Science with the ASTRI Mini-Array: From Experiment to Open Observatory

Abstract

Although celestial sources emitting in the few tens of GeV up to a few TeV are being investigated by imaging atmospheric Čerenkov telescope arrays such as H.E.S.S., MAGIC, and VERITAS, at higher energies, up to PeV, more suitable instrumentation is required to detect ultra-high-energy photons, such as extensive air shower arrays, as HAWC, LHAASO, Tibet AS-$\gamma$. The Italian National Institute for Astrophysics has recently become the leader of an international project, the ASTRI Mini-Array, with the aim of installing and operating an array of nine dual-mirror Čerenkov telescopes at the Observatorio del Teide in Spain starting in 2025. The ASTRI Mini-Array is expected to span a wide range of energies (1–200 TeV), with a large field of view (about 10 degrees) and an angular and energy resolution of ∼3 arcmin and ∼10 %, respectively. The first four years of operations will be dedicated to the exploitation of Core Science, with a small and selected number of pointings with the goal of addressing some of the fundamental questions on the origin of cosmic rays, cosmology, and fundamental physics, the time-domain astrophysics and non $\gamma$-ray studies (e.g., stellar intensity interferometry and direct measurements of cosmic rays). Subsequently, four more years will be dedicated to Observatory Science, open to the scientific community through the submission of observational proposals selected on a competitive basis. In this paper, I will review the Core Science topics and provide examples of possible Observatory Science cases, taking into account the synergies with current and upcoming observational facilities.

1. Introduction

About 300 celestial sources are currently known to emit in the $0.1<E<30$ TeV energy range (see the TeVCat Webpage1 [1]) based on their detection by the major imaging atmospheric Čerenkov telescope arrays (IACTs), such as H.E.S.S. [2], MAGIC [3], and VERITAS [4], whose energy range extends up to a few tens of TeV. Alternatively, extended air shower arrays (EAS) such as HAWC [5], LHAASO [6] and Tibet AS-$\gamma$ [7], adopt a different detection technique that allows us to investigate energies up to several hundreds of TeV, reaching the PeV limit. The sources detected by the current generation of EAS at energies $E>100$ TeV, and up to a few PeVs are a factor of ten fewer.

The Čerenkov Telescope Array Observatory (CTAO [8]) will be the next large scale Čerenkov array and will cover an energy range from a few tens of GeV up to a few hundreds of TeV by means of telescopes of different sizes (see, e.g., [9]). It will be deployed in both hemispheres to observe the full sky. A few telescope prototypes were developed in recent years, among them the ASTRI-Horn dual-mirror, Schwarzschild–Couder (SC) telescope [10], currently operating on Mount Etna in Sicily (Italy), which obtained the first-light optical qualification by means of observation of Polaris, using a dedicated optical camera [11], and the first detection of very high-energy $\gamma$-ray emission from the Crab Nebula by a Čerenkov telescope in dual-mirror SC configuration [12].

In this review, I will first describe the ASTRI Mini-Array characteristics and performance in the context of currently available very high- and ultra-high energy (VHE and UHE, respectively) instrumentation (Section 2), then briefly highlight the ASTRI Mini-Array science topics that will be pursued during the first four years of operation (Section 3). In Section 4, I will describe in detail the subsequent four years of operation and the Open Observatory Phase, when the scientific investigation is mainly driven by the community.

2. The ASTRI Mini-Array

The ASTRI Mini-Array [13,14] consists of nine ASTRI dual-mirror small-sized (SSTs) Čerenkov telescopes, currently being deployed at the Observatorio del Teide (Spain), which will commence its scientific operations in late 2025. The ASTRI Mini-Array will provide a large field of view (FoV) of about 10°, a wide energy range from 1 TeV to 200 TeV, an angular resolution of ∼3${}^{\prime }$, and an energy resolution of ∼10%.

Table 1. Performance of the ASTRI Mini-Array compared with the main current IACT arrays. References: ASTRI Mini-array [15], MAGIC [16], VERITAS [17] and https://veritas.sao.arizona.edu (accessed on 14 February 2024), H.E.S.S. [2].

