Svom detects one of the most distant bursts in the Universe and JWST observes the most distant supernova ever detected

On March 14, 2025, the French-Chinese satellite SVOM (Space-based multi-band astronomical Variable Objects Monitor) detected an exceptional gamma-ray burst, named GRB 250314A, originating from the outskirts of the Universe. As soon the ECLAIRs and GRM instruments triggered, the satellite slewed and positioned its narrow-field X-ray (MXT) and visible (VT) instruments to observe this source, which—thanks to joint observations from several observatories and satellites, including JWST—turned out to be one of the most distant bursts ever recorded. GRB 250314A is a long gamma-ray burst, produced by the explosion of a star when the Universe was only 730 million years old, and whose light traveled nearly 13 billion years before being detected by our instruments. 110 days after SVOM discovery, JWST targeted the galaxy hosting this burst. Early photometric analyses suggest that it may be associated with a supernova resulting from the violent gravitational collapse of a massive star at the end of its life, closely resembling local supernovae of the same type. This result may 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 SVOM ECLAIRs alert instrument detects a signal coming from the depths of the Universe and immediately issues an alert— the countdown begins. The operation starts immediately : the satellite reorients itself to observe the relevant region of the sky with its narrow-field MXT and VT instruments.

At 13:07 UTC, the Chinese GRM instrument confirms the detection.

At 13:23 UTC, the teams decide to send out a circular to alert the entire community.

The community on alert

Thanks to the alert transmitted by the burst advocates, other observatories step in to search for X-ray, optical, and infrared counterparts.

The Neil Gehrels Swift space observatory and the Einstein Probe satellite point their instruments toward the source. An X-ray counterpart is localized by the X-ray detector on board Swift. The Nordic Optical Telescope (NOT) discovers the near-infrared counterpart within the error box provided by Swift. Observations from Einstein Probe even confirm the transient nature of the X-ray counterpart— a key clue supporting the presence of a GRB afterglow.

On the SVOM side, however, the MXT and VT instruments did not detect a counterpart — an absence that may indicate that the event is particularly distant or heavily absorbed by surrounding gas and dust.

17 hours after the alert, using the precise position provided by NOT, the Very Large Telescope (VLT) equipped with its X-shooter spectrograph begins its observation. Very quickly, the observed optical/infrared spectrum shows a spectral shift corresponding to a redshift (z) of about 7.3. This measurement is then supported by photometric observations from the Gran Telescopio Canarias. GRB 250314A ultimately turns out to be the fifth most distant GRB ever identified. The previous GRB at such a high redshift (z > 7) had been identified roughly twelve years earlier!

110 days later, the JWST satellite observes the burst position and, using its NIRCam instrument, detects an infrared emission. In the JWST images, the emission seen in the reddest filters is interpreted as the combination of the GRB host galaxy and the emergence of the associated supernova. An additional observation, already scheduled in nine months, should confirm the disappearance of the supernova.

A possible homogeneity of supernovae since the dawn of the Universe?

The photometric observations carried out by the JWST on the field of GRB 250314A demonstrate the satellite ability to observe traces of extremely distant stellar explosions. These early results show a strong correspondence with the well-known model of supernova SN 1998bw, associated with a nearby gamma-ray burst (z = 0.0085), GRB 980425. Beyond the fact that SVOM has enabled the observation of the most distant supernova ever detected (by far), this discovery suggests that the collapse mechanism of massive stars (the interpretation for the origin of long gamma-ray bursts) may be the same in the early Universe as in our local Universe, very close to us in both distance and time.

The supernova associated with GRB250314A is by far the most distant supernova ever detected. The following table provides a comparison of the most distant supernovae detected to date, with their respective redshifts.

