Physics of Gamma-Ray Bursts Prompt Emission

Figure 14

The green line represents the (normalized) photospheric radius as a function of the angle to the line of sight, , for spherical explosion (see (20)). The red dots represent the last scattering locations of photons ejected in the center of relativistic expanding “fireball” (using a Monte-Carlo simulation). The black lines show contours. Clearly, photons can undergo their last scattering at a range of radii and angles, leading to the concept of “vague photosphere.” The observed photospheric signal is therefore smeared both in time and energy. Figure taken from Pe’er [241].