First Electromagnetic Pulse Associated with a Gravitational-Wave Event: Profile, Duration, and Delay

TL;DR

This paper analyzes the electromagnetic pulse following a gravitational wave event, showing how viewing angle affects the pulse profile and duration, and suggesting the pulse duration can inform gravitational wave search timing.

Contribution

It introduces a model linking electromagnetic pulse features to jet viewing angle and provides insights into jet ejection timing post-merger, especially for GW170817/GRB 170817A.

Findings

Steep decay phase varies with viewing angle.

On-axis jet's luminosity scales as t^{-3} during SD.

Pulse duration correlates with the time to search for gravitational waves.

Abstract

We study the first electromagnetic pulse after the gravitational wave chirp signal, focusing on the profile and duration. It is found that the light curve, especially the steep decay (SD) phase, can be very different by adopting different viewing angle $\theta_{\rm view}$ on the jet shell. For an on-axis jet with a power-law radiation spectrum, the observed flux in the SD is proportional to $t_{\rm{obs}}^{-2-\beta}$ with $\beta$ being the spectral index and $t_{\rm{obs}}$ being the observer time. Here, $t_{\rm{obs}}=0$ is set at the observed time of the jet ejected from the central engine. The SD may become steep by increasing $\theta_{\rm view}$. We also study the bolometric luminosity $L$ from a jet shell with a non-power-law radiation spectrum. For an on-axis jet, $L{\propto}t_{\rm{obs}}^{-3}$ is found in the SD. However, the SD is steeper than $L{\propto}t_{\rm{obs}}^{-3}$ for the radiation from an off-axis jet. The higher value of $\theta_{\rm view}$ is, the steeper of SD would be. Then, we suggest that the SD phase can be used to discriminate an off-axis jet from an on-axis jet. The reason for above behaviors is discussed. In addition, we find that the duration of first electromagnetic pulse is close to its peak time, especially for $\theta_{\rm{view}}\sim20^\circ$. This result is consistent with that found in GW~170817/GRB~170817A. Thus, the jet corresponding to the prompt emission of GRB~170817A should be immediately ejected after the merger. Our results also reveal that the duration of the first electromagnetic pulse can provide the information of the time to search gravitational waves.