The beginnings of H.E.S.S. - H.E.S.S. Collaboration

May 2022

Any large project grows for some time from first ideas to groundbreaking to completion and it is often difficult to define the ‘true’ start. Nonetheless history often considers the first preserved documents as an – arbitrary – definition, and going by this tradition, H.E.S.S. is celebrating the first recorded plans 25 years ago – in spring 1997. Ever since Messier 1 (the Crab Nebula) was firmly established as the first source of Very High Energy (VHE) gamma-ray emission (i.e. as an object that can be detected in observations of gamma-rays with energies above 100 GeV) [1] and subsequent pioneering experiments devised new techniques (see, e.g. [2]), several groups around the world developed plans towards a major facility that would be able to significantly improve the sensitivity and study a wide variety of source classes. A remarkable series of conferences on this topic was launched in 1992, when the first meeting ‘Towards a major Atmospheric Cherenkov Detector for TeV astroparticle physics’ was organized in Palaiseau, France [3]]. Subsequent meetings discussed and described further advancements towards the aim and different parts of the community formed groups that ultimately came to construct CANGAROO-III, MAGIC, VERITAS – and the High Energy Stereoscopic System (H.E.S.S.). In Spring 1997 a ‘Letter of Intent’ (LoI) [4] was published that summarized the scientific motivation and various options for the technical challenges that were anticipated. In the same year the fifth incarnation of the ‘Towards a major Atmospheric Cherenkov Detector’ series was held in Kruger Park, South Africa [5], paving the way towards the future H.E.S.S. consortium and the desire to locate this facility in Southern Africa.

**Fig. 1:** An initial installation of 4 telescopes was meant to be extended in a final phase to an array of sixteen telescopes, not very different from what is now foreseen as the MST subset of the CTA South array.

Despite a lot of experience gained by the groups that joined forces in the H.E.S.S. collaboration using their facilities HEGRA, CAT and Mark6, the new facility was planned to be quite an extrapolation into new terrain, and it is an interesting exercise to reflect upon technical choices and anticipated science with the benefit of hindsight. H.E.S.S. was conceived as an array of 16 telescopes, to be built in two phases (see Fig. 1). Interestingly, the LoI conceived the telescopes to be movable on rails (as are the telescopes of the VLA, built in the 1970s), and placed in shelters. The latter was considered in case the array would be erected in a location with severe weather conditions – such as the Calar Alto observatory in Spain. While the Gamsberg was mentioned as a possible site, the LoI listed several options and spelled out the necessity of proper testing and budgeting.

The size of the telescopes was not fixed precisely in the LoI. Fortunately the collection area of 82 square meters assumed as an indicative target was increased later to the 106 square meters of the telescopes that were constructed. The mount and dish structure were discussed in coarse terms.

The requirements for telescopes optics and performance were discussed in detail, while the technical design choice was not specified (Fig. 2).

**Fig. 2:** One design option for an array telescope depicted in the H.E.S.S. Letter of Intent.

The LoI clearly argued the case to improve the performance of the arrays that operated at the time by at least an order of magnitude, and set the design requirements accordingly (see figure 3).

**Fig. 3:** The intended sensitivity of the 16 telescope H.E.S.S. array allowed for 1% Crab fluxes in 100h of observations (a value that is easily reached by the four telescope array that is in operation since 2004). At the time the LoI was written, the facility *Fermi*-LAT was still known as GLAST.

The physics goals of the projected were described in an appendix, which is the most extensive part of the H.E.S.S. letter of Intent. It is obvious that a significant part of the very rich science program that was ultimately delivered by H.E.S.S. had been anticipated in the science motivation. Interestingly, the only science goal that is used to illustrate a design motivation in the main text of the LoI refers to the necessity of an (at the time low) threshold of 100 GeV in order to conduct cosmological observations (see Fig. 4).

**Fig. 4:** The only diagram that illustrates the science objectives of the H.E.S.S. array in the LoI appealed to the possibility of conducting cosmological observations out to a redshift of z~1 by allowing for a threshold of 100 GeV, low enough to avoid severe opacity by the diffuse infrared and optical background light. While a more precise determination of the level of diffuse background light was one of the highlights of the early H.E.S.S. science publications [6], reaching a redshift of z~1 has only been possible in recent months [7]).

While the H.E.S.S. LoI was clearly meant to spell out the motivation and feasibility of a’ High Energy Stereoscopic System’ rather than a detailed design, it is interesting to recollect that the initial motivation anticipated a great deal of the array that has now been operated successfully for nearly 20 years. In hindsight the most obvious aspect that was missing in the initial considerations was a layout of data management, which became an essential element of research infrastructure plans in the past two decades.

References

[1] Observation of TeV Gamma Rays from the Crab Nebula Using the Atmospheric Cerenkov Imaging Technique, T. Weekes et al., ApJ 342, p.379, 1989

[2] Cherenkov reflections, D. Fegan, 2019, World Scientific

[3] Towards a major atmospheric Cherenkov detector for TeV astroparticle physics. Proceedings, 1st Palaiseau Workshop, Palaiseau, France, June 11-12, 1992, P. Fleury, G. Vacanti (eds.), 1992

[4] H.E.S.S. (High Energy Stereoscopic System), Letter of Intent, F. Aharonian et al., 1997

[5] Towards a Major Atmospheric Cherenkov Detector V, Kruger Park, South Africa, 8.-11. Aug. 1997, O de Jager (Ed.), 1997

[6] A low level of extragalactic background light as revealed by gamma-rays from blazars, F. Aharonian et al. (H.E.S.S. collaboration), Nature, 440, 1018, 2006

[7] Enhanced HE and VHE gamma-ray activity from the FSRQ PKS 0346-27, S. Wagner (H.E.S.S. collaboration) & B. Rani (Fermi-LAT collaboration), ATEL 15092