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Register a new userElectron Spectrum for the Prompt Emission of Gamma-ray Bursts in the Synchrotron Radiation Scenario
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Growing evidence indicates that the synchrotron radiation mechanism may be responsible for the prompt emission of gamma-ray bursts (GRBs). In the synchrotron radiation scenario, the electron energy spectrum of the prompt emission is diverse in theoretical works and has not been estimated from observations in a general way (i.e., without specifying a certain physical model for the electron spectrum). In this paper, we creatively propose a method to directly estimate the electron spectrum for the prompt emission, without specifying a certain physical model for the electron spectrum in the synchrotron radiation scenario. In this method, an empirical function (i.e., a four-order Bezier curve jointed with a linear function at high-energy) is applied to describe the electron spectrum in log-log coordinate. It is found that our empirical function can well mimic the electron spectra obtained in many numerical calculations or simulations. Then, our method can figure out the electron spectrum for the prompt emission without specifying a model. By employing our method on observations, taking GRB 180720B and GRB 160509A as examples, it is found that the obtained electron spectra are generally different from that in the standard fast-cooling scenario and even a broken power law. Moreover, the morphology of electron spectra in its low-energy regime varies with time in a burst and even in a pulse. Our proposed method provides a valuable way to confront the synchrotron radiation mechanism with observations.
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Information on the spectral shape of prompt emission in gamma-ray bursts (GRB) is mostly available only at energies $gtrsim10$ keV, where the main instruments for GRB detection are sensitive. The origin of this emission is still very uncertain because of the apparent inconsistency with synchrotron radiation, which is the most obvious candidate, and the resulting need for considering less straightforward scenarios. The inclusion of data down to soft X-rays ($sim$ 0.5 keV), which are available only in a small fraction of GRBs, has firmly established the common presence of a spectral break in the low-energy part of prompt spectra, and the consistency of the overall spectral shape with synchrotron radiation in the moderately fast-cooling regime, the low-energy break being identified with the cooling frequency. In this work we further extend the range of investigation down to the optical band. In particular, we test the synchrotron interpretation by directly fitting a theoretically derived synchrotron spectrum and making use of optical to gamma-ray data. Secondly, we test an alternative model that considers the presence of a black-body component at $sim$keV energies, in addition to a non-thermal component that is responsible for the emission at the spectral peak (100 keV-1 MeV). We find that synchrotron radiation provides a good description of the broadband data, while models composed of a thermal and a non-thermal component require the introduction of a low-energy break in the non-thermal component in order to be consistent with optical observations. Motivated by the good quality of the synchrotron fits, we explore the physical parameter space of the emitting region. In a basic prompt emission scenario we find quite contrived solutions for the magnetic field strength (5 G $<B^prime<40$ G) and for the location of the region where the radiation is produced ($R_gamma>10^{16}$ cm).
Gamma-ray bursts (GRBs) were first detected thanks to their prompt emission, which was the only information available for decades. In 2010, while the high-energy prompt emission remains the main tool for the detection and the first localization of GRB sources, our understanding of this crucial phase of GRBs has made great progress. We discuss some recent advances in this field, like the occasional detection of the prompt emission at all wavelengths, from optical to GeV; the existence of sub-luminous GRBs; the attempts to standardize GRBs; and the possible detection of polarization in two very bright GRBs. Despite these advances, tantalizing observational and theoretical challenges still exist, concerning the detection of the faintest GRBs, the panchromatic observation of GRBs from their very beginning, the origin of the prompt emission, or the understanding of the physics at work during this phase. Significant progress on this last topic is expected with SVOM thanks to the observation of dozens of GRBs from optical to MeV during the burst itself, and the measure of the redshift for the majority of them. SVOM will also change our view of the prompt GRB phase in another way. Within a few years, the sensitivity of sky surveys at optical and radio frequencies, and outside the electromagnetic domain in gravitational waves or neutrinos, will allow them to detect several new types of transient signals, and SVOM will be uniquely suited to identify which of these transients are associated with GRBs. This radically novel look at GRBs may elucidate the complex physics producing these bright flashes.
