ABSTRACT The bright, short, and hard GRB 090510 was detected by all instruments aboard the Fermi and Swift satellites. The multiwavelength observations of this burst presented similar features to the ...Fermi-LAT-detected gamma-ray bursts. In the framework of the external shock model of early afterglow, a leptonic scenario that evolves in a homogeneous medium is proposed to revisit GRB 090510 and explain the multiwavelength light curve observations presented in this burst. These observations are consistent with the evolution of a jet before and after the jet break. The long-lasting LAT, X-ray, and optical fluxes are explained in the synchrotron emission from the adiabatic forward shock. Synchrotron self-Compton emission from the reverse shock is consistent with the bright LAT peak provided that the progenitor environment is entrained with strong magnetic fields. It could provide compelling evidence of magnetic field amplification in the neutron star merger.
Abstract
Gamma-ray bursts (GRBs) have been considered as potential very high energy photon emitters due to the large amount of energy released as well as the strong magnetic fields involved in their ...jets. However, the detection of teraelectronvolt photons is not expected from bursts beyond a redshift of
z
≳ 0.1, due to their attenuation with the extragalactic background light (EBL). For these reasons, the recent observation of photons with energies of 18 and 251 TeV from GRB 221009A (
z
= 0.151) last 2022 October 9 has challenged what we know about the teraelectronvolt-emission mechanisms and the extragalactic background. In order to explain the teraelectronvolt observations, recent works exploring candidates of dark matter have started to appear. In this paper, we discuss the required conditions and limitations within the most plausible scenario, synchrotron self-Compton radiation in the GRB afterglow, to interpret the one 18 TeV photon observation besides the EBL. To avoid the Klein–Nishina effect, we find an improbable value of the microphysical magnetic parameter below 10
−6
for a circumburst medium value >1 cm
−3
(expected in the collapsar scenario). Therefore, we explore possible scenarios in terms of axion-like particles (ALPs) and dark photon mechanisms to interpret this highly energetic photon and we discuss the implications in the GRB energetics. We find that the ALPs and dark photon scenarios can explain the 18 teraelectronvolt photon but not the 251 teraelectronvolt photon.
The BL Lac Markarian 501 exhibited two flaring activities in the very-high-energy (VHE) band in May 2009. The lack of correlation between X-rays and TeV gamma-rays without increasing in other bands ...suggested that more than one emission zone could be involved. Moreover, fast variability in the flaring state was observed, indicating that the emission zones responsible must have small sizes. We use a lepto-hadronic model with two-zone emission to explain the spectral energy distribution during quiescent and these flaring states. In the proposed scenario, the photopion processes explain the VHE flaring activities successfully, and variability constraints place the activity in a zone located near the jet’s base or named inner blob, while synchrotron self-Compton emission describing the X-ray signature during that flaring state occurs in the zone situated far the central engine or named outer blob.
Very-high-energy (VHE; ≥ 10 GeV) photons are expected from the nearest and brightest gamma-ray bursts (GRBs). VHE photons, at energies higher than 300 GeV, were recently reported by the MAGIC ...Collaboration for this burst. Immediately, GRB 190114C was followed up by a massive observational campaign covering a large fraction of the electromagnetic spectrum. In this Letter, we obtain the Large Area Telescope (LAT) light curve of GRB 190114C and show that it exhibits similar features to other bright LAT-detected bursts; the first high-energy photon (≥100 MeV) is delayed with the onset of the prompt phase and the flux light curve exhibits a long-lasting emission (much longer than the prompt phase) and a short-lasting bright peak (located at the beginning of long-lasting emission). Analyzing the multi-wavelength observations, we show that the short-lasting LAT and Gamma-Ray Burst Monitor bright peaks are consistent with the synchrotron self-Compton reverse-shock model, and that the long-lasting observations are consistent with the standard synchrotron forward-shock model that evolves from a stratified stellar-wind-like medium to a uniform interstellar-medium-like medium. Given the best-fit values, a bright optical flash produced by synchrotron reverse-shock emission is expected. From our analysis we infer that the high-energy photons are produced in the deceleration phase of the outflow, and some additional processes to synchrotron in the forward shocks should be considered to properly describe the LAT photons with energies beyond the synchrotron limit. Moreover, we claim that an outflow endowed with magnetic fields could describe the polarization and properties exhibited in the light curve of GRB 190114C.
