Xu-Hui Han

Probing the nature of high-z short GRB 090426 with its early optical and X-ray afterglows

GRB 090426 is a short-duration burst detected bySwift (T90 ∼ 1.28 s in the observer frame and T90 ∼ 0.33 s in the burst frame at z = 2.609). Its host galaxy properties and some gamma-rayrelated correlations are analogous to those seen in long-duration gamma-ray bursts (GRBs), which are believed to be of a massive star origin (so-called Type II GRBs). We present the results of its early optical observations with the 0.8-m Tsinghua University–National Astronomical Observatory of China Telescope (TNT) at Xinglong Observatory and the 1-m LOAO telescope at Mt Lemmon Optical Astronomy Observatory in Arizona. Our wellsampled optical afterglow light curve covers from ∼90 to 10 4 s after the GRB trigger. It shows two shallow decay episodes that are likely due to energy injection, which end at ∼230 and 7100 s, respectively. The decay slopes after the injection phases are consistent with each other (α � 1.22). The X-ray afterglow light curve appears to trace the optical, although the second energy-injection phase was missed due to visibility constraints introduced by the Swift orbit. The X-ray spectral index is βX ∼ 1.0 without temporal evolution. Its decay slope is consistent with the prediction of the forward shock model. Both X-ray and optical emission are consistent with being in the same spectral regime above the cooling frequency (νc). The fact that νc is below the optical band from the very early epoch of the observation provides a constraint on the burst environment, which is similar to that seen in classical long-duration GRBs. We therefore suggest that death of a massive star is the possible progenitor of this short burst.

The SVOM gamma-ray burst mission

We briefly present the science capabilities, the instruments, the operations, and the expected performance of the SVOM mission. SVOM (Space-based multiband astronomical Variable Objects Monitor) is a Chinese-French space mission dedicated to the study of Gamma-Ray Bursts (GRBs) in the next decade. The SVOM mission encompasses a satellite carrying four instruments to detect and localize the prompt GRB emission and measure the evolution of the afterglow in the visible band and in X-rays, a VHF communication system enabling the fast transmission of SVOM alerts to the ground, and a ground segment including a wide angle camera and two follow-up telescopes. The pointing strategy of the satellite has been optimized to favor the detection of GRBs located in the night hemisphere. This strategy enables the study of the optical emission in the first minutes after the GRB with robotic observatories and the early spectroscopy of the optical afterglow with large telescopes to measure the redshifts. The study of GRBs in the next decade will benefit from a number of large facilities in all wavelengths that will contribute to increase the scientific return of the mission. Finally, SVOM will operate in the era of the next generation of gravitational wave detectors, greatly contributing to searches for the electromagnetic counterparts of gravitational wave triggers at Xray and gamma-ray energies.

Variations of P2 in subpulse drifting pulsars

We develop a model for subpulse separation period, P2, taking into account both the apparent motion of the visible point as a function of pulsar phase, ψ, and the possibility of abrupt jumps between different rotation states in non-corotating pulsar magnetospheres. We identify three frequencies: (i) the spin frequency of the star, (ii) the drift frequency of the magnetospheric plasma in the source region and (iii) the angular frequency of the visible point around its trajectory. We show how the last of these, which is neglected in traditional models by implicitly assuming the line of sight through the centre of the star, affects the interpretation of P2. We attribute the subpulse structure to emission from m antinodes distributed uniformly in azimuthal angle about the magnetic axis. We show that variations of P2 as a function of rotational phase or observing frequency arise naturally when the motion of the visible point is taken into account. We discuss possible application of our model in signifying overall field-line distortion at the emitting region. Abrupt changes in P2 can occur during state switching in the magnetosphere. We demonstrate that the unique value of P2 in each rotation state can be used, in principle, to relate the rotation state of the magnetospheres to subpulse drifting.

The shallow-decay phase in both the optical and X-ray afterglows of Swift GRB 090529A: energy injection into a wind-type medium?

The energy injection model is the usual choice for interpreting the shallow-decay phase in Swift gamma-ray burst (GRB) X-ray afterglows. However, very few GRBs have simultaneous signatures of energy injection in their optical and X-ray afterglows. Here, we report on the optical observations of GRB 090529A from 2000 s to ∼10 6 s after the burst, in which an achromatic decay is seen at both wavelengths. The optical light curve shows a decay from 0.37 to 0.99, with a break at ∼10 5 s. In the same time interval, the decay indices of the X-ray light curve changed from 0.04 to 1.2. Comparing these values with the closure relations, the segment after 3 × 10 4 s is consistent with the prediction of the forward shock in an interstellar medium without any energy injection. The shallow-decay phase between 2000 and 3 × 10 4 s could be a result of the external shock in a wind-type medium with an energy injection under the condition of νo <ν c <ν x. However, the constraint of the spectral region is not consistent with the multiband observations. For this shallow-decay phase, other models are also possible, such as energy injection with evolving microphysical parameters, a jet viewed off-axis, etc.

Optical light curve of GRB 121011A： a textbook for the onset of GRB afterglow in a mixture of ISM and wind-type medium

We reported the optical observations of GRB 121011A by 0.8-m TNT telescope at Xinglong observatory, China. The light curve of optical afterglow shows a smooth and featureless bump during the epoch of

NUV Star Catalog from the Lunar-based Ultraviolet Telescope Survey: First Release

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A New Serial-direction Trail Effect in CCD Images of the Lunar-based Ultraviolet Telescope

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An unprecedented constraint on water content in the sunlit lunar exosphere seen by Lunar-based Ultraviolet Telescope of Chang׳e-3 mission

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Data processing pipeline for pointing observations of Lunar-based Ultraviolet Telescope

5000 sec with a rising index of

18-Months operation of Lunar-based Ultraviolet Telescope: a highly stable photometric performance

1.57\pm0.28