GRB spectra appear non-thermal, but recent observations of a few bursts with Fermi GBM have confirmed previous indications from BATSE of the presence of an underlying thermal component. Photospheric emission is indeed expected when the relativistic outflow emerging from the central engine becomes transparent to its own radiation, with a quasi-blackbody spectrum in absence of additional sub-photospheric dissipation. However, its intensity strongly depends on the acceleration mechanism - thermal or magnetic - of the flow. We aim to compute the thermal and non-thermal emissions produced by an outflow with a variable Lorentz factor, where the power injected at the origin is partially thermal (fraction epsilon_th) and partially magnetic (fraction 1-epsilon_th). The thermal emission is produced at the photosphere, and the non-thermal emission in the optically thin regime. Apart from the value of epsilon_th, we want to test how the other model parameters affect the observed ratio of the thermal to non-thermal emission. If the non-thermal emission is made by internal shocks, we self-consistently obtained the light curves and spectra of the thermal and non-thermal components for any distribution of the Lorentz factor in the flow. If the non-thermal emission results from magnetic reconnection we were unable to produce a light curve and could only compare the respective non-thermal and thermal spectra. In the different considered cases, we varied the model parameters to see when the thermal component in the light curve and/or spectrum is likely to show up or, on the contrary, to be hidden. We finally compared our results to the proposed evidence for the presence of a thermal component in GRB spectra. Focussing on GRB 090902B and GRB 10072B, we showed how these observations can be used to constrain the nature and acceleration mechanism of GRB outflows.
High-energy neutrinos are expected to originate from different stages in a gamma-ray burst (GRB) event. In this work we revisit the dissipative photospheric scenario, in which the GRB prompt emission is produced around the photospheric radius. Meanwhile, possible dissipation mechanisms (e.g., internal shocks or magnetic reconnection) could accelerate cosmic-rays (CRs) to ultra-high energies and then produce neutrinos via hadronuclear and photohadronic processes, which are referred to as prompt neutrinos. In this paper, we obtain the prompt neutrino spectrum of a single GRB within a self-consistent analytical framework, in which the jet-cocoon structure and possible collimation effects are included. We investigate a possible neutrino signal from the cocoon, which has been ignored in the previous studies. We show that if a GRB event happens at a distance of the order of Mpc, there is a great chance to observe the neutrino emission from the cocoon by IceCube, which is even more promising than jet neutrinos since the opening angle of the cocoon is much larger. We also figure out the diffuse neutrino flux of GRB cocoons and find that it could be comparable with that of the jets. Our results are consistent with the latest result reported by the IceCube collaboration that no significant correlation between neutrino events and observed GRBs is seen in the new data (Aartsen et al. 2017).
As gamma-ray burst (GRB) jet drills its way through the collapsing star, it traps a baryonic cork ahead of it. Here we explore a prompt emission model for GRBs in which the jet does not cross the cork, but rather photons that are emitted deep in the flow largely by pair annihilation are scattered inside the expanding cork and escape largely from the back end of it as they push it from behind. Due to the relativistic motion of the cork, these photons are easily seen by an observer close to the jet axis peaking at $varepsilon_{peak}sim~few times 100 keV$. We show that this model naturally explains several key observational features including: (1) High energy power law index $beta_1 sim -2 {~rm to~} -5$ with an intermediate thermal spectral region; (2) decay of the prompt emission light curve as $sim t^{-2}$; (3) Delay of soft photons; (4) peak energy - isotropic energy (the so-called Amati) correlation, $varepsilon_{peak} sim varepsilon_{iso}^m$, with $msim 0.45$, resulting from different viewing angles. At low luminosities, our model predicts an observable turn off in the Amati relation. (4) An anti-correlation between the spectral full width half maxima (FWHM) and time as $t^{-1}$. (5) Temporal evolution $varepsilon_{peak} sim t^{-1}$, accompanied by an increase of the high energy spectral slope with time. (6) Distribution of peak energies $varepsilon_{peak}$ in the observed GRB population. The model is applicable for a single pulse GRB lightcurves and respective spectra. We discuss the consequence of our model in view of the current and future prompt emission observations.
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