Quantity	ASTRI Mini-Array	MAGIC	VERITAS	H.E.S.S.
Location	28° 18${}^{\prime }$ 04${}^{\prime \prime }$ N	28° 45${}^{\prime }$ 22${}^{\prime \prime }$ N	31° 40${}^{\prime }$ 30${}^{\prime \prime }$ N	23° 16${}^{\prime }$ 18${}^{\prime \prime }$ S
	16° 30${}^{\prime }$ 38${}^{\prime \prime }$ W	17° 53${}^{\prime }$ 30${}^{\prime \prime }$ W	110° 57${}^{\prime }$ 7.8${}^{\prime \prime }$ W	16° 30${}^{\prime }$ 00${}^{\prime \prime }$ E
Altitude [m]	2390	2396	1268	1800
FoV	$\sim 10$°	$\sim 3.5$°	$\sim 3.5$°	$\sim 5$°
Angular Res.	$0.05$° (10 TeV)	0.07° (1 TeV)	0.07° (1 TeV)	0.06° (1 TeV)
Energy Res.	10% (10 TeV)	16% (1 TeV)	17% (1 TeV)	15% (1 TeV)
Energy Range	(0.5–200) TeV	(0.05–20) TeV	(0.08–30) TeV	(0.02–30) TeV ${}^{\left(\mathrm{a}\right)}$

Notes: ${}^{\left(\mathrm{a}\right)}$: considering the contribution of H.E.S.S.$--$II telescope unit [18].

compares the ASTRI Mini-Array performance with that of the current IACTs.

A detailed description of the ASTRI Mini-Array performance is reported in [19]. Figure 1 shows the ASTRI Mini-Array differential sensitivity (turquoise points, 50 h integration time, $5\sigma$ confidence level, C.L.) compared with those of the current major IACTs (H.E.S.S., MAGIC, and VERITAS) and of the planned CTAO. The ASTRI Mini-Array will improve the current IACT sensitivity at energies greater than a few TeVs and will be of the same order as that of CTAO North in the “alpha” (4 LSTs2 + 9 MSTs3 [9] configuration, and slightly better in the “science verification” (4 LSTs + 5 MSTs [20]) configuration, respectively, at energies greater than a few tens of TeVs.

The ASTRI Mini-Array will reserve, as we shall describe in Section 3, its first four years of operation for the investigation of a few specific science topics. This implies that most of the science operations will be performed as deep pointings, with exposures in the order of 200 h or even 500 h, towards specific sky regions. Figure 2 shows the ASTRI Mini-Array differential sensitivity curves for 200 h (turquoise squares) and 500 h (turquoise triangles) integration time, respectively. For such long integration times, the most appropriate comparison is with current EAS differential sensitivity curves, HAWC [21], Tibet AS-$\gamma$ (Takita M., priv. comm. based on [22]), and LHAASO [23].

Figure 1. ASTRI Mini-Array differential sensitivity for 50 h integration compared with those of MAGIC, H.E.S.S., VERITAS, and CTAO North. The differential sensitivity curves are drawn from [19] (ASTRI Mini-Array), [16] (MAGIC), the VERITAS official website https://veritas.sao.arizona.edu (accessed on 14 February 2024), and [24] (sensitivity curve for H.E.S.S.−I, stereo reconstruction). CTAO−N “alpha configuration” (alpha) sensitivity curve comes from [9]. The CTAO−N “science verification” (sv) sensitivity is drawn from [20].

Figure 1. ASTRI Mini-Array differential sensitivity for 50 h integration compared with those of MAGIC, H.E.S.S., VERITAS, and CTAO North. The differential sensitivity curves are drawn from [19] (ASTRI Mini-Array), [16] (MAGIC), the VERITAS official website https://veritas.sao.arizona.edu (accessed on 14 February 2024), and [24] (sensitivity curve for H.E.S.S.−I, stereo reconstruction). CTAO−N “alpha configuration” (alpha) sensitivity curve comes from [9]. The CTAO−N “science verification” (sv) sensitivity is drawn from [20].

