**1. Introduction**

On 17 August 2017, the *Fermi*/gamma-ray burst monitor (GBM) was triggered by a short gamma-ray burst (SGRB)-GRB 170817A [1–3]. Independently, the gravitational-wave (GW) event GW170817 produced by the double neutron star (NS) merger was detected by the advanced LIGO and Virgo detectors [4,5]. The joint detection of GW170817/GRB 170817A confirms that at least some SGRBs originate from NS mergers, and herald the multi-messenger astronomy [1–5]. It also enables better localization, which benefited the multi-wavelength follow-up observations. The detection of the associated kilonova, AT 2017gfo, led to the discovery of the host-galaxy NGC 4993 at a distance of ∼40 Mpc, which shed light on the physics of nucleosynthesis [1,6–11].

Joint detection can provide abundant information to study some fundamental physics. Using the GW data alone, constraints on the NS equation of state can be obtained (e.g., [4]). Combining with the electromagnetic (EM) observations, (1) the GW event can be treated as a standard siren to study cosmology [12]; (2) one can also constrain the difference between the speed of gravity and the light speed, test the violation of Lorentz invariance and the equivalence principle [5,13]. It can also be used to study the launching mechanism, structure, composition, and radiation mechanism of GRB jets (e.g., [3,14–23]).

Since the discovery of GW170817/GRB 170817A, many efforts have been put into the follow-up observations of GW events to search for their EM counterparts. No new confident joint detection is observed, except for a sub-threshold event: the sub-threshold GRB (GBM-190816) [20,24] was found to be possibly associated with a sub-threshold NS merger event GW190425 [25]. But in the archived *Fermi*/GBM data, a small sample of GRB 170817A-like events has been found [26]. In theory, many EM signals are expected for the NS merger. The observation and theory of SGRBs, afterglows, and kilonova were summarized in many reviews [27–32]. However, little attention has been placed on the pre-merger EM radiation. As a complement, we focused on the precursor emissions of SGRB in this review.

Precursors were initially identified as weak signals in long GRBs (e.g., [33–39]). Later precursors of SGRBs were found in the *Swift*/Burst Alert Telescope (BAT) data [40]. Within the standard fireball scenario, precursors are suggested to be associated with the transition of the fireball from optically thick to optically thin, leading to photospheric blackbody emissions [29,41–44]. This applies to both long and short GRBs. It is also suggested that a precursor can be generated by the shock breakout (SBO) of a jet or a cocoon. For long GRBs, this links to the SBO from the stellar surface [32,45–50], some research proposed that breakout of a radiation mediated shocks train can naturally generate a band-like spectrum [51,52]. For SGRB, this relates to the SBO from the ejecta produced during the NS merger [32,50,53,54]. Besides, there are two more scenarios proposed only for SGRB precursors. During the inspiral phase of the NS–NS/black hole (BH) binary, the magnetospheric interaction of the binary [55–63], or the crust crack of the NS [64–66] may also generate gamma-ray emissions. As such, precursors of SGRBs may shed light on the physical processes right before or shortly after the merger.

Moreover, the magnetospheric interaction model [62] and the SBO model [32,53,54] predict the precursor, although fainter than the main GRB, would have a much larger opening angle, as the radiation is generated by a mild relativistic component. In this case, the precursor can serve as an independent EM counterpart for GWs, even though the prompt GRB points away from the line of sight. It has been suggested that GRB 170817A can be such a case [32,53,54]. This feature would be greatly appreciated for follow-up observations. Thus, research on precursors is important for multi-messenger astronomy. This review aims to summarize the current understanding of SGRB precursors and discuss the possibility for future observations. In the next section, we review the feasible precursor models. Observations are summarized in Section 3. In Section 4, the discussion and prospects are presented.

## **2. Precursor Models**

Various research studies have shown that a gamma-ray precursor event can be produced prior to the main GRB event. Here, we divide the precursor models into two categories based on their relative time to the merger: pre-merger models and post-merger models. More specifically, in the pre-merger phase, magnetospheric interaction in the NS binary and the crustal failure triggered by tidal interactions could lead to precursor emissions. While during the post-merger phase, it is suggested that the photospheric emission from the fireball and the SBO can also result in precursors.

We summarize the luminosity, spectrum, duration, and opening angle of these precursor models below, which relate to their detectability. To make sure the precursor is detectable at an extra-galactic distance, its luminosity should satisfy *L* > 4*πD*2*S*, where *S* is the sensitivity of the detector, and *D* is the distance. Recently, researchers have searched for SGRBs in the local Universe in the *Swift* catalog and found that the four closest SGRBs could locate at *D* ∼ 100–200 Mpc [67]. Thus we adopt *D* > 100 Mpc. For sensitivity, we use the gravitational wave high-energy electromagnetic counterpart all-sky monitor (GECAM) as an example, which has *<sup>S</sup>* <sup>≈</sup> <sup>2</sup> <sup>×</sup> <sup>10</sup>−<sup>8</sup> erg cm−<sup>2</sup> s −1 in 8 − 2000 keV [68]. The corresponding lower limit on luminosity is then

$$L \gtrsim 2.4 \times 10^{46} \text{ erg s}^{-1}.\tag{1}$$
