X-Ray Afterglows of Gamma-Ray Bursts

The afterglow emission has become the main stream of Gamma-Ray burst research since its discovery three years ago. With the distance-scale enigma solved, the study of the late-time GRB emission is now the most promising approach to disclose the origin of these explosions and their relationship with the environment of the host galaxy in the early phase of the Universe. In this contribution I will review X-ray observations and their implication on our undertstanding on the GRB phenomenon. These measurements are providing a direct probe into the nature of the progenitor and a measurement of the GRB beaming properties, crucial to establish the total energy output. Some evidence of iron lines connects the GRB explosion with massive progenitors, thence with star-forming regions. Furthermore a comparison of the spectral properties with the temporal evolution indicates that the fireball expansion should not be - on average - highly collimated, with a jet angle $>10 \deg$.Comment: 14 pages, 8 figures, Proc.s. of "X-Ray Astronomy '99:Stellar Endpoints, AGN and the Diffuse X-ray Background", September 6-10, 1999, CNR Bologn

Global properties of X-ray afterglows of GRB

In this paper we review the general properties of X-ray afterglows. We discuss in particular on the powerful diagnostics provided by X-ray afterglows in constraining the environment and fireball in normal GRB, and the implications on the origin of dark GRB and XRF. We also discuss on the observed properties of the transition from the prompt to the afterglow phase, and present a case study for a late X-ray outburst interpreted as the onset of the afterglow stage.Comment: 8 pages, 1 color figure. Accepted for publication in "il nuovo cimento". Proceeding of the 4th Rome GRB conference, eds. L. Piro, L. Amati, S. Covino, B. Gendre. Corrected a typo in caption of Fig.

G-Mode Excitation During the Pre-explosive Simmering of Type Ia Supernovae

Prior to the explosive burning of a white dwarf (WD) that makes a Type Ia supernova (SN Ia), the star "simmers" for ~10^3 yrs in a convecting, carbon burning region. I estimate the excitation of g-modes by convection during this phase and explore their possible affect on the WD. As these modes propagate from the core of the WD toward its surface, their amplitudes grow with decreasing density. Once the modes reach nonlinear amplitudes, they break and deposit their energy into a shell of mass ~10^{-4}M_\odot. This raises the surface temperature by 6*10^8 K, which is sufficient to ignite a layer of helium, as is expected to exist for some SN Ia scenarios. This predominantly synthesizes 28Si, 32S, 40Ca, and some 44Ti. These ashes are expanded out with the subsequent explosion up to velocities of ~20,000 km/s, which may explain the high velocity features (HVFs) seen in many SNe Ia. The appearance of HVFs would therefore be a useful discriminant for determining between progenitors, since a flammable helium-rich layer will not be present for accretion from a C/O WD as in a merger scenario. I also discuss the implications of 44Ti production.Comment: Submitted for publication in The Astrophysical Journal Letters, 5 pages, 1 figure