Sub-GeV flashes in γ−\gamma-ray burst afterglows as probes of underlying bright UV flares

Bright optical and X-ray flares have been observed in many Gamma-ray Burst (GRB) afterglows. These flares have been attributed to late activity of the central engine. In most cases the peak energy is not known and it is possible and even likely that there is a significant far-ultraviolet component. These far-ultraviolet photons escape our detection because they are absorbed by the neutral hydrogen before reaching Earth. However, these photons cross the blast wave produced by the ejecta that have powered the initial GRB. They can be inverse Compton upscattered by hot electrons within this blast wave. This process will produce a strong sub-GeV flare that follows the high energy (soft X-ray) tail of the far-UV flare but lasts much longer and can be detected by the upcoming {\em Gamma-Ray Large Area Telescope} (GLAST) satellite. This signature can be used to probe the spectrum of the underlying far-ultraviolet flare. The extra cooling produced by this inverse Compton process can lower the X-ray emissivity of the forward shock and explain the unexpected low early X-ray flux seen in many GRBs.Comment: Accepted for publication in MNRAS Letters, minor revisio

GRB efficiency and Possible Physical Processes Shaping the Early Afterglow

The discovery by Swift that a good fraction of Gamma Ray Bursts (GRBs) have a slowly decaying X-ray afterglow phase led to the suggestion that energy injection into the blast wave takes place several hundred seconds after the burst. This implies that right after the burst the kinetic energy of the blast wave was very low and in turn the efficiency of production of $\gamma$-rays during the burst was extremely high, rendering the internal shocks model unlikely. We re-examine the estimates of kinetic energy in GRB afterglows and show that the efficiency of converting the kinetic energy into gamma-rays is moderate and does not challenge the standard internal shock model. We also examine several models, including in particular energy injection, suggested to interpret this slow decay phase. We show that with proper parameters, all these models give rise to a slow decline lasting several hours. However, even those models that fit all X-ray observations, and in particular the energy injection model, cannot account self-consistently for both the X-ray and the optical afterglows of well monitored GRBs such as GRB 050319 and GRB 050401. We speculate about a possible alternative resolution of this puzzle.Comment: Accepted for publication in MNRAS, minor revisio

Sciences for The 2.5-meter Wide Field Survey Telescope (WFST)

The Wide Field Survey Telescope (WFST) is a dedicated photometric survey facility under construction jointly by the University of Science and Technology of China and Purple Mountain Observatory. It is equipped with a primary mirror of 2.5m in diameter, an active optical system, and a mosaic CCD camera of 0.73 Gpix on the main focus plane to achieve high-quality imaging over a field of view of 6.5 square degrees. The installation of WFST in the Lenghu observing site is planned to happen in the summer of 2023, and the operation is scheduled to commence within three months afterward. WFST will scan the northern sky in four optical bands (u, g, r, and i) at cadences from hourly/daily to semi-weekly in the deep high-cadence survey (DHS) and the wide field survey (WFS) programs, respectively. WFS reaches a depth of 22.27, 23.32, 22.84, and 22.31 in AB magnitudes in a nominal 30-second exposure in the four bands during a photometric night, respectively, enabling us to search tremendous amount of transients in the low-z universe and systematically investigate the variability of Galactic and extragalactic objects. Intranight 90s exposures as deep as 23 and 24 mag in u and g bands via DHS provide a unique opportunity to facilitate explorations of energetic transients in demand for high sensitivity, including the electromagnetic counterparts of gravitational-wave events detected by the second/third-generation GW detectors, supernovae within a few hours of their explosions, tidal disruption events and luminous fast optical transients even beyond a redshift of 1. Meanwhile, the final 6-year co-added images, anticipated to reach g about 25.5 mag in WFS or even deeper by 1.5 mag in DHS, will be of significant value to general Galactic and extragalactic sciences. The highly uniform legacy surveys of WFST will also serve as an indispensable complement to those of LSST which monitors the southern sky.Comment: 46 pages, submitted to SCMP

GRB 080916C and GRB 090510:the high-energy emission and the afterglow

We constrain the physical composition of the outflows of GRBs 080916C and 090510 with the prompt emission data and find that the former is likely magnetic while the latter may be baryonic. The X-ray and optical afterglow emission of both GRBs can be reasonably fitted using the standard external shock model but the density profiles of the circum-burst medium are different. We also propose a simple method to estimate the number of the seed photons suppose the GeV afterglow photons are due to the inverse Compton radiation of external forward shock electrons. The seed photons needed in the modeling are too many to be realistic for both events. The synchrotron radiation of the forward shock seems able to account for the GeV afterglow data.Comment: 5 pages; accepted for publication in ApJ