Prompt Emission Spectra Research Articles

A NEW SPECTRAL ENERGY DISTRIBUTION OF PHOTONS IN THE FIRESHELL MODEL OF GRBS

The analysis of various Gamma-Ray Bursts (GRBs) having a low energetics (an isotropic energy E iso ≲ 1053 ergs) within the fireshell model has shown how the N(E) spectrum of their prompt emission can be reproduced in a satisfactory way by a convolution of thermal spectra. Nevertheless, from the study of very energetic bursts (E iso ≳ 1054 ergs) such as, for example, GRB 080319B, some discrepancies between the numerical simulations and the observational data have been observed. We investigate a different spectrum of photons in the comoving frame of the fireshell in order to better reproduce the spectral properties of GRB prompt emission within the fireshell model. We introduce a phenomenologically modified thermal spectrum: a thermal spectrum characterized by a different asymptotic power-law index in the low energy region. Such an index depends on a free parameter α, so that the pure thermal spectrum corresponds to the case α = 0. We test this spectrum by comparing the numerical simulations with the observed prompt emission spectra of various GRBs. From this analysis it has emerged that the observational data can be correctly reproduced by assuming a modified thermal spectrum with α = -1.8.

Spectral analysis of time-integrated Konus–Wind GRBs: Implication on radiative mechanisms

We have analysed a sample of 328 time-integrated GRB prompt emission spectra taken via the Konus instrument on board the US GGS-Wind spacecraft between 2002 and 2004 using a couple of two-components models, Cut-off Power Law (CPL) + Power Law (PL) and blackbody (BB) + PL. The spectra show clear deviation from the Band function. The PL term is interpreted as the low energy tail of a nonthermal emission mechanism. The distributions of corresponding index β give values β < −2/3 consistent with synchrotron and synchrotron self-Compton mechanisms. The distribution of low energy index α associated with the CPL term shows clear discordance with synchrotron models for 31.4% of the analysed GRBs with values exceeding that for the line of death, α = −2/3. Then, a set of nonthermal radiation mechanisms producing harder slopes, i.e., α > −2/3, are presented and discussed. For the remaining majority (68.6%) of GRBs with CPL index α < −2/3, we show that optically thin synchrotron produced by a power law electron distribution of type, N( γ) ∼ γ − p , γ 1 < γ < γ 2, for finite energy range ( γ 2 ≠ ∞) is a likely emission mechanism with α ∼−( p + 1)/2 in the frequency range ν 1 ≪ ν ≪ ν 2 (where ν 2 = η 2 ν 1 with η = γ 2/ γ 1), such that for p > 1/3, one gets α < −2/3. We also show that corresponding spectra in terms of F ν and νF ν functions are peaked around frequency ν 2 instead of ν 1, respectively for p < 1 and p < 3. Besides, thermal emission is examined taking a single Planck function for fitting the low energy range. It can be interpreted as an early emission from the GRB fireball photosphere with observed mean temperature, kT′ ∼ 16.8 keV. Furthermore, we have performed a statistical comparison between the CPL + PL and BB + PL models finding comparable χ 2-values for an important fraction of GRBs, which makes it difficult to distinguish which model and specific radiation mechanism (possible thermal or nonthermal γ-ray emissions) are best suitable for describing the reported data. Therefore, additional information for those bursts, such as γ-ray polarization, would be highly desirable in future determinations of GRBs observational data.

Using gamma-ray burst prompt emission to probe relativistic shock acceleration

It is widely accepted that the prompt transient signal in the 10 keV–10 GeV band from gamma-ray bursts (GRBs) arises from multiple shocks internal to the ultra-relativistic expansion. The detailed understanding of the dissipation and accompanying acceleration at these shocks is a currently topical subject. This paper explores the relationship between GRB prompt emission spectra and the electron (or ion) acceleration properties at the relativistic shocks that pertain to GRB models. The focus is on the array of possible high-energy power-law indices in accelerated populations, highlighting how spectra above 1 MeV can probe the field obliquity in GRB internal shocks, and the character of hydromagnetic turbulence in their environs. It is emphasized that diffusive shock acceleration theory generates no canonical spectrum at relativistic MHD discontinuities. This diversity is commensurate with the significant range of spectral indices discerned in prompt burst emission. Such system diagnostics are now being enhanced by the broad-band spectral coverage of bursts by the Fermi Gamma-Ray Space Telescope; while the Gamma-Ray Burst Monitor (GBM) provides key diagnostics on the lower energy portions of the particle population, the focus here is on constraints in the non-thermal, power-law regime of the particle distribution that are provided by the Large Area Telescope (LAT).