Event	Redshift	Age of the Univers at explosion time (Gyr)	Light travel time (Gyr)	Comoving distance (Glyr)
GRB 250314A	7,3	0,73	13,0	28,5
SN 1000+0216	3,899	1,6	12,1	25,6
DES16C2nm	2,0	3,3	10,5	16,4
SN UDS10Wil	1,914	3,5	10,3	16,0
SN SCP-0401	1,71	4,0	9,9	14,7

SVOM article : SVOM GRB 250314A at z ≃ 7.3: an exploding star in the era of reionization

JWST article : JWST reveals a supernova following a gamma-ray burst at z ≃ 7.3

The news in video by Andrea Saccardi (in French)

Contact: Andrea Saccardi

GRB 250706B: an “afterglow”-rise caught from space

On July 6, 2025, the Universe staged an impressive display. At 16:45:22 UTC, the Konus-WIND spacecraft was startled by a powerful 40-second flash of gamma rays, later catalogued as GRB 250706B. While this Gamma-Ray Burst (GRB) was impressive by its initial prompt brightness, it would be its afterglow that would steal the show.

Ten minutes after the initial blast, the fading light of the burst’s afterglow appeared above the Earth’s horizon and into the field of view of the LAT instrument onboard the Fermi satellite, which recorded photons carrying energies as high as 21 billion electronvolts. Then, another 10 minutes later at 17:06:04 UTC, as the burst emerged from behind the Earth for SVOM, its ECLAIRs instrument was triggered — not by the initial flash of prompt emission photons as is usually the case, but by the bright X-ray afterglow itself. This was the first time ECLAIRs had triggered on a GRB afterglow which was made possible thanks to its special image trigger mode and its lower energy threshold of 4 keV.

SVOM swiftly reacted, slewing toward the source in less than three minutes. Its onboard telescopes, MXT and VT, captured the afterglow pinning down the position of the burst to within a fraction of an arcsecond, enabling the X-shooter instrument installed at the Very Large Telescope in Chile to measure its redshift at z = 0.942, equivalent to a distance of 6.3 billion parsecs away, from a time when the universe was barely half its current age.

Finally, one month later on August 5th, after the afterglow finally faded, the NIRSpec instrument of the James Webb Space Telescope revealed the unmistakable signature of a broad-lined Type Ic supernova at the same location — the telltale mark of a massive star’s core collapse, confirming the collapsar origin of GRB 250706B. The wealth and quality of data collected on this event should allow scientists to get a detailed understanding of the physics at play and attempt to provide answers as to why it was so bright.

GRB 250706B was more than just another detection. It was a rare “afterglow”-rise: a cosmic sunrise, as the light of a cataclysm long past crested the Earth’s edge into our instruments’ view.  If a human with X-ray vision had been onboard the SVOM satellite, they too would have witnessed an extraordinary sight — not the familiar golden glow of dawn, but the sudden rising of a distant afterglow, born in the violent death of a star billions of years ago.

Contact: Jesse Palmerio

GRB 250403A, proof of the phenomenal power of gamma-ray bursts.

On 03 April 2025, at 09:42 UTC, SVOM was revisiting the gamma-ray burst detected the previous day, GRB 250402A. The optical axis of the instruments was pointing in the direction of the Virgo constellation (AD= 13.78° DEC= -5.94°).

Since ECLAIRs has a field of view of 2 steradians (approximately 90 degrees by 90 degrees), several sources were present within it, including Scorpius X-1.

Scorpius X-1, abbreviated Sco X-1, is an X-ray source located in the constellation of Scorpius about 9,000 light-years from Earth. Apart from the Sun, it is the most powerful source of X-rays in the sky. It was discovered by chance in 1962 by a team led by Riccardo Giacconi, using a sounding rocket equipped with an X-ray detector. At that time, Riccardo Giacconi was looking to study X-ray emissions from the Moon. The discovery of this object marked the birth of X-ray astronomy. Riccardo Giacconi was awarded the Nobel Prize in Physics 40 years later, in 2002.

Sco X-1 is likely to be a binary system hosting a neutron star that tears matter from its companion star, forming an accretion disc. The mass of the companion star is only 0.42 solar masses, hence the classification as a low-mass binary system.

In the ECLAIR energy range of 5 to 20 keV, Sco X-1 is therefore the brightest source in the sky. Normally we try to keep this source out of the field of view of ECLAIRS so as not to saturate its detectors. The presence of Sco X-1 in the ECLAIRs field of view disturbs the trigger and significantly reduces the sensitivity of the telescope.

