guilanz's Zhang:B [18 articles]

Abstract

The onset of GRB afterglow is characterized by a smooth bump in the early afterglow lightcurve. We make an extensive search for such a feature. Twenty optically selected GRBs and 12 X-ray selected GRBs are found, among which 17 optically selected GRBs and 2 X-ray-selected GRBs have redshift measurements. We fit the lightcurves with a smooth broken power-law and measure the temporal characteristic timescales of the bumps at FWHM. Strong mutual correlations among these timescales are found, and a dimmer and broader bump tends to peak at a ...

Abstract

Swift triggered on a precursor to the main burst of GRB 061121 (z=1.314), allowing observations to be made from the optical to gamma-ray bands. Many other telescopes, including Konus-Wind, XMM-Newton, ROTSE and the Faulkes Telescope North, also observed the burst. The gamma-ray, X-ray and UV/optical emission all showed a peak ~75s after the trigger, although the optical and X-ray afterglow components also appear early on - before, or during, the main peak. Spectral evolution was seen throughout the burst, with the prompt emission showing a clear positive correlation ...

Abstract

Long duration gamma-ray bursts (GRBs) are thought to be produced by the core-collapse of a rapidly-rotating massive star. This event generates a highly relativistic jet and prompt gamma-ray and X-ray emission arises from internal shocks in the jet or magnetised outflows. If the stellar core does not immediately collapse to a black hole, it may form an unstable, highly magnetised millisecond pulsar, or magnetar. As it spins down, the magnetar would inject energy into the jet causing a distinctive bump in the GRB light curve where the emission ...

Abstract

We have gathered optical photometry data from the literature on a large sample of Swift-era gamma-ray burst (GRB) afterglows including GRBs up to April 2009, for a total of 72 GRBs, and present an additional three pre-Swift GRBs not included in an earlier sample. Furthermore, we publish 791 additional new photometry data points on a total of 41 GRB afterglows, including large data sets for GRB 050319, GRB 050802, GRB 050820A, GRB 050922C, GRB 060418 and GRB 080810. We analyzed the light curves of all GRBs in ...

Abstract

We use a large sample of Gamma-Ray Burst (GRB) afterglow and prompt emission data to compare the optical afterglows (or the lack thereof) of “short/hard” Type I GRBs (those that are assumed not to be due to the death of massive stars, but, e.g., merger of compact objects) with those of “long/soft” Type II GRBs (those due to the core collapse of massive stars). In comparison to the afterglows of Type II GRBs, we find that those of Type I GRBs have a lower average luminosity and ...

Abstract

We analyze the properties of the shallow decay segment with a sample of 53 long Swift GRBs. We show that the distributions of its characteristics are log-normal or normal, and its isotropic X-ray energy (E_iso,X) is linearly correlated with the prompt gamma-ray energy, but with a steeper photon spectrum except for some X-ray flashes. No significant spectral evolution is observed from this phase to the follow-up phase, and the follow-up phase is usually consistent with the external shock models, implying that this segment likely due to a refreshed ...

Abstract

The cosmological gamma-ray burst (GRB) phenomenon is reviewed. The broad observational facts and empirical phenomenological relations of the GRB prompt emission and afterglow are outlined. A well-tested, successful fireball shock model is introduced in a pedagogical manner. Several important uncertainties in the current understanding of the phenomenon are reviewed, and prospects of how future experiments and extensive observational and theoretical efforts may address these problems are discussed. ...

Abstract

(Abridged) The Swift X-Ray Telescope (XRT) reveals some interesting features of early X-ray afterglows, including a distinct rapidly decaying component preceding the conventional afterglow component in many sources, a shallow decay component before the more “normal” decay component observed in a good fraction of GRBs (e.g. GRB 050128, GRB 050315, GRB 050319, and GRB 050401), and X-ray flares in nearly half of the afterglows (e.g. GRB 050406, GRB 050502B, GRB 050607, and GRB 050724). In this paper, we systematically analyze the possible physical processes that shape the properties ...

