GRB 241001A: from the look of SVOM to the deep eye of JWST

On 1 October 2024 at 17:08 UT, as SVOM continued its commissioning phase, a weak emission signal was detected by ECLAIRs: XRF 241001A (GCN37655). This gamma-ray burst had nothing of the flamboyant brilliance of the majority of its predecessors. It was an X-ray flash (XRF), a more discreet variant of the classic gamma-ray bursts. Identified in the early 2000s thanks to the BeppoSAX satellite, XRFs are an atypical subclass of GRBs. Their spectrum, dominated by X-rays rather than gamma rays, makes them more difficult to detect by the Swift or Fermi satellites, which are better suited to detecting gamma-ray bursts.

Light curve (number of counts as a function of time) measured by ECLAIRs in three energy bands. The active period of the burst is represented by the grey band. We can see that the burst is much more active below 20 keV. This is the signature of an X-ray rich burst (XRF). Credit: SVOM/IRAP, Marius Brunet.

In search of the afterglow emission

Shortly after the signal was detected, the SVOM and international multi-wavelength tracking system was activated. The SVOM teams immediately requested a rapid observation of the Swift satellite with its XRT instrument to detect the possible X-ray afterglow of the phenomenon. On Earth, the telescopes of the GRANDMA network and the Las Cumbres Observatory (LCO) quickly pointed their instruments at the source to obtain the first images of the optical counterpart and improve the location of the burst. (GCN37667, GCN37679).

Distance measurement

Unlike typical gamma-ray bursts, the afterglow of XRF 241001A was relatively modest, both in the X-ray and optical domains, and challenged one of the most powerful telescopes on the ground, the Very Large Telescope (VLT) in Chile. The VLT’s X-shooter instrument was able to capture a spectrum of the last moments of the optical counterpart in order to measure its distance. Although the signal in the spectrum appears weak, it is possible to distinguish both the absorption lines typical of matter swept along by the relativistic jet on the line of sight and the emission lines of the galaxy hosting the explosion. Both show the same distance with a redshift of z = 0.57, i.e. 5.6 billion years ago (GCN37677).

A supernova revealed by JWST and VT

The distance and atypical nature of this event aroused the interest of the community, including an international team of researchers who carried out observations with the James Webb Space Telescope (JWST) to better understand its origin. On 24 October, 24 days after ECLAIRs detected the burst, a spectrum of the source was obtained with JWST’s NIRSpec instrument, revealing emission consistent with that of a type Ic-BL supernova (GCN37867). This type of supernova, particularly energetic with very broad spectral lines produced by ejecta at extreme speeds and frequently associated with gamma-ray bursts, results from the explosion of a massive star that has lost its outer envelope of hydrogen and helium before collapsing. At the same time, the VT has revisited the position of the burst on several occasions and has tracked the emergence of the supernova using imagery. The association of a supernova with this XRF reinforces the idea that at least some of these phenomena are indeed linked to the explosion of a massive star.

Time evolution of XRF 241001A: a. Archive image taken before the explosion (Legacy Survey DR10, Dey et al. 2019): no object is visible at the location of the gamma-ray burst. b. Detection of the optical counterpart (OT) by the Visible Telescope (VT) on board SVOM, about 20 hours after the initial detection by ECLAIRs. c. The optical counterpart fades rapidly with time, becoming almost invisible to the VT after a few days. d. After about ten days, a light source reappears at the XRF location, corresponding to the emergence of the associated supernova (SN), confirming the explosion of a massive star. Credits: SVOM/VT, Huali li and Benjamin Schneider.

Exploring the origin of XRFs with ECLAIRs

However, XRF-type bursts are still poorly understood. Are they simply GRBs seen from a different angle? Or is it a physically distinct phenomenon, linked to less energetic jets or a brief luminous emission that occurs when the shock wave from a supernova pierces the star’s surface? The data collected on XRF 241001A provide a new anchor for better understanding this population of cosmic explosions. They also illustrate the ability of the ECLAIRs instrument to detect gamma-ray bursts that are spectrally softer and less energetic than those regularly observed by Swift or Fermi, offering a promising new perspective on these cosmic phenomena.

Contact: Benjamin Schneider