Optical Flash Research Articles

Pi of the Sky – all-sky, real-time search for fast optical transients

An apparatus to search for optical flashes in the sky is described. It has been optimized for optical counterparts of gamma ray bursts (GRB). It consists of 2×16 cameras covering all the sky. The sky is monitored continuously and the data are analyzed on-line. The apparatus has self-triggering capability and can react to external triggers with negative delay. The prototype with two cameras has been installed at Las Campanas (Chile) and is operational from July 2004. The paper presents the general idea of the experiment and describes the apparatus in detail. The performance of the prototype is briefly reviewed and perspectives for the future are outlined.

The physics of gamma-ray bursts

Gamma-ray bursts (GRB's), short and intense pulses of low-energy $\ensuremath{\gamma}$ rays, have fascinated astronomers and astrophysicists since their unexpected discovery in the late sixties. During the last decade, several space missions---BATSE (Burst and Transient Source Experiment) on the Compton Gamma-Ray Observatory, BeppoSAX and now HETE II (High-Energy Transient Explorer)---together with ground-based optical, infrared, and radio observatories have revolutionized our understanding of GRB's, showing that they are cosmological, that they are accompanied by long-lasting afterglows, and that they are associated with core-collapse supernovae. At the same time a theoretical understanding has emerged in the form of the fireball internal-external shocks model. According to this model GRB's are produced when the kinetic energy of an ultrarelativistic flow is dissipated in internal collisions. The afterglow arises when the flow is slowed down by shocks with the surrounding circumburst matter. This model has had numerous successful predictions, like the predictions of the afterglow itself, of jet breaks in the afterglow light curve, and of the optical flash that accompanies the GRB's. This review focuses on the current theoretical understanding of the physical processes believed to take place in GRB's.

GRB 990123 Revisited: Further Evidence of a Reverse Shock

Recently, we presented a new theoretical analysis of the reverse shock emission. We use this analysis here to revisit the early afterglow of GRB 990123. The first mechanism that was suggested as the source of the optical flash observed in GRB 990123 was the reverse shock. However, it was shown later that other processes can generate this flash as well. We find new and compelling evidence that the optical flash and the following radio flare of GRB 990123 did resulted from a reverse shock. This suggests that a significant fraction of the energy of the relativistic ejecta must have been carried by baryons. It also suggests that the external medium is an interstellar medium and that in this burst the reverse shock emission dominates at early time over other possible processes. We use the early optical emission to constrain the physical parameters of the original ejecta and the microscopic parameters in the emitting reverse-shocked region.

Optical and GeV-TeV Flashes from Gamma-Ray Bursts

The synchrotron optical flash caught in GRB 990123 overlaps with the MeV radiation front. Therefore, the optical-emitting electrons must also produce GeV-TeV emission by inverse Compton scattering of MeV photons. The ultra-high-energy flash can be much stronger than its optical counterpart. We also note that Compton cooling by MeV photons immediately terminates the optical emission unless the fireball Lorentz factor exceeds 103. Severe Compton losses may explain the nondetections of optical flashes in several long GRBs. Such failed optical flashes should be especially efficient GeV producers and likely to develop e± cascades. This probably happened in GRB 941017, and its mysterious high-energy component is well explained by Compton upscattering of GRB photons at the fireball deceleration radius. The proposed mechanism of GeV emission should not work for short GRBs that early decouple from the fireball and avoid interaction with the electrons in the deceleration flash. Observations by Swift and the Gamma-Ray Large Area Telescope will provide an opportunity to test these expectations. The existing data for GRB 990123 already impose interesting constraints on the explosion.