Figure 2. ASTRI Mini-Array differential sensitivity for 200 h (turquoise squares) and 500 h (turquoise triangles) integration times compared with those of HAWC (507 d), Tibet AS-$\gamma$ (1 yr), and LHAASO (1 yr). The differential sensitivity curves are drawn from ASTRI Mini-Array [19], HAWC [21], LHAASO [23], and Takita M. (priv. comm.) based on [22] (Tibet AS + MD). We note that the 507-day HAWC differential sensitivity curve corresponds to about 3000 h of acquisition on a source at a declination of 22° within its field of view [21].

Figure 2. ASTRI Mini-Array differential sensitivity for 200 h (turquoise squares) and 500 h (turquoise triangles) integration times compared with those of HAWC (507 d), Tibet AS-$\gamma$ (1 yr), and LHAASO (1 yr). The differential sensitivity curves are drawn from ASTRI Mini-Array [19], HAWC [21], LHAASO [23], and Takita M. (priv. comm.) based on [22] (Tibet AS + MD). We note that the 507-day HAWC differential sensitivity curve corresponds to about 3000 h of acquisition on a source at a declination of 22° within its field of view [21].

The main advantage of EAS with respect to IACTs is the former’s 2 sr FoV and their larger duty cycle. On the other hand, as reported in

Table 2. Summary of the performance of the current main particle sampling arrays compared with those of the ASTRI Mini-Array. References: ASTRI Mini-array [15], HAWC [5,25], LHAASO [6], Tibet AS-$\gamma$ [22,26].

Quantity	ASTRI Mini-Array	HAWC	LHAASO	Tibet AS-$\mathit{\gamma }$
Location	28° 18${}^{\prime }$ 04${}^{\prime \prime }$ N	18° 59${}^{\prime }$ 41${}^{\prime \prime }$ N	29° 21${}^{\prime }$ 31${}^{\prime \prime }$ N	30° 05${}^{\prime }$ 00${}^{\prime \prime }$ N
	16° 30${}^{\prime }$ 38${}^{\prime \prime }$ W	97° 18${}^{\prime }$ 27${}^{\prime \prime }$ W	100° 08${}^{\prime }$ 15${}^{\prime \prime }$ E	90° 33${}^{\prime }$ 00${}^{\prime \prime }$ E
Altitude [m]	2390	4100	4410	4300
FoV	∼0.024 sr	2 sr	2 sr	2 sr
Angular Res.	$0.05$° (10 TeV)	0.15° ${}^{\left(\mathrm{a}\right)}$ (10 TeV)	(0.24–0.32)° ${}^{\left(\mathrm{b}\right)}$ (100 TeV)	0.2° ${}^{\left(\mathrm{c}\right)}$ (100 TeV)
Energy Res.	10% (10 TeV)	30% (10 TeV)	(13–36)% (100 TeV) ${}^{\left(\mathrm{b}\right)}$	20% ${}^{\left(\mathrm{c}\right)}$ (100 TeV)
Energy Range	(0.5–200) TeV	(0.1–1000) TeV	(0.1–1000) TeV	(0.1–1000) TeV

Notes: ${}^{\left(\mathrm{a}\right)}$: (0.15–1)° as a function of the event size. ${}^{\left(\mathrm{b}\right)}$: angular resolution is (0.70–0.94)° at 10 TeV; (0.24–0.32)° at 100 TeV; 0.15° at 1000 TeV. Energy resolution is (30–45)% at 10 TeV; (13–36)% at 100 TeV; (8–20)% at 1000 TeV [27]. ${}^{\left(\mathrm{c}\right)}$: angular resolution is $\sim 0.5$° at 10 TeV and ∼0.2° at 10 TeV at 50% containment radius [22]. Energy resolution is ∼40% at 10 TeV and ∼20% at 100 TeV [26]. The different values of the LHAASO angular and energy resolution performance at a given energy have been computed at different Zenith angles, $0<\theta <20$, $20<\theta <35$, and $35<\theta <50$ degrees, respectively. At lower Zenith angles, the performance is better.

, their energy and angular resolution in the same energy range as the ASTRI Mini-Array (about 10 TeV) are at least a factor of 3 to 4 times worse. Clearly, this makes the ASTRI Mini-Array extremely competitive in studying the morphology of extended sources and crowded fields and accurately monitoring multiple targets in the same pointing.

