17,397 research outputs found

    Gamma-Ray Burst Prompt Emission

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    The origin of gamma-ray burst (GRB) prompt emission, bursts of gamma-rays lasting from shorter than one second to thousands of seconds, remains not fully understood after more than 40 years of observations. The uncertainties lie in several open questions in the GRB physics, including jet composition, energy dissipation mechanism, particle acceleration mechanism, and radiation mechanism. Recent broad-band observations of prompt emission with Fermi sharpen the debates in these areas, which stimulated intense theoretical investigations invoking very different ideas. I will review these debates, and argue that the current data suggest the following picture: A quasi-thermal spectral component originating from the photosphere of the relativistic ejecta has been detected in some GRBs. Even though in some cases (e.g. GRB 090902B) this component dominates the spectrum, in most GRBs, this component either forms a sub-dominant "shoulder" spectral component in the low energy spectral regime of the more dominant "Band" component, or is not detectable at all. The main "Band" spectral component likely originates from the optically thin region due to synchrotron radiation. The diverse magnetization in the GRB central engine is likely the origin of the observed diverse prompt emission properties among bursts.Comment: This invited review article is based on invited talks delivered by the author at several conferences, including the 13th Marcel Grossmann Meeting (Stockholm, July 1-7, 2012), "Gamma 2012" (Heidelberg, July 9-13, 2012), the 7th Huntsville GRB Symposium (Nashville, April 14-18, 2013), and SNe and GRBs 2013 (Kyoto, Nov. 11-14, 2013). Published in International Journal of Modern Physics

    FRB 121102: A Repeatedly Combed Neutron Star by a Nearby Low-luminosity Accreting Supermassive Black Hole

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    The origin of fast radio bursts (FRBs) remains mysterious. Recently, the only repeating FRB source, FRB 121102, was reported to possess an extremely large and variable rotation measure (RM). The inferred magnetic field strength in the burst environment is comparable to that in the vicinity of the supermassive black hole Sagittarius A* of our Galaxy. Here, we show that all of the observational properties of FRB 121102 (including the high RM and its evolution, the high linear polarization degree, an invariant polarization angle across each burst and other properties previously known) can be interpreted within the cosmic comb model, which invokes a neutron star with typical spin and magnetic field parameters whose magnetosphere is repeatedly and marginally combed by a variable outflow from a nearby low-luminosity accreting supermassive black hole in the host galaxy. We propose three falsifiable predictions (periodic on/off states, and periodic/correlated variation of RM and polarization angle) of the model and discuss other FRBs within the context of the cosmic comb model as well as the challenges encountered by other repeating FRB models in light of the new observations

    Fast Radio Burst Energetics and Detectability from High Redshifts

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    We estimate the upper limit redshifts of known fast radio bursts (FRBs) using the dispersion measure (DM)-redshift (z) relation and derive the upper limit peak luminosity L p and energy E of FRBs within the observational band. The average z upper limits range from 0.17 to 3.10, the average L p upper limits range from 1.24 × 1042 erg s−1 to 7.80 × 1044 erg s−1, and the average E upper limits range from 6.91 × 1039 erg to 1.94 × 1042 erg. FRB 160102 with DM = 2596.1 ± 0.3 pc cm−3 likely has a redshift greater than 3. Assuming that its intrinsic DM contribution from the host and FRB source is DMhost + DMscr ~ 100 pc cm−3, such an FRB can be detected up to z ~ 3.6 by Parkes and the Five-hundred-meter Aperture Spherical radio Telescope (FAST) under ideal conditions up to z ~ 10.4. Assuming the existence of FRBs that are detectable at z ~ 15 by sensitive telescopes such as FAST, the upper limit DM for FRB searches may be set to ~9000 pc cm−3. For single-dish telescopes, those with a larger aperture tend to detect more FRBs than those with a smaller aperture if the FRB luminosity function index α L is steeper than 2, and vice versa. In any case, large-aperture telescopes such as FAST are more capable of detecting high-z FRBs, even though most of FRBs detected by them are still from relatively low redshifts

    Physical origin of X-ray flares following GRBs

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    One of the major achievements of Swift is the discovery of the erratic X-ray flares harboring nearly half of gamma-ray bursts (GRBs), both for long-duration and short-duration categories, and both for traditional hard GRBs and soft X-ray flashes (XRFs). Here I review the arguments in support of the suggestion that they are powered by reactivation of the GRB central engine, and that the emission site is typically ``internal'', i.e. at a distance within the forward shock front. The curvature effect that characterizes the decaying lightcurve slope during the fading phase of the flares provides an important clue. I will then discuss several suggestions to re-start the GRB central engine and comment on how future observations may help to unveil the physical origin of X-ray flares.Comment: 6 pages, 2 figure, uses aipproc.cls; to appear in ``16th Annual October Astrophysics Conference in Maryland", eds. S. Holt, N. Gehrels and J. Nousek, AIP Conf.Proc

    Gamma-ray burst afterglows

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    Extended, fading emissions in multi-wavelength are observed following Gamma-ray bursts (GRBs). Recent broad-band observational campaigns led by the Swift Observatory reveal rich features of these GRB afterglows. Here we review the latest observational progress and discuss the theoretical implications for understanding the central engine, composition, and geometric configuration of GRB jets, as well as their interactions with the ambient medium.Comment: References added, accepted for publication in Advances in Space Researc

    Early X-ray and optical afterglow of gravitational wave bursts from mergers of binary neutron stars

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    Double neutron star mergers are strong sources of gravitational waves. The upcoming advanced gravitational wave detectors are expected to make the first detection of gravitational wave bursts (GWBs) associated with these sources. Proposed electromagnetic counterparts of a GWB include a short GRB, an optical macronova, and a long-lasting radio afterglow. Here we suggest that at least some GWBs could be followed by an early afterglow lasting for thousands of seconds, if the post-merger product is a short-lived massive neutron star rather than a black hole. This afterglow is powered by dissipation of a proto-magnetar wind. The X-ray flux is estimated to be as bright as 10^{-8}-10^{-7} erg/s/cm^2. The optical flux is subject to large uncertainties but could be as bright as 17th magnitude in R-band. We provide observational hints of such a scenario, and discuss the challenge and strategy to detect these signals.Comment: ApJL, in pres