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The ejecta composition is an open question in gamma-ray bursts (GRB) physics 1 . Some GRBs possess a quasi-thermal spectral component in the time-resolved spectral analysis 2 , suggesting a hot fireball origin. Others show a featureless non-thermal spectrum known as the "Band" function 3-5 , consistent with a synchrotron radiation origin 5,6 and suggesting that the jet is Poynting-flux-dominated at the central engine and likely in the emission region as well 7,8 . There are also bursts showing a subdominant thermal component and a dominant synchrotron component 9 , suggesting a likely hybrid jet composition 10 . 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 ∼ 0.8 s, 35 s, and 212 s, respectively. Its high brightness (with isotropic peak luminosity L p,iso ∼ 4 × 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.

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Mini‐MegaTORTORA is a nine‐channel wide‐field camera that continuously monitors the sky looking for rapid optical transients since mid‐2014. It is also performing a regular sky survey, and has already acquired nearly half a million images covering every point of Northern Sky hundreds to thousands of times. Photometric analysis of these data may provide a huge amount of information useful for the detection and characterization of different types of variable objects. Here we present a brief description of our activities related to the acquisition, processing, and calibration of these data, as well as examples of uncataloged variable stars of various types detected during the analysis.

Aims. We study the dark nature of GRB 130528A through multi-wavelength observations and conclude that the main reason for the optical darkness is local extinction inside of the host galaxy. Methods. Automatic observations were performed at the Burst Optical Observer and Transient Exploring System (BOOTES)-4/MET robotic telescope. We also triggered target of opportunity (ToO) observations at Observatorio de Sierra Nevada (OSN), IRAM Plateau de Bure Interferometer (PdBI) and Gran Telescopio Canarias (GTC + OSIRIS). The host galaxy photometric observations in optical to near-infrared (nIR) wavelengths were achieved through large ground-based aperture telescopes, such as 10.4 m Gran Telescopio Canarias (GTC), 4.2 m William Herschel Telescope (WHT), 6 m Bolshoi Teleskop Alt-azimutalnyi (BTA) telescope, and 2 m Liverpool Telescope (LT). Based on these observations, spectral energy distributions (SED) for the host galaxy and afterglow were constructed. Results. Thanks to millimetre (mm) observations at PdBI, we confirm the presence of a mm source within the XRT error circle that faded over the course of our observations and identify the host galaxy. However, we do not find any credible optical source within early observations with BOOTES-4/MET and 1.5 m OSN telescopes. Spectroscopic observation of this galaxy by GTC showed a single faint emission line that likely corresponds to [OII] 3727 Å at a redshift of 1.250 ± 0.001, implying a star formation rate (M /yr) > 6.18 M /yr without correcting for dust extinction. The probable line-of-sight extinction towards GRB 130528A is revealed through analysis of the afterglow SED, resulting in a value of A GRB V ≥ 0.9 at the rest frame; this is comparable to extinction levels found among other dark GRBs. The SED of the host galaxy is explained well (χ 2 /d.o.f. = 0.564) by a luminous (M B = −21.16), low-extinction (A V = 0, rest frame), and aged (2.6 Gyr) stellar population. We can explain this apparent contradiction in global and line-of-sight extinction if the GRB birth place happened to lie in a local dense environment. In light of having relatively small specific star formation rate ∼5.3 M /yr (L/L ) −1 , this also could explain the age of the old stellar population of host galaxy.