ALBERT

Description: Contemporaneous BAT and XRT observations of two recent well-covered GRBs observed by Swift, GRB 050315 and GRB 050319, show clearly a prompt component of the afterglow emission. The rapid slewing capability of the spacecraft enables X-ray observations immediately after the burst, typically approximately 100 s following the initiation of the prompt gamma-ray phase. By fitting a power law form to the gamma-ray spectrum, we extrapolate the time dependent fluxes measured by the BAT, in the energy band 15 - 350 keV, into the spectral regime observed by the XRT, 0.2 - 10 keV, and examine the functional form of the rate of decay of the two light curves. We find that the BAT and XRT light curves merge to form a unified curve. There is a period of steep decay up to approximately 300 s, followed by a flatter decay. The duration of the steep decay, approximately 100 s in the source frame after correcting for cosmological time dilation, agrees with a theoretical estimate for the deceleration time of the relativistic ejecta as it interacts with circumstellar material. For GRB 050315, the steep decay can be characterized by an exponential form, where T(sub e),(BAT)approximately equal to 24 plus or minus 2 s, and T(sub e)(XRT) approximately equal to 35 plus or minus 2 s. For GRB 050319 a power law decay -d lnf/d lnt = n, where n approximately equal to 3, provides a reasonable fit. The early time X-ray fluxes are consistent with representing the lower energy tail of the prompt emission, and provide our first quantitative measure of the decay of the prompt gamma-ray emission over a large dynamic range. The initial steep decay is expected from the high latitude emission from a curved shell of relativistic plasma illuminated only for a short interval. The overall conclusion is that the prompt phase of GRBs lasts for hundreds of seconds longer than previously thought.

Description: Correlation studies of prompt and afterglow emissions from gamma-ray bursts (GRBs) between different spectral bands has been difficult to do in the past because few bursts had comprehensive and intercomparable afterglow measurements. In this paper we present a large and uniform data set for correlation analysis based on bursts detected by the Swift mission. For the first time, short and long bursts can be analyzed and compared. It is found for both classes that the optical, X-ray and gamma-ray emissions are linearly correlated, but with a large spread about the correlation line; stronger bursts tend to have brighter afterglows, and bursts with brighter X-ray afterglow tend to have brighter optical afterglow. Short bursts are, on average, weaker in both prompt and afterglow emissions. No short bursts are seen with extremely low optical to X-ray ratio as occurs for 'dark' long bursts. Although statistics are still poor for short bursts, there is no evidence yet for a subgroup of short bursts with high extinction as there is for long bursts. Long bursts are detected in the dark category at the same fraction as for pre-Swift bursts. Interesting cases are discovered of long bursts that are detected in the optical, and yet have low enough optical to X-ray ratio to be classified as dark. For the prompt emission, short and long bursts have different average tracks on flux vs fluence plots. In Swift, GRB detections tend to be fluence limited for short bursts and flux limited for long events.

Description: POET (Polarimeters for Energetic Transients) is a Small Explorer mission concept proposed to NASA in January 2008. The principal scientific goal of POET is to measure GRB polarization between 2 and 500 keV. The payload consists of two wide FoV instruments: a Low Energy Polarimeter (LEP) capable of polarization measurements in the energy range from 2-15 keV and a high energy polarimeter (Gamma-Ray Polarimeter Experiment - GRAPE) that will measure polarization in the 60-500 keV energy range. Spectra will be measured from 2 keV up to 1 MeV. The POET spacecraft provides a zenith-pointed platform for maximizing the exposure to deep space. Spacecraft rotation will provide a means of effectively dealing with systematics in the polarization response. POET will provide sufficient sensitivity and sky coverage to measure statistically significant polarization for up to 100 GRBs in a two-year mission. Polarization data will also be obtained for solar flares, pulsars and other sources of astronomical interest.

Description: Contemporaneous BAT and XRT observations of two recent well-covered GRBs observed by Swift, GRB 050315 and GRB 050319, show clearly a prompt component joining the onset of the afterglow emission. The rapid slewing capability of the spacecraft enables X-ray observations immediately after the burst, typically ~ 100 s following the initiation of the prompt y-ray phase. By fitting a power law form to the y-ray spectrum, we extrapolate the time dependent fluxes measured by the BAT, in the energy band 15 - 350 keV, into the spectral regime observed by the XRT 0.2 - 10 keV, and examine the functional form of the rate of decay of the two light curves. We find that the BAT and XRT light curves merge to form a unified curve. There is a period of steep decay up to ~ 300 s, followed by a flatter decay. The duration of the steep decay, ~ 100 s in the source frame after correcting for cosmological time dilation, agrees roughly with a theoretical estimate for the deceleration time of the relativistic ejecta as it interacts with circumstellar material. For GRB 050315, the steep decay can be characterized by an exponential form, where one e-folding decay time Te (BAT)~ = 24 f 2 s, and Te,(XRT)~ = 35 f 2 s. For GRB 050319, a power law decay - d l n f / d l n t = n, where n approx. ~ = 3, provides a reasonable fit. The early time X-ray fluxes are consistent with representing the lower energy tail of the prompt emission, and provide our first quantitative measure of the decay of the prompt y-ray emission over a large dynamic range in flux. The initial steep decay is expected due to the delayed high latitude photons from a curved shell of relativistic plasma illuminated only for a short interval. The overall conclusion is that the prompt phase of GRBs remains observable for hundreds of seconds longer than previously thought.

