两类γ暴的谱形和光变曲线的统计差异的K-S检验

最近的研究表明：短γ射线暴（γ暴）的辐射特性和长γ暴的前两秒的辐射特征相似，这引发了对以前争论的问题，即：这两类暴是否是本质上相同的暴？本文主要对和γ暴谱形和光变曲线有关的两个量的分布进行了K—S检验。分析表明：这两类暴的分布是不同的，这和广为接受的两类γ暴事件产生于不同机制的观点是一致的。     

A comparison of temporal properties of short and long gamma-ray bursts

In this paper, we have performed a temporal analysis of single pulses from short (46) and long (51) gamma-ray bursts (GRBs) to investigate possible differences in their properties. In particular we pay detailed attention to the asymmetry of pulses. We find that the asymmetry ratio of short GRB pulses clusters around 0.81 and that these pulses are on average more symmetric than those from long GRBs, which have an average value of 0.47. In addition we note that the pulses in short GRBs display exponential rises and fast decays (ERFD) in comparison the fast rise exponential decays (FRED) pulses of long GRBs. The asymmetry ratio does not depend on the full width at half maximum (FWHM) and does not vary with energy channel. Moreover, there is a general trend for slower pulses to be more asymmetric. Finally, we deduce that the asymmetry could be used to probe the emission mechanisms of GRBs.     

Observational Characteristics of γ-ray Bursts and Their Early X-ray Afterglows

Data obtained by the on-board X-ray telescope of the Swift satellite show that a shallow decay component is present in the light curve of the early X-ray afterglows of some γ-ray bursts (GRBs), but not in others. The physical mechanism of this component is debatable. We have made a comparative study on the observational characteristics of the two kinds of GRBs for a sample of 29 GRBs. Our results demonstrate that the two kinds of GRBs have no significant difference in the burst duration, γ-ray flux, spectral index, hardness ratio and peak energy. However, a significant difference exists in the early X-ray afterglows of the bursts: the bursts with a shallow decay component tend to have a softer and fainter X-ray afterglow than those without a shallow decay component. The efficiency of the γ-ray radiation is also very different for the two kinds of bursts: it is obviously higher for the bursts with a shallow decay component than those without. These results seem to suggest that the progenitors and central engines of the two kinds of GRBs are similar, and that the appearance of the shallow decay component is probably due to the surrounding medium.     

The Early X-ray Afterglows of Optically Bright and Dark Gamma-Ray Bursts

A systematic study on the early X-ray afterglows of both optically bright and dark gamma-ray bursts (B-GRBs and D-GRBs) observed by Swift is presented. Our sample includes 25 GRBs of which 13 are B-GRBs and 12 are D-GRBs. Our results show that the distributions of the X-ray afterglow fluxes (Fx), the gamma-ray fluxes (5r), and the ratio (Rr,x.) are similar for the two kinds of GRBs, that any observed differences should be simply statistical fluctuation. These results indicate that the progenitors of the two kinds of GRBs are of the same population with comparable total energies of explosion. The suppression of optical emission in the D-GRBs should result from circumburst but not from their central engine.     

Consistency Analysis of the Data of GRBs Observed by Different Satellites

The BATSE (Compton Gamma-ray Observatory/Bursts and Transient Source Experiment), Swift (Swift Gamma-ray Burst Explorer) and Fermi (Fermi Gamma-ray Space Telescope) satellites have provided us with a large amount of GRB sample. After a systematical study and comparison of the properties of the GRBs detected by these 3 kinds of instruments, it is found that although significant differences exist in the number of red-shifted bursts, the proportion of long bursts in total bursts and the photon flux distribution (lgN − lg P), but no significant difference exists in the duration time, the total flux of Gamma-ray radiation and the spectral hardness ratio. Considering that the observational energy bands of the Swift- and Fermi-detected bursts are different, after making corrections the difference in the lgN − lg P distribution can be basically eliminated. The differences in the number of red-shifted bursts and the proportion of long bursts in total bursts are caused by the different sensitivities of instruments. Namely, various instruments give different observed characteristics of GRBs, but they are consistent in nature.     

