Xuchao Zhao

The Afterglow of GRB 130427A from 1 to 10^(16) GHz

We present multiwavelength observations of the afterglow of GRB 130427A, the brightest (in total fluence) gamma-ray burst (GRB) of the past 29 yr. Optical spectroscopy from Gemini-North reveals the redshift of the GRB to be z = 0.340, indicating that its unprecedented brightness is primarily the result of its relatively close proximity to Earth; the intrinsic luminosities of both the GRB and its afterglow are not extreme in comparison to other bright GRBs. We present a large suite of multiwavelength observations spanning from 300 s to 130 days after the burst and demonstrate that the afterglow shows relatively simple, smooth evolution at all frequencies, with no significant late-time flaring or rebrightening activity. The entire data set from 1 GHz to 10 GeV can be modeled as synchrotron emission from a combination of reverse and forward shocks in good agreement with the standard afterglow model, providing strong support to the applicability of the underlying theory and clarifying the nature of the GeV emission observed to last for minutes to hours following other very bright GRBs. A tenuous, wind-stratified circumburst density profile is required by the observations, suggesting a massive-star progenitor with a low mass-loss rate, perhaps due to low metallicity. GRBs similar in nature to GRB 130427A, inhabiting low-density media and exhibiting strong reverse shocks, are probably not uncommon but may have been difficult to recognize in the past owing to their relatively faint late-time radio emission; more such events should be found in abundance by the new generation of sensitive radio and millimeter instruments.

Tests for Standard Accretion Disk Models by Variability in Active Galactic Nuclei

In this paper, standard accretion disk models of active galactic nuclei (AGNs) are tested using light curves of 26 objects that have been well observed using reverberation mapping. Timescales of variations are estimated by the most common definition of the variability timescale and the zero-crossing time of the autocorrelation function of the optical light curves for each source. The timescales of variations measured by the two methods are consistent with each other. If the typical value of the viscosity parameter alpha similar to 0.1 is adopted, the measured optical variability timescales are closest to the thermal timescales of the standard disks. If alpha is allowed to range from similar to 0.03 to similar to 0.2, the measured timescales are consistent with the thermal timescales of the standard disks. There is a linear relation between the measured variability timescales and black hole masses; this linear relation is qualitatively consistent with expectation of the standard accretion disk models. The time lags measured by the z-transformed discrete correlation function (ZDCF) between different bands are on the order of days. The measured time lags of NGC 4151 and NGC 7469 are marginally consistent with the time lags estimated in the case of continuum thermal reprocessing for the standard accretion disk models. However, the measured time lags of NGC 5548 and Fairall 9 are unlikely to be the case of continuum thermal reprocessing. Our results are unlikely to be inconsistent with, or are likely to be conditionally in favor of, the standard accretion disk models of AGNs.

Identifying the location in the host galaxy of the short GRB 111117A with the Chandra subarcsecond position

We present our successful Chandra program designed to identify, with subarcsecond accuracy, the X-ray afterglow of the short GRB 111117A, which was discovered by Swift and Fermi. Thanks to our rapid target of opportunity request, Chandra clearly detected the X-ray afterglow, though no optical afterglow was found in deep optical observations. The host galaxy was clearly detected in the optical and near-infrared band, with the best photometric redshift of z = 1.31(-0.23)(+0.46) (90% confidence), making it one of the highest known short gamma-ray burst ( GRB) redshifts. Furthermore, we see an offset of 1.0 +/- 0.2 arcsec, which corresponds to 8.4 +/- 1.7 kpc, between the host and the afterglow position. We discuss the importance of using Chandra for obtaining subarcsecond X-ray localizations of short GRB afterglows to study GRB environments.

Stardust Investigation into the CR Chondrite Grove Mountain 021710

We report the presolar grain inventory of the CR chondrite Grove Mountain 021710. A total of 35 C-anomalous grains (∼236 ppm) and 112 O-anomalous grains (∼189 ppm) were identified in situ using NanoSIMS ion imaging. Of 35 C-anomalous grains, 28 were determined to be SiC grains by Auger spectroscopy. Seven of the SiC grains were subsequently measured for N and Si isotopes, allowing classification as one nova grain, one Y grain, one Z grain, and four mainstream grains. Eighty-nine out of 112 O-anomalous grains belong to Group 1, indicating origins in low-to-intermediate-mass red giant and asymptotic giant branch stars. Twenty-one are Group 4 grains and have origins in supernovae. Auger spectroscopic elemental measurements of 35 O-anomalous grains show that 33 of them are ferromagnesian silicates. They have higher Mg/(Mg+Fe) ratios than those reported in other meteorites, suggesting a lower degree of alteration in the nebula and/or asteroid parent bodies. Only two oxide grains were identified, with stoichiometric compositions of MgAl2O4 and SiO2, respectively. The presolar silicate/oxide ratio of GRV 021710 is comparable with those of the CR3 chondrites (QUE 99177 and MET 00426) and primitive interplanetary dust particles. In order to search for presolar sulfides, the meteorite was also mapped for S isotopes. However, no presolar sulfides were found, suggesting a maximum abundance of 2 ppm. The scarcity of presolar sulfides may be due to their much faster sputtering rate by cosmic rays compared to silicates.

