Information on the spectral shape of prompt emission in gamma-ray bursts (GRB) is mostly available only at energies ≳10 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 (∼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 even more importantly, 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 ∼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′< 40 G) and for the location of the region where the radiation is produced (Rγ > 1016 cm). We discuss which assumptions of the basic model would need to be relaxed in order to achieve a more natural parameter space.
Detection of prompt emission by Swift-XRT provides a unique tool to study how the prompt spectrum of gamma-ray bursts (GRBs) extends down to the soft X-ray band. This energy band is particularly ...important for prompt emission studies, since it is towards low energies that the observed spectral shape is in disagreement with the synchrotron predictions. Unfortunately, the number of cases where XRT started observing the GRB location during the prompt phase is very limited. In this work, we collect a sample of 34 GRBs and perform joint XRT+BAT spectral analysis of prompt radiation, extending a previous study focused on the 14 brightest cases. Fermi-GBM observations are included in the analysis when available (11 cases), allowing the characterization of prompt spectra from soft X-rays to MeV energies. In 62% of the spectra, the XRT data reveal a hardening of the spectrum, well described by introducing an additional, low-energy power-law segment (with index α1) into the empirical fitting function. The break energy below which the spectrum hardens has values between 3 keV and 22 keV. A second power-law (α2) describes the spectrum between the break energy and the peak energy. The mean values of the photon indices are 〈α1〉 = −0.51 (σ = 0.24) and 〈α2〉 = −1.56 (σ = 0.26). These are consistent, within one σ, with the synchrotron values in fast cooling regime. As a test, if we exclude XRT data from the fits we find typical results: the spectrum below the peak energy is described by a power law with 〈α〉 = −1.15. This shows the relevance of soft X-ray data in revealing prompt emission spectra consistent with synchrotron spectra. Finally, we do not find any correlation between the presence of the X-ray break energy and the flux, fluence, or duration of the prompt emission.
We discuss the new surprising observational results that indicate quite convincingly that the prompt emission of gamma-ray bursts (GRBs) is due to synchrotron radiation produced by a particle ...distribution that has a low-energy cut-off. The evidence of this is provided by the low-energy part of the spectrum of the prompt emission, which shows the characteristic
F
ν
∝
ν
1/3
shape followed by
F
ν
∝
ν
−1/2
up to the peak frequency. This implies that although the emitting particles are in fast cooling, they do not cool completely. This poses a severe challenge to the basic ideas about how and where the emission is produced, because the incomplete cooling requires a small value of the magnetic field to limit synchrotron cooling, and a large emitting region to limit the self-Compton cooling, even considering Klein–Nishina scattering effects. Some new and fundamental ingredient is required for understanding the GRBs prompt emission. We propose proton–synchrotron as a promising mechanism to solve the incomplete cooling puzzle.
The prompt emission spectra from gamma-ray bursts (GRBs) are often fitted with the empirical “Band” function, namely two smoothly connected power laws. The typical slope of the low-energy (sub-MeV) ...power law is
α
Band
≃ −1. In a small fraction of long GRBs this power law splits into two components such that the spectrum presents, in addition to the typical ∼MeV
νF
ν
peak, a break at the order of a few keV or hundreds of keV. The typical power law slopes below and above the break are −0.6 and −1.5, respectively. If the break is a common feature, the value of
α
Band
could be an “average” of the spectral slopes below and above the break in GRBs fitted with Band function. We analyze the spectra of 27 (9) bright long (short) GRBs detected by the
Fermi
satellite, finding a low-energy break between 80 keV and 280 keV in 12 long GRBs, but in none of the short events. Through spectral simulations we show that if the break is moved closer (farther) to the peak energy, a harder (softer)
α
Band
is found by fitting the simulated spectra with the Band function. The hard average slope
α
Band
≃ −0.38 found in short GRBs suggests that the break is close to the peak energy. We show that for 15 long GRBs best fitted by the Band function only, the break could be present but not identifiable in the
Fermi
/GBM spectrum, because either at low energies, close to the detector limit (
α
Band
≲ −1), or in the proximity of the energy peak (
α
Band
≳ −1). A spectrum with two breaks could be typical of GRB prompt emission, albeit hard to identify with current detectors. Instrumental design such as that conceived for the THESEUS space mission, extending from 0.3 keV to several MeV and featuring a larger effective area with respect to
Fermi
/GBM, could reveal a larger fraction of GRBs with spectral energy breaks.