Concept of High Speed Astronomical Instrumentation Based on Visible Light Photon Counters

The domain of high speed optical astrophysics is still quite unexplored. The availability of 10 meter diameter telescopes offers the unique possibility to investigate variability of faint objects at submillisecond time scales. In this paper I describe the concepts of a photometer and a spectrometer for high speed astronomical observations. The instruments are based on a photon counting detector developed for high energy physics, the Visible Light Photon Counter (VLPC). The detector has a quantum efficiency in the visible as high as 88% and performs photon counting with sub microsecond time resolution. The photometer is built using VLPC arrays. Adding a grating a VLPC array can be used in a time resolved spectrograph with medium resolution. This paper develops, starting from experimental data, the concept of the two VLPC based instruments and their application to time resolved photometry and spectroscopy of compact objects (pulsars, cataclysmic variables, low mass X-ray binary systems etc) and optical counterparts of Gamma Ray Bursts. The high speed optical observations are the ideal complement to X/γ rays and gravitational wave studies. The application of the instruments to the optical photometry of pulsars, the spectrophotometry of the prompt optical flash from Gamma Ray Bursts and the study of binary systems are discussed in detail: in the last two applications the instruments offer better opportunities than existing instruments.

Optical Observation of the 2003 Outburst of a Black Hole X-Ray Binary, V4641 Sagittarii

Abstract We report on the results of our optical photometric observations of a black hole X-ray binary, V4641 Sgr during an outburst in 2003 August. During the outburst, we detected several properties having noteworthy similarities to those observed during an outburst in 2002 May; the outburst lasted about 6 days, during which it exhibited flares and rapid fluctuations, having timescales of $10^{2\hbox{--}4} \,\mathrm{s}$. In complicated profiles of light curves, we identified several recurring patterns, which were also observed during the 2002 outburst. First, the object frequently experienced sudden temporary fadings, which we call “dips”, appearing just after the states characterized by strong short-term fluctuations. Second, we detected optical flashes, which are characterized by a brightening by $\sim 0.5 \,\mathrm{mag}$ within $\sim 50 \,\mathrm{s}$. Based on the similarity of the outbursts in 2002 and 2003, we conclude that they have the same nature. This is the first time that repetitive outburst phenomena have been confirmed in V4641 Sgr. The object probably has an outburst interval that can be as small as 1–2 years.

Early afterglow emission from a reverse shock as a diagnostic tool for gamma-ray burst outflows

The gamma-ray burst‐afterglow transition is one of the most interesting and least studied gamma-ray burst phases. During this phase, the relativistic ejecta begins interacting with the surrounding matter. A strong short-lived reverse shock propagates into the ejecta (provided that it is baryonic) while the forward shock begins to shape the surrounding matter into a Blandford‐McKee profile. We suggest a parametrization of the early afterglow light curve and we calculate (analytically and numerically) the observed parameters that result from a reverse shock emission (in an interstellar medium environment). We present a new fingerprint of the reverse shock emission that is added to the well-known t −2 optical decay. Observation of this signature would indicate that the reverse shock dominates the emission during the early afterglow. The existence of a reverse shock will in turn imply that the relativistic ejecta contains a significant baryonic component. This signature would also imply that the surrounding medium is an interstellar medium. We further show the following. (i) The reverse shock optical flash depends strongly on initial conditions of the relativistic ejecta. (ii) Previous calculations have generally overestimated the strength of this optical flash. (iii) If the reverse shock dominates the optical flash, then detailed observations of the early afterglow light curve would possibly enable us to determine the initial physical conditions within the relativistic ejecta and specifically to estimate its Lorentz factor and its width. Ke yw ords: hydrodynamics ‐ shock waves ‐ gamma-rays: bursts.