Furthermore, the ASTRI Mini-Array angular resolution will allow us to investigate the LHAASO uncertainty error box of Galactic sources, which is of the order of one degree [28] and study the different sources possibly associated with the PeV emission, in order to unambiguously identify them, when in synergy with GeV and X-ray facilities.

Figure 3 shows the ASTRI Mini-Array angular (left panel) and energy (right panel) resolution as a function of the energy as reported in [19].

3. Core Science Topics

The ASTRI Mini-Array science program will develop in two phases. During the first four years of operations, the ASTRI Mini-Array will be run as an experiment, while in the subsequent four years, it will gradually evolve into an observatory open to the scientific community.

A graphical description of the main Core Science topics that we plan to investigate during the first four years of operations is shown in Figure 4. Our Core Science Program is based on “Main Pillars”. They are science fields in which the ASTRI Mini-Array will contribute breakthrough pieces of evidence to improve our understanding of a few key science questions.

Recently, [15] discussed the ASTRI Mini-Array Core Science, which includes the study of the: (1) origin of cosmic rays; (2) cosmology and fundamental physics; (3) GRBs and time-domain astrophysics; (4) direct measurements of cosmic rays; (5) stellar-intensity interferometry. Here, I will review some of the results on all science topics, while Section 4.1 will focus on the Observatory Science ones, presented in [29,30].

3.1. The Origin of Cosmic Rays

The LHAASO Collaboration [28] reported the discovery of twelve Galactic sources emitting $\gamma$-rays at several hundreds TeV up to 1.4 PeV. These sources are able to accelerate particles up to ∼${10}^{15}$ eV, making them “PeVatron candidates”. We note that the majority of these sources are diffuse $\gamma$-ray structures with angular extensions up to 1°, which, together with the LHAASO limited angular resolution, make the identification of the actual sources responsible for the ultra high-energy $\gamma$-ray emission not univocal (except for the Crab Nebula). The recent publication of the First LHAASO Catalog of $\gamma$-ray Sources (1LHHASO [31]) containing 90 sources, 43 of them with emissions at energies $E>0.1$ PeV, marks a fundamental step for the astrophysics at very high- and ultra high-energies. This discovery is extremely important for the ASTRI Mini-Array science, especially because of its angular resolution, which, at energies of about 100 TeV, is a factor of 3 to 4 times better in radius than the LHAASO one: 0.08° vs. 0.24–0.32°. We should also mention that both the angular resolution and the energy resolution can be improved by means of specific analysis cuts, as shown in Figure 3. The ASTRI Mini-Array will investigate these and future PeVatron sources, providing important information on their morphology above 10 TeV. The ASTRI Mini-Array wide FoV will be extremely important in the investigation of extended regions and point-like sources. A single pointing will allow us to investigate the Galactic Center or the Cygnus regions. In these regions, we can accumulate several hundreds of hours by also including the epochs of moderate Moon condition [32]. This is crucial to investigate both the (energy-dependent) morphology of the sources in these regions and their possible variability on a long time scale. The ASTRI Mini-Array will investigate the Galactic Center at a high Zenith angle (maximum culmination angle of ∼${57}^{\circ }$). We expect to be able to study this region up to E∼200 TeV for an exposure time of 260 h, significantly improving the current results of other IACTs (see for further details [15]). The high-energy boundaries of the ASTRI Mini-Array will also be important to study the Crab Nebula, the only Galactic PeVatron4 currently known [33,34]. The origin of the Crab Nebula $\gamma$-ray emission detected by LHAASO does not require a hadronic contribution but cannot exclude it either. A deep ASTRI Mini-Array observation lasting about 500 h in the $E>100$ TeV energy range should definitely be able to provide constraints on the proton component in this source.

