GRB Afterglow Polarimetry: Past, Present and Future

Gamma-ray bursts and their afterglows are thought to be produced by an ultrarelativistic jet. One of the most important open questions is the outflow composition: the energy may be carried out from the central source either as kinetic energy (of baryons and/or pairs), or in electromagnetic form (Poynting flux). While the total observable flux may be indistinguishable in both cases, its polarization properties are expected to differ markedly. The later time evolution of afterglow polarization is also a powerful diagnostic of the jet geometry. Again, with subtle and hardly detectable differences in the output flux, we have distinct polarization predictions.Comment: Proceedings of the conference "The coming of age of X-ray polarimetry", Rome, Italy, 27-30 April 200

Polarization of prompt and afterglow emission of Gamma-Ray Bursts

Gamma-ray bursts and their afterglows are thought to be produced by an ultra-relativistic jet. One of the most important open questions is the outflow composition: the energy may be carried out from the central source either as kinetic energy (of baryons and/or pairs), or in electromagnetic form (Poynting flux). While the total observable flux may be indistinguishable in both cases, its polarization properties are expected to differ markedly. The prompt emission and afterglow polarization are also a powerful diagnostic of the jet geometry. Again, with subtle and hardly detectable differences in the output flux, we have distinct polarization predictions. In this review we briefly describe the theoretical scenarios that have been developed following the observations, and the now large observational datasets that for the prompt and the afterglow phases are available. Possible implications of polarimetric measurements for quantum gravity theory testing are discussed, and future perspectives for the field briefly mentioned.Comment: Invited review talk presented at the Ioffe Workshop on GRBs and other transient sources: 20 years of Konus-Wind Experiment (St. Pertersburg, Russia) to be published in Astronomical and Astrophysical Transactions. 34 pages, 7 figures, 8 tables. Referee comments included, and some references adde

Bright Stars and Metallicity Spread in the Globular Cluster Omega Centauri

The globular cluster Omega Centauri (NGC 5139) is the most massive and brightest cluster in our Galaxy. It has also a moderately high mass to light ratio (3.6) and an anomalous flattening (0.83) for a globular cluster. This cluster is also very interesting because it is one of a few examples of globular clusters with a measurable spread in the metal abundance (see Da Costa & Willumsen 1981, Norris et al. 1996, and Suntzeff and Kraft 1996 and references therein) and then it offers a unique, big sample of nearby stars having all the same distance and reddening but showing different metallicity (and age ?) effects. A recent paper by Norris et al. (1997) shows also an interesting correlation between kinematics and metal abundance.Comment: 5 pages, 3 eps figure, pasconf.sty included, Proceedings of the Third Mount Stromlo Symposium: the Galactic Hal

The detection of linear polarization in the afterglow of GRB 990510 and its theoretical implications

We present the recent discovery of linear polarization of the optical afterglow of GRB 990510. Effects that could introduce spurious polarization are discussed, showing that they do not apply to the case of GRB 990510, which is then intrinsically polarized. It will be shown that this observation constrains the emission mechanism of the afterglow radiation, the geometry of the fireball and degree of order of the magnetic field. We then present the theoretical interpretations of this observation with particular emphasis on the possibility of observing polarization in beamed fireballs.Comment: 5 pages, 3 postscript figures. To appear in the proceedings of the 5th Huntsville GRBs symposium (Huntsville AL, Oct. 1999

GRB Redshift determination in the X-ray band

If gamma-ray bursts originate in dense stellar forming regions, the interstellar material can imprint detectable absorption features on the observed X-ray spectrum. Such features can be detected by existing and planned X-ray satellites, as long as the X-ray afterglow is observed after a few minutes from the burst. Detection of these X-ray features will make possible the determination of the redshift of gamma-ray bursts even when their optical afterglows are severely dimmed by extinction.Comment: 2 pages, A&AS in press, proceedings of the Workshop "Gamma Ray Bursts in the Afterglow Era" held in Rome, November 199

Gamma-Ray Bursts and Afterglow Polarisation

Polarimetry of Gamma-Ray Burst (GRB) afterglows in the last few years has been considered one of the most effective tool to probe the geometry, energetic, dynamics and the environment of GRBs. We report some of the most recent results and discuss their implications and future perspectives.Comment: 6 pages, 4 figures. Proceeding for the "Interacting Binaries: Accretion, Evolution and Outcome" worshop held in Cefalu' (Italy) in July 200