On 03 April 2025, at 15:15 UTC, a flash appeared in the ECLAIRs data, GRB 250403A. This burst lasted around twenty seconds and triggered the SVOM machinery with the emission of a GRB alert, the drafting of a circular (GCN 40026) and the conduct of a ground follow-up campaign. This follow-up campaign resulted in an estimate of the distance of the burst by measuring its redshift. With a redshift equal to 1.847 (GCN 40162), this burst occurred when the universe was only 3.58 billion years old. The photons that reached our detectors travelled for 10.14 billion years.

The image produced by ECLAIRs is very interesting and shows us the power involved in a gamma-ray burst. In Figure 2, at the moment of the flash, we can clearly distinguish the two mark patterns projected by Sco X-1 and GRB 250403A (for more information “play with ECLAIRs“).  The two sources are also clearly visible in the deconvolved image with almost the same intensity, except that Sco X1 is in our galaxy and GRB 250403A is at cosmological distances…

Since the universe is expanding, using standard cosmological parameters, we can calculate the luminous distance of GRB 250304A, which is equal to 46.78 billion light-years. If we remember that the distance of Sco X1 is only 9000 light-years, then to impregnate our detector with the same intensity for 20 seconds, GRB 250403A was 26 million million times more intense than the brightest source in the sky in this energy range. That’s amazing !

GRB 250314A : A message from the depths of time

Just imagine. Around 13 billion years ago, one of the first stars in the Universe collapses into a black hole, releasing a flash of light of unprecedented power. This light travelled through the expanding Universe, past the first galaxies, then drifted through the ages… all the way to us. And on 14 March 2025, SVOM captured it.

A race against time

At 12:56pm UTC, a signal emerges from the depths of the Universe. ECLAIRs, SVOM’s watchful eye, detected it immediately and gave the alert. The machine is in motion: the satellite slews, points its small-field instruments MXT and VT, and begins to observe what could be one of the most distant gamma-ray bursts ever detected.

Thanks to the data transmitted in real time via the SVOM VHF network, our team of scientists on call are trying to determine whether this is a genuine alert or just a fluctuation in the background noise. The task is proving difficult for several reasons:

• At the time of the alert, the satellite was flying over a zone without coverage in the VHF network in the Pacific Ocean, preventing full reception of the data.
• The signal received by ECLAIRs was weak and below the detection threshold required to trigger an automatic alert to the international community.

At 1.07pm, we were truly alerted. Despite these difficulties, the simultaneous detection of the signal by the GRM instrument strengthened the scientists’ confidence. At 1.23pm, we decided to inform the international community by sending a brief message to observatories around the world.

A journey into the youth of our Universe

Thanks to the quick response of the SVOM scientists, other observatories quickly mobilised to search for the optical and near-infrared counterpart of the gamma-ray burst. The Nordic Optical Telescope, as well as the Neil Gehrels Swift Observatory and the Einstein Probe satellite, pointed their instruments at the source and pinpointed its location. Einstein Probe’s observations even confirm the transient nature of the optical counterpart, a key clue in favour of a gamma-ray burst. On the SVOM side, however, the MXT and VT instruments did not detect the counterpart. This could indicate that the event is particularly distant…

Excitement rises when, on the ground, the Very Large Telescope (VLT) and its X-shooter spectrograph take us back to the youth of the Universe. Around 17 hours after the SVOM alert, the spectrum acquired by the VLT revealed a spectral shift estimated at z ~7.3. The measurement was then confirmed by the Gran Telescopio Canarias.

Such a spectral shift means that the light from this gamma-ray burst was emitted when the Universe was only 730 million years old, compared with 13.7 billion years old today. This gamma-ray burst could well be a sign that one of the very first stars in the Universe has evolved into a black hole. It is the fifth most distant gamma-ray burst ever detected! As with the GRB 090423 and GRB 120923A bursts, the redshift of our 14 March burst was determined from its spectrum. For GRB 090429B and GRB 100905A, on the other hand, it was estimated without spectral measurement, by comparing the intensity through different filters. This method, which determines a photometric redshift, is less accurate than the spectroscopic method. Taking this into account, our gamma-ray burst is even the third most distant whose distance has been accurately measured using its spectrum.