Early and late multiwavelength observations play an important role in determining the nature of the progenitor, circumburst medium, physical processes, and emitting regions associated with the ...spectral and temporal features of bursts. GRB 180720B is a long and powerful burst detected by a large number of observatories at multiple wavelengths that range from radio bands to sub-TeV gamma-rays. The simultaneous multiwavelength observations were presented over multiple periods of time beginning just after the trigger time and extending to more than 30 days. The temporal and spectral analysis of Fermi Large Area Telescope (LAT) observations suggests that it presents similar characteristics to other bursts detected by this instrument. Coupled with X-ray and optical observations, the standard external shock model in a homogeneous medium is favored by this analysis. The X-ray flare is consistent with the synchrotron self-Compton (SSC) model from the reverse-shock region evolving in a thin shell and previous LAT, X-ray, and optical data with the standard synchrotron forward-shock model. The best-fit parameters derived with Markov chain Monte Carlo simulations indicate that the outflow is endowed with magnetic fields and that the radio observations are in the self-absorption regime. The SSC forward-shock model with our parameters can explain the LAT photons beyond the synchrotron limit as well as the emission recently reported by the HESS Collaboration.
Abstract
Observing gravitationally lensed objects in the time domain is difficult, and well-observed time-varying sources are rare. Lensed gamma-ray bursts (GRBs) offer improved timing precision for ...this class of objects, complementing observations of quasars and supernovae. The rate of lensed GRBs is highly uncertain, approximately one in 1000. The Gamma-ray Burst Monitor on board the Fermi Gamma-ray Space Telescope has observed more than 3000 GRBs, making it an ideal instrument to uncover lensed bursts. Here we present observations of GRB 210812A showing two emission episodes, separated by 33.3 s and with a flux ratio of about 4.5. An exhaustive temporal and spectral analysis shows that the two emission episodes have the same pulse and spectral shape, which poses challenges to GRB models. We report multiple lines of evidence for a gravitational lens origin. In particular, modeling the lightcurve using nested sampling, we uncover strong evidence in favor of the lensing scenario. Assuming a point-mass lens, the mass of the lensing object is about 1 million solar masses. High-resolution radio imaging is needed for future lens candidates to derive tighter constraints.
Abstract
Very high energy (VHE) emission is usually interpreted in the synchrotron self-Compton scenario and expected from the low-redshift and high-luminosity gamma-ray bursts (GRBs), such as GRB ...180720B and GRB 190114C. Recently, the H.E.S.S. telescopes reported VHE emission from one of the closest bursts, GRB 190829A, which was associated with the supernova 2019oyw. In this paper, we present a temporal and spectral analysis from optical bands to the Fermi-LAT energy range over multiple observational periods beginning after the trigger time and extending for almost 3 months. We show that the X-ray and optical observations are consistent with synchrotron forward-shock emission evolving between the characteristic and cooling spectral breaks during the early and late afterglow in a uniform-density medium. Modeling the light curves together with the spectral energy distribution, we show that the outflow expanded with an initial bulk Lorentz factor of Γ ∼ 30, which is high for low-luminosity GRBs and low for high-luminosity GRBs. The values of the initial bulk Lorentz factor and the isotropic-equivalent energy suggest that GRB 190829A is an intermediate-luminosity burst; consequently, it becomes the first burst of this class to be detected in the VHE gamma-ray band by an imaging atmospheric Cherenkov telescope and, in turn, the first event to not be simultaneously observed by the Fermi-LAT instrument. Analyzing the intermediate-luminosity bursts with
z
≲ 0.2, such as GRB 130702A, we show that bursts with intermediate luminosities are potential candidates to be detected in VHEs.
