arXiv subjects

T. Lu

Publications and source records attributed to T. Lu.

75 records · Page 5Linked to original sources

arXiv1998

Gamma-Ray Burst Afterglows: Effects of Radiative Corrections and Nonuniformity of the Surrounding Medium

The afterglow of a gamma-ray burst (GRB) is commonly thought to be due to continuous deceleration of a relativistically expanding fireball in the surrounding medium. Assuming that the expansion of the fireball is adiabatic and that the density of the medium is a power-law function of shock radius, viz., $n_{ext}\propto R^{-k}$, we analytically study the effects of the first-order radiative correction and the nonuniformity of the medium on a GRB afterglow. We first derive a new relation among the observed time, the shock radius and the fireball's Lorentz factor: $t_\oplus=R/4(4-k)γ^2c$, and also derive a new relation among the comoving time, the shock radius and the fireball's Lorentz factor: $t_{co}=2R/(5-k)γc$. We next study the evolution of the fireball by using the analytic solution of Blandford and McKee (1976). The radiation losses may not significantly influence this evolution. We further derive new scaling laws both between the X-ray flux and observed time and between the optical flux and observed time. We use these scaling laws to discuss the afterglows of GRB 970228 and GRB 970616, and find that if the spectral index of the electron distribution is $p=2.5$, implied from the spectra of GRBs, the X-ray afterglow of GRB970616 is well fitted by assuming $k=2$.

astro-ph↗

arXiv1998

Is GRO J1744-28 a Strange Star?

The unusal hard x-ray burster GRO J1744-28 recently discovered by the Compton Gamma-ray Observatory (GRO) can be modeled as a strange star with a dipolar magnetic field $\le 10^{11} $Gauss. When the accreted mass of the star exceeds some critical mass, its crust may break, resulting in conversion of the accreted matter into strange matter and release of energy. Subsequently, a fireball may form and expand relativistically outward. The expanding fireball may interact with the surrounding interstellar medium, causing its kinetic energy to be radiated in shock waves, producing a burst of x-ray radiation. The burst energy, duration, interval and spectrum derived from such a model are consistent with the observations of GRO J1744-28.

astro-ph↗

arXiv1997

The evolution of GRB remnants

The detection of the delayed emission in X-ray, optical and radio band, i.e. the afterglow of $γ$-ray bursts (GRBs), suggestes that the sources of GRBs are likely to be at cosmological distances. Here we explore the interaction of a relativistic shell with a uniform interstellar medium (ISM) and obtain the exact solution of the evolution of $γ$-ray burst remnants, including the radiative losses. We show that in general the evolution of bulk Lorentz factor $γ$ satisfies $γ\propto t^{-α_{t}}$ when $γ\gg 1$, here $α_{t}$ is mainly in the range $9/22 \sim 3/8$, the latter corresponds to adiabatic expansion. So it is clear that adiabatic expansion is a good approximation even when radiative loss is considered. However, in fact, $α_{t}$ is slightly larger than 3/8, which may have some effects on a detailed data analysis. Synchrotron-self-absorption is also calculated and it is demonstrated that the radio emission may become optically thin during the afterglow. Our solution can also apply to the nonrelativistic case ($γ\sim 1$), at that time the observed flux decrease more rapidly than that in the relativistic case.

astro-ph↗