A Closer Look at a Gamma-Ray Burst

A study of gamma rays produced when stars collapse or collide reveals details of the explosion mechanism, particularly the role of magnetic fields.Comment: 2 pages, 1 figure; Science perspective to Mundell et al. 2007, Science, 315, 182

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

Strongly Polarized Optical Afterglows of Gamma-Ray Bursts

The optical afterglows of the gamma ray bursts can be strongly polarized, in principle up to tens of percents, if: (i) the afterglow is synchrotron radiation from an ultra-relativistic blast, (ii) the blast is beamed during the afterglow phase, i.e. the shock propagates within a narrow jet, (iii) we observe at the right time from the right viewing angle, (iv) magnetic fields parallel and perpendicular to the jet have different proper strengths.Comment: 5 pages, submitted to Ap

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

Polarization in the prompt emission of gamma-ray bursts and their afterglows

Synchrotron is considered the dominant emission mechanism in the production of gamma-ray burst photons in the prompt as well as in the afterglow phase. Polarization is a characteristic feature of synchrotron and its study can reveal a wealth of information on the properties of the magnetic field and of the energy distribution in gamma-ray burst jets. In this paper I will review the theory and observations of gamma-ray bursts polarization. While the theory is well established, observations have prove difficult to perform, due to the weakness of the signal. The discriminating power of polarization observations, however, cannot be overestimated.Comment: 16 pages, 9 figures, accepted for publication in the New Journal of Physics focus issue on Gamma Ray Burst