47 research outputs found
Sub-GeV flashes in 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
Research Progress on Antagonistic Antimicrobial Prevention and Control of Grape Postharvest Diseases
During the storage and transportation of grapes after picking, diseases occurs generally, including gray mold, aspergillus rot, rhizopus rot, penicillium rot, etc., which seriously affect its production and commodity value. At present, the control of postharvest diseases of grape still depends on chemical control. Long-term application of chemical fungicides can lead to pesticide residues, environmental pollution, harm to human health, and enhance the drug resistance of pathogenic fungal. Using antagonistic bacteria to control postharvest diseases of grape is a green, environmental and safe method instead of chemical fungicides. In this paper, the postharvest infection diseases and symptoms of grape, the source, application effect and their mechanism of action of antagonistic bacteria, the combination of antagonists with other methods to enhance the biocontrol effect are introduced. The prospect of controlling postharvest diseases of grape by antagonistic bacteria is also discussed
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 -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