ABSTRACT
We report on our observing campaign of the compact binary merger GW190814, detected by the Advanced LIGO and Advanced Virgo detectors on 2019 August 14. This signal has the best localization ...of any observed gravitational wave (GW) source, with a 90 per cent probability area of 18.5 deg2, and an estimated distance of ≈240 Mpc. We obtained wide-field observations with the Deca-Degree Optical Transient Imager (DDOTI) covering 88 per cent of the probability area down to a limiting magnitude of w = 19.9 AB. Nearby galaxies within the high probability region were targeted with the Lowell Discovery Telescope (LDT), whereas promising candidate counterparts were characterized through multicolour photometry with the Reionization and Transients InfraRed (RATIR) and spectroscopy with the Gran Telescopio de Canarias (GTC). We use our optical and near-infrared limits in conjunction with the upper limits obtained by the community to constrain the possible electromagnetic counterparts associated with the merger. A gamma-ray burst seen along its jet’s axis is disfavoured by the multiwavelength data set, whereas the presence of a burst seen at larger viewing angles is not well constrained. Although our observations are not sensitive to a kilonova similar to AT2017gfo, we can rule out high-mass (>0.1 M⊙) fast-moving (mean velocity ≥0.3c) wind ejecta for a possible kilonova associated with this merger.
The Fermi-LAT collaboration presented the second gamma-ray burst (GRB) catalog covering its first 10 years of operations. A significant fraction of afterglow-phase light curves in this catalog cannot ...be explained by the closure relations of the standard synchrotron forward-shock model, suggesting that there could be an important contribution from another process. In view of the above, we derive the synchrotron self-Compton (SSC) light curves from the reverse shock in the thick- and thin-shell regime for a uniform-density medium. We show that this emission could explain the GeV flares exhibited in some LAT light curves. Additionally, we demonstrate that the passage of the forward shock synchrotron cooling break through the LAT band from jets expanding in a uniform-density environment may be responsible for the late time ( 102 s) steepening of LAT GRB afterglow light curves. As a particular case, we model the LAT light curve of GRB 160509A that exhibited a GeV flare together with a break in the long-lasting emission, and also two very high energy photons with energies of 51.9 and 41.5 GeV observed 76.5 and 242 s after the onset of the burst, respectively. Constraining the microphysical parameters and the circumburst density from the afterglow observations, we show that the GeV flare is consistent with an SSC reverse-shock model, the break in the long-lasting emission with the passage of the synchrotron cooling break through the Fermi-LAT band, and the very energetic photons with SSC emission from the forward shock, when the outflow carries a significant magnetic field (RB 30) and it decelerates in a uniform-density medium with a very low density ( ).
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.
ABSTRACT
We derive limits on any electromagnetic counterpart to the compact binary merger S190814bv, whose parameters are consistent with the merger of a black hole and a neutron star. We present ...observations with the new wide-field optical imager DDOTI and also consider Swift/BAT observations reported by Palmer et al. We show that Swift/BAT would have detected a counterpart with similar properties to a typical on-axis short GRB at the 98 per cent confidence level, whereas our DDOTI observations only rule out such a counterpart at the 27 per cent confidence level. Neither has sufficient sensitivity to rule out an off-axis counterpart like GW 170817. We compare the efficiency of Swift/BAT and DDOTI for future observations, and show that DDOTI is likely to be about twice as efficient as Swift/BAT for off-axis events up to about 100 Mpc.
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.
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
Gamma-ray bursts (GRBs) have been classified traditionally based on their duration. The increasing number of extended emission (EE) GRBs, lasting typically more than 2s but with properties ...similar to those of short GRBs, challenges the traditional classification criteria. In this work, we use the t-distributed stochastic neighbor embedding (t-SNE), a machine-learning technique, to classify GRBs. We present the results for GRBs observed until 2022 July by the Swift/BAT (Burst Alert Telescope) instrument in all its energy bands. We show the effects of varying the learning rate and perplexity parameters as well as the benefit of preprocessing the data by a nonparametric noise-reduction technique. Consistently with previous works, we show that the t-SNE method separates GRBs into two subgroups. We also show that EE GRBs reported by various authors under different criteria tend to cluster in a few regions of our t-SNE maps and identify seven new EE GRB candidates by using the gamma-ray data provided by the automatic pipeline of Swift/BAT and the proximity with previously identified EE GRBs.