The gamma-ray burst (GRB) 180720B is one of the brightest events detected by the
Fermi
satellite and the first GRB detected by the H.E.S.S. telescope above 100 GeV, at around ten hours after the ...trigger time. We analysed the
Fermi
(GBM and LAT) and
Swift
(XRT and BAT) data and describe the evolution of the burst spectral energy distribution in the 0.5 keV–10 GeV energy range over the first 500 s of emission. We reveal a smooth transition from the prompt phase, dominated by synchrotron emission in a moderately fast cooling regime, to the afterglow phase whose emission has been observed from the radio to the gigaelectronvolts energy range. The LAT (0.1–100 GeV) light curve initially rises (
F
LAT
∝
t
2.4
), peaks at ∼78 s, and falls steeply (
F
LAT
∝
t
−2.2
) afterwards. The peak, which we interpret as the onset of the fireball deceleration, allows us to estimate the bulk Lorentz factor Γ
0
∼ 150 (300) under the assumption of a circum-burst medium with a wind-like (homogeneous) density profile. We derive a flux upper limit in the LAT energy range at the time of H.E.S.S. detection, but this does not allow us to unveil the nature of the high-energy component observed by H.E.S.S. We fit the prompt spectrum with a physical model of synchrotron emission from a non-thermal population of electrons. The 0–35 s spectrum after its
E
F
(
E
) peak (at 1–2 MeV) is a steep power law extending to hundreds of megaelectronvolts. We derive a steep slope of the injected electron energy distribution
N
(
γ
) ∝
γ
−5
. Our fit parameters point towards a very low magnetic field (
B
′ ∼ 1 G) in the emission region.
We present time-resolved spectral analysis of prompt emission from GRB 160625B, one of the brightest bursts ever detected by Fermi in its nine years of operations. Standard empirical functions fail ...to provide an acceptable fit to the GBM spectral data, which instead require the addition of a low-energy break to the fitting function. We introduce a new fitting function, called 2SBPL, consisting of three smoothly connected power laws. Fitting this model to the data, the goodness of the fits significantly improves and the spectral parameters are well constrained. We also test a spectral model that combines non-thermal and thermal (black body) components, but find that the 2SBPL model is systematically favoured. The spectral evolution shows that the spectral break is located around Ebreak ~100 keV, while the usual νFν peak energy feature Epeak evolves in the 0.5–6 MeV energy range. The slopes below and above Ebreak are consistent with the values –0.67 and –1.5, respectively, expected from synchrotron emission produced by a relativistic electron population with a low-energy cut-off. If Ebreak is interpreted as the synchrotron cooling frequency, the implied magnetic field in the emitting region is ~10 Gauss, i.e. orders of magnitudes smaller than the value expected for a dissipation region located at ~1013−14 cm from the central engine. The low ratio between Epeak and Ebreak implies that the radiative cooling is incomplete, contrary to what is expected in strongly magnetized and compact emitting regions.
