1,745 research outputs found
Can life survive Gamma-Ray Bursts in the high-redshift universe?
Nearby Gamma-Ray Bursts (GRBs) have been proposed as a possible cause of mass
extinctions on Earth. Due to the higher event rate of GRBs at higher redshifts,
it has been speculated that life as we know it may not survive above a certain
redshift (e.g. ). We examine the duty cycle of lethal (life-threatening)
GRBs in the solar neighborhood, in the Sloan Digital Sky Survey (SDSS) galaxies
and GRB host galaxies, with the dependence of the long GRB rate on star
formation and metallicity properly taken into account. We find that the number
of lethal GRBs attacking Earth within the past 500 Myr ( epoch of the
Ordovician mass extinction) is . The number of lethal GRBs hitting a
certain planet increases with redshift, thanks to the increasing star formation
rate and decreasing metallicity in high- galaxies. Taking 1 per 500 Myr as a
conservative duty cycle for life to survive, as evidenced by our existence, we
find that there are still a good fraction of SDSS galaxies beyond where
the GRB rate at half-mass radius is lower than this value. We derive the
fraction of such benign galaxies as a function of redshift through Monte Carlo
simulations, and find that the fraction is at and even at . The mass distribution of benign galaxies is dominated
by Milky-Way-like ones, thanks to their commonness, relatively large mass, and
low star formation rate. GRB host galaxies are among the most dangerous ones.Comment: Accepted for publication in The Astrophysical Journal, 7 pages, 4
figures, 1 tabl
The FRB 121102 Host Is Atypical among Nearby Fast Radio Bursts
We search for host galaxy candidates of nearby fast radio bursts (FRBs), FRB 180729.J1316+55, FRB 171020, FRB 171213, FRB 180810.J1159+83, and FRB 180814.J0422+73 (the second repeating FRB). We compare the absolute magnitudes and the expected host dispersion measure DMhost of these candidates with that of the first repeating FRB, FRB 121102, as well as those of long gamma-ray bursts (LGRBs) and superluminous supernovae (SLSNe), the proposed progenitor systems of FRB 121102. We find that while the FRB 121102 host is consistent with those of LGRBs and SLSNe, the nearby FRB host candidates, at least for FRB 180729.J1316+55, FRB 171020, and FRB 180814.J0422+73, either have a smaller DMhost or are fainter than FRB 121102 host, as well as the hosts of LGRBs and SLSNe. In order to avoid the uncertainty in estimating DMhost due to the line-of-sight effect, we propose a galaxy-group-based method to estimate the electron density in the intergalactic regions, and hence, DMIGM. The result strengthens our conclusion. We conclude that the host galaxy of FRB 121102 is atypical, and LGRBs and SLSNe are likely not the progenitor systems of at least most nearby FRB sources. The recently reported two FRB hosts differ from the host of FRB 121102 and also the host candidates suggested in this paper. This is consistent with the conclusion of our paper and suggests that the FRB hosts are very diverse
Detectable MeV neutrinos from black hole neutrino-dominated accretion flows
Neutrino-dominated accretion flows (NDAFs) around rotating stellar-mass black
holes (BHs) have been theorized as the central engine of relativistic jets
launched in massive star core collapse events or compact star mergers. In this
work, we calculate the electron neutrino/anti-neutrino spectra of NDAFs by
fully taking into account the general relativistic effects, and investigate the
effects of viewing angle, BH spin, and mass accretion rate on the results. We
show that even though a typical NDAF has a neutrino luminosity lower than that
of a typical supernova (SN), it can reach
peaking at MeV, making them potentially detectable with the upcoming
sensitive MeV neutrino detectors if they are close enough to Earth. Based on
the observed GRB event rate in the local universe and requiring that at least 3
neutrinos are detected to claim a detection, we estimate a detection rate up to
(0.10-0.25) per century for GRB-related NDAFs by the Hyper-Kamiokande
(Hyper-K) detector if one neglects neutrino oscillation. If one assumes that
all Type Ib/c SNe have an engine-driven NDAF, the Hyper-K detection rate would
be (1-3) per century. By considering neutrino oscillations, the
detection rate may decrease by a factor of 2-3. Detecting one such event would
establish the observational evidence of NDAFs in the universe.Comment: 7 pages, 2 figures, 2 tables, accepted for publication in PR
A simple proof of exponential decay in the two dimensional percolation model
Kesten showed the exponential decay of percolation probability in the
subcritical phase for the two-dimensional percolation model. This result
implies his celebrated computation that for bond percolation in the
square lattice, and site percolation in the triangular lattice, respectively.
In this paper, we present a simpler proof for Kesten's theorem.Comment: 9 pages and one figur
Blind Image Restoration via the Integration of Stochastic and Deterministic Methods
This paper addresses the image restoration problem which remains a significant field of image processing.
The fields of experts- (FoE-) based image restoration has been discussed and some open issues including noise estimation and parameter selection have been approached. The stochastic method FoE performs fairly well; meanwhile it might also produce unsatisfactory outcome especially when the noise is grave. To improve the final performance, we introduce the integration with deterministic method K-SVD. The FoE-treated image has been used to obtain the dictionary, and with the help of sparse and redundant
representation over trained dictionary, the K-SVD algorithm can dramatically solve the problem, even though the pretreated result is of poor quality under severe noise condition. The experimental results via our proposed method are demonstrated and compared in detail. Meanwhile the test results from both qualitative and quantitative aspects are given, which present the better performance over current
state-of-art related restoration algorithms
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