We present observations of the possible short GRB 180418A in γ-rays, X-rays, and in the optical. Early optical photometry with the TAROT and RATIR instruments shows a bright peak ( 14.2 AB mag) ...between T + 28 and T + 90 s that we interpret as the signature of a reverse shock. Later observations can be modeled by a standard forward shock model and show no evidence of a jet break, allowing us to constrain the jet collimation to θj > 7°. Using deep late-time optical observations, we place an upper limit of r > 24 AB mag on any underlying host galaxy. The detection of the afterglow in the Swift UV filters constrains the GRB redshift to z < 1.3 and places an upper bound on the γ-ray isotropic equivalent energy Eγ,iso < 3 × 1051 erg. The properties of this GRB (e.g., duration, hardness ratio, energy, and environment) lie at the intersection between short and long bursts, and we cannot conclusively identify its type. We estimate that the probability that it is drawn from the population of short GRBs is 10%-30%.
Abstract
Gamma-ray bursts (GRBs) are fascinating events due to their panchromatic nature. We study optical plateaus in GRB afterglows via an extended search into archival data. We comprehensively ...analyze all published GRBs with known redshifts and optical plateaus observed by many ground-based telescopes (e.g., Subaru Telescope, RATIR) around the world and several space-based observatories such as the Neil Gehrels Swift Observatory. We fit 500 optical light curves, showing the existence of the plateau in 179 cases. This sample is 75% larger than the previous one, and it is the largest compilation so far of optical plateaus. We discover the 3D fundamental plane relation at optical wavelengths using this sample. This correlation is between the rest-frame time at the end of the plateau emission,
T
opt
*
, its optical luminosity,
L
opt
, and the peak in the optical prompt emission,
L
peak,opt
, thus resembling the three-dimensional (3D) X-ray fundamental plane (the so-called 3D Dainotti relation). We correct our sample for redshift evolution and selection effects, discovering that this correlation is indeed intrinsic to GRB physics. We investigate the rest-frame end-time distributions in X-rays and optical (
T
opt
*
,
T
X
*
), and conclude that the plateau is achromatic only when selection biases are not considered. We also investigate if the 3D optical correlation may be a new discriminant between optical GRB classes and find that there is no significant separation between the classes compared to the Gold sample plane after correcting for evolution.
GRB 160625B, one of the brightest bursts in recent years, was simultaneously observed by Fermi and Swift satellites, and ground-based optical telescopes in three different events separated by long ...periods of time. In this paper, the non-thermal multiwavelength observations of GRB 160625B are described and a transition phase from wind-type-like medium to interstellar medium (ISM) between the early (event II) and the late (event III) afterglow is found. The multiwavelength observations of the early afterglow are consistent with the afterglow evolution starting at ∼150 s in a stellar wind medium, whereas the observations of the late afterglow are consistent with the afterglow evolution in ISM. The wind-to-ISM transition is calculated to be at s when the jet has decelerated, at a distance of ∼1 pc from the progenitor. Using the standard external shock model, the synchrotron and synchrotron self-Compton emission from reverse shock is required to model the GeV γ-ray and optical observations in the early afterglow, and synchrotron radiation from the adiabatic forward shock to describe the X-ray and optical observations in the late afterglow. The derived values of the magnetization parameter, the slope of the fast decay of the optical flash, and the inferred magnetic fields suggest that Poynting flux-dominated jet models with arbitrary magnetization could account for the spectral properties exhibited by GRB 160625B.
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