X-ray emission from the PSR B1259--63 system near apastron

The PSR B1259--63 system contains a 47 ms radio pulsar in a highly eccentric binary with a Be-star companion. Strongly time variable X-ray emission was reported from this system as the pulsar was near apastron in 1992-early 1993. The variability was primarily deduced from an apparent non-detection of the \psr system during a first pre-apastron \ros observation in February~1992. We have re-analyzed the \ros observations of the \psr system. Contrary to the results of a previous analysis, we find that the \psr system was detected by \ros during the first off-axis February~1992 observation. The intensity of the soft X-ray emission of the \psr system before and after the 1992 apastron appears to vary at most by a factor $\sim 2$. Our results sensibly constrain theoretical models of X-ray emission from the \psr system.Comment: LATEX, Accepted for publ. in ApJ

Time Resolved GRB Spectroscopy

We present the main results of a study of time-resolved spectra of 43 intense GRBs detected by BATSE. We considered the 4-parameter Band model and the Optically Thin Synchrotron Shock model (OTSSM). We find that the large majority of time-resolved spectra of GRBs are in remarkable agreement with the OTSSM. However, about 15 % of initial GRB pulses show an apparent low-energy photon suppression. This phenomenon indicates that complex radiative conditions modifying optically thin emission may occur during the initial phases of some GRBs.Comment: 5 pages, 3 figures, Paper presented at the 5th Huntsville Symposium, Huntsville (Alabama) Oct. 199

Modelling the Kinked Jet of the Crab Nebula

We investigate the dynamical propagation of the South-East jet from the Crab pulsar interacting with supernova ejecta by means of three-dimensional relativistic MHD numerical simulations with the PLUTO code. The initial jet structure is set up from the inner regions of the Crab Nebula. We study the evolution of hot, relativistic hollow outflows initially carrying a purely azimuthal magnetic field. Our jet models are characterized by different choices of the outflow magnetization ($\sigma$ parameter) and the bulk Lorentz factor ($\gamma_{j}$). We show that the jet is heavily affected by the growth of current-driven kink instabilities causing considerable deflection throughout its propagation length. This behavior is partially stabilized by the combined action of larger flow velocities and/or reduced magnetic field strengths. We find that our best jet models are characterized by relatively large values of $\sigma$ ($\gtrsim 1$) and small values of $\gamma_{j}\simeq 2$. Our results are in good agreement with the recent X-ray (\textit{Chandra}) data of the Crab Nebula South-East jet indicating that the jet changes direction of propagation on a time scale of the order of few years. The 3D models presented here may have important implications in the investigation of particle acceleration in relativistic outflows.Comment: 15 pages, 20 figure

X-ray afterglows from gamma-ray bursts

We consider possible interpretations of the recently detected X- ray afterglow from the gamma-ray burst source GRB 970228. Cosmological and Galactic models of gamma-ray bursts predict different flux and spectral evolution of X-ray afterglows. We show that models based on adiabatic expansion of relativistic forward shocks require very efficient particle energization or post-burst re-acceleration during the expansion. Cooling neutron star models predict a very distinctive spectral and flux evolution that can be tested in current X-ray data.Comment: 15 pages, 2 postscript files with figures submitted to Astrophys. J. Letters, March 18, 199

Euclidean vs. non-Euclidean Gamma-Ray Bursts

We classify gamma-ray bursts (GRBs) according to their observed durations and physical properties of their spectra. We find that long/hard bursts (of duration T_90 > 2.5 s, and typical photon energy E_p > 0.8 MeV corresponding to BATSE's energy fluence hardness H^e_{32} > 3) show the strongest deviation from the three-dimensional Euclidean brightness distribution. The majority of GRBs, i.e., short bursts (T_90 2.5 s, and H^e_{32} < 3) show little, if any, deviations from the Euclidean distribution. These results contradict the prediction of simple extragalactic GRB models that the most distant bursts should be the most affected by cosmological energy redshift and time-dilation (long/soft GRBs). The strongly non-Euclidean GRB subclass has very hard spectra of typical photon energy above 1 MeV, i.e., outside the ideal energy range for optimal detection by BATSE. We discuss possible explanations of this puzzling feature of GRBs.Comment: 15 pages, LATEX text plus two postscript figures included. Submitted to ApJ Letters on November 24, 1997. Accepted on February 13, 199

Phase transition for the Maki-Thompson rumour model on a small-world network

We consider the Maki-Thompson model for the stochastic propagation of a rumour within a population. We extend the original hypothesis of homogenously mixed population by allowing for a small-world network embedding the model. This structure is realized starting from a $k$-regular ring and by inserting, in the average, $c$ additional links in such a way that $k$ and $c$ are tuneable parameter for the population architecture. We prove that this system exhibits a transition between regimes of localization (where the final number of stiflers is at most logarithmic in the population size) and propagation (where the final number of stiflers grows algebraically with the population size) at a finite value of the network parameter $c$. A quantitative estimate for the critical value of $c$ is obtained via extensive numerical simulations.Comment: 24 pages, 4 figure