*2.2. Next Achievements*

In the next years, with the second generation GW interferometer network (i.e., LIGO, Virgo, KAGRA and by 2025 also LIGO-India, see [19]) we expect to detect other cases like GRB 170817A from BNS mergers and possibly short GRBs associated with NS-BH merger systems up to distances *z* < 0.1 − 0.2. The joint detection rate is still very uncertain but possibly limited to a few cases within the 2020s. This low rate is mainly due to the collimated nature of GRB emission that confines simultaneous detections to a tiny fraction of the total GW events associated with BNS or NS-BH, for which GWs are emitted isotropically.

Interesting predictions concern the possible formation of a NS remnant with large magnetic field (magnetar) after a BNS merger. In this case, the dipole radiation, expected mainly in the X-ray band and with low level of collimation, increases the chances of joint detection. The presence of a spinning-down, new-born magnetar is among the possible scenarios invoked to explain the "Extended Emission" detected after some short GRBs, a ∼100 s lasting component with softer spectrum than the main short burst, that may represents a potential X-ray counterpart of a BNS merger [20]. Another potential multimessenger X-ray target expected to be less collimated than short GRBs is represented by the afterglow emission during the so called "plateau" phase, characterized by a nearly constant flux level lasting on a timescale that goes from a few hundreds of seconds up to ∼1 day. The origin of the plateaus is still debated: among the possible scenarios is the presence of a magnetar pumping energy into the forward shock. In this case, a long-transient continuous GW emission might be simultaneously detected [21]. An alternative scenario invokes the high-latitude prompt emission or afterglow emission from a structured jet for an observer line of sight slightly offset with respect to the jet axis (e.g., [22,23]). In this case, GW continuous emission is not necessarily expected. We note that X-ray plateaus are observed also for long GRBs: in this case, if a magnetar is the origin of this feature, long-transient continuous GWs may be detected also from this other class of GRBs.

During the 2030s, the third generation GW detectors are expected to be operational, with sensitivity nearly one order of magnitude higher. By that time, the distance up to which a BNS can be detected is *z* & 1, thus implying a huge detection rate, of the order of O(10<sup>5</sup> ) per year [24].

With such large detection rate, the fraction of joint detection as short GRBs will be high and will allow statistical studies on large samples. Among the possible issues that can be tackled with a statistical approach there are: (i) jet launching mechanisms and efficiency, (ii) the universality of the jet structure, (iii) differences/commonalities among BNS and NS-BH systems; (iv) accurate cosmological parameter measurements. The high sensitivity of 3G detectors, in addition, will make the detection of the faint GW emission from cc-SN a realistic proposition, possibly up to Mpc scales, allowing us to gain crucial insights on the still uncertain explosion mechanisms.
