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Register a new userA long optical plateau in the afterglow of the Extended Emission short GRB 150424A: Evidence for energy injection by a magnetar?
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Short-duration GRBs with extended emission form a subclass of short GRBs, comprising about 15% of the short-duration sample. Afterglow detections of short GRBs are also rare (about 30%) due to their smaller luminosity. We present a multi-band data set of the short burst with extended emission GRB 150424A, comprising of GROND observations, complemented with data from Swift/UVOT, Swift/XRT, HST, Keck/LRIS and data points from the literature. The GRB 150424A afterglow shows an extended plateau phase, lasting about 8hrs. The analysis of this unique GRB afterglow might shed light on the understanding of afterglow plateau emission, the nature of which is still under debate. We present a phenomenological analysis by applying fireball closure relations, and interpret the findings in the context of the fireball model. We discuss the plausibility of a magnetar as a central engine, being responsible for additional and prolonged energy injection into the fireball. We find convincing evidence for energy injection into the afterglow of GRB 150424A. We find that a magnetar spin down as source for a prolonged energy injection requires that at least 4% of the spin-down energy is converted to radiation.
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We present a search for prompt radio emission associated with the short-duration gamma-ray burst (GRB) 150424A using the Murchison Widefield Array (MWA) at frequencies from 80-133 MHz. Our observations span delays of 23 s-30 min after the GRB, corresponding to dispersion measures of 100-7700 pc/cm^3. We see no excess flux in images with timescales of 4 s, 2 min, or 30 min, and set a 3 sigma flux density limit of 3.0 Jy at 132 MHz on the shortest timescales: some of the most stringent limits to date on prompt radio emission from any type of GRB. We use these limits to constrain a number of proposed models for coherent emission from short-duration GRBs, although we show that our limits are not particularly constraining for fast radio bursts because of reduced sensitivity for this pointing. Finally, we discuss the prospects for using the MWA to search for prompt radio emission from gravitational wave transients and find that while the flux density and luminosity limits are likely to be very constraining, the latency of the gravitational wave alert may limit the robustness of any conclusions.
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 early optical emission of gamma-ray bursts gives an opportunity to understand the central engine and first stages of these events. About 30% of GRBs present flares whose origin is still a subject of discussion. We present optical photometry of GRB 180620A with the COATLI telescope and RATIR instrument. COATLI started to observe from the end of prompt emission at $T+39.3$~s and RATIR from $T+121.4$~s. We supplement the optical data with the X-ray light curve from emph{Swift}/XRT. %The optical and X-ray light curves show very unusual behavior with features clearly beyond the standard fireball model. We observe an optical flare from $T+110$ to $T+550$~s, with a temporal index decay $alpha_mathrm{O,decay}=1.32pm 0.01$, and a $Delta t/t=1.63$, which we interpret as the signature of a reverse shock component. After the initial normal decay the light curves show a long plateau from $T+500$ to $T+7800$~s both in X-rays and the optical before decaying again after an achromatic jet break at $T+7800$~s. Fluctuations are seen during the plateau phase in the optical. Adding to the complexity of GRB afterglows, the plateau phase (typically associated with the coasting phase of the jet) is seen in this object after the ``normal decay phase (emitted during the deceleration phase of the jet) and the jet break phase occurs directly after the plateau. We suggest that this sequence of events can be explained by a rapid deceleration of the jet with $t_dlesssim 40$ s due to the high density of the environment ($approx 100$ cm$^{-3}$) followed by reactivation of the central engine which causes the flare and powers the plateau phase.
We study the high-energy properties of GRB 181123B, a short gamma-ray burst (sGRB) at redshift $zapprox$1.75. We show that, despite its nominal short duration with $T_{90}<$2 s, this burst displays evidence of a temporally extended emission (EE) at high energies and that the same trend is observed in the majority of sGRBs at $zgtrsim$1. We discuss the impact of instrumental selection effects on the GRB classification, stressing that the measured $T_{90}$ is not an unambiguous indicator of the burst physical origin. By examining their environment (e.g. stellar mass, star formation, offset distribution), we find that these high-$z$ sGRBs share many properties of long GRBs at a similar distance and are consistent with a short-lived progenitor system. If produced by compact binary mergers, these sGRBs with EE may be easier to localize at large distances and herald a larger population of sGRBs in the early universe.
The afterglow of GRB 170817A has been detected for more than three years, but the origin of the multi-band afterglow light curves remains under debate. A classical top-hat jet model is faced with difficulties in producing a shallow rise of the afterglow light curves as observed $(F_{ u} propto T^{0.8})$. Here we reconsider the model of stratified ejecta with energy profile of $E(>Gamma beta)=E_0(Gamma beta)^{-k}$ as the origin of the afterglow light curves of the burst, where $Gamma$ and $beta$ are the Lorentz factor and speed of the ejecta, respectively. $k$ is the power-law slope of the energy profile. We consider the ejecta are collimated into jets. Two kinds of jet evolutions are investigated, including a lateral-spreading jet and a non-lateral-spreading jet. We fit the multi-band afterglow light curves, including the X-ray data at one thousand days post-burst, and find that both the models of the spreading and non-spreading jets can fit the light curves well, but the observed angular size of the source and the apparent velocity of the flux centroid for the spreading jet model are beyond the observation limits, while the non-spreading jet model meets the observation limits. Some of the best-fit parameters for the non-spreading jet model, such as the number density of the circumburst medium $sim10^{-2}$ cm$^{-3}$ and the total jet kinetic energy $E sim 4.8times 10^{51}$ erg, also appear plausible. The best-fit slope of the jet energy profile is $k sim 7.1$. Our results suggest that the afterglow of GRB 170817A may arise from the stratified jet and that the lateral spreading of the jet is not significant.
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