ABSTRACT
Gamma-ray bursts (GRBs) are known to be highly collimated events, and are mostly detectable when they are seen on-axis or very nearly on-axis. However, GRBs can be seen from off-axis angles, ...and the recent detection of a short GRB associated with a gravitational wave event has conclusively shown such a scenario. The observer viewing angle plays an important role in the observable spectral shape and the energetic of such events. We present a numerical model that is based on the single-pulse approximation with emission from a top-hat jet and has been developed to investigate the effects of the observer viewing angle. We assume a conical jet parametrized by a radius Rjet, half-opening angle θjet, a comoving-frame emissivity law and an observer viewing angle θobs, and then study the effects for the conditions θobs < θjet and θobs > θjet. We present results considering a smoothly broken power-law emissivity law in jet comoving frame, albeit the model implementation easily allows to consider other emissivity laws. We find that the relation $E^{\rm i}_{\rm p}\propto E_{\rm iso}^{0.5}$ (Amati relation) is naturally obtained from pure relativistic kinematic when $\Gamma \gtrsim 10$ and θobs < θjet; on the contrary, when θobs > θjet it results $E^{\rm i}_{\rm p}\propto E_{\rm iso}^{0.25}$. Using data from literature for a class of well-know sub-energetic GRBs, we show that their position in the $E^{\rm i}_{\rm p}\!-\!E_{\rm iso}$ plane is consistent with event observed off-axis. The presented model is developed as a module to be integrated in spectral fitting software package xspec and can be used by the scientific community.
The origin and phenomenology of the Fast Radio Burst (FRB) remains unknown despite more than a decade of efforts. Though several models have been proposed to explain the observed data, none is able ...to explain alone the variety of events so far recorded. The leading models consider magnetars as potential FRB sources. The recent detection of FRBs from the galactic magnetar SGR J1935+2154 seems to support them. Still, emission duration and energetic budget challenge all these models. Like for other classes of objects initially detected in a single band, it appeared clear that any solution to the FRB enigma could only come from a coordinated observational and theoretical effort in an as wide as possible energy band. In particular, the detection and localisation of optical/NIR or/and high-energy counterparts seemed an unavoidable starting point that could shed light on the FRB physics. Multiwavelength (MWL) search campaigns were conducted for several FRBs, in particular for repeaters. Here we summarize the observational and theoretical results and the perspectives in view of the several new sources accurately localised that will likely be identified by various radio facilities worldwide. We conclude that more dedicated MWL campaigns sensitive to the millisecond–minute timescale transients are needed to address the various aspects involved in the identification of FRB counterparts. Dedicated instrumentation could be one of the key points in this respect. In the optical/NIR band, fast photometry looks to be the only viable strategy. Additionally, small/medium size radiotelescopes co-pointing higher energies telescopes look a very interesting and cheap complementary observational strategy.
We present detailed multiwavelength observations of GRB 161219B at z = 0.1475, spanning the radio to X-ray regimes, and the first Atacama Large Millimeter/submillimeter Array (ALMA) light curve of a ...γ-ray burst (GRB) afterglow. The centimeter- and millimeter-band observations before 8.5 days require emission in excess of that produced by the afterglow forward shock (FS). These data are consistent with radiation from a refreshed reverse shock (RS) produced by the injection of energy into the FS, signatures of which are also present in the X-ray and optical light curves. We infer a constant-density circumburst environment with an extremely low density, , and show that this is a characteristic of all strong RS detections to date. The Karl G. Lansky Very Large Array (VLA) observations exhibit unexpected rapid variability on roughly minute timescales, indicative of strong interstellar scintillation. The X-ray, ALMA, and VLA observations together constrain the jet break time, days, yielding a wide jet opening angle of , implying beaming-corrected γ-ray and kinetic energies of erg and erg, respectively. Comparing the RS and FS emission, we show that the ejecta are only weakly magnetized, with relative magnetization, , compared to the FS. These direct, multifrequency measurements of a refreshed RS spanning the optical to radio bands highlight the impact of radio and millimeter data in probing the production and nature of GRB jets.
The Radio to GeV Afterglow of GRB 221009A Laskar, Tanmoy; Alexander, Kate D.; Margutti, Raffaella ...
Astrophysical journal. Letters,
03/2023, Letnik:
946, Številka:
1
Journal Article
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Abstract
GRB 221009A (
z
= 0.151) is one of the closest known long
γ
-ray bursts (GRBs). Its extreme brightness across all electromagnetic wavelengths provides an unprecedented opportunity to study a ...member of this still-mysterious class of transients in exquisite detail. We present multiwavelength observations of this extraordinary event, spanning 15 orders of magnitude in photon energy from radio to
γ
-rays. We find that the data can be partially explained by a forward shock (FS) from a highly collimated relativistic jet interacting with a low-density, wind-like medium. Under this model, the jet’s beaming-corrected kinetic energy (
E
K
∼ 4 × 10
50
erg) is typical for the GRB population. The radio and millimeter data provide strong limiting constraints on the FS model, but require the presence of an additional emission component. From equipartition arguments, we find that the radio emission is likely produced by a small amount of mass (≲6 × 10
−7
M
⊙
) moving relativistically (Γ ≳ 9) with a large kinetic energy (≳10
49
erg). However, the temporal evolution of this component does not follow prescriptions for synchrotron radiation from a single power-law distribution of electrons (e.g., in a reverse shock or two-component jet), or a thermal-electron population, perhaps suggesting that one of the standard assumptions of afterglow theory is violated. GRB 221009A will likely remain detectable with radio telescopes for years to come, providing a valuable opportunity to track the full lifecycle of a powerful relativistic jet.