Where areSwiftγ-ray bursts beyond the ‘synchrotron deathline’?

We study time-resolved spectra of the prompt emission of Swift Gamma-ray bursts (GRB). Our goal is to see if previous BATSE claims of the existence of a large amount of spectra with the low energy photon indices harder than 2/3 are consistent with Swift data. We perform a systematic search of the episodes of the spectral hardening down to the photon indices below 2/3 in the prompt emission spectra of Swift GRBs. We show that the data of the BAT instrument on board of Swift are consistent with BATSE data, if one takes into account differences between the two instruments. Much lower statistics of the very hard spectra in Swift GRBs is explained by the smaller field of view and narrower energy band of the BAT telescope.

WE‐C‐BRC‐06: Determining Changes to Tissues Irradiated During Proton Treatment Delivery by Measuring Secondary Prompt Gamma‐Ray Spectra

Purpose: Recent studies have investigated the use of secondary ‘prompt’ gamma‐rays emitted from patients during proton therapy as a possible method to verify dose delivery. The purpose of this study was to determine if the prompt gamma‐ray spectra emitted from biological tissues could be used as a method to analyze changes to tissues irradiated during proton treatment delivery. Method and Materials: We preformed measurements and Monte Carlo calculations of the prompt gamma emission spectra from tissue equivalent phantoms and tissue targets. The emission spectra from several types of tissue where characterized according to the emission lines from the individual elemental constituents of the target tissue. Next, we studied the individual emission lines from the tissue targets as a function of elemental concentration and density. Results: These results show that the prompt gamma emission lines from the major elemental components can be identified in the measured and calculated spectra. The intensities of these emission lines were found to be a function of concentration of each element and the physical density of the tissue. Conclusion: Based on the results of these preliminary studies, we conclude that it may be possible to determine both the atomic composition and physical density of irradiated tissues by measuring the prompt gamma ray spectra emitted during proton treatment delivery. Development of a technique to measure prompt emission spectra during daily treatment delivery would allow for the direct measurement of changes to irradiated tissues, such as changes to tumor hypoxia or tissue densities over the course of proton treatment delivery.

EpeakESTIMATOR FOR GAMMA-RAY BURSTS OBSERVED BY THESWIFTBURST ALERT TELESCOPE

We report a correlation based on a spectral simulation study of the prompt emission spectra of gamma-ray bursts (GRBs) detected by the Swift Burst Alert Telescope (BAT). The correlation is between the Epeak energy, which is the peak energy in the \nu F_\nu spectrum, and the photon index (\Gamma) derived from a simple power-law model. The Epeak - \Gamma relation, assuming the typical smoothly broken power-law spectrum of GRBs, is \log Epeak = 3.258 - 0.829\Gamma (1.3 < \Gamma < 2.3). We take into account not only a range of Epeak energies and fluences, but also distributions for both the low-energy photon index and the high-energy photon index in the smoothly broken power-law model. The distribution of burst durations in the BAT GRB sample is also included in the simulation. Our correlation is consistent with the index observed by BAT and Epeak measured by the BAT, and by other GRB instruments. Since about 85% of GRBs observed by the BAT are acceptably fit with the simple power-law model because of the relatively narrow energy range of the BAT, this relationship can be used to estimate Epeak when it is located within the BAT energy range.

Global characteristics of GRBs observed with INTEGRAL and the inferred large population of low-luminosity GRBs

INTEGRAL has two sensitive gamma-ray instruments that have detected 46 gamma-ray bursts (GRBs) up to July 2007. We present the spectral, spatial, and temporal properties of the bursts in the INTEGRAL GRB catalogue using data from the imager, IBIS, and spectrometer, SPI. Spectral properties of the GRBs are determined using power-law, Band model and quasithermal model fits to the prompt emission. Spectral lags, i.e. the time delay in the arrival of low-energy gamma-rays with respect to high-energy gamma-rays, are measured for 31 of the GRBs. The photon index distribution of power-law fits to the prompt emission spectra is consistent with that obtained by Swift. The peak flux distribution shows that INTEGRAL detects proportionally more weak GRBs than Swift because of its higher sensitivity in a smaller field of view. The all-sky rate of GRBs above ~0.15 ph cm^-2 s^-1 is ~1400 yr^-1 in the fully coded field of view of IBIS. Two groups are identified in the spectral lag distribution, one with short lags <0.75 s (between 25-50 keV and 50-300 keV) and one with long lags >0.75 s. Most of the long-lag GRBs are inferred to have low redshifts because of their long spectral lags, their tendency to have low peak energies and their faint optical and X-ray afterglows. They are mainly observed in the direction of the supergalactic plane with a quadrupole moment of Q=-0.225+/-0.090 and hence reflect the local large-scale structure of the Universe. The rate of long-lag GRBs with inferred low luminosity is ~25% of Type Ib/c supernovae. Some of these bursts could be produced by the collapse of a massive star without a supernova or by a different progenitor, such as the merger of two white dwarfs or a white dwarf with a neutron star or black hole, possibly in the cluster environment without a host galaxy.