The Kolmogorov-Smirnov test for three redshift distributions of long gamma-ray bursts in the Swift Era

We investigate redshift distributions of three long burst samples, with the first sample containing 131 long bursts with observed redshifts, the second including 220 long bursts with pseudo-redshifts calculated by the variability-luminosity relation, and the third including 1194 long bursts with pseudo-redshifts calculated by the lag-luminosity relation, respectively. In the redshift range 0-1 the Kolmogorov-Smirnov probability of the observed redshift distribution and that of the variability-luminosity relation is large. In the redshift ranges 1-2, 2-3, 3-6.3 and 0-37, the Kolmogorov-Smimov probabilities of the redshift distribution from lag-luminosity relation and the observed redshift distri-bution are also large. For the GRBs, which appear both in the two pseudo-redshift burst samples, the KS probability of the pseudo-redshift distribution from the lag-luminosity relation and the observed reshifi distribution is 0.447, which is very large. Based on these results, some conclusions are drawn: I) the V-Liso relation might be more believable than the τ-Liso relation in low redshift ranges and the τ-Liso relation might be more real than the V-Liso relation in high redshift ranges; ii) if we do not consider the redshift ranges, the τ-Liso relation might be more physical and intrinsical than the V-Liso relation.     

Population synthesis of gamma-ray bursts with precursor activity and the spinar paradigm

We study statistical properties of long gamma-ray bursts (GRBs) produced by the collapsing cores of WR stars in binary systems. Fast rotation of the cores enables a two-stage collapse scenario, implying the formation of a spinar-like object. A burst produced by such a collapse consists of two pulses, whose energy budget is enough to explain observed GRBs. We calculate models of spinar evolution using results from a population synthesis of binary systems (done by the 'Scenario Machine') as initial parameters for the rotating massive cores. Among the resulting bursts, events with a weaker first peak, namely precursor, are identified, and the precursor-to-main-pulse time separations fully agree with the range of the observed values. The calculated fraction of long GRBs with precursor (about 10 per cent of the total number of long GRBs) and the durations of the main pulses are also consistent with observations. Precursors with lead times greater by up to one order of magnitude than those observed so far are expected to be about a factor of 2 less numerous. Independently of the GRB model assumed, we predict the existence of precursors that arrive up to  ≳10³ s  in advance of the main events of GRBs.     

Analysis of TTE/BATSE time profiles for short gamma-ray bursts

The TTE/BATSE time profiles for short gamma-ray bursts (GRBs) are analyzed. A sample of 287 short GRBs and a sample of 143 background regions are studied. Bursts similar to BRBs with precursors and bursts with time profiles that are not encountered among the bursts whose time profiles were investigated by using a combination of DISCSC and PREB data. In addition, there are fewer events with single-peak time profiles among short GRBs than among long GRBs (many double and triple bursts). A fractal analysis of the TTE time profiles for short GRBs is performed. According to the TTE data, the range of fractal dimensions for short bursts is 0.80≤D≤2.25. The derived fractal-dimension distribution exhibits two peaks that correspond to a similar distribution obtained previously by reducing the DISCSC data for short GRBs (D=1.44±0.02 and D=1.90±0.03) and a third peak (D=1.05±0.03). The bursts with 〈D〉=1.90±0.03 correspond to events in whose sources the processes with long-term variations and (or) quasi-periodicity take place, while the event time profiles with a fractal dimension 〈D〉=1.05±0.03 correspond to events in whose sources many extremely short random processes take place. A more detailed analysis of a subgroup of bursts with D=1.44±0.02 shows that its fractal dimension distribution is broader than that for a group with the same (within the limits of the measurement errors) D. At least two more GRB subgroups can be distinguished in this subgroup: (1) bursts with 〈D〉=1.51±0.04; according to the TTE data, their fractal dimensions correspond to those of the background; i.e., these are events with smooth time profiles without any variability in the sources on the time scales on which the fractal dimension is analyzed; and (2) bursts with 〈D〉=1.31±0.05, whose time profiles can correspond to those of the events obtained from the fireball model with internal shock waves. We present time profiles for the events obtained by using this model. The range of fractal dimensions for the modeled time profiles is 1.213≤D≤1.400, with the fractal dimensions for such an event and for the real GRB 990208 being equal, within the error limits, for some model parameters. A study of the TTE and DISCSC fractal-dimension distributions for the background indicates that the fractal dimension distributions obtained by analyzing these two types of data can be processed simultaneously.     