Description: We have investigated particle acceleration and shock structure associated with an unmagnetized relativistic jets propagating into an unmagnetized plasmas. Strong magnetic fields generated in the trailing shock contribute to the electrons transverse deflection and acceleration. We have calculated, self-consistently, the radiation from electrons accelerated in the turbulent magnetic fields. We found that the synthetic spectra depend on the Lorentz factor of the jet, its thermal temperature and strength of the generated magnetic fields. The properties of the radiation may be important for understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets in general, and supernova remnants

Description: Plasma instabilities excited in collisionless shocks are responsible for particle acceleration. We have investigated the particle acceleration and shock structure associated with an unmagnetized relativistic electron-positron jet propagating into an unmagnetized electron-positron plasma. Cold jet electrons are thermalized and slowed while the ambient electrons are swept up to create a partially developed hydrodynamic-like shock structure. In the leading shock, electron density increases by a factor of about 3.5 in the simulation frame. Strong electromagnetic fields are generated in the trailing shock and provide an emission site. This simulation corresponds to a case for gamma-ray burst afterglows. We will simulate colliding shells as an internal shock model for prompt emission. Turbulent magnetic fields generated by a slower shell will be collided by a faster shell. These magnetic fields contribute to the electron s transverse deflection behind the shock. We calculate the radiation from deflected electrons in the turbulent magnetic fields. The properties of this radiation may be important to understanding the complex time evolution and/or spectral structure in gamma-ray bursts

Description: Plasma instabilities excited in collisionless shocks are responsible for particle acceleration, generation of magnetic fields , and associated radiation. We have investigated the particle acceleration and shock structure associated with an unmagnetized relativistic jet propagating into an unmagnetized plasma. Cold jet electrons are thermalized and slowed while the ambient electrons are swept up to create a partially developed hydrodynamic-like shock structure. The shock structure depends on the composition of the jet and ambient plasma (electron-positron or electron-ions). Strong electromagnetic fields are generated in the reverse , jet shock and provide an emission site. These magnetic fields contribute to the electron's transverse deflection behind the shock. We have calculated, self-consistently, the radiation from electrons accelerated in the turbulent magnetic fields. We found that the synthetic spectra depend on the Lorentz factor of the jet, its thermal temperature and strength of the generated magnetic fields. The detailed properties of the radiation are important for understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jet shocks, and supernova remnants

Description: We present the second Swift Burst Alert Telescope (BAT) catalog of gamma-ray bursts. (GRBs), which contains 476 bursts detected by the BAT between 2004 December 19 and 2009 December 21. This catalog (hereafter the BAT2 catalog) presents burst trigger time, location, 90% error radius, duration, fluence, peak flux, time-averaged spectral parameters and time-resolved spectral parameters measured by the BAT. In the correlation study of various observed parameters extracted from the BAT prompt emission data, we distinguish among long-duration GRBs (L-GRBs), short-duration GRBs (S-GRBs), and short-duration GRBs with extended emission (S-GRBs with E.E.) to investigate differences in the prompt emission properties. The fraction of L-GRBs, S-GRBs and S-GRBs with E.E. in the catalog are 89%, 8% and 2% respectively. We compare the BAT prompt emission properties with the BATSE, BeppoSAX and HETE-2 GRB samples.. We also correlate the observed prompt emission properties with the redshifts for the GRBs with known redshift. The BAT T(sub 90) and T(sub 50) durations peak at 70 s and 30 s, respectively. We confirm that the spectra of the BAT S-GRBs are generally harder than those of the L-GRBs.

Description: We present a comprehensive analysis of a bright, long duration (T(sub 90) approx. 257 s) GRB 110205A at redshift z = 2.22. The optical prompt emission was detected by Swift/UVOT, ROTSE-IIIb and BOOTES telescopes when the GRB was still radiating in the gamma-ray band. Thanks to its long duration, nearly 200 s of observations were obtained simultaneously from optical, X-ray to gamma-ray (1 eV - 5 MeV), which makes it one of the exceptional cases to study the broadband spectral energy distribution across 6 orders of magnitude in energy during the prompt emission phase. In particular, by fitting the time resolved prompt spectra, we clearly identify, for the first time, an interesting two-break energy spectrum, roughly consistent with the standard GRB synchrotron emission model in the fast cooling regime. Although the prompt optical emission is brighter than the extrapolation of the best fit X/ -ray spectra, it traces the -ray light curve shape, suggesting a relation to the prompt high energy emission. The synchrotron + synchrotron self-Compton (SSC) scenario is disfavored by the data, but the models invoking a pair of internal shocks or having two emission regions can interpret the data well. Shortly after prompt emission (approx. 1100 s), a bright (R = 14.0) optical emission hump with very steep rise ( alpha approx. 5.5) was observed which we interpret as the emission from the reverse shock. It is the first time that the rising phase of a reverse shock component has been closely observed.

Description: Plasma instabilities are responsible not only for the onset and mediation of collisionless shocks but also for the associated acceleration of particles. We have investigated particle acceleration and shock structure associated with an unmagnetized relativistic electron-positron jet propagating into an unmagnetized electron-positron plasma. Cold jet electrons are thermalized and slowed while the ambient electrons are swept up to create a partially developed hydrodynamic-like shock structure. In the leading shock, electron density increases by a factor of about 3.5 in the simulation frame. Strong electromagnetic fields are generated in the trailing shock and provide an emission site. These magnetic fields contribute to the electrons transverse deflection and, more generally, relativistic acceleration behind the shock. We have calculated, self-consistently, the radiation from electrons accelerated in the turbulent magnetic fields. We found that the synthetic spectra depend on the Lorentz factor of the jet, its thermal temperature and strength of the generated magnetic fields. We are currently investigating the specific case of a jet colliding with an anti-parallel magnetized ambient medium. The properties of the radiation may be important for understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets in general, and supernova remnants.