3.2. Cosmology and Fundamental Physics

IACT arrays detected extra-galactic sources since the early nineties [35]. Since then, among the 280 sources listed in TeVCat, 93 are found to be extra-galactic: 55 high-peaked BL Lacs (HBLs), 10 intermediate-peaked BL Lacs (IBLs), 9 flat-spectrum radio quasars (FSRQs), 4 Blazars, 4 Fanaroff-Riley Type (FR-I) galaxies, 2 star-bursting galaxies (SBGs), 2 BL Lacertae objects with class unclear (BL Lacs), 2 unknown type AGNs, and 5 $\gamma$-ray bursts (GRBs). Extra-galactic jetted sources are excellent probes for several science cases. They can be used to investigate the extra-galactic background light, as well as to probe, by means of variability studies, the properties of the $\gamma$-ray emitting region. They can also be useful to investigate peculiar physical phenomena, such as the existence of the axion-like particle, to test the Lorentz invariance violation, and to study the intergalactic magnetic fields.

The Extra-galactic background light (EBL)—The EBL significantly affects the spectra of jetted sources at energies that can be explored by the ASTRI Mini-Array. Moreover, the EBL direct measurement in the infra-red (IR) portion of the spectrum is particularly challenging because of the dominant contribution of our Galaxy at these wavelengths. Nevertheless, the ASTRI Mini-Array can contribute to the study of the EBL IR component given the well-known relation ${\lambda }_{\max }\simeq 1.24\times E_{\mathrm{TeV}}$ $\mathsf{\mu }$m, between the wavelength of the target EBL photon, ${\lambda }_{\max }$  and the energy of the $\gamma$-ray, $E_{\mathrm{TeV}}$. The IR component in the ($10<\lambda <100$) $\mathsf{\mu }$m regime represents a challenge because of the dominance of local emission from both the Galaxy and our Solar system. The preferred candidates for observations with the ASTRI Mini-Array are TeV-emitting low-redshift radio-galaxies and local star-bursting galaxies. Among the sources fulfilling these criteria, we investigated the low-redshift radio-galaxies IC 310 (z∼0.0189) and M 87 (z∼0.00428). For IC 310 we assumed three different spectral states: flare [36], high and low [37]. Short (5 h) and deep (200 h) observations will allow us to detect these sources in different spectral states at energies $E>10$ TeV, thus probing the IR EBL component. Similar results can be obtained for M 87 in different spectral states, as reported by [38] (low state), [39] (high state), and [40] (flaring state).

Fundamental and exotic physics—Blazar spectra above a few TeV are excellent probes of non-standard $\gamma$-ray propagation effects such as the presence of hadron beams (HB) in the jet of extreme BL Lac objects (E-HBL and references therein [41]), the existence of axion-like particles [42] (ALP), or the effects of the Lorentz invariance violation (LIV and references therein [41]) and the properties of inter-galactic magnetic fields (IGMF and references therein [43]). The most promising sources to detect spectral signatures induced by these effects are 1ES 0229+200 (E-HBL, z∼0.139) and Mrk 501 (HBL, z∼0.03298). The presence of HB implies that the spectrum of blazars extends at energies above those allowed by the standard EBL model [44]. A detection of a $\gamma$-ray photons at energies of a few tens of TeV, when the standard EBL model would imply a roll-off at a few TeV, could be the signature for the presence of the HB scenario. On the other hand, this excess at energies above a few tens of TeV could also be induced by both the ALPs and LIV effects. ALPs produce a distinctive oscillation pattern in the blazar spectrum, that could represent the unique marker for this process, but it would require a much finer energy resolution than that of the ASTRI Mini-Array to be revealed. Also, in the context of the widely studied dark matter (DM) weakly-interacting massive particles scenario, the ASTRI Mini-Array may provide interesting DM-related insights from dwarf spheroidal galaxies and Galactic center observations, particularly for the case of monochromatic $\gamma$-ray emission lines [30]. In order to address all these studies we can plan deep (in the order of 200 h) dedicated pointing, a typical exposure that could be accumulated during the first years, with the described ASTRI Mini-Array observing strategy.

3.3. Multi-Messenger and Time-Domain Astrophysics

Transients and multi-messenger studies such as $\gamma$-ray bursts (GRBs), gravitational waves (GWs), and neutrino emission (${\nu }_{\mathrm{s}}$) from VHE sources are indeed the new frontiers of high-energy astrophysics.

