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      Light curves and spectra from off-axis gamma-ray bursts

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          Abstract

          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 \(\theta_{\rm view} \gtrsim \theta_{\rm jet} + \Gamma^{-1}\), 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 coincidence, we estimate the fraction of off-axis gamma-ray bursts detectable by \textit{Swift} as a function of redshift, finding that \(\sim 40\%\) of nearby (\(z<0.1\)) bursts are observed with \(\theta_{\rm view} \gtrsim \theta_{\rm jet} + \Gamma^{-1}\). Based on these results, we argue that low luminosity \GRBs are consistent with being ordinary bursts seen off-axis.

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          Author and article information

          Journal
          2016-01-14
          2016-03-31
          Article
          1601.03735
          e27df62b-ad59-47f8-a8c5-6bc0af1b4d4c

          http://arxiv.org/licenses/nonexclusive-distrib/1.0/

          History
          Custom metadata
          13 pages, 17 figures, submitted to MNRAS main journal
          astro-ph.HE

          High energy astrophysical phenomena
          High energy astrophysical phenomena

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