Analyses of the data collected by SVOM and its partners will make it possible to establish the physical properties of this primordial burst and its local environment, in order to compare them with the populations of bursts detected throughout the different ages of the Universe.

A great night for hunting gamma-ray bursts

The burst advocate’s diary …

On Wednesday 5 February 2025, it was 10.25pm when I received a notification on my phone. The ECLAIRs telescope on board the SVOM satellite had detected a burst of X-rays and gamma rays and immediately triggered the satellite’s automatic re-pointing to observe the region of sky with the MXT and VT small field of view instruments.

We very quickly organised an emergency videoconference with the other scientists on call, during which we interpreted the first data sent in real time by the satellite via the VHF antenna network. This data, which includes a portion of the image of the sky acquired by the ECLAIRs telescope, gives us confidence in the astrophysical nature of the phenomenon detected: it is very probably a gamma-ray burst.

In order to alert the community, we drafted a circular summarising our detection, including the position needed to follow the phenomenon in optical. Initially, this position was provided solely by ECLAIRs, but after a few minutes we received a more precise location, obtained thanks to rapid observations with the MXT telescope on board SVOM. Our circular is published at 23:18 (T0+53 min): https://gcn.nasa.gov/circulars/39154. The burst was named GRB 250205A.

Meanwhile, the VT instrument is locating the source with an excellent accuracy of the order of one arcsecond (https://gcn.nasa.gov/circulars/39159). These observations confirm the transient nature of the phenomenon, as the luminosity is already starting to decrease. Time is running out to continue observations before it’s too late!

For the specialists on the ECLAIRs and MXT instruments observing at high energy, it’s time to go to sleep, but for the colleagues in charge of optical follow-up, it’s only the beginning. In the hours that followed, several ground-based observatories began observing the sky around the position provided by SVOM: the Observatoire de Haute-Provence (T193), the Roque de los Muchachos Observatory on the island of La Palma (LT, GTC), and the National Astronomical Observatory of San Pedro Mártir in Mexico (Colibri). Space telescopes are also observing the target (Swift, Einstein Probe).

These various optical and near-infrared observations also confirm the transient nature of the phenomenon, with the luminous flux diminishing rapidly over time.

Thanks to spectroscopic observations made at just T0+1h45 with the Large Telescope on the Canary Islands (10.4 m in diameter), the distance of the gamma-ray burst has even been determined. Its redshift is 3.55 (https://gcn.nasa.gov/circulars/39160), which means that the light emitted by this gamma-ray burst – which undoubtedly marked the birth of a black hole during the collapse of a very massive star – was emitted at a time when the Universe was only 1.8 billion years old. Its light travelled to us for almost 12 billion years before being detected by the instruments aboard SVOM!

There is still a lot of analytical work to be done to fully understand the precise nature of the phenomenon and to determine its characteristics, but we can already be delighted that the sequence of multi-wavelength observations, both onboard SVOM and on the ground within the community, has been carried out so quickly. This made it possible to observe the burst at several wavelengths and even to obtain its distance.

Contact : Andrea Saccardi

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.

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.

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

The combination of two worldviews

In the past, Europe and China have had two radically different visions of the cosmos. While the former has long thought of the fixed and immutable sky as revealing only silent perfection, the latter has, on the contrary, relentlessly tracked down the slightest celestial changes, sometimes almost imperceptible phenomena, betrayed only by infinitesimal manifestations. Unchanging sky or transient sky therefore remained for a long time two opposing visions. Today the transient sky has become an irreplaceable source of information for astrophysicists and China and Europe are joining forces to unlock the secrets of this constantly changing world where some cataclysms can occur in just a few fractions of a second.