$\gamma$-ray bursts—GRBs have only been detected by IACTs starting from 2018 and, at the time of writing, we only count six5 GRBs detected at energies in excess of 0.1 TeV: GRB 160821B ($z=0.162,$ MAGIC] [45]), GRB 180720B ($z=0.653,$ H.E.S.S. [46]), GRB 190114C ($z=0.424,$ MAGIC [47]), GRB 190829A ($z=0.078,$ H.E.S.S. [48]), GRB 201015A ($z=0.42,$ MAGIC [49]), GRB 201216C ($z=1.1,$ MAGIC [50]), GRB 221009A ($z=0.151,$ LHAASO] [51]). Two of them are particularly relevant for the ASTRI Mini-Array. GRB 190114C is the first GRB detected at VHE within one minute from the $T_0$, up to an energy of about 1 TeV. GRB 221009A, described as “to be a once-in-10,000-year event” [52,53] was detected by LHAASO up to 13 TeV [54], challenging the standard emission scenario for the canonical EBL absorption [55,56]. Taking into account the energetics observed in GRB 221009A, we can estimate that this event, placed at a different redshift ($z=0.078,0.25,0.42$), can be detected up and above 10 TeV by the ASTRI Mini-Array within a few minutes from the event (L. Nava, Priv. Comm.).

Neutrinos—AGNs can be sources of extra-galactic ${\nu }_{\mathrm{s}}$. The IceCube data [57] seem to indicate that there could be an association between ${\nu }_{\mathrm{s}}$ emission and a few AGNs: a Seyfert-2 galaxy (NGC 1068, $D=14.4$ Mpc), and two BL Lac objects (TXS 0506 + 056, $z=0.3365$; PKS 1424 + 240, $z=0.16$). Although the two latter sources are known TeV emitters, NGC 1068 shows prominent emission only in the 0.1–300 GeV energy band [58,59]. NGC 1068 could show emission above 10 TeV under the assumption of a dominant contribution of relativistic particles accelerated by the AGN-driven wind, as discussed in Section 4.4.

3.4. Non $\gamma$-ray Astrophysics

Stellar intensity interferometry—Stellar intensity interferometry (SII) is based on the second-order coherence of light, which allows imaging sources at the level of 100 $\mathsf{\mu }$as. This means that it is possible to reveal details on the surface and of the environment surrounding bright stars in the sky, which typically have angular diameters of 1–10 mas. The SII observing mode will take advantage of an additional, dedicated instrument that is being designed and will be installed on the ASTRI Mini-Array telescopes [60].

Direct measurements of cosmic rays—More than 99% of the signal acquired by the ASTRI Mini-Array is hadronic in nature. In particular, this hadronic component could be useful to investigate the cosmic ray composition in the TeV–PeV energy range and the measurement of the cosmic ray spectrum at energies characteristics of its “knee”.

3.5. Synergies with Other Facilities

The ASTRI Mini-Array will be operating during a period when several facilities will cover the whole electromagnetic spectrum, from radio to PeV. The Sardinia radio telescope (SRT) will complement VHE observation with radio data at different frequencies, both for Galactic and extra-galactic objects. Galactic sources already observed with SRT are W 44, IC 433, and Tycho [61,62]. In the optical energy band, the Telescopio Nazionale Galileo [63] and the GASP/WEBT Consortium [64] can provide excellent coverage, as well as several facilities managed by Instituto de Astrofísica de Canarias (IAC) at the Canary Island. In the X-ray energy band, in addition to the long-standing ESA and NASA legacy Observatories, we can now exploit the eROSITA [65] surveys and, in particular, the IXPE [66] X-ray polarimetric data. At the extreme energy boundary, LHAASO, HAWC, and Tibet AS-$\gamma$ will extend data at energies of a few PeV.

The ASTRI Mini-Array location at the Observatorio del Teide and its collaboration with the IAC will allow us to investigate sources synergically with both the MAGIC and CTAO−N arrays. In particular, they will be of paramount importance for their capability to investigate not only the local Universe but also to reach redshifts well beyond one and perform cosmological studies on extra-galactic sources. Moreover, both MAGIC and CTAO−N will allow us to extend the ASTRI Mini-Array spectral performance in the sub-TeV regime, with almost no breaks from a few tens of GeV up to hundreds of TeV.

