Very early observations with the Swift satellite of g-ray burst (GRB) afterglows reveal that the optical component is not detected in a large number of cases. This is in contrast to the bright ...optical flashes previously discovered in some GRBs (e.g., GRB 990123 and GRB 021211). Comparisons of the X-ray afterglow flux to the optical afterglow flux and prompt g-ray fluence is used to quantify the seemingly deficient optical, and in some cases X-ray, light at these early epochs. This comparison reveals that some of these bursts appear to have higher than normal g-ray efficiencies. We discuss possible mechanisms and their feasibility for explaining the apparent lack of early optical emission. The mechanisms considered include, foreground extinction, circumburst absorption, Lya blanketing and absorption due to high-redshift, low-density environments, rapid temporal decay, and intrinsic weakness of the reverse shock. Of these, foreground extinction, circumburst absorption, and high redshift provide the best explanations for most of the nondetections in our sample. There is tentative evidence of suppression of the strong reverse shock emission. This could be because of a Poynting flux-dominated flow or a pure nonrelativistic hydrodynamic reverse shock.
Time-Domain Astronomy with Swift, Fermi and Lobster Gehrels, Neil; Barthelmy, Scott D.; Cannizzo, John K.
Proceedings of the International Astronomical Union,
09/2011, Volume:
7, Issue:
S285
Journal Article
Peer reviewed
Open access
The dynamic transient gamma-ray sky is revealing many interesting results, largely due to findings by Fermi and Swift. The list includes new twists on gamma-ray bursts (GRBs), a GeV flare from a ...symbiotic star, GeV flares from the Crab Nebula, high-energy emission from novae and supernovae, and, within the last year, a new type of object discovered by Swift—a jetted tidal disruption event. In this review we present highlights of these exciting discoveries. A new mission concept called Lobster is also described; it would monitor the X-ray sky at order-of-magnitude higher sensitivity than current missions can.
The hard x-ray polarimeter X-Calibur Kislat, Fabian; Beilicke, Matthias; Baring, Matthew G. ...
2013 IEEE Nuclear Science Symposium and Medical Imaging Conference (2013 NSS/MIC),
10/2013
Conference Proceeding
X-ray polarimetry promises unique information about astrophysical objects such as binary black hole systems, neutron stars, microquasars, active galactic nuclei, and gamma-ray bursts. We designed and ...built X-Calibur, a hard x-ray polarimeter to be flown in the focal plane of the InFOCS grazing incidence mirror in the fall of 2014 from Fort Sumner (NM) 1 . During the one-day flight X-Calibur will measure spectrum and polarization of x-rays in the 20-80keV range from up to five selected sources. X-Calibur exploits the fact that polarized photons scatter preferentially in a direction orthogonal to the orientation of their electric field vector. By combining a low-Z scatterer with high-Z Cadmium Zinc Telluride (CZT) detectors to photo-absorb the scattered x-rays, X-Calibur achieves a high detection efficiency of almost unity and a large modulation factor close to the theoretical limit given by the physics of Compton scattering. We have calibrated and tested X-Calibur extensively in the laboratory at Washington University and at the Cornell High-Energy Synchrotron Source (CHESS). Measurements using the highly polarized synchrotron beam at CHESS confirm the polarization sensitivity of the instrument. In this paper we report on the design of X-Calibur and results of laboratory calibration measurements characterizing the performance of the instrument.
We report on an investigation that confronts the lag-luminosity relation (LLR), i.e. a surrogate redshift parameterization based upon an empirically determined anti-correlation between the observed ...isotropic luminosity and the temporal lag between the arrivals of photon pulses in low (25-50 keV) and high (100-300 keV) energy bands, with observations of gamma-ray bursts (GRBs) with Swift's Burst Alert Telescope (BAT). Our initial methodology is based upon a cross-correlation function (CCF) analysis of 64 ms BAT light curves (temporal spectra), for a small subset of GRBs with observed spectroscopic redshift (zobs). Our preliminary results span 4 temporal decades and indicate an agreement with LLR, within the margins of error (excluding GRBs 980425 and 060218).
The Burst Alert Telescope (BAT) on the Swift gamma ray burst mission will perform the first new all sky hard X-ray survey since 1977. Swift will perform pointings covering > 64% of the sky each day ...and achieve an integrated systematics limited sensitivity in three years of 0.6 milliCrabs for sources well off the galactic plane. This survey is expected to identify hundreds of new highly obscured AGN. BAT will also serve as a sensitive rapid response X-ray outburst and transient monitor.
PDF
