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Existing theory and models suggest that a Type I (merger) GRB should have a larger jet beaming angle than a Type II (collapsar) GRB, but so far no statistical evidence is available to support this suggestion. In this paper, we obtain a sample of 37 beaming angles and calculate the probability that this is true. A correction is also devised to account for the scarcity of Type I GRBs in our sample. The probability is calculated to be 83% without the correction and 71% with it.
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We investigate the shape of the jet break in within-beam gamma-ray burst (GRB) optical afterglows for various lateral jet structure profiles. We consider cases with and without lateral spreading and a range of inclinations within the jet core half-opening angle, $theta_c$. We fit model and observed afterglow lightcurves with a smoothly-broken power-law function with a free-parameter $kappa$ that describes the sharpness of the break. We find that the jet break is sharper ($kappa$ is greater) when lateral spreading is included than in the absence of lateral spreading. For profiles with a sharp-edged core, the sharpness parameter has a broad range of $0.1lesssimkappalesssim4.6$, whereas profiles with a smooth-edged core have a narrower range of $0.1lesssimkappalesssim2.2$ when models both with and without lateral spreading are included. For sharp-edged jets, the jet break sharpness depends strongly on the inclination of the system within $theta_c$, whereas for smooth-edged jets, $kappa$ is more strongly dependent on the size of $theta_c$. Using a sample of 20 GRBs we find nine candidate smooth-edged jet structures and eight candidate sharp-edged jet structures, while the remaining three are consistent with either. The shape of the jet break, as measured by the sharpness parameter $kappa$, can be used as an initial check for the presence of lateral structure in within-beam GRBs where the afterglow is well-sampled at and around the jet-break time.
The quick and precise localization of GRBs by the Swift telescope allows the early evolution of the afterglow light curve to be captured by ground-based telescopes. With GROND measurements we can investigate the optical/near-infrared light curve of the afterglow of gamma-ray burst 080413B in the context of late rebrightening. Multi-wavelength follow-up observations were performed on the afterglow of GRB 080413B. X-ray emission was detected by the X-ray telescope onboard the Swift satellite and obtained from the public archive. Optical and near-infrared photometry was performed with the seven-channel imager GROND mounted at the MPG/ESO 2.2 m telescope and additionally with the REM telescope, both in La Silla, Chile. The light curve model was constructed using the obtained broad-band data. The broad-band light curve of the afterglow of GRB 080413B is well fitted with an on-axis two-component jet model. The narrow ultra-relativistic jet is responsible for the initial decay, while the rise of the moderately relativistic wider jet near its deceleration time is the cause of the rebrightening of the light curve. The later evolution of the optical/NIR light curve is then dominated by the wide component, the signature of which is almost negligible in the X-ray wavelengths. These components have opening angles of theta(narrow) ~1.7 degrees and theta(wide) ~9 degrees, and Lorentz factors of Gamma(narrow) >188 and Gamma(wide) ~18.5. We calculated the beaming-corrected energy release to be 7.9 x 10^48 erg.
We present multiwavelength modeling of the afterglow from the long gamma-ray burst GRB 160625B using Markov Chain Monte Carlo (MCMC) techniques of the afterglowpy Python package. GRB 160625B is an extremely bright burst with a rich set of observations spanning from radio to gamma-ray frequencies. These observations range from ~0.1 days to >1000 days, thus making this event extremely well-suited to such modeling. In this work we compare top-hat and Gaussian jet structure types in order to find best fit values for the GRB jet collimation angle, viewing angle, and other physical parameters. We find that a Gaussian-shaped jet is preferred (2.7-5.3 sigma) over the traditional top-hat model. Our estimate for the opening angle of the burst ranges from 1.26 to 3.90 degrees, depending on jet shape model. We also discuss the implications that assumptions on jet shape, viewing angle, and particularly the participation fraction of electrons have on the final estimation of GRB intrinsic energy release and the resulting energy budget of the relativistic outflow. Most notably, allowing the participation fraction to vary results in an estimated total relativistic energy of ~$10^{53}$ erg. This is two orders of magnitude higher than when the total fraction is assumed to be unity, thus this parameter has strong relevance for placing constraints on long GRB central engines, details of the circumburst media, and host environment.
In light of the most recent observations of late afterglows produced by the merger of compact objects or by the core-collapse of massive dying stars, we research the evolution of the afterglow produced by an off-axis top-hat jet and its interaction with a surrounding medium. The medium is parametrized by a power law distribution of the form $n(r)propto r^{-k}$ is the stratification parameter and contains the development when the surrounding density is constant ($k=0$) or wind-like ($k=2$). We develop an analytical synchrotron forward-shock model when the outflow is viewed off-axis, and it is decelerated by a stratified medium. Using the X-ray data points collected by a large campaign of orbiting satellites and ground telescopes, we have managed to apply our model and fit the X-ray spectrum of the GRB afterglow associated to SN 2020bvc with conventional parameters. Our model predicts that its circumburst medium is parametrized by a power law with stratification parameter $k=1.5$.
We present X-ray and multi-band optical observations of the afterglow and host galaxy of GRB 180418A, discovered by ${it Swift}$/BAT and ${it Fermi}$/GBM. We present a reanalysis of the GBM and BAT data deriving durations of the prompt emission of $T_{90}approx$2.56s and $approx$1.90s, respectively. Modeling the ${it Fermi}$/GBM catalog of 1405 bursts (2008-2014) in the Hardness-$T_{90}$ plane, we obtain a probability of $approx$60% that GRB 180418A is a short-hard burst. From a combination of ${it Swift}$/XRT and ${it Chandra}$ observations, the X-ray afterglow is detected to $approx$38.5 days after the burst, and exhibits a single power-law decline with $F_{rm X} propto t^{-0.98}$. Late-time Gemini observations reveal a faint r$approx$25.69 mag host galaxy at an angular offset of $approx$0.16. At the likely redshift range of z$approx$1-2.25, we find that the X-ray afterglow luminosity of GRB 180418A is intermediate between short and long GRBs at all epochs during which there is contemporaneous data, and that GRB 180418A lies closer to the $E_{gamma,{rm peak}}-E_{gamma,{rm iso}}$ correlation for short GRBs. Modeling the multi-wavelength afterglow with the standard synchrotron model, we derive the burst explosion properties and find a jet opening angle of $theta_{rm j} gtrsim 9-14^{circ}$. If GRB 180418A is a short GRB that originated from a neutron star merger, it has one of the brightest and longest-lived afterglows along with an extremely faint host galaxy. If instead the event is a long GRB that originated from a massive star collapse, it has among the lowest luminosity afterglows, and lies in a peculiar space in terms of the Hardness-$T_{90}$ and $E_{gamma,{rm peak}}-E_{gamma,{rm iso}}$ planes.
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