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The coincidental detection of the gravitational wave event GW 170817 and the gamma-ray burst GRB 170817A marked the advent of multi-messenger astronomy and represented a milestone in the study of GRBs. Significant progress in this field is expected in the coming years with the increased sensitivity of gravitational waves detectors and the launch of new facilities for the high-energy survey of the sky. In this context, the launch of SVOM in mid-2022, with its two wide-field high-energy instruments ECLAIRs and GRM, will foster the possibilities of coincidental transient detection with gravitational waves and gamma-rays events. The purpose of this paper is to assess the ability of SVOM/ECLAIRs to detect and quickly characterize high-energy transients in the local Universe (z ≤ 0.3), and to discuss the contribution of this instrument to multi-messenger astronomy and to gamma-ray burst (GRB) astrophysics in the 2020’s. A list of local HE transients, along with their main characteristics, is constructed through an extensive literature survey. This list includes 41 transients: 24 long GRBs, 10 short GRBs and 7 SGR Giant Flares. The detectability of these transients with ECLAIRs is assessed with detailed simulations using tools developed for the SVOM mission, including a GEANT4 simulation of the energy response and a simulated trigger algorithm representative of the onboard trigger algorithm. SVOM/ECLAIRs would have been able to detect 88% of the short high-energy transients in our list: 22 out of 24 long GRBs, 8 out of 10 short GRBs and 6 out of 7 SGR Giant Flares. The SNR for almost all detections will be sufficiently high to allow the on-board ECLAIRs trigger algorithm to derive the localisation of the transient, transmitting it to the SVOM satellite and ground-based instruments. Coupled with the anti-solar pointing strategy of SVOM, this will enable an optimal follow-up of the events, allowing the observation of their afterglows, supernovae/kilonovae counterparts, and host galaxies. We conclude the paper with a discussion of the unique contribution expected from SVOM and of the possibility of simultaneous GW detection for each type of transient in our sample.

Swift response The Swift satellite, launched in November last year, is designed to study γ-ray bursts (GRBs) as soon as they happen. GRBs are the most powerful explosions known in the Universe, and Swift's ability to study the early phases of the X-ray afterglow was expected to yield exciting results. Swift has now bagged its first two long GRBs: in both, the X-ray afterglow emission declined rapidly in the first few hundred seconds, then flattened out. The steep decline was unexpected, and neither it nor the spectral properties of the afterglow can be explained by current models. ‘Long’ γ-ray bursts (GRBs) are commonly accepted to originate in the explosion of particularly massive stars, which give rise to highly relativistic jets. Inhomogeneities in the expanding flow result in internal shock waves that are believed to produce the γ-rays we see 1 , 2 . As the jet travels further outward into the surrounding circumstellar medium, ‘external’ shocks create the afterglow emission seen in the X-ray, optical and radio bands 1 , 2 . Here we report observations of the early phases of the X-ray emission of five GRBs. Their X-ray light curves are characterised by a surprisingly rapid fall-off for the first few hundred seconds, followed by a less rapid decline lasting several hours. This steep decline, together with detailed spectral properties of two particular bursts, shows that violent shock interactions take place in the early jet outflows.

The Swift X-ray Telescope (XRT) has discovered that flares are quite common in early X-ray afterglows of gamma-ray bursts (GRBs), being observed in roughly 50% of afterglows with prompt follow-up observations. The flares range in fluence from a few per cent to approximately 100% of the fluence of the prompt emission (the GRB). Repetitive flares are seen, with more than four successive flares detected by the XRT in some afterglows. The rise and fall times of the flares are typically considerably smaller than the time since the burst. These characteristics suggest that the flares are related to the prompt emission mechanism, but at lower photon energies. We conclude that the most likely cause of these flares is late-time activity of the GRB central engine.

