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Register a new userDetection of a very bright optical flare from a gamma-ray burst at redshift 6.29
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In this letter we discuss the flux and the behavior of the bright optical flare emission detected by the 25 cm TAROT robotic telescope during the prompt high-energy emission and the early afterglow. We combine our data with simultaneous observations performed in X-rays and we analyze the broad-band spectrum. These observations lead us to emphasize the similarity of GRB 050904 with GRB 990123, a remarkable gamma-ray burst whose optical emission reached 9th magnitude. While GRB 990123 was, until now, considered as a unique event, this observation suggests the existence of a population of GRBs which have very large isotropic equivalent energies and extremely bright optical counterparts. The luminosity of these GRBs is such that they are easily detectable through the entire universe. Since we can detect them to very high redshift even with small aperture telescopes like TAROT, they will constitute powerful tools for the exploration of the high-redshift Universe and might be used to probe the first generation of stars.
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Future missions for long gammma-ray burst (GRB) observations at high redshift such as HiZ-GUNDAM and THESEUS will provide clue to the star formation history in our universe. In this paper focusing on high redshift (z>8) GRBs, we calculate the detection rate of long GRBs by future observations, considering both Population (Pop) I&II stars and Pop III stars as GRB progenitors. For the Pop I&II star formation rate (SFR), we adopt an up-to-date model of high-redshift SFR based on the halo mass function and dark matter accretion rate obtained from cosmological simulations. We show that the Pop I&II GRB rate steeply decreases with redshift. This would rather enable us to detect the different type of GRBs, Pop III GRBs, at very high redshift. If 10% or more Pop III stars die as an ultra-long GRB, the future missions would detect such GRBs in one year in spite of their low fluence. More luminous GRBs are expected from massive compact Pop III stars produced via the binary merger. In our conventional case, the detection rate of such luminous GRBs is 3-20 /yr (z>8). Those future observations contribute to revealing of the Pop III star formation history.
Long $rm gamma$-ray bursts (GRBs) are produced by the dissipation of ultra-relativistic jets launched by newly-born black holes after a collapse of massive stars. Right after the luminous and highly variable $gamma$-ray emission, the multi-wavelength afterglow is released by the external dissipation of the jet in circumburst medium. We report the discovery of very bright ($rm sim 10$ mag) optical emission $rm sim 28$ s after the explosion of the extremely luminous and energetic GRB 210619B located at redshift 1.937. Early multi-filter observations allowed us to witness the end of the shock wave propagation into the GRB ejecta. We observed the spectral transition from a bright reverse to the forward shock emission, demonstrating that the GRB multi-wavelength emission is originated from a narrow and highly magnetised jet propagating into a rarefied interstellar medium. We also find evidence of an additional component of radiation, coming from the jet wings which is able explain the uncorrelated optical/X-ray emission.
High-redshift ($z>2$) blazars are the most powerful members of the blazar family. Yet, only a handful of them have both X-ray and $gamma$-ray detection, thereby making it difficult to characterize the energetics of the most luminous jets. Here, we report, for the first time, the Fermi-Large Area Telescope detection of the significant $gamma$-ray emission from the high-redshift blazar DA 193 ($z=2.363$). Its time-averaged $gamma$-ray spectrum is soft ($gamma$-ray photon index = $2.9pm0.1$) and together with a relatively flat hard X-ray spectrum (14$-$195 keV photon index = $1.5pm0.4$), DA 193 presents a case to study a typical high-redshift blazar with inverse Compton peak being located at MeV energies. An intense GeV flare was observed from this object in the first week of 2018 January, a phenomenon rarely observed from high-redshift sources. What makes this event a rare one is the observation of an extremely hard $gamma$-ray spectrum (photon index = $1.7pm0.2$), which is somewhat unexpected since high-redshift blazars typically exhibit a steep falling spectrum at GeV energies. The results of our multi-frequency campaign, including both space- (Fermi, NuSTAR, and Swift) and ground-based (Steward and Nordic Optical Telescope) observatories, are presented and this peculiar $gamma$-ray flare is studied within the framework of a single-zone leptonic emission scenario.
GRB050904 is very interesting since it is by far the most distant GRB event known to date($z=6.29$). It was reported that during the prompt high energy emission phase, a very bright optical flare was detected, and it was temporal coincident with an X-ray flare. Here we use two models to explain the optical flare, One is the late internal shock model, in which the optical flare is produced by the synchrotron radiation of the electrons accelerated by the late internal shock, and the X-ray flare is produced by the synchrotron-self-Compton mechanism. The other is the external forward-reverse shock model, in which the optical flare is from the reverse shock emission and the X-ray flare is attributed to the central engine activity. We show that with proper parameters, a bright optical flare can appear in both models. We think the late internal shock model is more favored since in this model the optical flash and the X-ray flare have the same origin, which provides a natural explanation of the temporal coincidence of them. In the forward-reverse shock scenario, fits to the optical flare and the late afterglow suggests that the physical parameters of the reverse shock are much different from that of forward shock, as found in modeling the optical flash of GRB 990123 previously.
The giant flares of soft gamma-ray repeaters (SGRs) have long been proposed to contribute to at least a subsample of the observed short gamma-ray bursts (GRBs). In this paper, we perform a comprehensive analysis of the high-energy data of the recent bright short GRB 200415A, which was located close to the Sculptor galaxy. Our results suggest that a magnetar giant flare provides the most natural explanation for most observational properties of GRB 200415A, including its location, temporal and spectral features, energy, statistical correlations, and high-energy emissions. On the other hand, the compact star merger GRB model is found to have difficulty reproducing such an event in a nearby distance. Future detections and follow-up observations of similar events are essential to firmly establish the connection between SGR giant flares and a subsample of nearby short GRBs.
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