Transition from fireball to Poynting-flux-dominated outflow in three-episode GRB 160625B [submitted version]

The ejecta composition of gamma-ray bursts (GRBs) is an open question in GRB physics. Some GRBs possess a quasi-thermal spectral component in the time-resolved spectral analysis, suggesting a hot fireball origin. Some others show an essentially feature-less non-thermal spectrum known as the "Band" function, which can be interpreted as synchrotron radiation in an optically thin region, suggesting a Poynting-flux-dominated jet composition. Here we report an extraordinarily bright GRB 160625B, simultaneously observed in gamma-rays and optical wavelengths, whose prompt emission consists of three dramatically different isolated episodes separated by long quiescent intervals, with the durations of each "sub-burst" being ∼ 0.8 s, 35 s, and 212 s, respectively. The high brightness (with isotropic peak luminosity Lp,iso∼4×1053 erg/s) of this GRB allows us to conduct detailed time-resolved spectral analysis in each episode, from precursor to the main burst and extended emission. Interestingly, the spectral properties of the first two sub-bursts are distinctly different, which allow us for the first time to observe the transition from thermal to non-thermal radiation in a single GRB. Such a transition is a clear indication of the change of jet composition from a fireball to a Poynting-flux-dominated jet

A922 Sequential measurement of 1 hour creatinine clearance (1-CRCL) in critically ill patients at risk of acute kidney injury (AKI)

Meeting abstrac

Searching for Hα-emitting sources in the gaps of five transitional disks: SPHERE/ZIMPOL high-contrast imaging

Context. (Pre-)Transitional disks show gaps and cavities that can be related to ongoing planet formation. According to theory, young embedded planets can accrete material from the circumplanetary and circumstellar disks and can be detected using accretion tracers, such as the Hα emission line. Aims. We aim to detect accreting protoplanets within the cavities of five (pre-)transitional disks through adaptive-optics(AO)-assisted spectral angular differential imaging in the optical regime. Methods. We performed simultaneous AO observations in the Hα line and the adjacent continuum using the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) with the Zurich Imaging Polarimeter (ZIMPOL) at the Very Large Telescope (VLT). We combined spectral and angular differential imaging techniques to increase the contrast in the innermost regions close to the star and search for the signature of young accreting protoplanets. Results. The reduced images show no clear Hα point source around any of the targets. We report the presence of faint Hα emission around TW Hya and HD163296: while the former is most probably an artifact related to a spike, the nature of the latter remains unclear. The spectral and angular differential images yield contrasts of 6- 8 magnitudes at ∼ 100 mas from the central stars, except in the case of LkCa15, with values of ∼ 3 mag. We used the contrast curves to estimate average upper limits to the Hα line luminosity of LHα ∼ 5 × 10-6 L· at separations ¥200 mas for TW Hya, RXJ1615, and T Cha, while for HD163296 and LkCa15 we derive values of ∼ 3 × 10- 5 L·. We estimated upper limits to the accretion luminosity of potential protoplanets, obtaining that planetary models provide an average value of Lacc ∼ 10- 4 L· at 200 mas, which is about two orders of magnitude higher than the Lacc estimated from the extrapolation of the LHα - Lacc stellar relationship. Conclusions. When considering all the objects observed with SPHERE/ZIMPOL in the Hα line, 5 in this work and 13 from the literature, we can explain the lack of protoplanet detections by a combination of factors, such as a majority of low-mass, low-accreting planets; potential episodic accretion; significant extinction from the circumstellar and circumplanetary disks; and the fact that the contrast is less favorable at separations of smaller than 100 mas, where giant planets are more likely to form.ISSN:0004-6361ISSN:1432-074