The generation, annihilation and re-radiation of e± pairs in fireballs of internal shock waves

In the internal shock wave model of gamma-ray bursts (GRBs), the central energy source ejects a series of material envelopes with comparable masses but very different global Lorentz factors. Strong collisions of these envelopes with various velocities generate relativistic shock waves. The electrons in the envelopes are heated by the shock waves, and via synchrotron radiation as well as inverse Compton scattering they emit high-energy γ photons. Due to the electron pairs produced by γ — γ collisions, photons with energies on orders as high as GeV (in the observer's system) are absorbed by the fireball. As revealed by Pilla & Leob's numerical calculation, the number of electron pairs is much larger than that of electrons in the fireball. Recently, Li et al. obtained a similar result, which was used to interpret the deficiency of optical flashes in the early afterglow. From an analytical study of the time evolution of the processes of generation and annihilation of electron pairs, it is discovered that for a typical pulse the number of e ± pairs produced in the early phase in the high-energy part of synchrotron radiation is large and the rate of annihilation is high. However, in the late phase, owing to the limitation of the maximum frequency of the synchrotron radiation, the high-energy part no longer contributes to the generation of e ± pairs. Unlike this, the contribution of inverse Compton scattering to the production of e ±'s is approximately proportional to the time scale of the pulse duration. For typical parameter values, the number of electron pairs generated jointly by the two components of radiation may be as high as more than 10 times the number of electrons carried by the original fireball. Because the Lorentz factor of the generated e ±'s is rather small, the corresponding synchrotron radiation has no influence on the observed spectrum (this is so at least for the energy range of the BATSE detector). But after the secondary inverse Compton scattering, there is a possible influence on the observed spectrum. Because the mass of electron pair is far smaller than that of proton, the effect on the late phase of the dynamic evolution of the fireball is not large. At least for a uniform medium environment, the existence of electron pairs has no great influence on the optical radiation of afterglow in its early phase.

Outburst and Post-Outburst Active Phase of the Black Hole X-Ray Binary V4641 Sagittarii in 2002

Abstract The black hole X-ray binary V4641 Sgr experienced an outburst in 2002 May, which was detected at X-ray, optical, and radio wavelengths. The outburst lasted for only 6 days, but the object remained active for the next several months. Here, we report on the detailed properties of light curves during the outburst and the post-outburst active phase. We reveal that rapid optical variations of $\sim 100 \,\mathrm{s}$ became more prominent when a thermal flare weakened and the optical spectrum flattened in the $I_{\mathrm{c}}$, $R_{\mathrm{c}}$, and $V$-band regions. In conjunction with the flat spectrum in the radio range, this strongly indicates that the origin of rapid variations is not thermal emission, but synchrotron emission. Just after the outburst, we detected repeated flares at optical and X-ray wavelengths. The optical and X-ray light curves exhibited a strong correlation with the X-rays lagging by about 7 min. The X-ray lag can be understood in terms of a hot region propagating into the inner region of the accretion flow. The short X-ray lag, however, requires modifications of this simple scenario to account for the short propagation time. We also detected high-amplitude rapid variations 50 days after the outburst, which we call optical flashes. During the most prominent optical flash, the object brightened by 1.2 mag only within 30 s. The released energy indicates that the emission source should be at the innermost region of the accretion flow.

News on Multifrequency Behaviour of GRBs: Polarized Emission and Optical Flashes

In this presentation the main advances occurred in the past years concerning the observational features of Gamma-Ray Burst (GRB) prompt event and afterglow polarization in the optical, as well as in other bands, are reviewed. Also, the observed cases of an `optical flash' simultaneous with the GRB itself are presented, along with their theoretical interpretation and the description of present and future observational fast-response techniques to chase this emission.

Linear Polarization in Gamma-Ray Bursts: The Case for an Ordered Magnetic Field

Linear polarization at the level of $\sim 1-3%$ has by now been measured in several gamma-ray burst afterglows. Whereas the degree of polarization, $P$, was found to vary in some sources, the position angle, $\theta_p$, was roughly constant in all cases. Until now, the polarization has been commonly attributed to synchrotron radiation from a jet with a tangled magnetic field that is viewed somewhat off axis. However, this model predicts either a peak in $P$ or a $90^\circ$ change in $\theta_p$ around the ``jet break'' time in the lightcurve, for which there has so far been no observational confirmation. We propose an alternative interpretation, wherein the polarization is attributed, at least in part, to a large-scale, ordered magnetic field in the ambient medium. The ordered component may dominate the polarization even if the total emissivity is dominated by a tangled field generated by postshock turbulence. In this picture, $\theta_p$ is roughly constant because of the uniformity of the field, whereas $P$ varies as a result of changes in the ratio of the ordered-to-random mean-squared field amplitudes. We point out that variable afterglow light curves should be accompanied by a variable polarization. The radiation from the original ejecta, which includes the prompt gamma-ray emission and the emission from the reverse shock (the `optical flash' and `radio flare'), could potentially exhibit a high degree of polarization (up to $\sim 60%$) induced by an ordered transverse magnetic field advected from the central source.

