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Register a new userA luminous blue kilonova and an off-axis jet from a compact binary merger at z=0.1341
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The recent discovery of a faint gamma-ray burst (GRB) coincident with the gravitational wave (GW) event GW 170817 revealed the existence of a population of low-luminosity short duration gamma-ray transients produced by neutron star mergers in the nearby Universe. These events could be routinely detected by existing gamma-ray monitors, yet previous observations failed to identify them without the aid of GW triggers. Here we show that GRB150101B was an analogue of GRB170817A located at a cosmological distance. GRB 150101B was a faint short duration GRB characterized by a bright optical counterpart and a long-lived X-ray afterglow. These properties are unusual for standard short GRBs and are instead consistent with an explosion viewed off-axis: the optical light is produced by a luminous kilonova component, while the observed X-rays trace the GRB afterglow viewed at an angle of ~13 degrees. Our findings suggest that these properties could be common among future electromagnetic counterparts of GW sources.
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GRB 060505 was the first well-known nearby (at redshift 0.089) hybrid gamma-ray burst that has a duration longer than 2 seconds but without the association of a supernova down to very stringent limits. The prompt $gamma-$ray flash lasting $sim 4$ sec could consist of an intrinsic short burst and its tail emission, but the sizable temporal lag ($sim 0.35$ sec) as well as the environment properties led to the widely-accepted classification of a long duration gamma-ray burst originated from the collapse of a massive star. Here for the $ first$ time we report the convincing evidence for a thermal-like optical radiation component in the spectral energy distribution of the early afterglow emission. In comparison to AT2017gfo, the thermal radiation is $sim 2$ times brighter and the temperature is comparable at similar epochs. The optical decline is much quicker than that in X-rays, which is also at odds with the fireball afterglow model but quite natural for the presence of a blue kilonova. Our finding reveals a neutron star merger origin of the hybrid GRB 060505 and strongly supports the theoretical speculation that some binary neutron stars can merge ultra-quickly (within $sim 1$ Myr) after their formation when the surrounding region is still highly star-forming and the metallicity remains low. Gravitational wave and electromagnetic jointed observations are expected to confirm such scenarios in the near future.
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 $k=1.5$.
CDF-S XT1 is a fast-rising non-thermal X-ray transient detected by textit{Chandra} in the Deep-Field South Survey. Although various hypotheses have been suggested, the origin of this transient remains unclear. Here, we show that the observations of CDF-S XT1 are well explained as the X-ray afterglow produced by a relativistic structured jet viewed off-axis. We measure properties of the jet, showing that they are similar to those of GRB170817A, albeit at cosmological distances. We measure the observers viewing angle to be $theta_{textrm{obs}} = 10^{circ}pm3^{circ}$ and the core of the ultra-relativistic jet to be $theta_{textrm{core}} = 4.4^{circ}pm0.9^{circ}$, where the uncertainties are the $68%$ credible interval. The inferred properties and host galaxy combined with Hubble, radio, and optical non detections favour the hypothesis that CDF-S XT1 is the off-axis afterglow of a binary neutron star merger. We find that other previously suggested hypotheses are unable to explain all properties of CDF-S XT1. At a redshift of $z=2.23$, this is potentially the most distant observed neutron star merger to date and the first orphan afterglow of a short gamma-ray burst. We discuss the implications of a binary neutron star merger at such a high redshift for the star-formation rate in the early Universe, the nucleosynthesis of heavy elements, and the prospect of identifying other off-axis afterglows.
We review current understanding of kilonova/macronova emission from compact binary mergers (mergers of two neutron stars or a neutron star and a black hole). Kilonova/macronova is optical and near-infrared emission powered by radioactive decays of r-process nuclei. Emission from the dynamical ejecta with ~0.01 Msun is likely to have a luminosity of ~10^{40}-10^{41} erg s^{-1} with a characteristic timescale of about 1 week. The spectral peak is located in red optical or near-infrared wavelengths. A subsequent accretion disk wind may provide an additional luminosity, or an earlier/bluer emission if it is not absorbed by the precedent dynamical ejecta. The detection of near-infrared excess in the afterglow of short GRB 130603B and possible optical excess in GRB 060614 supports the concept of the kilonova/macronova scenario. At 200 Mpc distance, a typical brightness of kilonova/macronova with 0.01 Msun ejecta is expected to be about 22 mag and the emission rapidly fades to >24 mag within ~10 days after the merger. Kilonova/macronova candidates can be distinguished from supernovae by (1) the faster time evolution, (2) fainter absolute magnitudes, and (3) redder colors. To effectively search for such objects, follow-up survey observations with multiple visits within <10 days and with multiple filters will be important. Since the high expansion velocity (v ~ 0.1-0.2c) is a robust outcome of compact binary mergers, the detection of smooth spectra will be the smoking gun to conclusively identify the GW source.
The binary neutron star merger event GW170817 was detected through both electromagnetic radiation and gravitational waves. Its afterglow emission may have been produced by either a narrow relativistic jet or an isotropic outflow. High spatial resolution measurements of the source size and displacement can discriminate between these scenarios. We present Very Long Baseline Interferometry observations, performed 207.4 days after the merger, using a global network of 32 radio telescopes. The apparent source size is constrained to be smaller than 2.5 milliarcseconds at the 90% confidence level. This excludes the isotropic outflow scenario, which would have produced a larger apparent size, indicating that GW170817 produced a structured relativistic jet. Our rate calculations show that at least 10% of neutron star mergers produce such a jet.
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