The long-lasting optical afterglow plateau of short burst GRB 130912A

The short burst GRB 130912A was detected by Swift, Fermi satellites and several ground-based optical telescopes. Its X-ray light curve decayed with time normally. The optical emission, however, displayed a long term plateau, which is the longest one in current short GRB observations. In this work we examine the physical origin of the X-ray and optical emission of this peculiar event. We find that the canonical forward shock afterglow emission model can account for the X-ray and optical data self-consistently and the energy injection model that has been widely adopted to interpret the shallowly-decaying afterglow emission is not needed. We also find that the burst was born in a very-low density interstellar medium, consistent with the compact object merger model. Significant fractions of the energy of the forward shock have been given to accelerate the non-thermal electrons and amplify the magnetic fields (i.e., $\epsilon_{\rm e}\sim 0.37$ and $\epsilon_{\rm B}\sim 0.16$, respectively), which are much larger than those inferred in most short burst afterglow modeling and can explain why the long-lasting optical afterglow plateau is rare in short GRBs.Comment: 5 pages, 2 figure

Is the late near-infrared bump in short-hard GRB 130603B due to the Li-Paczynski kilonova?

Short-hard gamma-ray bursts (GRBs) are widely believed to be produced by the merger of two binary compact objects, specifically by two neutron stars or by a neutron star orbiting a black hole. According to the Li-Paczynski kilonova model, the merger would launch sub-relativistic ejecta and a near-infrared/optical transient would then occur, lasting up to days, which is powered by the radioactive decay of heavy elements synthesized in the ejecta. The detection of a late bump using the {\em Hubble Space Telescope} ({\em HST}) in the near-infrared afterglow light curve of the short-hard GRB 130603B is indeed consistent with such a model. However, as shown in this Letter, the limited {\em HST} near-infrared lightcurve behavior can also be interpreted as the synchrotron radiation of the external shock driven by a wide mildly relativistic outflow. In such a scenario, the radio emission is expected to peak with a flux of $\sim 100 \mu$Jy, which is detectable for current radio arrays. Hence, the radio afterglow data can provide complementary evidence on the nature of the bump in GRB 130603B. It is worth noting that good spectroscopy during the bump phase in short-hard bursts can test validity of either model above, analogous to spectroscopy of broad-lined Type Ic supernova in long-soft GRBs.Comment: 4 pages, 2 figures, published in ApJ Lette

The lightcurve of the macronova associated with the long-short burst GRB 060614

The {\it Swift}-detected GRB 060614 was a unique burst that straddles an imaginary divide between long- and short-duration gamma-ray bursts (GRBs), and its physical origin has been heavily debated over the years. Recently, a distinct very-soft F814W-band excess at $t\sim 13.6$ days after the burst was identified in a joint-analysis of VLT and HST optical afterglow data of GRB~060614, which has been interpreted as evidence for an accompanying Li-Paczynski macronova (also called a kilonova). Under the assumption that the afterglow data in the time interval of $1.7-3.0$ days after the burst are due to external forward shock emission, when this assumption is extrapolated to later times it is found that there is an excess of flux in several multi-band photometric observations. This component emerges at $\sim$4 days after the burst, and it may represent the first time that a multi-epoch/band lightcurve of a macronova has been obtained. The macronova associated with GRB 060614 peaked at $t\lesssim 4$ days after the burst, which is significantly earlier than that observed for a supernova associated with a long-duration GRB. Due to the limited data, no strong evidence for a temperature evolution is found. We derive a conservative estimate of the macronova rate of $\sim 16.3^{+16.3}_{-8.2}~{\rm Gpc^{-3}}{\rm yr^{-1}}$, implying a promising prospect for detecting the gravitational wave radiation from compact object mergers by upcoming Advanced LIGO/VIRGO/KAGRA detectors (i.e., the rate is ${\cal R}_{\rm GW} \sim 0.5^{+0.5}_{-0.25}(D/200~{\rm Mpc})^{3}~{\rm yr^{-1}}$).Comment: The version published in ApJL. Fig.1 has been updated, main conclusions are unchange

Herders’ Attitude and Decision Making in Stocking Rates and Implication for Grassland Management in China

Overgrazing is widely acknowledged to be the main driver of grassland degradation. Governments seeking to address the grassland degradation problem have therefore focused on policies designed to reduce overgrazing. The Chinese government has implemented a series of policies with the aim of protecting the grasslands from more serious degradation. The efficacy of these policies has been questioned given that, since their introduction, stocking rates have remained high in many affected areas. It has been suggested that the government should enhance grassland monitoring and the punishment of overgrazing. Increasing penalties would reduce stocking rates, however that is likely to cause more social and economic problems. A survey of 1588 herders found that 40% consider the government should set a grass-animal balance rule and implement it strictly, and 30% think the government should only recommend a grass-animal balance rule for herders to follow，only 30% of herders were likely to set the stocking rate by themselves. An analysis of the survey data showed that household expenditure (food, house, clothing, medical expenses) was an important driver of overgrazing. Most herders depend upon their livestock for income; only 3% have a part-time job, compared to over 30% of all Chinese farmers. On average 70% of a herders’ income comes from livestock production and around 26% comes from subsidies. The contingent model developed found that, increasing subsidies with punishment for non-compliance, should reduce stocking rates and help maintain herders’ income, but not to the level required to alleviate poverty and unsustainable. We suggest that herders need training to improve their business skills so they can move from a focus on survival to one where optimising production and better marketing, are the aims of their livestock enterprise. Demonstration farms need to be part of this training