S. Karpov

FAST OPTICAL VARIABILITY OF A NAKED-EYE BURST—MANIFESTATION OF THE PERIODIC ACTIVITY OF AN INTERNAL ENGINE

We imaged the position of the naked-eye burst, GRB080319B, before, during, and after its gamma-ray activity with sub-second temporal resolution using the TORTORA wide-field camera. The burst optical prompt emission, which reached 5.3 mag, has been detected, and its periodic optical variability has been discovered in the form of four equidistant flashes with a duration of several seconds. We also detected a strong correlation (r ≈ 0.82) between optical and gamma-ray light curves with a 2 s delay of the optical emission with respect to the gamma-ray emission. The revealed temporal structure of the optical light curve in comparison with the gamma-ray light curve can be interpreted in the framework of the model of shell collisions in the ejecta containing a significant neutron component. All observed emission features reflect the non-stationary behavior of the burst internal engine—supposedly, a hyperaccreting solar-mass black hole formed in the collapse of a massive stellar core.

Temperature-Dependent Internal Quantum Efficiency of Blue High-Brightness Light-Emitting Diodes

Internal quantum efficiency (IQE) of a blue high-brightness InGaN/GaN light-emitting diode (LED) was evaluated from the external quantum efficiency measured as a function of current at various temperatures ranged between 13 and 440 K. Processing the data with a novel evaluation procedure based on the ABC-model, we have determined the temperature-dependent IQE of the LED structure and light extraction efficiency of the LED chip. Separate evaluation of these parameters is helpful for further optimization of the heterostructure and chip designs. The data obtained enable making a guess on the temperature dependence of the radiative and Auger recombination coefficients, which may be important for identification of dominant mechanisms responsible for the efficiency droop in III-nitride LEDs. Thermal degradation of the LED performance in terms of the emission efficiency is also considered.

Modeling of III-nitride light-emitting diodes: progress, problems, and perspectives

Recent progress in III-nitride LED modeling is reviewed with the focus on physical models that provide a better understanding of such hot issues, as factors limiting the internal quantum efficiency of light emission and high-current efficiency droop, polarization doping in graded-composition III-nitride alloys and its utilization in LEDs, current crowding in LED dice and its impact on the light extraction efficiency, and optimal light conversion in white LED lamps. Specific features of III-nitride materials, their impact on the LED operation, and models accounting for these features are considered. Insufficient understanding of transport mechanisms of non-equilibrium electrons and holes and their localization in InGaN inhomogeneous active regions are discussed along with other still unsolved problems. Influence of technological factors on LED heterostructures and their operation is argued in the context of further model developments.

Temperature-dependent recombination coefficients in InGaN light-emitting diodes: Hole localization, Auger processes, and the green gap

We obtain temperature-dependent recombination coefficients by measuring the quantum efficiency and differential carrier lifetimes in the state-of-the-art InGaN light-emitting diodes. This allows us to gain insight into the physical processes limiting the quantum efficiency of such devices. In the green spectral range, the efficiency deteriorates, which we assign to a combination of diminishing electron-hole wave function overlap and enhanced Auger processes, while a significant reduction in material quality with increased In content can be precluded. Here, we analyze and quantify the entire balance of all loss mechanisms and highlight the particular role of hole localization.

Determination of recombination coefficients in InGaN quantum-well light-emitting diodes by small-signal time-resolved photoluminescence

We suggest a novel technique for the evaluation of the recombination coefficients corresponding to Shockley–Read–Hall, radiative, and Auger recombination that occur in InGaN/GaN-based light-emitting diodes (LEDs). This technique combines the measurement of the LED efficiency as a function of LED drive current with a small-signal time-resolved photoluminescence measurement of the differential carrier life time (DLT). Using the relationships between the efficiency and DLT following from the empirical ABC-model, one can evaluate all three recombination coefficients. The suggested technique is applied to a number of single- and multiple-quantum well LEDs to gain a deeper insight into the mechanisms ultimately limiting their efficiency.

