Transition from Fireball to Poynting-flux-dominated Outflow in Three-Episode GRB 160625B

The ejecta composition is an open question in gamma-ray bursts (GRB) physics. Some GRBs possess a quasi-thermal spectral component in the time-resolved spectral analysis, suggesting a hot fireball origin. Others show a featureless non-thermal spectrum known as the "Band" function, consistent with a synchrotron radiation origin and suggesting that the jet is Poynting-flux-dominated at the central engine and likely in the emission region as well. There are also bursts showing a sub-dominant thermal component and a dominant synchrotron component suggesting a likely hybrid jet composition. Here we report an extraordinarily bright GRB 160625B, simultaneously observed in gamma-rays and optical wavelengths, whose prompt emission consists of three isolated episodes separated by long quiescent intervals, with the durations of each "sub-burst" being $\sim$ 0.8 s, 35 s, and 212 s, respectively. Its high brightness (with isotropic peak luminosity L$_{\rm p, iso}\sim 4\times 10^{53}$ erg/s) allows us to conduct detailed time-resolved spectral analysis in each episode, from precursor to main burst and to extended emission. The spectral properties of the first two sub-bursts are distinctly different, allowing us to observe the transition from thermal to non-thermal radiation between well-separated emission episodes within 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.Comment: Revised version reflecting the referees' comments. 27 pages, 11 figures, 5 tables. The final edited version will appear in Nature Astronom

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

Photometric observations of LO peg in 2014-2015

We performed new observations of an ultra-fast rotator of the spectral class K - the LO Peg star - in SAO RAS in 2014 and in Zvenigorod Observatory of INASAN in 2015. The light curves were used to build the maps of temperature inhomogeneities on the LO Peg surface in order to determine the longitudes corresponding to the location of active regions. The obtained measurements suggest the ongoing evolution of movements of active regions and probably the cyclic character of such movements. According to our estimations, the area of the star surface covered with spots decreases and by now it reached 14% of the total visible area of its surface. New observations of the star in V filter allowed us to specify LO Peg long-term variability cycles. Based on spectropolarimetric observations of LO Peg the null result for measurements of mean longitudinal component of magnetic field is obtained

ИЗМЕНЕНИЕ СТРУКТУРЫ И СВОЙСТВ ПРИ ОТПУСКЕ ЗАКАЛЕННОЙ СРЕДНЕУГЛЕРОДИСТОЙ СТАЛИ

The questions change of structure and properties are considered at tempering of quenched on martensite mediumcarbon steel are considered. At the temperatures of tempering from 250–300°C effects of softening in quenching carbon steel exceed influence from block of dislocations with atoms of carbon and development of processes dispertion hardening. Рассмотрены вопросы изменения структуры и свойств при отпуске закаленной на мартенсит среднеуглеродистой стали. При отпуске от температур 250–300 °С эффекты разупрочнения в закаленной углеродистой стали превышают влияние от закрепления дислокаций атомами углерода и развития процессов дисперсионного упрочнения.