FIRST LABORATORY OBSERVATION OF SILICA GRAINS FROM CORE COLLAPSE SUPERNOVAE

We report the discovery of two supernova silica (SiO2) grains in the primitive carbonaceous chondrites LaPaZ 031117 and Grove Mountains 021710. Only five presolar silica grains have been previously reported from laboratory measurements but they all exhibit enrichments in 17O relative to solar, indicating origins in the envelopes of asymptotic giant branch stars. The two SiO2 grains identified in this study are characterized by moderate enrichments in 18O relative to solar, indicating that they originated in Type II supernova ejecta. If compared to theoretical models, the oxygen isotopic compositions of these grains can be reproduced by mixing of different supernova zones. While both theoretical models of grain condensation and recent NASA Spitzer Space Telescope observations have suggested the presence of silica in supernova ejecta, no such grains had been identified, until now, in meteorites. The discovery of these two silica grains provides definitive evidence of the condensation of silica dust in supernova ejecta.

THE TEMPORAL AND SPECTRAL CHARACTERISTICS OF “FAST RISE AND EXPONENTIAL DECAY” GAMMA-RAY BURST PULSES

In this paper, we have analyzed the temporal and spectral behavior of 52 fast rise and exponential decay (FRED) pulses in 48 long-duration gamma-ray bursts (GRBs) observed by the CGRO/BATSE, using a pulse model with two shape parameters and the Band model with three shape parameters, respectively. It is found that these FRED pulses are distinguished both temporally and spectrally from those in the long-lag pulses. In contrast to the long-lag pulses, only one parameter pair indicates an evident correlation among the five parameters, which suggests that at least four parameters are needed to model burst temporal and spectral behavior. In addition, our studies reveal that these FRED pulses have the following correlated properties: (1) long-duration pulses have harder spectra and are less luminous than short-duration pulses and (2) the more asymmetric the pulses are, the steeper are the evolutionary curves of the peak energy (E(p)) in the nu f(nu) spectrum within the pulse decay phase. Our statistical results give some constraints on the current GRB models.

ENERGY-DEPENDENT GAMMA-RAY BURST PULSE WIDTH DUE TO THE CURVATURE EFFECT AND INTRINSIC BAND SPECTRUM

Previous studies have found that the width of the gamma-ray burst (GRB) pulse is energy dependent and that it decreases as a power-law function with increasing photon energy. In this work we have investigated the relation between the energy dependence of the pulse and the so-called Band spectrum by using a sample including 51 well-separated fast rise and exponential decay long-duration GRB pulses observed by BATSE (Burst and Transient Source Experiment on the Compton Gamma Ray Observatory). We first decompose these pulses into rise and decay phases and find that the rise widths and the decay widths also behave as a power-law function with photon energy. Then we investigate statistically the relations between the three power-law indices of the rise, decay, and total width of the pulse (denoted as delta(r), delta(d), and delta(w), respectively) and the three Band spectral parameters, high-energy index (alpha), low-energy index (beta), and peak energy (E-p). It is found that (1) alpha is strongly correlated with delta(w) and delta(d) but seems uncorrelated with delta(r); (2) beta is weakly correlated with the three power-law indices, and (3) E-p does not show evident correlations with the three power-law indices. We further investigate the origin of delta(d)-alpha and delta(w)-alpha. We show that the curvature effect and the intrinsic Band spectrum could naturally lead to the energy dependence of the GRB pulse width and also the delta(d)-alpha and delta(w)-alpha correlations. Our results hold so long as the shell emitting gamma rays has a curved surface and the intrinsic spectrum is a Band spectrum or broken power law. The strong delta(d)-alpha correlation and inapparent correlations between delta(r) and the three Band spectral parameters also suggest that the rise and decay phases of the GRB pulses have different origins.

Stardust material in the paired enstatite chondrites: SAH 97096 and SAH 97159

Stardust grains, more commonly referred to as presolar grains, are solid condensates of stars that are studied in terrestrial laboratories with a variety of analytical techniques. Here we report on submicrometer silicate, oxide and carbonaceous stardust grains identified in the paired enstatite chondrites SAH 97096 and SAH 97159. A majority of the grains with O isotopic anomalies exhibit O excesses and probably originated in the dusty envelopes of low-mass AGB or RG stars. One grain is highly Orich and has a normal Si isotopic composition; based on its O and Si isotopic composition, an origin in a nova is most likely. However, another scenario that may explain this grain’s O isotopic composition is a binary star system consisting of an evolved or mainstream star accreting material from its nova companion. Elemental characterization of the O-anomalous grains shows the presence of eleven magnesian silicate grains with or without Fe and three Fe-oxide grains; none of the grains contain Ca or Al. Carbon-anomalous grains have C/C ratios from 19–78; most are probably SiC. The abundances of the Oand C-anomalous grains are 98±34 and 51±13 ppm, respectively, which is much higher than previously observed in other enstatite chondrites, and close to that of some carbonaceous chondrites.