UR: Viewing angle effects in the gamma-ray burst internal shock scenario

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Marco Dalla Ca di Dio

Università degli Studi di Milano-Bicocca

Marco Dalla Ca di Dio graduated in Physics in July 2021 at Università degli Studi di Milano-Bicocca. This research has been the focus of his thesis under the supervision of Dr. Om Sharan Salafia.

Gamma-ray bursts are intense pulses of γ-rays emitted in a short time, and some of the most energetic phenomena known in the universe. GRBs are associated with the merger of binary neutron stars and the collapse of massive stars. In order to use GRBs as standard candles, we need a model for the geometry of the event that explains the great irregularities of their light curves.

In this project we adopt the internal shock model to represent the dynamics of the jet that produces the observed light curve. The progenitor of GRBs ejects packets of relativistically accelerated matter through complex magnetohydrodynamical processes, progressing as independent separated shells with individual Lorentz factors, Γ, which determines both their speed and their beaming (see Figure 1). Colliding shells convert part of their kinetic energy into internal energy, which is quickly re-emitted and this pulse is a single peak in the GRB light curve we observe.

modelled diagram representing the internal shock scenario
Figure 1: Geometric representation of the internal shock scenario: the jet is modeled as a series of shells, each defined by its Lorentz factor, traveling in a common direction. In the collisions, part of the kinetic energy is converted into internal energy and then radiated as γ-rays.

The light curve we receive is affected by the viewing angle, the angle between the direction of the observer and the direction of travel of the jet. When the viewing angle is wider than the beaming angle, we observe a much fainter jet as it is severely suppressed by the relativistic Doppler effect, while the duration is dilated. This affects the relative relations between the light curve peaks, both in height and width. We measure the high irregularities in the GRB light curve using the variability measure, as formalized in Reichart et al. 2001. We can then analyze the correlation between the average luminosity of a burst and its variability measure. 

We simulate a population of progenitor events with plausible characteristics from which 1000 GRBs are emitted. We plot the luminosity-variability correlation for these GRBs, as seen from 100 different viewing angles, as a distribution in Figure 2.

Distribution of simulated GRBs on the Variability - Average luminosity plane
Figure 2: Distribution of our simulated GRBs on the Variability-Average luminosity plane. Distribution at 1σ (dark blue) and 2σ (light blue) of 1000 Long GRBs viewed at 100 different angles; in black, experimental data as analyzed by Reichart et al 2001. The red dashed line delimits the region in which 1σ bursts are seen on-axis.

We compare the distribution of average luminosity and variability of our simulated GRBs with experimental data analyzed by Reichart et al. 2001. We find that GRBs seen on-axis (top-right corner in Figure 2) have the highest average luminosity and variability measure, both decreasing at wider viewing angles. We conclude that the great irregularities in observed GRBs light curves can be explained as dominated by the effect of the viewing angle rather than pertaining to exotic progenitor events.

Astrobite edited by: Emma Foxell

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