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

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 θ 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 −2−β obs with β being the spectral index and t obs being the observer time. Here, t obs = 0 is set at the observed time of the jet ejected from the central engine. The SD may become steep by increasing θ 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∝t −3 obs is found in the SD. However, the SD is steeper than L∝t −3 obs for the radiation from an off-axis jet. The higher value of θ 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 θ view ∼ 20 • . 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.

Short Duration Gamma-Ray Bursts and Their Outflows in Light of GW170817

“…The meaning of all the symbols is explained in Section 2 which allowed for the measuring of the time delay between the GWs and the gamma-ray signals. This delay, which we indicate as Δt GW−c can be due to several reasons, as detailed below and shown in Figure 1 (Granot et al, 2017;Lin et al, 2018;Zhang, 2019;Lazzati et al, 2020;Lucca and Sagunski 2020).…”

The delay time of gravitational wave — gamma-ray burst associations

“…The origin of the ∼ 1.7 s delay in GW170817/GRB 170817A has been dis-Zhang cussed in the literature (e.g. Granot et al 2017;Zhang et al 2018;Shoemaker & Murase 2018;Veres et al 2018;Lin et al 2018;Salafia et al 2018). In this mini-review, we systematically investigate several physical processes that contribute to the observed time delay ∆t (Section 2).…”

The Photosphere Emission Spectrum of Hybrid Relativistic Outflow for Gamma-ray Bursts

“…After decades of researches, the radiation mechanism of GRB prompt emission is still unclear (e.g., Zhang & Yan 2011;Geng et al 2018Geng et al , 2019Lin et al 2018;Zhang et al 2018aZhang et al ,b, 2021Duan, & Wang 2019a,b;Huang et al 2019;Li et al 2019b;Yang et al 2020;Zhang 2020). The photospheric emission model seems to be a promising scenario (e.g., Abramowicz et al 1991;Thompson 1994;Mészáros & Rees 2000;Rees & Mészáros 2005;Pe'er & Ryde 2011;Fan et al 2012;Lazzati et al 2013;Ruffini et al 2013;Gao & Zhang 2015;Bégué & Pe'er 2015;Pe'er et al 2015;Ryde et al 2017;Acuner & Ryde 2018;Hou et al 2018;Meng et al 2018Meng et al , 2019Li 2019a,c,d;Acuner et al 2020;Wang et al 2020).…”