when the Universe was 380,000 years old, redshift ~1000) to the present day (redshift 0). The stars represent bursts detected by SVOM with measured redshifts. The pentagons indicate the four most distant bursts ever detected, with GRB 250314A being the third whose distance was determined spectroscopically. These bursts are associated with the first stars in the Universe, formed less than a billion years after the Big Bang.

Possible Homogeneity of Supernovae Since the Early Universe?

Photometric observations conducted by JWST in the field of GRB 250314A demonstrate the satellite’s ability to detect remnants of extremely distant stellar explosions. These initial results show a strong correspondence with the well-known model of supernova SN 1998bw, linked to a nearby gamma-ray burst (GRB 980425, z = 0.0085). Beyond the fact that SVOM enabled the observation of the farthest supernova ever detected, this discovery suggests that the mechanism of massive star collapse (interpreted as the origin of long gamma-ray bursts) may have been the same in the early Universe as it is in our local Universe, both in distance and time.

Contacts : B. Cordier, A. Saccardi, D. Gotz, S. Schanne, D. Turpin, E. Le Floc’h

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