The X-ray emission of
γ
-ray burst (GRBs) is often characterized by an initial steep decay followed by a nearly constant emission phase (so-called “plateau”) which can extend up to thousands of ...seconds. While the steep decay is usually interpreted as the tail of the prompt
γ
-ray flash, the long-lasting plateau is commonly associated to the emission from the external shock sustained by energy injection from a long-lasting central engine. A recent study proposed an alternative interpretation, ascribing both the steep decay and the plateau to high-latitude emission (HLE) from a “structured jet” whose energy and bulk Lorentz factor depend on the angular distance from the jet symmetry axis. In this work we expand on this idea and explore more realistic conditions: (a) the finite duration of the prompt emission, (b) the angular dependence of the optical depth, and (c) the dependence of the light curve on the observer viewing angle. We find that, when viewed highly off-axis, the structured jet HLE light curve is smoothly decaying with no clear distinction between the steep and flat phases, as opposed to the on-axis case. For a realistic choice of physical parameters, the effects of a latitude-dependent Thomson opacity and finite duration of the emission have a marginal effect on the overall light-curve evolution. We discuss the possible HLE of GW170817, showing that the emission would have faded away long before the first
Swift
-XRT observations. Finally, we discuss the prospects for the detection of HLE from off-axis GRBs by present and future wide-field X-ray telescopes and X-ray surveys, such as eROSITA and the mission concept THESEUS.
GRB 190114C: from prompt to afterglow? Ravasio, M. E.; Oganesyan, G.; Salafia, O. S. ...
Astronomy and astrophysics (Berlin),
06/2019, Letnik:
626
Journal Article
Recenzirano
Odprti dostop
GRB 190114C is the first gamma-ray burst detected at very high energies (VHE, i.e., > 300 GeV) by the MAGIC Cherenkov telescope. The analysis of the emission detected by the Fermi satellite at lower ...energies, in the 10 keV–100 GeV energy range, up to ∼50 s (i.e., before the MAGIC detection) can hold valuable information. We analyze the spectral evolution of the emission of GRB 190114C as detected by the Fermi Gamma-Ray Burst Monitor (GBM) in the 10 keV–40 MeV energy range up to ∼60 s. The first 4 s of the burst feature a typical prompt emission spectrum, which can be fit by a smoothly broken power-law function with typical parameters. Starting on ∼4 s post-trigger, we find an additional nonthermal component that can be fit by a power law. This component rises and decays quickly. The 10 keV–40 MeV flux of the power-law component peaks at ∼6 s; it reaches a value of 1.7 × 10−5 erg cm−2 s−1. The time of the peak coincides with the emission peak detected by the Large Area Telescope (LAT) on board Fermi. The power-law spectral slope that we find in the GBM data is remarkably similar to that of the LAT spectrum, and the GBM+LAT spectral energy distribution seems to be consistent with a single component. This suggests that the LAT emission and the power-law component that we find in the GBM data belong to the same emission component, which we interpret as due to the afterglow of the burst. The onset time allows us to estimate that the initial jet bulk Lorentz factor Γ0 is about 500, depending on the assumed circum-burst density.
Electrical measurements on heavily doped
n
-type germanium subjected to gamma-irradiation show that the features of impurity-related defect formation before
n
–
p
conversion of conductivity type are ...the same as those previously observed in lightly and moderately doped materials, thus extending the range of doping from ≈10
14
to ≈10
16
cm
–3
. It is clear now that the presently adopted model of the dominant impurity-related defects as simple vacancy-impurity pairs in irradiated
n
-Ge, in analogy to such defects reliably identified in irradiated
n
-Si, appears to be inconsistent with the experimental information collected so far. As a consequence, the impurity diffusion simulations in heavily doped Ge based on this model need to be reconsidered. The requirements to be met while modeling impurity-related defects in irradiated
n
-Ge in accordance with the reliable experimental data are established.
The effect of proton irradiation (proton energy 15 MeV) on the parameters of high-voltage 4H-SiC integrated Schottky diodes (JBS) was studied for the first time in the operating temperature range Ti ...(23 and 175°C). The blocking voltage of the diodes under study,
U
b
, was 600 and 1700 V. For devices with
U
b
= 600 V, the fluence range was 5 × 10
13
–1 × 10
14
cm
–2
; for devices with
U
b
= 1700 V, the fluence range was 3 × 10
13
–6 × 10
13
cm
–2
. An increase in the irradiation temperature leads to a noticeable decrease in the effect of irradiation on the current–voltage characteristics of the diodes. The effect of annealing on the current-voltage characteristics of irradiated devices is studied.
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