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.

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

The Einstein Equivalence Principle (EEP) can be probed with astrophysical sources emitting simultaneously different types of neutral particles, or particles with varying energies, by testing their time of flight through the same gravitational field. Here we use the time delays between correlated photons from cosmological transients to constrain the accuracy of the EEP. We take data from two gamma-ray bursts as an example, and use, as a lower limit to the theoretical time delays between different energies, delays arising from only the gravitational field of our own galaxy. We then show that the parameterized post-Newtonian parameter $\gamma$ is the same for photons over energy ranges between eV and MeV and between MeV and GeV to a part in $10^{-7}$, which is at least one order of magnitude better than previous limits. Combining this bound on the wavelength dependence of $\gamma$ with the absolute bound $|\gamma-1|<0.3\%$ from light-deflection measurements at optical (eV) wavelengths, we thus extend this absolute bound on $\gamma$ to GeV energies.

Cosmic Transients Test Einstein's Equivalence Principle out to GeV Energies

The Einstein Equivalence Principle (EEP) can be probed with astrophysical sources emitting simultaneously different types of neutral particles, or particles with varying energies, by testing their time of flight through the same gravitational field. Here we use the time delays between correlated photons from cosmological transients to constrain the accuracy of the EEP. We take data from two gamma-ray bursts as an example and, as a lower limit to the theoretical time delays between different energies, we use delays arising from only the gravitational field of our own galaxy. We then show that the parameterized post-Newtonian parameter gamma is the same for photons over energy ranges between eV and MeV and between MeV and GeV to a part in 10(-7), which is at least one order of magnitude better than previous limits. Combining this bound on the wavelength dependence of gamma with the absolute bound vertical bar gamma - 1 vertical bar < 0.3% from light-deflection measurements at optical (eV) wavelengths, we thus extend this absolute bound on gamma to GeV energies.

http://iopscience.iop.org/article/10.1088/0004-637X/810/2/121/pdf

COSMIC TRANSIENTS TEST EINSTEIN'S EQUIVALENCE PRINCIPLE OUT to GeV ENERGIES

On 2008 March 19, the northern sky was the stage of a spectacular optical transient that for a few seconds remained visible to the naked eye. The transient was associated with GRB 080319B, a gamma-ray burst (GRB) at a luminosity distance of about 6 Gpc (standard cosmology), making it the most luminous optical object ever recorded by humankind. We present comprehensive sky monitoring and multicolor optical follow-up observations of GRB 080319B collected by the RAPTOR telescope network covering the development of the explosion and the afterglow before, during, and after the burst. The extremely bright prompt optical emission revealed features that are normally not detectable. The optical and gamma-ray variability during the explosion are correlated, but the optical flux is much greater than can be reconciled with single-emission mechanism and a flat gamma-ray spectrum. This extreme optical behavior is best understood as synchrotron self-Compton model (SSC). After a gradual onset of the gamma-ray emission, there is an abrupt rise of the prompt optical flux, suggesting that variable self-absorption dominates the early optical light curve. Our simultaneous multicolor optical light curves following the flash show spectral evolution consistent with a rapidly decaying red component due to large-angle emission and the emergence of a blue forward-shock component from interaction with the surrounding environment. While providing little support for the reverse shock that dominates the early afterglow, these observations strengthen the case for the universal role of the SSC mechanism in generating GRBs.

http://iopscience.iop.org/article/10.1088/0004-637X/691/1/495/pdf

Gamma-ray burst at the extreme : the naked-eye burst grb 080319b

NSF grant [AST-1411763]; NASA ADA grant [NNX15AE50G]; NASA through Einstein Postdoctoral Fellowship [PF4-150121]; VLA Large Program [15A-235]

/pdf/a-reverse-shock-and-unusual-radio-properties-in-grb-160625b-iut4qc21rh.pdf

A Reverse Shock and Unusual Radio Properties in GRB 160625B

A new astronomical complex intended to study fast optical transients is presented. It is placed at the La Silla observatory in Chile. Here we present first results of its operation.

TORTOREM: Two-telescope complex for detection and investigation of optical transients

Wide-field subsecond temporal resolution optical monitoring systems for the detection and study of cosmic hazards

Mini-MegaTORTORA Wide-Field Monitoring System with Subsecond Temporal Resolution: Observation of Transient Events

GRB 160625B: Mini-MegaTORTORA detection of bright optical flash simultaneous with LAT event.

Here we briefly summarize our long period experience of constructing and operating wide-field monitoring cameras with sub-second temporal resolution to look for optical components of GRBs, fast-moving satellites and meteors. General requirements for hardware for such systems are discussed along with algorithms of real-time detection and classification of various kinds of short optical transients. We also give a status report on the next generation, multi-objective and transforming monitoring system, the MegaTORTORA, whose 6-channel (Mini-MegaTORTORA-Spain) and 9-channel prototypes (Mini-MegaTORTORA-Kazan) we are building now at SAO RAS. This system combines a wide field of view with subsecond temporal resolution in monitoring regime, and is able to reconfigure itself, in a fractions of second, to follow-up mode which has better sensitivity and provides us with multi-color and polarimetric information on detected transients simultaneously.

/pdf/status-and-perspectives-of-mini-megatortora-wide-field-vc2j0i5k35.pdf

Status and perspectives of Mini-MegaTORTORA wide-field monitoring system with high temporal resolution

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