Iron line afterglows: how to produce them

We discuss how a powerful iron line emission can be produced if ~1-5 iron rich solar masses are concentrated in the close vicinity of the burst. Recombination, thermal and fluorescent reflection are discussed. We find that recombination suffers the high Compton temperature of the plasma while the other two scenarios are not mutually exclusive and could account for the claimed iron line detected in two afterglows.Comment: 2 pages, A&AS in press, proceedings of the Workshop "Gamma Ray Bursts in the Afterglow Era" held in Rome, November 199

Iron line in the afterglow: a key to unveil Gamma-Ray Burst progenitors

The discovery of a powerful and transient iron line feature in the X-ray afterglow spectra of gamma-ray bursts would be a major breakthrough for understanding the nature of their progenitors. Piro et al. (1999) and Yoshida et al. (1999) report such a detection in the afterglow of GRB 970508 and GRB 970828, respectively. We discuss how such a strong line could be produced in the various scenarios proposed for the event progenitor. We show that the observed line intensity requires a large iron mass, concentrated in the vicinity of the burst. The previous explosion of a supernova, predicted in the Supranova scenario, is the most straightforward way to account for such a large amount of matter. We discuss three different physical processes that could account for the line: recombination, reflection and thermal emission. Among these, reflection and thermal emission may explain the observed line features: reflection should be important if the remnant is optically thick, while thermal lines can be produced only in a thin plasma. The recombination process requires extremely high densities to efficiently reprocess the burst photons, whereas this process could work during the X-ray afterglow. Future key observations for discriminating the actual radiating process are discussed.Comment: 5 pages, 1 figure, MNRAS letters in pres

Iron line afterglows: general constraints

The discovery of a powerful and transient iron line feature in the X-ray afterglow spectra of gamma-ray bursts would be a major breakthrough for understanding the nature of their progenitors, strongly suggesting the presence of a large, iron rich, mass in the vicinity of the burst event. Model-independent limits to the size and the mass of the the iron line emitting region are derived and discussed. We also discuss how these results can be used to constrain the amount of beaming or anisotropy of the burst emission.Comment: 2 pages, A&AS in press, proceedings of the Workshop "Gamma Ray Bursts in the Afterglow Era" held in Rome, November 199

X-ray transients in quiescence

Transient X-ray binaries remain in their quiescent state for a long time (months to hundred years) and then bright up as the most powerful sources of the X-ray sky. While it is clear that, when in outbursts, transient binaries are powered by accretion, the origin of the low luminosity X-ray emission that has been detected in the quiescent state has different interpretations and provides the unique opportunity for testing different accretion regimes. In this paper we concentrate on the various aspects of the accretion physics at low rates onto compact objects. We describe the observational panorama of quiescent emission for the three classes of X-ray transients and try to interpret these data in light of the different regimes accessible at such low mass inflow rates.Comment: 10 pages 2 figures, invited review at Bologna X-ray Astronomy 1999. To appear in Astrophysical Letters and Communication

Emission processes in quiescent neutron star transients

We review the observational properties of transient systems made by a neutron star primary and a late dwarf companion (known also as Soft X-ray Transients) during their quiescent state. We focus on the several emission mechanisms proposed and try to compare them with observations. Finally, we review new tools to improve our comprehension of the physics of the emission processes.Comment: 6 pages, talk presented at the Symposium: "Plasmas in the Laboratory and in the Universe: new insights and new challenges", September 16-19, 2003, Como, Italy. Macros include