Short time scale pulse stability of the Crab pulsar in the optical band

AbstractnThe fine structure and the variations of the optical pulse shape and phase of the Crab pulsar are studied on various time scales. The observations have been carried out on 4-m William Hershel and 6-m BTA telescopes with APD photon counter, photomultiplier based 4-channel photometer and PSD based panoramic spectrophotopolarimeter with 1xa0μs time resolution inxa01994, 1999, 2003 and 2005–2006xa0years. The upper limit on the pulsar precession on Decxa02, 1999 is placed in the 10xa0s–2xa0hr time range. The evidence of a varying from set to set fine structure of the main pulse is found in the 1999 and 2003 years data. No such fine structure is detected in the integral pulse shape ofxa01994, 1999 and 2003xa0years.nnThe drastic change of the pulse shape in the 2005–2006 years set is detected along with the pulse shape variability and quasi-periodic phase shifts.n

Monotonic and cyclic components of radio pulsar spin-down

In this article we revise the problem of anomalous values of pulsars’ braking indices nobs and frequency second derivatives ¨ ν arising in observations. The intrinsic evolutionary braking is buried deep under the superimposed irregular processes, that prevent direct estimations of its parameters for the majority of pulsars. We re-analyze the distribution of “ordinary” radio pulsars on a ¨ ν u ν, ¨ ν, u ν ν and nobs τch diagrams assuming their spin-down to be a superposition of a “true” monotonous and a symmetric oscillatory terms. We demonstrate that their effects may be clearly separated using simple ad hoc arguments. Using maximum likelihood estimator we derive the parameters of both components. We find characteristic timescales of such oscillations to be of order of 10 3 ...10 4 years, while its amplitudes are large enough to modulate the observed spin-down rate up to 0.5...5 times and completely dominate the second frequency derivatives. On the other hand, pulsars’ secular evolution is consistent with classical magnetodipolar model with braking index n � 3 So, observed pulsars’ characteristic ages (and similar estimators that depend on the observed u ν) are biased up to 0.5...5 times. This fact naturally resolves the discrepancy of characteristic and independently estimated physical ages of several objects, as well as explains very large, up to 10 8 years, characteristic ages of some pulsars. We suggest that these oscillations are caused by the long-term precession of neutron stars around its magnetic axis, probably driven by the anomalous braking torque.

Light-emitting diodes for solid-state lighting: searching room for improvements

State-of-the art light-emitting diodes (LEDs) for solid-state lighting (SSL) are reviewed with the focus on their efficiency and ways for its improvement. Mechanisms of the LED efficiency losses are considered on the heterostructure, chip, and device levels, including high-current efficiency droop, recombination losses, “green gap”, current crowding, Stokes losses, etc. Materials factors capable of lowering the LED efficiency, like composition fluctuations in InGaN alloys and plastic stress relaxation in device heterostructures, are also considered. Possible room for the efficiency improvement is discussed along with advanced schemes of color mixing and LED parameters optimal for generation of high-quality white light.

TORTORA discovery of Naked‐Eye Burst fast optical variability

Features characterizing gamma‐ray bursts in the different spectral bands may be a clue for the nature of their inner engine. Up to now, only several bursts have been observed in optical band during the gamma activity, and the only one—GRB080319B—was covered from rise till fall with high temporal resolution. Here we discuss these data, acquired with TORTORA fast wide‐field monitoring optical camera, as well as results of its analysis.The camera observed the position of Naked‐Eye Burst, GRB080318B, before, during and after the trigger. It detected the fast rise of optical emission, which reached the peak of V 5.3 at the eighteenth second, had a complex evolution till T+43s and monotonously faded then. The brightest part of the light curve contains two 15–20 s segments with different fluxes, each having two clearly‐seen peaks of 5–8 s duration; all four peaks look quasi‐periodic with separation of 9 s. There is no clear evidence of any sub‐second variability. However, there are signs of quasi‐periodic variab...

Optically Thick Outflows of Supercritical Accretion Discs: Radiative Diffusion Approach

Highly supercritical accretion discs are probable sources of dense optically thick axisymmetric winds. We introduce a new approach based on diffusion approximation radiative transfer in a funnel geometry, and obtain an analytical solution for the energy density distribution inside the wind, assuming that all of the mass, momentum and energy are injected well inside the spherization radius. This allows us to derive the spectrum of the emergent emission for various inclination angles. We show that self-irradiation effects play an important role in altering the temperature of the outcoming radiation by about 20%, and the apparent X-ray luminosity by a factor of 2–3. The model has been successfully applied to two ULXs. The basic properties of the high ionization He II-regions found around some ULXs are also easily reproduced in our models.