4. The Observatory Phase

The ASTRI Mini-Array science program will gradually evolve from an experiment towards an Observatory Phase, built on the experience and results from the Core Science phase, and open to observational proposals from the scientific community at large. We foresee important synergies with the above-mentioned facilities to yield the best scientific return from the proposed observations.

An example of such synergies is illustrated in Figure 5, where we show, in Galactic coordinates and Aitoff projection, the First LHAASO Catalog of $\gamma$-ray Sources (1LHHASO, [31]) plotted as orange dots and, superimposed, the ASTRI Mini-Array Core Science target regions for both Pillar-1 (blue circles) and Pillar-2 (red circles) targets. Some 1LHAASO sources already overlap the ASTRI Mini-Array selected Pillar regions, in particular along the Galactic Plane. We also note that Mrk 501 and Mrk 421 are natural candidates for common variability studies.

We can now discuss a few examples of foreseeable investigations that can be performed during the Observatory Phase. Since this phase will be open to the scientific community through competitive proposals, these examples represent ideas on how to best employ the ASTRI Mini-Array capabilities.

4.1. Cygnus Region Mini-Survey

The ASTRI Mini-Array wide FoV is well suited to perform mini-surveys of selected sky regions. One of the most important ones, as shown in Figure 5, is the Cygnus Region (${60}^{\circ }<l<{90}^{\circ }$), which contains several sources emitting above a few TeV, as reported in the First LHAASO Catalog. Figure 6 shows the results of a possible ASTRI Mini-Array mini-survey of this region with different exposure times, 50 h, 100 h, and 200 h from top to bottom, respectively.

The simulations, discussed in [29], combined fifty different pointings, at the same Galactic latitude and spaced by 0.4° in Galactic longitude, from (l, b) = (64, 0) to (l, b) = (84, 0), and lasting 1 h, 2 h, and 4 h hours each, respectively. The very high-energy simulated sources were drawn from the Third HAWC Catalog of very high-energy $\gamma$-ray sources (3HWC [67]). Thirteen of them fall inside the area considered and were simulated according to their published spectral parameters. Ten of these very high-energy sources are always significantly detected by the ASTRI Mini-Array, even at the shortest (50 h) exposure time. Recently, [68] reported the detection of an extend (about $6^{\circ }$ in diameter) $\gamma$-ray emission centered on Cygnus-X (l,b)≈(80${}^{\circ }$, $0^{\circ }$) with 66 photon-like events with energies greater than 400 TeV (see Figure 1 in [68]). The ASTRI Mini-Array wide FoV (∼${10}^{\circ }$ in diameter) and the stable off-axis performance (see Section 2) will allow us to investigate this region with a single pointing and prolonged exposure, performing a more accurate morphological measurement on the core region of the bubble discovered by LHAASO.

4.2. Gamma-Ray Binaries—LS 5039

A recent study [69] discusses the properties of $\gamma$-ray binaries. We currently know ten non-transient $\gamma$-ray binaries: seven have a compact source (six are located in our Galaxy and one, LMC P3, in the Large Magellanic Cloud), while three are colliding-wind binaries. We discuss the results of ASTRI Mini-Array simulations of LS 5039 to show the ASTRI Mini-Array capabilities of reproducing both the folded light-curve and the spectrum in different orbital phases for this source. We simulated 300 h of total exposure, 250 h in the low state, and 50 h in the high state (see [70] for a detailed description of the different flux levels). The orbit-averaged spectrum6, described in [71], is a cut-off power-law with $\Gamma =2.06\pm 0.05$ and $E_{\mathrm{cut}}=13.0\pm 4.1$ TeV. We simulated a fixed exposure time of 10 h for each phase bin. Figure 7 shows the simulation results, the flux (left panel), and the 1 $\sigma$ uncertainty ($\delta \Gamma$) on the spectral index (right panel) as a function of the orbital phase. The simulated source flux is fully consistent with the flux expected from the model. Moreover, for 90% of the orbital phase, the uncertainty on the photon index, $\delta \Gamma$, is between 0.1 and 0.25, while only in the case of the lowest-flux bin (phase range 0.1–0.2) its value rises up to about 0.4.