Diagnosing the site of gamma-ray burst prompt emission with spectral cut-off energy

Abstract The site and mechanism of gamma-ray burst (GRB) prompt emission is still unknown. Although internal shocks have been widely discussed as the emission site of GRBs, evidence supporting other emission sites, including the closer-in photosphere where the fireball becomes transparent and further-out radii near the fireball deceleration radius where magnetic dissipation may be important, have been also suggested recently. With the successful operation of the GLAST experiment, prompt high-energy emission spectra from many GRBs would be detected in the near future. We suggest that the cut-off energy of the prompt emission spectrum from a GRB depends on both the fireball bulk Lorentz factor and the unknown emission radius from the central engine. If the bulk Lorentz factor could be independently measured (e.g. from early afterglow observations), the observed spectral cut-off energy can be used to diagnose the emission site of gamma-rays. This would provide valuable information to understand the physical origin of GRB prompt emission.

Synchrotron Emission in Small‐Scale Magnetic Fields as a Possible Explanation for Prompt Emission Spectra of Gamma‐Ray Bursts

Synchrotron emission is believed to be a major radiation mechanism during gamma-ray bursts' (GRBs) prompt emission phase. A significant drawback of this assumption is that the theoretical predicted spectrum, calculated within the framework of the scenario using the standard assumption that the magnetic field maintains a steady value throughout the shocked region, leads to a slope Fν ∝ ν-1/2 below 100 keV, which is in contradiction to the much harder spectra observed. This is due to the electron cooling time being much shorter than the dynamical time. In order to overcome this problem, we propose here that the magnetic field created by the internal shocks decays on a length scale much shorter than the comoving width of the plasma. We show that under this assumption synchrotron radiation can reproduce the observed prompt emission spectra of the majority of the bursts. We calculate the required decay length of the magnetic field, and find it to be ~104-105 cm (equivalent to 105-106 skin depths), much shorter than the characteristic comoving width of the plasma, ~3 × 109 cm. We implement our model to the case of GRB 050820A, where a break at 4 keV was observed, and show that this break can be explained by synchrotron self-absorption. We discuss the consequences of the small-scale magnetic field scenario on current models of magnetic field generation in shock waves.

Prompt emission spectra from the photosphere of a GRB

I explore the observational appearance of the photosphere of an ultrarelativistic flow with internal dissipation of energy (“dissipative” GRB model). As a case study, I use the magnetic reconnection model (AC model) that makes robust predictions on the energy dissipation rates at different radii in the flow. With analytical and numerical tools for the radiative transfer problem, I show that the flow develops a hot photosphere where inverse Compton scattering leads to highly non-thermal spectrum. For a wide range of luminosities and baryon loadings of the flow, this spectrum is very close to the observed prompt GRB emission. Its luminosity ranges from ~3 to 20% of that of the total energy input.

Discovery of a tight correlation among the prompt emission properties of long gamma-ray bursts