The SVOM (Space-based multi-band Variable Objects Monitor) space mission, to be launched in 2021, will be the second Sino-French scientific mission from space after the launch of CFOSAT (China-France Oceanography SATellite) on 29 October 2018. Its purpose is to detect the brutal sudden end of the very first stars, located at the edge of the Universe, and also to help locate super-powerful cosmic phenomena generated by the fusion of compact stars. This is an undeniable tribute to the long Chinese tradition.

Unchanging sky and transient sky

Europe, and more broadly the Mediterranean basin, has mainly received its astronomical tradition from the ancient Greek world. From an often very limited and superficial observation of the heavens, the ancient Greek thinkers had imagined a cosmic world divided into two distinct domains. On the one hand, the sub-lunar world close to the Earth where all the changing cosmic phenomena (meteors, comets,…) were confined and on the other hand the supra-lunar world where the errant bodies that are the planets orbit in perfect circles. This world itself was entirely contained in a last celestial sphere, the fixed, immutable and eternal sphere that carried the stars. In this highly idealized vision, it was therefore impossible to think of discovering any change whatsoever in this last starred sphere, which, according to common experience, indeed seemed largely untroubled.

After the collapse of classical Greek culture at the beginning of the modern era, this same idealized vision was taken up by the two monotheistic religions, Christianity and Islam, which were to develop in turn. This time, the perfect and immutable character of the heavens was no longer a aesthetic vision from philosophical thinkers, but became a strict religious prescription that sought to describe the perfection of the great Creator. To identify elements that could question this cosmic perfection then became a frontal opposition to the religious powers that also dominated the political world of the time. This religious dogma has thus acted for nearly 1500 years on astronomical science in Europe.

In contrast, other parts of the world have not been subjected to this religious censorship, and in the case of China, on the contrary, it was the political power itself that has favoured since the dawn of time, the deep examination of the changing sky, permanently scanning the heavens in search of transitional phenomena.
In a premonitory way, China is the civilization that has given the most importance to the changes in the skies. At the origin of this quest, the permanent concern to preserve the harmony between Earth and Heaven. The two terrestrial and celestial worlds were seen as two complementary worlds in constant interaction and any disruption of balance in the sky heralded a similar disruption on Earth that needed to be identified. In this vision of Heaven as a mirror of the Earth, the main role was assigned to the sovereign himself, called “Son of Heaven” (Tianzi), because his essential responsibility was to guarantee this Earth-Sky harmony. The Chinese emperor did not receive his mandate and legitimacy from a simple family line or even a conquest, but above all he had to justify a “celestial mandate” (Tianming) granted to him if he could predict and anticipate astronomical phenomena. In this sense, China is the only country in the world to have elevated astronomy to the rank of state science.

On the orders of the emperor and the central power, everything that could disturb the harmony of the heavens was hunted down, discovered and interpreted. Entire bodies of astronomers and astrologers were mobilized night after night in imperial astronomical observatories gathering hundreds of people (observers, timekeepers and clepsydra, calendar specialists, mathematicians,…) who had nothing to envy our great modern scientific institutes.
From the beginning of the Hans, in the second century BC, the specifications were clearly established and reported to us by the great historian-astronomer of ancient China, Sima Qian, in his encyclopedic work “The Historical Memories”: “If in the whole cycle from beginning to end and from antiquity to modern times the changes that occur at fixed times have been observed deeply and if we have examined their details and the whole, then the science of the Governors of Heaven is complete. “Shiji, Historical Memoirs of Astronomer Historian Sima Qian (109 to 91 BCE).

This perfect organization, which has covered more than forty centuries of Chinese civilization, has provided the world with fundamental astronomical discoveries, often still greatly underestimated.
The existence of sunspots on the Sun’s surface was thus clearly established as early as the Hans dynasty (206 BCE to 220 CE) while their discovery in Europe is attributed to Galileo (1613 CE). Even more spectacularly, the first mention of a star explosion, resulting in the temporary appearance of a new star (novae or supernovae), seems to have been documented in China as early as the 15th century BC and precise catalogues and reports of these spectacular events are available since the Han period. In Europe, the first description of such phenomena was given only by the Danish astronomer Tycho Brahe in 1573.