4.3. Spectral Features—Mrk 501

Mrk 501 ($z=0.032983\pm 0.00005$) is the second extra-galactic source detected at VHE [72]. It is classified as a high-synchrotron-peaked BL Lac object, which means that the synchrotron peak of the usual double-humped blazar spectral energy distribution reaches the ultra-violet energy band or even higher frequencies (${\nu }_{\mathrm{peak}}\gtrsim {10}^{15}$ Hz). This source is extremely variable, at almost all frequencies. Recently MAGIC detected a peculiar spectral feature during the highest X-ray ($E>0.3$ KeV) flux ever recorded from this source [73].

The spectral feature, emerging during the highest X-ray flux state at about 3 TeV with a significance of ≈$4\sigma$, can be modeled both as a curved narrow-band log-parabola or a Gaussian function superimposed to a broad-band simple log-parabola, respectively. The physical interpretation is still debated and three possible scenarios can be invoked: a two-zone emitting region model, a pile-up in the electron energy distribution, or a pair cascade from electrons accelerated in a black hole magnetospheric vacuum gap. Figure 8 shows the ASTRI Mini-Array simulations performed to investigate its capabilities in terms of energy resolution to detect such spectral features. Simulations (see [30] for a detailed discussion) were performed to investigate the percentage of number of detections of the spectral feature with respect to a broad-band log-parabola above $5\sigma$ confidence level for 200 realizations. In order to have at least a ≈50% probability of detection of the feature, 1.5 h of observation time would be required, increasing up to ≈80% probability for 2 h of exposure.

4.4. Disentangling Spectral Models in Misaligned Jetted Sources—NGC 1068

NGC 1068 ($z=0.00379\pm 0.00001$) is a powerful $\gamma$-ray Seyfert-2 galaxy detected by Fermi-LAT. It also hosts starburst activity in its central region and AGN-driven winds. The origin of the $\gamma$-ray emission is still debated because of the presence of different particle acceleration sites, such as the starburst ring, the circum-nuclear disk, and the jet [58,74]. Moreover, it has recently been associated with a possible source of neutrino emission [57]. While the canonical jet model does not extend above 10 TeV (see [74]), the AGN wind model predicts a hard spectrum that extends in the very high energy band. Figure 9 shows the results of a simulated deep observation (200 h) to test if we can detect VHE emission expected by the AGN wind model. The ASTRI Mini-Array is able to measure the source spectrum in the energy bins ∼2–5 TeV and ∼5–13 TeV at about 5 $\sigma$ level.

5. Conclusions

The ASTRI Mini-Array will commence scientific observations at the end of 2025 from the Observatorio del Teide, collecting data that will create a natural connection between current and future VHE facilities and other multi-wavelength observatories by providing light-curves, spectra, and high-resolution images of point-like and extended sources. Its 10° field of view will allow us to investigate both extended sources (e.g., supernova remnants) and crowded/rich fields (e.g., the Galactic Center) with a single pointing, while its 3${}^{\prime }$ angular resolution at 10 TeV will allow us to perform detailed morphological studies of extended sources. Moreover, its sensitivity, extending above 100 TeV with a moderate degradation (about a factor of 2) up to the edge of the FoV, will make it the most sensitive IACT in the 5–200 TeV energy range in the Northern Hemisphere before the advent of CTAO−N. The ASTRI Mini-Array will join the energy domain typical of EASs with the precision domain (excellent angular and energy resolutions) typical of IACTs, allowing several synergies with LHAASO, HAWC, and Tibet AS-$\gamma$, investigating PeV-only sources, obtaining broad-band spectra, and detailed source morphology. For the first four years, the ASTRI Mini-Array will be run as an experiment with dedicated pointings in order to address specific Core Science Topics. Afterward, we expect a smooth transition toward an Observatory Phase open to observational proposals from the scientific community.