Comparison between Swift and pre-Swift gamma-ray bursts

The gamma-ray burst (GRB) mission Swift has made a much deeper GRBsurvey than any previous one. I present a systematical comparison between GRB samples detected with pre-Swift missions and those from Swift, in order to investigate whether they show any statistical difference. Our Swift GRB sample includes the bursts detected by Swift/BAT before 2007 September. With both flux-limited surveys and redshift-known GRB samples, I show that, apparently, the observed distributions of the redshifts, T90, and log N-log P are significantly different, but not for the spectral hardness ratio, fluence and Eiso. The redshifts of the Swift GRB sample are statistically larger than those of pre-Swift GRBs, with a mean of 1.95±0.17 compared to ～ 1 for pre-Swift GRBs. The cosmological effect on the observables is thus considerable. This effect on the spectral hardness ratio, fluence and Eiso is cancelled out, and the distributions of these quantities indeed do not show significant differences between the Swift and pre-Swift GRBs. Taking this effect into account, I found that the corrected distributions of T90 for long GRBs and log N - log P observed with Swift/BAT are also consistent with those observed with CGRO/BATSE. These results indicate that the Swift and pre-Swift GRBs are from the same population.     

Classification of BeppoSAX’s gamma-ray bursts

The BeppoSAX Catalog has been very recently published. In this paper we analyze—using the Maximum Likelihood (ML) method—the duration distribution of the 1003 GRBs listed in the catalog with duration. The ML method can identify the long and the intermediate duration groups. The short population of the bursts is identified only at a 96% significance level. MC simulation has been also applied and gives a similar significance level; 95%. However, the existence of the short bursts is not a scientific question after the Compton Gamma-Ray Observatory’s observation. Our minor result is this well-known fact that in the BeppoSAX data the short bursts are under-represented, mainly caused by the different triggering system. Our major result is the identification of the intermediate group in the BeppoSAX data. Therefore, four different satellites (CGRO, Swift, RHESSI and BeppoSAX) observed the intermediate type Gamma-Ray Burst.     

伽玛暴及其早期X射线余辉的观测特征

Swift卫星的X射线望远镜观测揭示部分伽玛暴的早期余辉光变曲线有一个缓慢衰减的成分,而相当一部分却没有这样的成分.研究比较这两种暴的观测性质发现两类暴的持续时间、伽玛辐射总流量、谱指数、谱硬度比峰值能量等物理量均没有显著差异.然而有该成分的那些伽玛暴谱比较软、早期X射线余辉比较弱、伽玛射线辐射效率显著高于没有这个成分的那些暴.结果表明两类暴的前身星和中心机制一致,是否呈现这个缓慢衰减成分可能取决于外部介质.     

The correlation between black hole mass and bulge velocity dispersion in hierarchical galaxy formation models

Popular models for the origin of gamma-ray bursts (GRBs) include short-lived massive stars as the progenitors of the fireballs. Hence the redshift distribution of GRBs should track the cosmic star formation rate of massive stars accurately. A significant proportion of high-mass star formation activity appears to occur in regions that are obscured from view in the optical waveband by interstellar dust. The amount of dust-enshrouded star formation activity taking place has been estimated by observing the thermal radiation from the dust that has been heated by young stars in the far-infrared and submillimetre wavebands. Here we discuss an alternative probe – the redshift distribution of GRBs. GRBs are detectable at the highest redshifts, and because gamma-rays are not absorbed by dust, the redshift distribution of GRBs should therefore be unaffected by dust extinction. At present the redshifts of GRBs can only be determined from the associated optical transient emission; however, useful information about the prevalence of dust-obscured star formation can also be obtained from the ratio of GRBs with and without an associated optical transient. Eight GRBs currently have spectroscopic redshifts. Once about a hundred redshifts are known, the population of GRBs will provide an important test of different models of the star formation history of the Universe.     