X-Rated Supernova A link between long γ-ray bursts (GRBs) and supernovae has been established, but whether there is a similar relationship between the weaker and softer X-ray flashes and supernovae is unclear. GRB/XRF 060218, spotted by the Swift satellite on 18 February this year, may supply that missing link. In the first of four papers on this novel burster, Campana et al . report the sighting of the X-ray signature of a shock break-out, possible evidence of a supernova in progress. Pian et al . report the optical discovery of a type Ic supernova 2006aj associated with GRB/XRF 060218. Soderberg et al . report radio and X-ray observations that show that XRF 060218 is 100 times less energetic than, but of a type that is ten times more common than cosmological GRBs. Mazzali et al . modelled the spectra and light curve of SN 2006aj to show that it had a much smaller explosion energy and ejected much less mass than other GRB-supernovae, suggesting that it was produced by a star with a mass was only about 20 times that of the Sun, leaving behind a neutron star, rather than a black hole. Observations of the close gamma-ray burst GRB 060218 and its connection to supernova SN 2006aj reveal the break-out of a shock wave driven by a mildly relativistic shell into the dense wind surrounding the GRB progenitor. These observation catch a supernova in the act of exploding. Although the link between long γ-ray bursts (GRBs) and supernovae has been established 1 , 2 , 3 , 4 , hitherto there have been no observations of the beginning of a supernova explosion and its intimate link to a GRB. In particular, we do not know how the jet that defines a γ-ray burst emerges from the star's surface, nor how a GRB progenitor explodes. Here we report observations of the relatively nearby GRB 060218 (ref. 5 ) and its connection to supernova SN 2006aj (ref. 6 ). In addition to the classical non-thermal emission, GRB 060218 shows a thermal component in its X-ray spectrum, which cools and shifts into the optical/ultraviolet band as time passes. We interpret these features as arising from the break-out of a shock wave driven by a mildly relativistic shell into the dense wind surrounding the progenitor 7 . We have caught a supernova in the act of exploding, directly observing the shock break-out, which indicates that the GRB progenitor was a Wolf–Rayet star.

SAGE (SagnAc interferometer for Gravitational wavE) is a project for a space observatory based on multiple 12-U CubeSats in geosynchronous orbit. The objective is a fast track mission which would fill the observational gap between LISA and ground based observatories. With albeit a lower sensitivity, it would allow early investigation of the nature and event rate of intermediate-mass black hole (IMBH) mergers, constraining our understanding of the universe formation by probing the building up of IMBH up to supermassive black holes. Technically, the CubeSats would create a triangular Sagnac interferometer with 140.000km roundtrip arm length, optimized to be sensitive to gravitational waves at frequencies between 10mHz and 2Hz. The nature of the Sagnac measurement makes it almost insensitive to position error, enabling the use of spacecrafts in ballistic trajectories. The light source and recombination units of the interferometer are based on compact fibered technologies without bulk optics. A peak sensitivity of 23 pm/sqrt(Hz) is expected at 1Hz assuming a 200mW internal laser source and 10-centimeter diameter apertures. Because of the absence of a test mass, the main limitation would come from the non-gravitational forces applied on the spacecrafts. However, conditionally upon our ability to partially post-process the effect of solar wind and solar pressure, SAGE would allow detection of gravitational waves with strains as low as a few 1e-19 within the 0.1 to 1Hz range. Averaged over the entire sky, and including the antenna gain of the Sagnac interferometer, the SAGE observatory would sense equal mass black hole mergers in the 1e4 to 1e6 solar masses range up to a luminosity distance of 800Mpc. Additionally, coalescence of stellar black holes (10Msun) around SMBH (IMBH) forming extreme (intermediate) mass ratio inspirals could be detected within a sphere of radius 200Mpc.

Most ultra-luminous X-ray sources (ULXs) are now thought to be powered by stellar-mass compact objects accreting at super-Eddington rates. While the discovery of evolutionary cycles have marked a breakthrough in our understanding of the accretion flow changes in the sub-Eddington regime in Galactic Black Hole Binaries, their evidence in the super-Eddington regime remained elusive. However, recent circumstantial evidence had hinted the presence of a recurrent evolutionary cycle in two archetypal ULXs: Holmberg II X-1 and NGC 5204 X-1. Here we build on our previous work and exploit the long-term high-cadence monitoring of Swift-XRT in order to provide evidence of the evolutionary cycle in these two sources and investigate the main physical parameters inducing their spectral transitions. We study the long-term evolution of both sources using hardness-intensity diagrams (HID) and by means of Lomb-Scargle periodograms and Gaussian processes modelling to look for periodic variability. We show that both sources follow a recurrent evolutionary pattern in the HID that can be characterized by the hard ultraluminous (HUL) and soft ultraluminous (SUL) spectral regimes, and a third state with characteristics similar to the supersoft ultraluminous (SSUL) state. The transitions between the soft states seem aperiodic, as revealed by timing analysis of the light curve of Holmberg II X-1, albeit further investigation is warranted. The light curve of NGC 5204 X-1 shows a periodicity of \\(\\sim\\) 200 days, possibly associated with the duration of the evolutionary cycle. We support a scenario in which the spectral changes from HUL to SUL are due to a periodic increase of the mass-transfer rate and subsequent narrowing of the opening angle of the supercritical funnel. The narrower funnel, combined with stochastic variability imprinted by the wind, might explain the SUL--SSUL spectral changes.

