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Possible violations of Lorentz invariance (LIV) have been investigated for a long time using the observed spectral lags of gamma-ray bursts (GRBs). However, these generally have relied on using a single photon in the highest energy range. Furthermore, the search for LIV lags has been hindered by our ignorance concerning the intrinsic time lag in different energy bands. GRB 160625B, the only burst so far with a well-defined transition from positive lags to negative lags provides a unique opportunity to put new constraints on LIV. Using multiphoton energy bands we consider the contributions to the observed spectral lag from both the intrinsic time lag and the lag by LIV effects, and assuming the intrinsic time lag to have a positive dependence on the photon energy, we obtain robust limits on LIV by directly fitting the spectral lag data of GRB 160625B. Here we show that these robust limits on the quantum gravity energy scales are E QG,1 ≥ 0.5 × 10 16 GeV for the linear, and E QG,2 ≥ 1.4 × 10 7 GeV for the quadratic LIV effects, respectively. In addition, we give for the first time a reasonable formulation of the intrinsic energy-dependent time lag.

The afterglows of gamma-ray bursts (GRBs) have commonly been assumed to be due to shocks sweeping up the circum-stellar medium. However, most GRBs have been found in dense star-forming regions where a significant fraction of the prompt X-ray emission can be scattered by dust grains. Here we revisit the behavior of dust scattering of X-rays in GRBs. We find that the features of some X-ray afterglows from minutes to days after the gamma-ray triggers are consistent with the scattering of prompt X-ray emission from GRBs off host dust grains. This implies that some of the observed X-ray afterglows (especially those without sharp rising and decaying flares) could be understood with a dustscattering-driven emission model. Subject headings: dust, extinction -gamma rays: bursts -interstellar medium -X-rays: generalRecently, an X-ray halo around the short GRB 050724 was detected by XRT, with both a radial temporal evolution and intensity distribution that are consistent with the properties of Galactic dust-scattering (Vaughan et al. 2006). Previously, GRB 031203 (Vaughan et al.

We perform a statistical analysis of the temporal and spectral properties of the latest Fermi gamma-ray bursts (GRBs) to revisit the classification of GRBs. We find that the bimodalities of duration and the energy ratio (E peak /Fluence) and the anti-correlation between spectral hardness (hardness ratio (HR), peak energy and spectral index) and duration (T 90 ) support the long/soft − short/hard classification scheme for Fermi GRBs. The HR − T 90 anti-correlation strongly depends upon the spectral shape of GRBs and energy bands, and the bursts with the curved spectra in the typical BATSE energy bands show a tighter anti-correlation than those with the power-law spectra in the typical BAT energy bands. This might explain why the HR − T 90 correlation is not evident for those GRB samples detected by instruments like Swift with a narrower/softer energy bandpass. We also analyze the intrinsic energy correlation for the GRBs with measured redshifts and well defined peak energies. The current sample suggests E p,rest = 2455 × (E iso /10 52 ) 0.59 for short GRBs, significantly different from that for long GRBs. However, both the long and short GRBs comply with the same E p,rest −L iso correlation.