Gamma-Ray Burst Prompt Emission

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

GRB Progenitors and Observational Criteria

Phenomenologically, two classes of GRBs (long/soft vs. short/hard) are identified based on their gamma-ray properties. The boundary between the two classes is vague. Multi-wavelength observations lead to identification of two types of GRB progenitor: one related to massive stars (Type II), and another related to compact stars (Type I). Evidence suggests that the majority of long GRBs belong to Type II, while at least the majority of nearby short GRBs belong to Type I. Nonetheless, counter examples do exist. Both long-duration Type I and short-duration Type II GRBs have been observed. In this talk, I review the complications in GRB classification and efforts in diagnosing GRB progenitor based on multiple observational criteria. In particular, I raise the caution to readily accept that all short/hard GRBs detected by BATSE are due to compact star mergers. Finally, I propose to introduce "amplitude" as the third dimension (besides "duration" and "hardness") to quantify burst properties, and point out that the "tip-of-iceberg" effect may introduce confusion in defining the physical category of GRBs, especially for low-amplitude, high-redshift GRBs.Comment: Invited talk at IAU Symposium 279: "Death of Massive Stars: Supernovae and Gamma-Ray Bursts". To appear in the Proceedings IAU Symposium 279, (eds. P. Roming, N. Kawai, E. Pian). 8 page

The Delay Time of Gravitational Wave — Gamma-Ray Burst Associations

The first gravitational wave (GW) — gamma-ray burst (GRB) association, GW170817/GRB 170817A, had an offset in time, with the GRB trigger time delayed by ∼1.7 s with respect to the merger time of the GW signal. We generally discuss the astrophysical origin of the delay time, Δt, of GW-GRB associations within the context of compact binary coalescence (CBC) — short GRB (sGRB) associations and GW burst — long GRB (lGRB) associations. In general, the delay time should include three terms, the time to launch a clean (relativistic) jet, Δtjet; the time for the jet to break out from the surrounding medium, Δtbo; and the time for the jet to reach the energy dissipation and GRB emission site, ΔtGRB. For CBC-sGRB associations, Δtjet and Δtbo are correlated, and the final delay can be from 10 ms to a few seconds. For GWB-lGRB associations, Δtjet and Δtbo are independent. The latter is at least ∼10 s, so that Δt of these associations is at least this long. For certain jet launching mechanisms of lGRBs, Δt can be minutes or even hours long due to the extended engine waiting time to launch a jet. We discuss the cases of GW170817/GRB 170817A and GW150914/GW150914-GBM within this theoretical framework and suggest that the delay times of future GW/GRB associations will shed light into the jet launching mechanisms of GRBs

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

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

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

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

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