Optical flashes and very early afterglows in wind environments

The interaction of a relativistic fireball with its ambient medium is described through two shocks: a reverse shock that propagates into the fireball, and a forward shock that propagates into the medium. The observed optical flash of GRB 990123 has been considered to be the emission from such a reverse shock. The observational properties of afterglows suggest that the progenitors of some γ-ray bursts (GRBs) may be massive stars and their surrounding media may be stellar winds. We here study very early afterglows from the reverse and forward shocks in winds. An optical flash mainly arises from the relativistic reverse shock, while a radio flare is produced by the forward shock. The peak flux densities of optical flashes are larger than 1 Jy for typical parameters, if we do not take into account some appropriate dust obscuration along the line of sight. The radio flare always has a long-lasting constant flux, which will not be covered up by interstellar scintillation. The non-detections of optical flashes brighter than about ninth magnitude may constrain the GRB isotropic energies to be no more than a few 1052 erg and wind intensities to be relatively weak.

Early optical emission from the gamma-ray burst of 4 October 2002.

Observations of the long-lived emission--or 'afterglow'--of long-duration gamma-ray bursts place them at cosmological distances, but the origin of these energetic explosions remains a mystery. Observations of optical emission contemporaneous with the burst of gamma-rays should provide insight into the details of the explosion, as well as into the structure of the surrounding environment. One bright optical flash was detected during a burst, but other efforts have produced negative results. Here we report the discovery of the optical counterpart of GRB021004 only 193 seconds after the event. The initial decline is unexpectedly slow and requires varying energy content in the gamma-ray burst blastwave over the course of the first hour. Further analysis of the X-ray and optical afterglow suggests additional energy variations over the first few days.

Optical Flash and Radio Flare in Wind Environment

The interaction of a relativistic fireball with its ambient medium is described through two shocks: a reverse shock that propagates into the fireball, and a forward shock that propagates into the medium. We here study very early afterglows from the reverse and forward shocks in winds. An optical flash arises from both the relativistic reverse and forward shock while a radio flare is produced by the forward shock.

Flashes in Be/X-ray Binary A0535+26

A luminosity of the transient optical flashes of HMXB A0535+26 amounted to 1.4 × 1038 erg s−1, the same order of magnitudes as a power of giant X-ray outbursts of the system. The flashes are very similar to ultrafast bright (on the timescale of 1 msec) flashes which have been detected in LMXBs in the MANIA experiment. Apparently the flashes arise as a result of non-thermal processes which excited in both type of binaries. The program of searches of the high energy optical transients among HMXBs is proposed.

The ROTSE‐III Robotic Telescope System

ABSTRACTThe observation of a prompt optical flash from GRB 990123 convincingly demonstrated the value of autonomous robotic telescope systems. Pursuing a program of rapid follow‐up observations of gamma‐ray bursts, the Robotic Optical Transient Search Experiment (ROTSE) has developed a next‐generation instrument, ROTSE‐III, that will continue the search for fast optical transients. The entire system was designed as an economical robotic facility to be installed at remote sites throughout the world. There are seven major system components: optics, optical tube assembly, CCD camera, telescope mount, enclosure, environmental sensing and protection, and data acquisition. Each is described in turn in the hope that the techniques developed here will be useful in similar contexts elsewhere.