Broadband power density spectra offer a window to understanding turbulent behavior in the emission mechanism and, at the highest frequencies, in the putative inner engines powering long gamma-ray ...bursts (GRBs). We describe a chirp search method alongside Fourier analysis for signal detection in the Poisson noise-dominated, 2 kHz sampled, BeppoSAX light curves. An efficient numerical implementation is described in O(Nn log n) operations, where N is the number of chirp templates and n is the length of the light-curve time series, suited for embarrassingly parallel processing. For the detection of individual chirps over a 1 s duration, the method is one order of magnitude more sensitive in signal-to-noise ratio than Fourier analysis. The Fourier-chirp spectra of GRB 010408 and GRB 970816 show a continuation of the spectral slope with up to 1 kHz of turbulence identified in low-frequency Fourier analysis. The same continuation is observed in an average spectrum of 42 bright, long GRBs. An outlook on a similar analysis of upcoming gravitational wave data is included.
We discuss the results of the analysis of multi-wavelength data for the afterglows of GRB 081007 and GRB 090424, two bursts detected by Swift. One of them, GRB 081007, also shows a spectroscopically ...confirmed supernova, SN 2008hw, which resembles SN 1998bw in its absorption features, while the maximum magnitude may be fainter, up to 0.7 mag, than observed in SN 1998bw. Bright optical flashes have been detected in both events, which allows us to derive solid constraints on the circumburst-matter density profile. This is particularly interesting in the case of GRB 081007, whose afterglow is found to be propagating into a constant-density medium, yielding yet another example of a gamma-ray burst (GRB) clearly associated with a massive-star progenitor which did not sculpt the surroundings with its stellar wind. There is no supernova component detected in the afterglow of GRB 090424, likely due to the brightness of the host galaxy, comparable to the Milky Way. We show that the afterglow data are consistent with the presence of both forward- and reverse-shock emission powered by relativistic outflows expanding into the interstellar medium. The absence of optical peaks due to the forward shock strongly suggests that the reverse-shock regions should be mildly magnetized. The initial Lorentz factor of outflow of GRB 081007 is estimated to be Gamma ~ 200, while for GRB 090424 a lower limit of Gamma > 170 is derived. We also discuss the prompt emission of GRB 081007, which consists of just a single pulse. We argue that neither the external forward-shock model nor the shock-breakout model can account for the prompt emission data and suggest that the single-pulse-like prompt emission may be due to magnetic energy dissipation of a Poynting-flux-dominated outflow or to a dissipative photosphere.
Early Optical Polarization of a Gamma-Ray Burst Afterglow Mundell, Carole G; Steele, Iain A; Smith, Robert J ...
Science (American Association for the Advancement of Science),
03/2007, Letnik:
315, Številka:
5820
Journal Article
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We report the optical polarization of a gamma-ray burst (GRB) afterglow, obtained 203 seconds after the initial burst of γ-rays from GRB 060418, using a ring polarimeter on the robotic Liverpool ...Telescope. Our robust (2σ) upper limit on the percentage of polarization, less than 8%, coincides with the fireball deceleration time at the onset of the afterglow. The combination of the rate of decay of the optical brightness and the low polarization at this critical time constrains standard models of GRB ejecta, ruling out the presence of a large-scale ordered magnetic field in the emitting region.
In review of the White Papers from the Voyage 2050 process
1
and after the public presentation of a number of these papers in October 2019 in Madrid, we as White Paper lead authors have identified a ...coherent science theme that transcends the divisions around which the Topical Teams are structured. This note aims to highlight this synergistic science theme and to make the Topical Teams and the Voyage 2050 Senior Committee aware of the wide importance of these topics and the broad support that they have across the worldwide science community.
The power-density spectrum of a light curve is often calculated as the average of a number of spectra derived on individual time intervals the light curve is divided into. This procedure implicitly ...assumes that each time interval is a different sample function of the same stochastic ergodic process. While this assumption can be applied to many astrophysical sources, there remains a class of transient, highly non-stationary and short-lived events, such as gamma-ray bursts, for which this approach is often inadequate. The power spectrum statistics of a constant signal affected by statistical (Poisson) noise are known to be a χ2
2 in the Leahy normalization. However, this is no more the case when a non-stationary signal is also present. As a consequence, the uncertainties on the power spectrum cannot be calculated on the basis of the χ2
2 properties, as assumed by tools such as xronos
powspec. We generalize the result in the case of a non-stationary signal affected by uncorrelated white noise and show that the new distribution is a non-central χ2
2(λ), whose non-central value λ is the power spectrum of the deterministic function describing the non-stationary signal. Finally, we test these results in the case of synthetic curves of gamma-ray bursts. We end up with a new formula for calculating the power spectrum uncertainties. This is crucial in the case of non-stationary short-lived processes affected by uncorrelated statistical noise, for which ensemble averaging does not make any physical sense.
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