Synchrotron Origin of the Typical GRB Band Function—a Case Study of GRB 130606b

Zhang, Bin-Bin; Uhm, Z. Lucas; Connaughton, V.; Briggs, M. S.; Zhang, Bing

doi:10.3847/0004-637x/816/2/72

Cited by 106 publications

(98 citation statements)

References 49 publications

(75 reference statements)

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“…We consider a fit to be poor when all NaI detectors deviate from the one-to-one line over a significant portion of the 95% region (see Extended Data Figs. [10][11][12].…”

Section: Model Checkingmentioning

confidence: 99%

“…(4) A fourth implication is that other physical synchrotron models like, e.g. one which incorporates a temporally decaying magnetic field and increasing electron number injection 11 , are not required: though a plausible physical scenario, we find that we do not need this feature to describe the data as we can model the spectra simply by allowing for the electrons to cool with time. Implementing a radially (and thus temporally) decaying magnetic field, as possible in a relativistically expanding blast wave of a GRB, provides curved low-energy spectra below the spectral peak energy most of which are consistent with the Band function 15 .…”

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confidence: 95%

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Gamma-ray bursts as cool synchrotron sources

et al. 2019

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Gamma-ray bursts are the most energetic electromagnetic sources in the Universe. Their prompt gamma-ray radiation, lasting between a fraction of a second to several thousand seconds, corresponds to an energy release of 10 42 -10 47 J [1,2]. Fifty years after their discovery and several dedicated space-based instruments, the physical origin of this emission is still unknown. Synchrotron emission has been one of the early contenders 3,4 , but was criticized because spectral fits of empirical models (such as a smoothly-connected broken power law or a cut-off power law) suggest too hard a slope of the low-energy power law, violating the so-called synchrotron line-of-death 5,6 , reviving models of photospheric emission 7-9 . Fitting proper synchrotron spectra 10-12 (rather than heuristic functions) was first shown to work for individual GRBs 13,14 , though without tracking electron cooling. When the latter was taken into account, several GRB spectra could be fit successfully 10 . Here we show that idealized synchrotron emission, when properly incorporating time-dependent cooling of the electrons, is capable of fitting~95% of all time-resolved spectra of single-peaked GRBs as measured with Fermi/GBM. The comparison with spectral fit results based on previous empirical models demonstrates that the past exclusion of synchrotron radiation as an emission mechanism derived via the line-of-death was misleading. Our analysis probes the physics of these ultra-relativistic outflows and the microphysical processes which cause them to shine, and for the first time provides estimates of magnetic field strength and Lorentz factors of the emitting region directly from spectral fits. The parameter distributions that we find are grossly compatible with theoretical spectral 15-17 and outflow 18 predictions. The emission energetics remain challenging for all theoretical models. As synchrotron radiation alone can explain the observed emission, it is difficult to reconcile the time scales, efficiencies, and microphysics predicted by relativistic Fermi shock acceleration 19 and the fireball model 20 with the observations. Thus, our modeling of the Fermi/GBM observations provides evidence that GRBs are produced by moderately magnetized jets in which relativistic mini-jets emit optically-thin synchrotron radiation at large emission radii.We perform time-resolved spectral analysis of the prompt spectra of gamma-ray bursts (GRBs) detected with the Fermi Gamma-ray Burst Monitor (GBM). We select a subset of GRBs which exhibit a single contiguous, pulse-like structure which is justified by the assumption that the emission originates from a single dissipation episode 10 . In addition, we require that all GRBs have a measured redshift to ascertain their energetics. This results in a sample of 19 out of the 81 GRBs in the GBM time-resolved spectral catalog 21 . After background modeling, we employ the Bayesian blocks algorithm to create spectral datasets in optimised temporal bins for each of those GRBs, leading to a total of 162 bins, each with a P...

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“…We consider a fit to be poor when all NaI detectors deviate from the one-to-one line over a significant portion of the 95% region (see Extended Data Figs. [10][11][12].…”

Section: Model Checkingmentioning

confidence: 99%

mentioning

confidence: 95%

Gamma-ray bursts as cool synchrotron sources

et al. 2019

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“…(3) in the burst frame vary, so that η X should be calculated case by case. Taking (A, α, L 0 , n, τ ) as parameters we can do a Bayesian Monte-Carlo fitting using MCurveFit package (Zhang et al 2016). Figure 4 gives two examples of afterglow fitting results, that are GRB 100615A with normal β > −2 and GRB 150910A with β < −2.…”

Section: Impact On the Temporal Decay Index After Plateaumentioning

confidence: 99%

Determining the Efficiency of Converting Magnetar Spindown Energy into Gamma-Ray Burst X-Ray Afterglow Emission and Its Possible Implications

Xiao

Dai

2019

ApJ

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Plateaus are common in X-ray afterglows of gamma-ray bursts. Among a few scenarios for the origin of them, the leading one is that there exists a magnetar inside and persistently injects its spin-down energy into an afterglow. In previous studies, the radiation efficiency of this process is assumed to be a constant 0.1, which is quite simple and strong. In this work we obtain the efficiency from a physical point of view and find that this efficiency strongly depends on the injected luminosity. One implication of this result is that those X-ray afterglow light curves which show steeper temporal decay than t −2 after the plateau phase can be naturally understood now. Also, the braking indexes deduced from afterglow fitting are found to be larger than those in previous studies, which are more reasonable for newborn magnetars.

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“…The dependence of X-ray efficiency on the injected luminosity will influence the X-ray temporal decay index after plateau phase. Taking (t 0 , L 0 , n, τ ) as parameters, now we can do a Bayesian Monte-Carlo fitting using MCurveFit package (Zhang et al 2016). The best-fitting parameters are shown in Table 1.…”

Section: Fitting the Light Curve Of Cdf-s Xt2mentioning

confidence: 99%

On the Properties of a Newborn Magnetar Powering the X-Ray Transient CDF-S XT2

Xiao

Zhang

Dai

2019

ApJL

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Very recently Xue et al. (2019) reported an important detection of the X-ray transient, CDF-S XT2, whose light curve is analogous to X-ray plateau features of gamma-ray burst afterglows. They suggested that this transient is powered by a remnant stable magnetar from a binary neutron star merger since several pieces of evidence (host galaxy, location, and event rate) all point toward such an assumption. In this paper, we revisit this scenario and confirm that this X-ray emission can be well explained by the internal gradual magnetic dissipation process in an ultra-relativistic wind of the newborn magnetar. We show that both the light curve and spectral evolution of CDF-S XT2 can be well fitted by such a model. Furthermore, we can probe some key properties of the central magnetar, such as its initial spin period, surface magnetic field strength and wind saturation Lorentz factor.

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Synchrotron Origin of the Typical GRB Band Function—a Case Study of GRB 130606b

Cited by 106 publications

References 49 publications

Gamma-ray bursts as cool synchrotron sources

Gamma-ray bursts as cool synchrotron sources

Determining the Efficiency of Converting Magnetar Spindown Energy into Gamma-Ray Burst X-Ray Afterglow Emission and Its Possible Implications

On the Properties of a Newborn Magnetar Powering the X-Ray Transient CDF-S XT2

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