Doppler effect of gamma-ray bursts in the fireball framework

The influence of the Doppler effect in the fireball framework on the spectrum of gamma-ray bursts is investigated. The study shows that the shape of the expected spectrum of an expanding fireball remains almost the same as that of the corresponding rest frame spectrum for constant radiations of the bremsstrahlung, Comptonized, and synchrotron mechanisms as well as for that of the GRB model. The peak flux spectrum and the peak frequency are obviously correlated. When the value of the Lorentz factor becomes 10 times larger, the flux of fireballs would be several orders of magnitude larger. The expansion speed of fireballs is a fundamental factor of the enhancement of the flux of gamma-ray bursts.Comment: 19 pages, 13 figure

Distinguishing RBL-like objects and XBL-like objects with the peak emission frequency of the overall energy spectrum

We investigate quantitatively how the peak emission frequency of the overall energy spectrum is at work in distinguishing RBL-like and XBL-like objects. We employ the sample of Giommi et al. (1995) to study the distribution of BL Lacertae objects with various locations of the cutoff of the overall energy spectrum. We find that the sources with the cutoff located at lower frequency are indeed sited in the RBL region of the $\alpha_{ro}-\alpha_{ox}$ plane, while those with the cutoff located at higher frequency are distributed in the XBL region. For a more quantitative study, we employ the BL Lacertae samples presented by Sambruna et al. (1996), where, the peak emission frequency, $\nu _p$, of each source is estimated by fitting the data with a parabolic function. In the plot of $\alpha_{rx}-\log \nu_p$ we find that, in the four different regions divided by the $\alpha_{rx}=0.75$ line and the $\log \nu_p=14.7$ line, all the RBL-like objects are inside the upper left region, while most XBL-like objects are within the lower right region. A few sources are located in the lower left region. No sources are in the upper right region. This result is rather quantitative. It provides an evidence supporting what Giommi et al. (1995) suggested: RBL-like and XBL-like objects can be distinguished by the difference of the peak emission frequency of the overall energy spectrum.Comment: 7 pages, 2 figure

The disappearance of a narrow Mg II absorption system in quasar SDSS J165501.31+260517.4

In this letter, we present for the first time, the discovery of the disappearance of a narrow Mg II $\lambda\lambda2796,2803$ absorption system from the spectra of quasar SDSS J165501.31+260517.4 ($z_{\rm e}=1.8671$). This absorber is located at $z_{\rm abs} =1.7877$, and has a velocity offset of $8,423\rm ~km~s^{-1}$ with respect to the quasar. According to the velocity offset and the line variability, this narrow Mg II $\lambda\lambda2796,2803$ absorption system is likely intrinsic to the quasar. Since the corresponding UV continuum emission and the absorption lines of another narrow Mg II $\lambda\lambda2796,2803$ absorption system at $z_{\rm abs}=1.8656$ are very stable, we think that the disappearance of the absorption system is unlikely to be caused by the change in ionization of absorption gas. Instead, it likely arises from the motion of the absorption gas across the line of sight