Today, modern astronomers know that everything that changes in the sky reveals the most extraordinary cosmic phenomena.
Thus the appearance of a supernovae, a new transitional star, reveals an event that is crucial to the history of the universe: the explosion of a massive star that will spread in space all the complex cosmic elements (carbon, nitrogen, oxygen, etc.) produced in the star’s heart that can contribute later to the emergence of life on a planet.
In other areas of light such as gamma rays, the appearance of very short bursts of gamma radiation also marks the end of life of very large stars, which can be detected thanks to this signal up to the far limits of the Universe.
Finally, beyond light itself, the very structure of space-time can be transiently altered by the monstrous fusion of black holes or compact stars and reach us as gravitational waves, new messengers now detectable on Earth by laser-based sensors. In short, we are learning much more today from the transient sky than from the permanent sky
All these transient phenomena are the main objectives of the Franco-Chinese SVOM mission. Their detection, location and precise description will be recorded to better interpret the sky, just as the astronomers of the Han dynasty did more than 2000 years ago. Thus the same concern to better read the changes in Heaven, a quest shared today by China and France thanks to SVOM, is renewed centuries later.

References

Jean-Marc Bonnet-Bidaud, « 4000 ans d’astronomie chinoise », Ed. Belin, 2017.
Joseph Needham and Wang Ling. « Science and Civilisation in China, vol. 3, Mathematics and the Sciences of the Heavens and the Earth », Cambridge University Press, 1959.
Xiaochun Sun and Jacob Kistemaker, « The Chinese Sky during the Han – Constellating Stars & Society », Ed. Brill, 1997.
Zezong Xi, Shuren Bo, « Ancient Oriental Records of Novae and Supernovæ », Science, vol. 154, p. 597, 1965.

The third SVOM scientific workshop, entitled “Disentangling the merging Universe with SVOM” and bringing together 70 participants, was held from 14 to 18 May 2018 at the Houches physics school in the Chamonix valley. This year, a scientific theme was particularly highlighted, the contribution of the SVOM mission to the study of gravitational waves at the dawn of the next decade. The observation strategy during a gravitational wave warning was at the heart of the debates.

On August 17, 2017 the Ligo-Virgo collaboration detected a packet of gravitational waves from the fusion of two neutron stars lurking in the galaxy NGC 4993. Less than two seconds later, the Gamma ray Burst Monitor (GBM) of the Fermi mission detected for the first time an electromagnetic counterpart to this event, a signal confirmed by the Integral satellite. This correlated detection in gravitational waves and electromagnetic waves is a major scientific fact of the last decade. The SVOM mission, whose primary objective is the detection and study of gamma-ray bursts, is particularly well suited to contribute to this major scientific theme in view of its array of instruments equipping its space platform and the ground resources at its disposal.

During this workshop, several sessions described the state of the art of gravitational wave detection by active detectors, in terms of scientific results and future projections. The event of 17 August 2017 (fusion of two neutron stars) was the subject of special attention. The potential of the SVOM mission was highlighted, informed by simulations of the detection of such an event by the mission instruments and their contribution to the community.

Beyond this theme, the scientists exchanged on the more practical aspects of the technical constraints of the mission (orbit, accessible sky zone in accordance with the main program, opportunity and limit of modification of the satellite point linked to an alert, etc.).

The participants also took advantage of the majestic setting of the Houches School of Physics to address new research themes and update and complete the scientific document describing the mission in a broad sense (the White Paper).

The event of August 2017

On August 17, 2017, ground-based gravitational wave detectors (LIGO and Virgo) detected an event, called GW 170817, interpreted as the fusion of two neutron stars. This is the first time that LIGO detects a signal whose profile is similar to the fusion of two neutron stars, but what made the event even more unique is the temporal and spatial coincident detection of a short gamma burst GRB170817A by the GBM detector onboard the Fermi satellite. The detection was confirmed and announced in the following minutes by the SPI spectrometer onboard the INTEGRAL satellite.

The characteristics of GRB170817A were published after a few hours on the site of the Fermi / GBM mission. This is a low-intensity short gamma burst lasting about 2 seconds. Taking into account the distance estimated by the gravitational wave analyses, as well as its observed flux, GRB 170817A is an unusual burst, particularly sub-luminous (see Figure 1). It could be a gamma burst seen from the side.