Abstract
The Fermi Large Area Telescope (Fermi-LAT) Collaboration reported the Second Gamma-ray Burst Catalog (2FLGC), which comprises a subset of 29 bursts with photon energies above 10 GeV. ...Although the standard synchrotron forward-shock model has successfully explained the gamma-ray burst (GRB) afterglow observations, energetic photons higher than 10 GeV from these transient events can hardly be described in this scenario. We present the closure relations (CRs) of the synchrotron self-Compton (SSC) afterglow model in the adiabatic and radiative scenario, and when the central engine injects continuous energy into the blast wave to study the evolution of the spectral and temporal indexes of those bursts reported in 2FLGC. We consider the SSC afterglow model evolving in stellar-wind and the interstellar medium (ISM), and the CRs as a function of the radiative parameter, the energy injection index, and the electron spectral index for 1 <
p
< 2 and 2 ≤
p
. We select all GRBs that have been modeled with both a simple or a broken power law in the 2FLGC. We found that the CRs of the SSC model can satisfy a significant fraction of the burst that cannot be interpreted in the synchrotron scenario, even though those that require an intermediate density profile (e.g., GRB 130427A) or an atypical fraction of total energy given to amplify the magnetic field (
ε
B
). The value of this parameter in the SSC model ranges (
ε
B
≈ 10
−5
− 10
−4
) when the cooling spectral break corresponds to the Fermi-LAT band for typical values of GRB afterglow. The analysis shows that the ISM is preferred for the scenario without energy injection and the stellar-wind medium for an energy injection scenario.
GRB 110721A was detected by the Gamma-ray Burst Monitor and the Large Area Telescope (LAT) on board the Fermi satellite and the Gamma-ray Burst Polarimeter on board the IKAROS solar mission. Previous ...analysis of this burst showed: (i) a linear polarization signal with position angle stable ( φ p = 160 ° 11 ) and high degree of = 84 − 28 + 16 % , (ii) an extreme peak energy of a record-breaking 15 2 MeV, and (iii) a subdominant prompt thermal component observed right after the onset of this burst. In this paper, the LAT data around the reported position of GRB 110721A are analyzed with the most recent software and then, the LAT light curve above 100 MeV is obtained. The LAT light curve is modeled in terms of adiabatic early-afterglow external shocks when the outflow propagates into a stellar wind. Additionally, we discuss the possible origins and also study the implications of the early-afterglow external shocks on the extreme peak energy observed at 15 2 MeV, the polarization observations, and the subdominant prompt thermal component.
Abstract
Gamma-ray bursts (GRBs) are extremely high-energy events that can be observed at very high redshift. In addition to
γ
rays, they can emit in X-ray, optical, and sometimes radio wavelengths. ...Here, following the approach in Srinivasaragavan et al.; Dainotti et al.; and Dainotti et al., we consider 82 GRBs from Dainotti et al. that have been observed in optical wavelengths and fitted with a broken power law. We consider the relations between the spectral and temporal indices (closure relations; CRs) according to the synchrotron forward-shock model evolving in the constant-density interstellar medium (ISM;
k
= 0) and the stellar wind environment (
k
= 2) in both slow- and fast-cooling regimes, where the density profile is defined as
n
∝
r
−
k
. We find the
ν
>
max
{
ν
c
,
ν
m
}
regime is most favored, where
ν
c
and
ν
m
are the cooling and characteristic frequencies, respectively. Finally, we test the 2D Dainotti correlation between the rest-frame end time of the plateau and the luminosity at that time on GRBs that fulfill the most-favored CRs. When we compare the intrinsic scatter
σ
int
of those 2D correlations to the scatter presented in Dainotti et al., we see the scatters of our correlations generally agree with the previous values within 1
σ
, both before and after correction for selection bias. This new information has helped us to pinpoint subsamples of GRBs with features that could drive the GRB emission mechanism, and eventually allow for GRBs to be used as standard candles.
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