Bimodal distribution of short gamma-ray bursts: Evidence for two distinct types of short gamma-ray bursts

GRB 170817A was confirmed to be associated with GW170817, which was produced by a neutron star - neutron star merger. It indicates that at least some short gamma-ray bursts come from binary neutron star mergers. Theoretically, it is widely accepted that short gamma-ray bursts can be produced by two distinctly different mechanisms, binary neutron star mergers and neutron star - black hole mergers. These two kinds of bursts should be different observationally due to their different trigger mechanisms. Motivated by this idea, we collect a universal data set constituted of 51 short gamma-ray bursts observed by Swift/BAT, among which 14 events have extended emission component. We study the observational features of these 51 events statistically. It is found that our samples consist of two distinct groups. They clearly show a bimodal distribution when their peak photon fluxes at 15–150 keV band are plotted against the corresponding fluences. Most interestingly, all the 14 short bursts with extended emission lie in a particular region of this plot. When the fluences are plotted against the burst durations, short bursts with extended emission again tend to concentrate in the long duration segment. These features strongly indicate that short gamma-ray bursts really may come from two distinct types of progenitors. We argue that those short gamma-ray bursts with extended emission come from the coalescence of neutron stars, while the short gamma-ray bursts without extended emission come from neutron star - black hole mergers.     

Quiescent times in gamma-ray bursts – I. An observed correlation between the durations of subsequent emission episodes

Although more than 2000 astronomical gamma-ray bursts (GRBs) have been detected, the precise progenitor responsible for these events is unknown. The temporal phenomenology observed in GRBs can significantly constrain the different models. Here we analyse the time histories of a sample of bright, long GRBs, searching for the ones exhibiting relatively long (more than 5 per cent of the total burst duration) 'quiescent times', defined as the intervals between adjacent episodes of emission during which the gamma-ray count rate drops to the background level. We find a quantitative relation between the duration of an emission episode and the quiescent time elapsed since the previous episode. We suggest here that the mechanism responsible for the extraction and the dissipation of energy has to take place in a metastable configuration, such that the longer the accumulation period, the higher the stored energy available for the next emission episode.     

Evidence for luminosity evolution of long gamma-ray bursts in Swift data

We compute the luminosity function (LF) and the formation rate of long gamma-ray bursts (GRBs) by fitting the observed differential peak flux distribution obtained by the Burst and Transient Source Experiment (BATSE) in two different scenarios: (i) the GRB luminosity evolves with redshift and (ii) GRBs form preferentially in low-metallicity environments. In both cases, model predictions are consistent with the Swift number counts and with the number of detections at   z > 2.5  and >3.5. To discriminate between the two evolutionary scenarios, we compare the model results with the number of luminous bursts (i.e. with isotropic peak luminosity in excess of 10⁵³ erg s⁻¹) detected by Swift in its first 3 yr of mission. Our sample conservatively contains only bursts with good redshift determination and measured peak energy. We find that pure luminosity evolution models can account for the number of sure identifications. In the case of a pure density evolution scenario, models with   Z _th > 0.3 Z_⊙  are ruled out with high confidence. For lower metallicity thresholds, the model results are still statistically consistent with available lower limits. However, many factors can increase the discrepancy between model results and data, indicating that some luminosity evolution in the GRB LF may be needed also for such low values of Z _th. Finally, using these new constraints, we derive robust upper limits on the bright end of the GRB LF, showing that this cannot be steeper than ∼2.6.     

Non-thermal transient sources from rotating black holes

Rotating black holes can power the most extreme non-thermal transient sources. They have a long-duration viscous time-scale of spin-down, and produce non-thermal emissions along their spin-axis, powered by a relativistic capillary effect. We report on the discovery of exponential decay in Burst and Triensient Source Experiment (BATSE) light curves of long gamma-ray bursts (GRBs) by matched filtering, consistent with a viscous time-scale, and identify ultra-high energy cosmic rays (UHECRs) about the Greisen–Zatsepin–Kuzmin (GZK) threshold with linear acceleration of ion contaminants along the black hole spin-axis, consistent with black hole masses and lifetimes of Fanaroff–Riley type II (FR II) active galactic nuclei (AGN). We explain the absence of UHECRs from BL Lac objects due to UHECR emissions preferably at appreciable angles away from the black hole spin-axis. Black hole spin may be the key to unification of GRBs and their host environments, and to AGN and their host galaxies. Our model points to long-duration bursts in radio from long GRBs without supernovae and gravitational waves from all long GRBs.     

