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At a redshift of 8.3 GRB 090423 marks the highest known redshift object in the Universe. By combining our radio measurements with existing X-ray and infrared observations, we estimated the kinetic energy of the afterglow, the geometry of the outflow and the density of the circumburst medium. Our best fit model is a quasi-spherical, high-energy explosion in a low, constant-density medium. We compare the properties of GRB 090423 with a sample of GRBs at moderate redshifts. We find that the high energy and afterglow properties of GRB 090423 are not sufficiently different from other GRBs to suggest a different kind of progenitor, such as a Population III star. However, we argue that it is not clear that the afterglow properties alone can provide convincing identification of Population III progenitors. We suggest that the millimeter and centimeter radio detections of GRB 090423 at early times contained emission from a reverse shock component. This has important implications for the detection of high redshift GRBs by the next generation of radio facilities. [ABSTRACT FROM AUTHOR]

Long-duration gamma-ray bursts (GRBs) provide a premier tool for studying high-redshift star-forming galaxies thanks to their extreme brightness and association with massive stars. Here we use GRBs to study the galaxy mass-metallicity (M*-Z) relation at z∼3-5, where conventional direct metallicity measurements are extremely challenging. We use the interstellar medium metallicities of long-duration GRB hosts derived from afterglow absorption spectroscopy (Z≈0.01-1 Z⊙), in conjunction with host galaxy stellar masses determined from deep Spitzer 3.6 μm observations of 20 GRB hosts. We detect about 1/4 of the hosts with MAB(I)≈-21.5 to -22.5 mag, and place a limit of MAB(I)>=-19 mag on the remaining hosts from a stacking analysis. Using a conservative range of mass-to-light ratios for simple stellar populations (with ages of 70 Myr to ∼2 Gyr), we infer the host stellar masses and present the galaxy mass-metallicity measurements at z∼3-5 (≈3.5). We find that the detected GRB hosts, with M*≈2×1010 M⊙, display a wide range of metallicities, but that the mean metallicity at this mass scale, Z≈0.1 Z⊙, is lower than measurements at z<=3. Combined with stacking of the non-detected hosts (with M*<=4×109 M⊙ and Z<=0.03 Z⊙), we find evidence for the existence of an M*-Z relation at z∼3.5 and continued evolution of this relation to systematically lower metallicities from z∼2. [ABSTRACT FROM AUTHOR]

Today, the generation of magnetic fields in solar-type stars and its relation to activity and rotation can coherently be explained, although it is certainly not understood in its entirety. Rotation facilitates the generation of magnetic flux that couples to the stellar wind, slowing down the star. There are still many open questions, particularly at early phases (young age), and at very low mass. It is vexing that rotational braking becomes inefficient at the threshold to fully convective interiors, although no threshold in magnetic activity is seen, and the generation of large scale magnetic fields is still possible for fully convective stars. This article briefly outlines our current understanding of the rotation-magnetic field relation. [ABSTRACT FROM AUTHOR]

Despite a rich diversity in observational properties, gamma-ray bursts (GRBs) can be divided into two broad categories based on their duration and spectral hardness — the long-soft and the short-hard GRBs. The discovery of afterglows from long GRBs in 1997, and their localization to arcsecond accuracy, was a watershed event. The ensuing decade of intense study led to the realization that long-soft GRBs are located in star forming galaxies, produce about 1051 erg in collimated relativistic ejecta, are accompanied by supernovae, and result from the death of massive stars. While theoretical arguments suggest that short GRBs have a different physical origin, the lack of detectable afterglows prevented definitive conclusions. The situation changed dramatically starting in May 2005 with the discovery of the first afterglows from short GRBs localized by Swift and HETE-2. Here I summarize the discovery of these afterglows and the underlying host galaxies, and draw initial conclusions about the nature of the progenitors and the properties of the bursts. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The nearby (z=0.1685) gamma-ray burst of 29 March 2003 has presented us with a unique opportunity to study an event with unprecedented physical resolution. This burst reached flux density levels at centimeter wavelengths more than 50 times brighter than any previously studied event. Here we present the results of VLBI observations that have resolved the radio afterglow, and constrain its proper motion in the sky to <0.3 mas. The size of the afterglow is measured to be ∼0.08 mas 24 days after the burst, consistent with expectations of the standard fireball model. In observations taken 51 days after the burst we detect an additional compact, “jet”, component at a distance from the main component of 0.28 ± 0.05 mas. The presence of this jet component is not consistent with the standard model. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]