A new camera on the large telescope of H.E.S.S.

October 2020

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Together with the continuation of H.E.S.S. operations into an extension phase starting October 2019, the H.E.S.S. collaboration has upgraded its 600 square metre Cherenkov telescope CT5 with a new high-performance camera with fully-digital trigger and readout system, and high quantum efficiency photon detectors. The main goals of the upgrade are a reduction in the energy threshold of the telescope, improved sensitivity, and better stability of operation. The new camera is based on the FlashCam design, which has been developed for the use in the Cherenkov Telescope Array (CTA). After extensive tests of the complete camera with its hardware, firmware and software, it was partially disassembled and shipped to Namibia, where it arrived one year ago. After reassembly (see Fig. 1) and adaption of the mechanical telescope interfaces, the camera was installed in October 2019 (Fig. 2) and the H.E.S.S. collaboration celebrates the first anniversary this month.

Fig. 1: Installation of the photodetector modules into the camera body during the upgrade campaign in October 2019 in Namibia.
Fig. 2: CT5 observing the night sky with the new camera in the prime focus of the large, 28m diameter telescope. Image Credit: Christian Föhr (MPIK)

The new camera and the CT5 telescope were ready to make astrophysical observations just two days after mechanical installation on October 20, 2019. In its first night of operation, the telescope was pointed at the Crab Nebula, among other targets. A clear detection at the position of the Crab Nebula was obtained with operations and preliminary analysis straight out-of-the-box. Subsequent to technical installation, periods of technical commissioning and science verification happened while the four 12 m Davies-Cotton-telescopes ("CT1"–"CT4") with the HESS 1U cameras continued standard operations. Science verification observations were interleaved with routine observations following the H.E.S.S. schedule and 'Target-of-Opportunity (ToO)' observations including CT5 with the newly installed camera. Following the completion of science verification, the upgraded CT5 participates in routine operations of the H.E.S.S. array, including, e.g. the monitoring of Eta Carina during its periastron in early 2020.

One example of the data obtained with the new camera in given in Figure 3. In April, flaring activity of the active galaxy PKS 0903-57 (a BL Lacertae object with a reported redshift of z = 0.695) in MeV-GeV gamma-rays triggered H.E.S.S. observations. An extremely strong signal was detected using CT5 alone ('mono mode'), as shown in the statistical significance map in Fig. 3. This signal also provides a good opportunity to test the consistence of our simulated model of the telescope and camera, and real life. Also shown in Fig. 3. hence also shows a comparison of the point-spread-function obtained with the PKS 0903-57 data collected over six nights in April 2020 with the simulations. The H.E.S.S. detection of the source was reported in Astronomer's Telegram #13632

Fig. 3: Observation of PKS 0903-57 with HESS CT5 (in mono mode) over six nights in April 2020, 13 hours of data were collected. The upper plot shows the statistical significance for a point-like source calculated over the field of view, and the lower plot the radial profile of emission, compared to the expectations for a point-like source of gamma-rays.

The new camera allows H.E.S.S. to trigger on significantly fainter flashes of Cherenkov light, enabling the collection of data at about twice the rate compared to previous studies and reach lower gamma-ray energies. Pointing straight up the rate of collection of air-shower events is about 3000 per second. Figure 4 shows two videos/movies of air-showers captured with the new camera. The movies are 100 nanoseconds long and show large showers initiated by charged cosmic rays, a ring formed by a shower muon passing through the telescope is seen at the beginning of the second movie.

Fig. 4: Videos of two gamma-ray events recorded with the new camera.

The ability to record fainter air showers is is in part due to the higher efficiency of the photosensors used in the new camera. With the current conservative trigger settings the effective collection area for triggering gamma-rays has increased by a factor of 1.4 in the energy range 20-30 GeV with respect to the original system. This is illustrated in Figure 5, which compares the efficiencies as "effective areas". The effective area is the nominal size of the collection area of the specific combination of telescope and instrument. The effective area is larger for photons of higher energies, as the air showers generated by high energy gamma rays are brighter and can hence be detected at larger distance from the telescope.

Fig. 5: The sensitivity of CT5 with its previous and new cameras shown as a function of gamma-ray photon energy (illustrated as "effective area"). The lower panel illustrates the gain in sensitivity which is as high as 40% for 30 GeV gamma rays.

Integration and technical verification was completed in December 2019. Since then the new camera has been in routine operation with a fixed configuration, available for science observations 98.7% of the time when observations are possible. So far only 1.3% of observation time lost due to technical issues associated to the camera, and this high efficiency might even improve as further experience is gained. By now (the end of September 2020) over 1000 hours of Cherenkov observations have been obtained with the upgraded system. We are very excited about the next years of operation of the world's largest Cherenkov Telescope and the list of scientific targets is very long! Beyond the gain for H.E.S.S. operations and the H.E.S.S. science plans, the smooth installation and operation of the new camera is a very positive sign for CTA: With the high fraction of flawless operations of its prototype in H.E.S.S., FlashCam clearly meets the requirement for CTA operations.