An unprecedented observational campaign followed, mobilizing dozens of observatories on Earth and in space to detect and characterize the electromagnetic counterpart of the gravitational wave source. Thanks to the contribution of Virgo, the error circle was considerably reduced which allowed a fast tracking on the ground by small telescopes. By selecting galaxies located at the distance deduced from the gravitational signal, the 1 m Swope telescope at the Las Campanas observatory detected a new source SSS17A near the galaxy NGC 4993 at 40 Mpc. In the hours that followed other telescopes (DLT 40, Vista, Master, DECam, LCOGT) confirmed this detection, motivating the whole community to focus on SSS17A and leading to the detection of the electromagnetic counterpart of GW 170817 in all the wavelengths of the ultraviolet to the radio domain. This unprecedented campaign was rewarded by the discovery of a kilonova [1].

The succession of observations that have made the detection and localisation of the electromagnetic counterparts of GW 170817 possible reinforces the observational strategy chosen for the SVOM mission. With its set of interconnected multi-wavelength detectors, covering the electromagnetic spectrum from gamma rays to infra-red, SVOM will be able to detect and study these gravitational wave sources resulting from the fusion of two neutron stars and producing short bursts.

What would have SVOM seen if the mission had been online on the 17th of August 2017 ?

If the burst had appeared in the field of view of its high-energy detectors (ECLAIRs and GRM), SVOM would have detected it with a high probability and would have sent a trigger warning to the ground (see figure 2), all the while measuring the energy emitted in gamma rays. At the same time, the satellite would have slewed automatically to put the burst in the center of the field of view of its X-ray and optical instruments (MXT and VT), initiating an observing sequence of several hours. The optical counterpart of GW 170817, the emission produced by the kilonova associated with the fusion of the two neutron stars, would have been easily detected by the visible telescope VT [2] (see Figure 3).

But what would SVOM have seen if GRB170817A had not been in the field of view of its high-energy instruments (ECLAIRs and GRM) ?

Upon receiving the LIGO-Virgo alert at the French scientific center, SVOM would have triggered its robotic telescopes (F-GFT and C-GFT). By selecting nearby galaxies compatible with the probable distance from the event and contained in the gravitational wave detector error circle, SVOM’s robotic telescopes would have started a systematic search for the optical counterpart by performing several observation cycles. This strategy is particularly suited for the discovery of transient objects in their brightening phase. Thanks to its sensitivity but also thanks to its infra-red capabilities, the French GFT would certainly have detected the kilonova associated with GW 170817 (see figure 3).

In parallel, the French scientific center would have prepared a request for a multi-messenger opportunity target, asking the satellite to interrupt its observing sequence and point at various regions of the sky within the error circle of the gravitational wave detectors. It typically takes about ten hours to reprogram the satellite. Although arriving later, the VT would have easily detected the kilonova associated with the fusion of the two neutron stars (see Figure 3).

And now ?

In the beginning of the next decade, thanks to its ground and space-based instruments, the SVOM mission will certainly be a major player in the study of the transient sky and should significantly contribute to the study of gravitational wave sources. Starting next year, some of the SVOM ground-based telescopes (GWAC) should be operational and should validate the strategies implemented within the consortium at the next LIGO-Virgo data collection scheduled for autumn 2018.

[1] Kilonova: during the coalescence of two neutron stars, neutron-rich material is suddenly released under temperature and density conditions very favorable for the nucleosynthesis of heavy elements by the fast neutron capture process (process r). This is expected to result in the quasi-isotropic ejection of heavy-element-enriched material. This material is heated by the radioactivity of freshly synthesised elements and radiate thermally, with a color evolution from blue to red due to progressive cooling. This emission called kilonova has a physical origin quite distinct from the gamma ray burst and its afterglow.

[2] For this event no short-term X-ray observations occurred. It is therefore difficult to predict a possible detection by the MXT X-ray telescope, despite the fact that it is expected by various models.