We report the discovery of a correlation among three prompt emission properties of gamma-ray bursts (GRBs). These are the isotropic peak luminosity Liso, the peak energy (in νLν) of the time-integrated prompt emission spectrum Epk, and the ‘high signal’ time-scale T0.45, previously used to characterize the variability behaviour of bursts. In the rest frame of the source, the discovered correlation reads Liso∝E1.62pkT−0.490.45. We find other strong correlations, but at the cost of increasing the number of variables, involving the variability and the isotropic energy of the prompt emission. With respect to the other tight correlations found in GRBs (i.e. between the collimation corrected energy Eγ and Epk, the so-called Ghirlanda correlation, and the phenomenological correlation among the isotropic emitted energy Eiso,-Epk and the jet break time tbreak), the newly found correlation does not require any information from the afterglow phase of the bursts, nor any model-dependent assumption. In the popular scenario in which we are receiving beamed radiation originating in a fireball pointing at us, the discovered correlation preserves its form in the comoving frame. This helps to explain the small scatter of the correlation and underlines the role of the local brightness (i.e. the brightness of the visible fraction of the fireball surface). This correlation has been found with a relatively small number of objects and it is hard to establish if any selection bias affects it. Its connection with the prompt local brightness is promising, but a solid physical understanding is still to be found. Despite all that, we find that some properties of the correlation, which we discuss, support its true existence, and this has important implications for the GRB physics. Furthermore, it is possible to use such correlation as an accurate redshift estimator, and, more importantly, its tightness will allow us to use it as a tool to constrain the cosmological parameters.

X-ray flares in the early Swift observations of the possible naked gamma-ray burst 050421

We present the Swift observations of the faint GRB 050421. The X-ray light-curve shows at least two flares: the first flare peaking at ∼110 s after the BAT trigger (To) and the second one peaking at ∼154 s. The first flare presents a flux variation of δF/F peak ∼ 3.7 and a short timescale ratio δt/t peak ∼ 0.07. The second flare is smaller and presents a flux variation of δF/F peak ∼ 1.7 and a short timescale ratio δt/t peak ∼ 0.03. We argue that the mechanism producing these flares is probably late internal shocks. The X-ray light-curve is consistent with a rapid decline with a temporal index α ∼ 3.1, which decays from ∼10 -9 erg cm -2 s -1 at To + 100s to <7 x 10 -13 erg cm -2 s -1 at To + 900 s. A possible spectral softening is also observed with time, from β ∼ 0.1 to ∼1.2. A good joint fit to the BAT and XRT spectra before To + 171 s with β XRT-BAT ∼ 0.2 indicates that the early X-ray and Gamma-ray emissions are likely produced by the same mechanism. We argue that the X-ray spectral softening, if any, is due to a shift of the peak of the prompt emission spectrum down to lower energies, and that the rapid decline of the X-ray emission is probably the tail of the prompt emission. This suggests that the X-ray emission is completely dominated by high latitude radiation and the external shock, if any, is extremely faint and below the detection threshold. GRB 050421 is likely the first naked burst detected by Swift.

The Observable Effects of a Photospheric Component on GRB and XRF Prompt Emission Spectrum

A thermal radiative component is likely to accompany the first stages of the prompt emission of gamma-ray bursts (GRBs) and X-ray flashes (XRFs). We analyze the effect of such a component on the observable spectrum, assuming that the observable effects are due to a dissipation process occurring below or near the thermal photosphere. We consider both the internal shock model and a slow heating model as possible dissipation mechanisms. For comparable energy densities in the thermal and leptonic components, the dominant emission mechanism is Compton scattering. This leads to a nearly flat energy spectrum (νFν ∝ ν0) above the thermal peak at ≈10-100 keV and below 10-100 MeV, for a wide range of optical depths 0.03 τγe 100, regardless of the details of the dissipation mechanism or the strength of the magnetic field. At lower energies steep slopes are expected, while above 100 MeV the spectrum depends on the details of the dissipation process. For higher values of the optical depth, a Wien peak is formed at 100 keV-1 MeV, and no higher energy component exists. For any value of τγe, the number of pairs produced does not exceed the baryon-related electrons by a factor of larger than a few. We conclude that dissipation near the thermal photosphere can naturally explain both the steep slopes observed at low energies and a flat spectrum above 10 keV, thus providing an alternative scenario to the optically thin synchrotron-SSC model.

Promptes und verzögertes Spektrum des reinen Dibenzofuran-Kristalls / Prompt and, Delayed Spectrum of the Pure Dibenzofuran-crystal

Abstract Prompt and, Delayed Spectrum of the Pure Dibenzofuran-crystal The mirror symmetry of the prompt emission and the transmission spectrum allows to estimate the S1→S0 O-O transition to be at 32.64 · 103 cm-1. The correspondence of the prompt emission spectrum of the DBF-crystal with the spectra of the solutions of DBF in bencene and methyl-alcohol as well as measurements of the delayed spectrum indicate a high purity of the crystal. Decay-time measurements of the delayed fluorescence and the delayed spectrum as a function of intensity of excitation light demonstrate the long wave emission in the delayed to be the phosphorescence of DBF.