Tuning FlaSh: Redesign of the Dynamics, Voltage Range, and Color of the Genetically Encoded Optical Sensor of Membrane Potential

The optical voltage sensor FlaSh, made from a fusion of a GFP “reporter domain” and a voltage-gated Shaker K + channel “detector domain,” has been mutagenically tuned in both the GFP reporter and channel detector domains. This has produced sensors with improved folding at 37 °C, enabling use in mammalian preparations, and yielded variants with distinct spectra, kinetics, and voltage dependence, thus expanding the types of electrical signals that can be detected. The optical readout of FlaSh has also been expanded from single wavelength fluorescence intensity changes to dual wavelength measurements based on both voltage-dependent spectral shifts and changes in FRET. Different versions of FlaSh can now be chosen to optimize the detection of either action potentials or synaptic potentials, to follow high versus low rates of activity, and to best reflect electrical activity in cell types with distinct voltages of operation.

Lightning effects at high altitudes: sprites, elves, and terrestrial gamma ray flashes

A fascinating set of newly discovered complex phenomena indicate that thunderstorms and lightning discharges are strongly coupled to the overlying upper atmospheric regions. Lightning discharges at cloud altitudes (<20 km) affect altitudes >40 km either via the release of intense electromagnetic pulses (EMPs) and/or the production of intense quasi-static electric (QE) fields. The intense transient QE fields of up to ∼1 kV·m −1, which for positive CG discharges is directed downwards, can avalanche accelerate upward-driven runaway MeV electron beams, producing brief (∼1 ms) flashes of gamma radiation. A spectacular manifestation of these intense fields is the so-called ‘Sprites’, large luminous discharges in the altitude range of ∼40 km to 90 km, which are produced by the heating of ambient electrons for a few to tens of milliseconds following intense lightning flashes. The so-called ‘Elves’ are optical flashes which last much shorter (<1 ms) than sprites, and are typically limited to 80–95 km altitudes with much larger (up to 600 km) lateral extent, being produced by the heating, ionization, and optical emissions due to the EMPs radiated by both positive and negative lightning discharges. To cite this article: U.S. Inan, C. R. Physique 3 (2002) 1411–1421.

Optical Flash of GRB 990123: Constraints on the Physical Parameters of the Reverse Shock

The optical flash accompanying GRB 990123 is believed to be powered by the reverse shock of a thin shell. With the best fitted physical parameters for GRB 990123 (Panaitescu & Kumar 2001) and the assumption that the parameters in the optical flash are the same as those in the afterglow, we show that: 1) the shell is thick but not thin, and we have provided the light curve for the thick shell case which coincides with the observation; 2) the theoretical peak flux of the optical flash accounts for only 3\times10^{-4} of the observed. In order to compensate this divergency, the physical parameters the electron energy ration and the magnetic ratio $\epsilon_e$, $\epsilon_B$ should be 0.61, 0.39 respectively, which are much different from those in the late afterglow.

Research aims to develop artificial photosynthesis

The paper describes a project aiming at constructing functional mimics of the oxygen evolving complex in photosystem II, coupled to photoinduced charge separation. Biomimetic electron donors, manganese complexes and tyrosine, have been linked to a Ru(II)-polypyridine photosensitiser. Oxidation of the donors by intramolecular electron transfer from the photooxidised Ru(III) complex was demonstrated using optical flash photolysis and EPR experiments. A step-wise electron transfer Mn(III,III)→tyrosine→Ru(III) was demonstrated, in analogy to the reaction on the donor side of photosystem II. Electron transfer from the tyrosine to Ru(III) was coupled to tyrosine deprotonation. This resulted in a large reorganisation energy and thus a slow reaction rate, unless the tyrosine was hydrogen bonded or already deprotonated. A comparison with analogous reaction in photosystem II is made. Finally, light-induced oxidation of a manganese dimer linked to a Ru(II)-photosensitiser was observed. Preliminary results suggest the possibility of photooxidising manganese dimers in several steps, which is an important step towards water oxidation.