*3.3. Mission Profile*

The baseline launcher / orbit configuration is a launch with a Vega-C to a low inclination (<6°) Low Earth Orbit (LEO, 550–640 km altitude), which has the unique advantages of granting a low and stable background level in the high-energy instruments, allowing the exploitation of the Earth's magnetic field for spacecraft fast slewing and facilitating the prompt transmission of transient triggers and positions to the ground. The mission profile will include: (a) a spacecraft autonomous slewing capability >7°/min; (b) the capability of promptly (within a few tens of seconds at most) transmitting to the ground the trigger time and positions of GRBs (and other transients of interest) through the Trigger Broadcasting Unit (TBU) transmitter (via inter-satellite systems like, e.g., ORBCOMM, Iridium and, in case of US contribution, the NASA/TDRSS) and the THESEUS Burst Alert Ground Segment (TBAGS). The main ground station will be 10 m antenna (X-band receiver) at ASI "Luigi Broglio Space Centre" in Malindi (Kenya). As assessed during ESA/M5 Phase A study through a sophisticated Mission Observation Simulator (MOS), the mission scientific goals could be achieved with a nominal duration of 4 years (about 3.5 years of scientific operations). The Mission Operation Control (MOC) and Science Operations Centre (SOC) will be managed by ESA, while the Science Data Centre (SDC) will be under the responsibility of the Consortium.

The thorough R&D activities carried on by the THESEUS Consortium and ESA during the M5 Phase A study, grant a Technology Readiness Level (TRL) already close to that required for mission adoption for the main payload elements. The technical feasibility of the spacecraft, including payload accommodation and thermal control, the required pointing accuracy and stability, the reliability of reaching TRL at mission adoption, the compatibility of a launch with Vega-C in LEO, as well of the overall mission profile, within the M-class mission boundaries and according to the THESEUS scientific requirements, has also already been successfully assessed by M5 Phase A study.

The baseline mission operation concept includes a Survey mode, during which the monitors are waiting for GRBs and other transients of interest. Following a GRB (or transient of interest) trigger validated by the Data Handling Unit (DHU) system, the spacecraft enters a Burst mode (improved data acquisition and spacecraft slewing), followed by a pre-determined (but flexible) IRT observing sequence (Follow-up and Characterization or Deep Imaging modes). The pointing strategy during the Survey mode will be such as to maximize the combined efficiency of the sky monitoring by SXI and XGIS and that of the follow-up with the IRT. Small deviations (of the order of a few degrees until core science goals are achieved) from the Survey mode pointing strategy will be possible so to point the IRT on sources of interest pre-selected through a Guest Observer (GO) programme. Scientific modes also include an external trigger (or Target of Opportunity) mode, in which the IRT and high-energy monitors will be pointed to the direction of a GRB, transient or, e.g., to the error region of a GW or neutrino signal, provided by an external facility.

#### **4. Multi-Messenger Astrophysics with THESEUS**

The large THESEUS/XGIS and SXI field of view and sky localization accuracy will secure independent triggers on the electromagnetic counterparts of several GW and neutrino sources and their localization down to arcmin/arcsec level. The synergies of THESEUS with next generation neutrino and GW observatories will significantly increase the number of multi-messenger detections, enabling unprecedented robust statistical studies of multi-messenger sources [30].

By the end of the 2030s, 3G GW interferometers as the Einstein Telescope [24,34] and the Cosmic Explorer [35], are expected to operate at full sensitivity and likely within a network configuration. In the left panel of Figure 4 we show the THESEUS/XGIS short GRB redshift distribution, where the expected detection rate is of the order of 12/year. These numbers are obtained from simulations of THESEUS pointing strategy, considering all observational constraints, and a random set of short GRB triggers based on the population model of Ghirlanda et al. (2016). This model is built on past short GRBs observed with *Swift* and *Fermi* (i.e., before GRB170817A), considered to be "aligned" (for which the line of sight falls inside the narrow core of the corresponding jet). By taking into account the BNS merger detection efficiency of the 3G interferometers, we also show in each redshift bin the expected fraction that will be jointly detected with ET only and by a network of ET located in Europe plus 2 CEs assumed to be located one in USA and the other in Australia. Expected joint detection rates are quoted in Table 1.

**Figure 4.** (**Left**): Short GRB redshift distribution detected with THESEUS/XGIS assuming an on-axis configuration (blue) and jointly with ET (green) and a network of 3G composed by 2CE and ET (pink). (**Right**): Same figure but now Short GRB with off-axis configurations are included.

THESEUS/XGIS is suitable to detect soft-faint bursts as those we expect to observe from large viewing angles ("misaligned") [30]: in the right panel of Figure 4 we included "misaligned" short GRBs by assuming a structured jet model from [18,36]. We find that the most nearby events can be detected up to large viewing angles [30]. As a consequence, the number of THESEUS short GRB detections at small redshifts is significantly increased with respect to the "aligned" only case. Since at low redshift the GW interferometers BNS merger detection efficiency is near 100%, this improvement is also reflected in the number of joint detections (see third column of Table 1).

With such joint detection rates, THESEUS will allow us to build statistical samples of multi-messenger sources with which a number of fundamental issues can be investigated [30]. To mention some examples:


**Figure 5.** The X-raylong monitoring from prompt emission to the afterglow of relatively bright short GRB with THESEUS/XGIS and SXI. The linear temporal-scale plot on the left shows the short GRB prompt with the "Extended Emission" as it would be detected by THESEUS/XGIS possibly explained by invoking a magnetar remnant. The logarithmic temporal-scale plot on the right shows the X-ray plateau phase with the indication of two possible scenarios invoked for its origin, i.e., High-Latitude Emission of the prompt assuming a structured jet [22] and a spinning-down magnetar pumping energy into the external shock [37] [Credit: S. Vinciguerra].

Last but not least, from a large sample of sources with independent measurement of the cosmological redshift and luminosity distance, the Hubble constant can be measured with the sufficient accuracy to solve the current tensions among different measurements. In Figure 6 we plot the gaussian probability distributions of the current estimates of *H*<sup>0</sup> and the expected improvement of the GW170817 distribution by assuming a realistic number of 20 joint detected events (Table 2).

**Figure 6.** Gaussian probability distribution of the current *H*<sup>0</sup> measurements, including the one obtained from the BNS GW170817, and the expected one with ∼20 joint GW+GRB detections. During the end of the 2030s, 20 joint detection is a conservative estimate for three years of synergies between THESEUS and a network of ET and 2 CE by taking into account the expected fraction of short GRB for which a redshift can be measured.


**Table 2.** Expected joint detections of short GRBs with THESEUS and the 3G GW interferometers (ET = Einstein Telescope, CE = Cosmic Explorer) expected to be operational in the second half of the 2030s, by assuming 3.45 years of joint observations [30].

THESEUS will disseminate accurate sky localization (arcmin/arcsecond uncertainties) within seconds/minutes to the astronomical community, thus enabling large ground and space-based telescopes available by the end of 2030s to observe and deeply characterise the nature of large sample of multi-messenger sources [26,38]. Figure 2 illustrates the main large multi-wavelength and multi-messenger facilities expected to operate in synergy with THESEUS and Figure 3 shows the planned timeline.