Abstract

The composition of gamma-ray burst (GRB) ejecta is still a mystery. The standard model invokes an initially hot "fireball" composed of baryonic matter. Here we analyze the broad band spectra of GRB 080916C detected by the Fermi satellite. The featureless Band-spectrum of all five epochs as well as the detections of >~ 10 GeV photons in this burst place a strong constraint on the prompt emission radius >~ 10^15 cm, independent on the details of the emission process. The lack of the detection of a thermal component ...

Abstract

(Abridged) The highest redshift (z=6.7) gamma-ray burst (GRB) 080913 recently detected by Swift is an intrinsically short, hard GRB. It would be recognized by BATSE as a short/hard GRB should it have occurred at z ≤ 1. We perform a more thorough investigation on the physical classification scheme of cosmological GRBs and their observational characteristics. We reiterate the physical definitions of Type I/II GRBs and then review the observational criteria and their physical motivations. Contrary to the traditional approach of assigning the physical category based on the gamma-ray ...

Abstract

GRB 060614 is a remarkable GRB observed by Swift with puzzling properties, which challenge current progenitor models. The lack of any bright SN down to very strict limits and the vanishing spectral lags are typical of short GRBs, strikingly at odds with the long (102s) duration of this event. Here we present spectral and temporal analysis of the Swift observations. We show that the burst presents standard optical, UV and X-ray afterglows. An achromatic break is observed simultaneously in optical and X-rays, at a time consistent with ...

Abstract

Since the successful launch of NASA's dedicated gamma-ray burst (GRB) mission, Swift, the study of cosmological GRBs has entered a new era. Here I review the rapid observational and theoretical progress in this dynamical research field during the first two-year of the Swift mission, focusing on how observational breakthroughs have revolutionized our understanding of the physical origins of GRBs. Besides summarizing how Swift helps to solve some pre-Swift mysteries, I also list some outstanding problems raised by the Swift observations. An outlook of GRB science in the future, ...

Abstract

By analyzing the Swift/XRT lightcurves detected before 2008 October, we find 17 cases that decay as a single power law (SPL) from tens of seconds to ~10^5 seconds post the GRB trigger. They are apparently different from the canonical ones that are characterized by a shallow-to-normal decay transition. We compare the distributions of the observables of the prompt gamma-rays and X-rays for the two groups of GRBs, but no statistical difference is found. Interestingly, the SPL XRT lightcurves in the burst frame merge into a conflux. The ...

Abstract

Gamma-ray bursts (GRBs) have been an enigma since their discovery forty years ago. However, considerable progress unraveling their mysteries has been made in recent years. Developments in observations, theory, and instrumentation have prepared the way so that the next decade can be the one in which we finally answer the question, "What are gamma-ray bursts?" This question encompasses not only what the progenitors are that produce the GRBs, but also how the enormous luminosity of the GRBs, concentrated in gamma rays, is achieved. Observations across the electromagnetic spectrum, ...

Abstract

We present a systematic temporal and spectral study of all Swift-XRT observations of GRB afterglows discovered between 2005 January and 2007 December. After constructing and fitting all light curves and spectra to power-law models, we classify the components of each afterglow in terms of the canonical X-ray afterglow and test them against the closure relations of the forward shock models for a variety of parameter combinations. The closure relations are used to identify potential jet breaks with characteristics including the uniform jet model with and without lateral spreading ...

Abstract

The emission mechanism and the origin and structure of magnetic fields in gamma-ray burst (GRB) jets are among the most important open questions concerning the nature of the central engine of GRBs. In spite of extensive observational efforts, these questions remain to be answered and are difficult or even impossible to infer with the spectral and lightcurve information currently collected. Polarization measurements will lead to unambiguous answers to several of these questions. Recent developments in X-ray and gamma-ray polarimetry techniques have demonstrated a significant increase in sensitivity enabling several ...

Abstract

We present observations of GRB 060124, the first event for which both the prompt and the afterglow emission could be observed simultaneously and in their entirety by the three Swift instruments. Indeed, Swift-BAT triggered on a precursor ~570s before the main burst peak, and this allowed Swift to repoint the narrow field instruments to the burst position ~350s before the main burst occurred. GRB 060124 also triggered Konus-Wind, which observed the prompt emission in a harder gamma-ray band (up to 2MeV). Thanks to these exceptional circumstances, the temporal ...