Superlong gamma-ray bursts

We searched for anomalously long gamma-ray bursts (GRBs) in the archival records of the Burst and Transient Sources Experiment (BATSE). Ten obvious superlong (>500 s) GRBs with almost continuous emission episodes were found. Nine of these events were known from the BATSE catalog, but five had no duration estimates; we found one burst for the first time. We also detected events with emission episodes separated by a long period of quiescence (up to ～1000 s) with a total duration of 1000–2000 s. In the latter case, we cannot reach an unequivocal conclusion about a common origin of the episodes due to the BATSE poor angular resolution. However, for most of these pairs, the probability of independent GRBs coinciding is much lower than unity, and the probability that all of these are coincidences is ～10^?8. All of the events have a hardness ratio (the ratio of the count rates in different energy channels) typical of GRBs, and their unique duration is unlikely to be related to their high redshifts. Superlong bursts do not differ in their properties from typical long (>2 s) GRBs. We estimated the fraction of superlong GRBs (>500 s) among the long (>2 s)GRBs in the BATSE sample with fluxes up to 0.1 ph cm^?2 s^?1 to be between 0.3 and 0.5%, which is higher than the estimate based on the BATSE catalog.     

Catalog of short gamma-ray transients detected in the SPI/INTEGRAL experiment

We analyzed the data obtained by the SPI telescope onboard the INTEGRAL observatory to search for short transient events with a duration from 1 ms to a few tens of seconds. An algorithm for identifying gamma-ray events against the background of a large number of charged particle interactions with the detector has been developed. The classification of events was made. Apart from the events associated with cosmic gamma-ray bursts (GRBs) confirmed by other space experiments and the activity of known soft gamma repeaters (for example, SGR 1806-20), previously unreported GRBs have been found. GRB candidates and short gamma-ray events probably associated with the activity of known SGRs and AXPs have been selected. The spectral evolution of 28 bright GRBs from the catalog has been studied extensively. A new method for investigating the spectral evolution is proposed. The energy dependence of the spectral lag for bursts with a simple structure of their light curves and for individual pulses of multipulse events is shown to be described by a logarithmic function, lag ～ Alog(E). It has been established that the parameter A depends on the pulse duration, with the dependence being universal for all of the investigated GRBs. No negative spectral lags have been detected for bursts with a simple structure of their light curves.     

Torus formation in neutron star mergers and well-localized short gamma-ray bursts

Merging neutron stars (NSs) are hot candidates for the still enigmatic sources of short gamma-ray bursts (GRBs). If the central engines of the huge energy release are accreting relic black holes (BHs) of such mergers, it is important to understand how the properties of the BH–torus systems, in particular disc masses and mass and rotation rate of the compact remnant, are linked to the characterizing parameters of the NS binaries. For this purpose, we present relativistic smoothed particle hydrodynamic simulations with conformally flat approximation of the Einstein field equations and a physical, non-zero temperature equation of state. Thick disc formation is highlighted as a dynamical process caused by angular momentum transfer through tidal torques during the merging process of asymmetric systems or in the rapidly spinning triaxial post-merger object. Our simulations support the possibility that the first well-localized short and hard GRBs 050509b, 050709, 050724, 050813 have originated from NS merger events and are powered by neutrino-antineutrino annihilation around a relic BH–torus system. Using model parameters based on this assumption, we show that the measured GRB energies and durations lead to estimates for the accreted masses and BH mass accretion rates which are compatible with theoretical expectations. In particular, the low-energy output and short duration of GRB 050509b set a very strict upper limit of less than 100 ms for the time interval after the merging until the merger remnant has collapsed to a BH, leaving an accretion torus with a small mass of only  ∼0.01 M_⊙  . This favours a (nearly) symmetric NS+NS binary with a typical mass as progenitor system.     

Precursors of short gamma-ray bursts in the SPI-ACS/INTEGRAL experiment

We have analyzed the light curves of 519 short gamma-ray bursts (GRBs) detected in the SPI-ACS/INTEGRAL experiment from December 2002 toMay 2014 to search for precursors (a possible activity of the GRB source before the beginning of themain episode). Both the light curves of 519 individual events and the averaged light curve of 372 brightest bursts have been analyzed. In several cases, we have found and thoroughly studied precursor candidates based on SPI-ACS/INTEGRAL, GBM/Fermi, and LAT/Fermi data. A statistical analysis of the averaged light curve for the entire sample of short bursts has revealed no regular precursor. Upper limits for the relative intensity of precursors have been estimated. We have compared our results with those of other authors; no convincing evidence for the existence of precursors of short GRBs has been found. We show that the fraction of short GRBs with precursors is less than 0.4% of all short bursts.