The majority of energetic long-duration gamma-ray bursts (GRBs) are thought to arise from the collapse of massive stars, making them powerful tracers of star formation across cosmic time. Evidence for this origin comes from the presence of supernovae in the aftermath of the GRB event, whose properties in turn link back to those of the collapsing star. In principle, with GRBs we can study the properties of individual stars in the distant universe. Here, we present JWST/NIRCAM observations that detect both the host galaxy and likely supernova in the SVOM GRB 250314A with a spectroscopically measured redshift of z \\(\\simeq\\) 7.3, deep in the era of reionisation. The data are well described by a combination of faint blue host, similar to many z \\(\\sim\\) 7 galaxies, with a supernova of similar luminosity to the proto-type GRB supernova, SN 1998bw. Although larger galaxy contributions cannot be robustly excluded, given the evidence from the blue afterglow colours of low dust extinction, supernovae much brighter than SN 1998bw can be. These observations suggest that, despite disparate physical conditions, the star that created GRB 250314A was similar to GRB progenitors in the local universe.

Since their discovery by the Beppo-SAX satellite in 1997, gamma-ray burst afterglows have attracted an ever-growing interest. They have allowed redshift measurements that have confirmed that gamma-ray bursts are located at cosmological distances. Their study covers a huge range both in time (from one minute to several months after the trigger) and energy (from the GeV to radio domains). The purpose of this review is first to give a short historical account of afterglow research and describe the main observational results with a special attention to the early afterglow revealed by Swift. We then present the standard afterglow model as it has been developed in the pre-Swift era and show how it is challenged by the recent Swift and Fermi results. We finally discuss different options (within the standard framework or implying a change of paradigm) that have been proposed to solve the current problems.

Gamma-Ray Bursts (GRBs) are often referred to as the most luminous explosions in the Universe, due to their short and highly luminous prompt emission. This apparent luminosity, however, does not reflect the true energy budget of the prompt emission, which is strongly beamed. Accurate estimations of the energy radiated during the prompt phase require taking into account the geometry of GRB jets, which remains poorly known. Nevertheless, one may establish the distribution of well measured quantities, like Eiso, the GRB isotropic equivalent energy, which encrypts crucial information about GRB jets, with the aim of providing constraints on the jets radiated energy. In this work, we study the bright end of the GRB isotropic equivalent energy distribution (hereafter called \"apparent energy\"), using an updated sample of 185 apparently energetic GRBs with Eiso \\(\\geq 10^{53}\\) erg. This new sample includes GRB 221009A, allowing to discuss this apparently super-energetic GRB in the context of the general Eiso distribution of long GRBs. We describe the construction of the sample and compare fits of the Eiso distribution with a simple power law, a cutoff power law and a broken power law. Our study confirms the existence of a cutoff around Eiso = \\(4\\times10^{54}\\) erg, even when GRB 221009A is included in the sample. Based on this finding, we discuss the possible reasons behind the rapid decrease of the number of apparently energetic gamma-ray bursts beyond Eiso = \\(4\\times10^{54}\\) erg and the interpretation of GRB 221009A, the most apparently energetic GRB detected to date, in this context.

Detection of stellar flares at hard X-ray is still rare at the current stage. A transient was recently detected by the hard X-ray camera, ECLAIRs onboard the SVOM mission at 11:39:01.2UT on 2025, January 09. Simultaneous monitor in the optical band on the ground by SVOM/GWAC and follow-up spectroscopy enable us to confirm that the transient is caused by a superflare on HD~22468, a RS CVn-type star. The bolometric energy released in the flare is estimated to be \\(\\sim7.2\\times10^{37}-1.7\\times10^{38}\\ \\mathrm{erg}\\). The hard X-ray spectra of the event at the peak can be reproduced by the ``apec'' model of a hot plasma with a temperature of \\(106^{+27}_{-22}\\)~MK. In the optical range, the H\\(\\alpha\\) emission-line profile obtained at \\(\\sim1.7\\) hrs after the trigger shows a bulk blueshift of \\(-96\\pm20\\ \\mathrm{km\\ s^{-1}}\\), which can be explained by either a chromospheric evaporation or a prominence eruption. The ejected mass is estimated to be \\(3.9\\times10^{20}\\) g for the evaporating plasma, and to be \\(3.2\\times10^{21}\\ \\mathrm{g}

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