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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.
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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.
We report the results of our observing campaign on GRB140903A, a nearby (z=0.351) short duration (T90~0.3 s) gamma-ray burst discovered by Swift. We monitored the X-ray afterglow with Chandra up to 21 days after the burst, and detected a steeper decay of the X-ray flux after approximately 1 day. Continued monitoring at optical and radio wavelengths showed a similar decay in flux at nearly the same time, and we interpret it as evidence of a narrowly collimated jet. By using the standard fireball model to describe the afterglow evolution, we derive a jet opening angle of 5 deg and a collimation-corrected total energy release of 2E50 erg. We further discuss the nature of the GRB progenitor system. Three main lines disfavor a massive star progenitor: the properties of the prompt gamma-ray emission, the age and low star-formation rate of the host galaxy, and the lack of a bright supernova. We conclude that this event was likely originated by a compact binary merger.
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 broad-band observations of the afterglow and environment of the short GRB 111020A. An extensive X-ray light curve from Swift/XRT, XMM-Newton and Chandra, spanning ~100 seconds to 10 days after the burst, reveals a significant break at t~2 days with pre- and post-break decline rates of alphaX,1 ~ -0.78 and alphaX,2<-1.7, respectively. Interpreted as a jet break, we infer a collimated outflow with an opening angle of ~3-8 degrees. The resulting beaming-corrected gamma-ray (10-1000 keV band) and blastwave kinetic energies are (2-3)e48 erg and (0.3-2)e49 erg, respectively, with the range depending on the unknown redshift of the burst. We report a radio afterglow limit of <39 microJy (3-sigma) from EVLA observations which, along with our finding that vc<vX, constrains the circumburst density to n~0.01-0.1 cm^(-3). Optical observations provide an afterglow limit of i>24.4 mag at 18 hours after the burst, and reveal a potential host galaxy with i~24.3 mag. The sub-arcsecond localization from Chandra provides a precise offset of 0.80+/-0.11 (1-sigma) from this galaxy corresponding to an offset of 5-7 kpc for z=0.5-1.5. We find a high excess neutral Hydrogen column density of (7.5+/-2.0)e21 cm^(-2) (z=0). Our observations demonstrate that a growing fraction of short GRBs are collimated which may lead to a true event rate of >100-1000 Gpc^(-3) yr^(-1), in good agreement with the NS-NS merger rate of ~200-3000 Gpc^(-3) yr^(-1). This consistency is promising for coincident short GRB-gravitational wave searches in the forthcoming era of Advanced LIGO/VIRGO.
We investigate the long GRB140629A through multiwavelength observations, which cover optical, infrared and X-rays between 40s and 3yr after the burst, to derive the properties of the dominant jet and its host galaxy. Polarisation observations by the MASTER telescope indicate that this burst is weakly polarised. The optical spectrum contains absorption features, from which we confirm the redshift of the GRB as originating at z=2.276. We performed spectral fitting of the X-rays to optical afterglow data and find there is no strong spectral evolution. We determine the hydrogen column density to be 7.2x10^21cm^-2 along the line of sight. The afterglow in this burst can be explained by a blast wave jet with a long-lasting central engine expanding into a uniform medium in the slow cooling regime. At the end of energy injection, a normal decay phase is observed in both the optical and X-ray bands. An achromatic jet break is also found in the afterglow light curves 0.4d after trigger. We fit the multiwavelength data simultaneously with a model based on a numerical simulation and find that the observations can be explained by a narrow uniform jet in a dense environment with an opening angle of 6.7deg viewed 3.8deg off-axis, which released a total energy of 1.4x10^54erg. Using the redshift and opening angle, we find GRB 140629A follows both the Ghirlanda and Amati relations. From the peak time of the light curve, identified as the onset of the forward shock (181s after trigger), the initial Lorentz factor is constrained in the range 82-118. Fitting the host galaxy photometry, we find the host to be a low mass, star-forming galaxy with a star formation rate of logSFR=1.1^+0.9_-0.4Myr^-1. We obtain a value of the neutral hydrogen density by fitting the optical spectrum, logN(HI)=21.0+-0.3, classifying this host as a damped Lyman-alpha. High ionisation lines are also detected in the spectrum.
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