2016/01/31 by O. S. Salafia, Om S. Salafia, Gabriele Ghisellini +7
Medicine · Physics and Astronomy · #Astronomy #Astrophysics #Autopsy Techniques and Outcomes #Curvature #Detector #Doppler effect #Galaxy #Gamma-ray burst #Gamma-ray bursts and supernovae #Geometry #Light curve #Optics #Physics #Pulsars and Gravitational Waves Research #Pulse (music) #RADIUS #Radius of curvature #Redshift #Spectral line #Superposition principle #Viewing angle #astro-ph.HE
paper · pdf · doi:10.1093/mnras/stw1549
13 pages, 17 figures, submitted to MNRAS main journal; updated estimate of the fraction of off-axis grbs seen by Swift
arxiv created 2016/06/23 · arxiv updated 2016/06/28 · openalex publication_date 2016/06/28 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
If gamma-ray burst prompt emission originates at a typical radius, and if material producing the emission moves at relativistic speed, then the variability of the resulting light curve depends on the viewing angle. This is due to the fact that the pulse evolution time-scale is Doppler contracted, while the pulse separation is not. For off-axis viewing angles θview ≳ θjet + Γ−1, the pulse broadening significantly smears out the light-curve variability. This is largely independent of geometry and emission processes. To explore a specific case, we set up a simple model of a single pulse under the assumption that the pulse rise and decay are dominated by the shell curvature effect. We show that such a pulse observed off-axis is (i) broader, (ii) softer and (iii) displays a different hardness–intensity correlation with respect to the same pulse seen on-axis. For each of these effects, we provide an intuitive physical explanation. We then show how a synthetic light curve made by a superposition of pulses changes with increasing viewing angle. We find that a highly variable light curve (as seen on-axis) becomes smooth and apparently single-pulsed (when seen off-axis) because of pulse overlap. To test the relevance of this fact, we estimate the fraction of off-axis gamma-ray bursts detectable by Swift as a function of redshift, finding that a sizeable fraction (between 10 per cent and 80 per cent) of nearby (z < 0.1) bursts are observed with θview ≳ θjet + Γ−1. Based on these results, we argue that low-luminosity gamma-ray bursts are consistent with being ordinary bursts seen off-axis.