Performance of the MAGIC telescopes under moonlight

Abstract

MAGIC, a system of two imaging atmospheric Cherenkov telescopes, achieves its best performance under dark conditions, i.e. in absence of moonlight or twilight. Since operating the telescopes only during dark time would severely limit the duty cycle, observations are also performed when the Moon is present in the sky. Here we develop a dedicated Moon-adapted analysis to characterize the performance of MAGIC under moonlight. We evaluate energy threshold, angular resolution and sensitivity of MAGIC under different background light levels, based on Crab Nebula observations and tuned Monte Carlo simulations. This study includes observations taken under non-standard hardware configurations, such as reducing the camera photomultiplier tubes gain by a factor â\u88¼ 1.7 (reduced HV settings) with respect to standard settings (nominal HV) or using UV-pass filters to strongly reduce the amount of moonlight reaching the cameras of the telescopes. The Crab Nebula spectrum is correctly reconstructed in all the studied illumination levels, that reach up to 30 times brighter than under dark conditions. The main effect of moonlight is an increase in the analysis energy threshold and in the systematic uncertainties on the flux normalization. The sensitivity degradation is constrained to be below 10%, within 15â\u80\u9330% and between 60 and 80% for nominal HV, reduced HV and UV-pass filter observations, respectively. No worsening of the angular resolution was found. Thanks to observations during moonlight, the maximal duty cycle of MAGIC can be increased from â\u88¼ 18%, under dark nights only, to up to â\u88¼ 40% in total with only moderate performance degradation

Optical intensity interferometry observations using the MAGIC imaging atmospheric Cherenkov telescopes

“…The sensitivity of the detector is proportional to the photon flux detected by the PMT and in first However we have added a filter in front of the central pixels to make the anode currents of the PMTs (DCs) low enough that PMTs are not damaged when pointing at bright stars or during bright Moon observations. The filter is held 15 mm in front of the Winston cone using a mechanical frame previously developed for bright Moon VHE observations (M. Ahnen et al 2017). This frame must be mounted and dismounted manually, which generally prevents standard VHE observations for the whole night.…”

A photomultiplier tube test stand and on-site measurements to characterise the performance of Photonis XP3062 photomultiplier tubes at increased background light conditions and lower gain

“…The flasher board is adapted from the one originally designed for FD calibration measurements with an airborn and remotely controlled light source [15]. A Schott MUG-6 UV bandpass filter, absorbing most parts of the NSB with wavelength >400 nm, is mounted in front of the output port of the Ulbricht sphere 5 . The distance between the light source and PMT wheel (97.5 cm) was chosen based on an optimisation of light uniformity and intensity of continuous and flashed light at the PMT wheel.…”

“…The light flux, measured at the PMT positions, can be set in the range of 10 and 2000 photons/50 ns (0.2 -40 GHz) for the continuous background light and in the range of 1.5×10 5 and 4.5×10 6 photons for a 14 µs light flash. 5 The same type of filter is used at the FD aperture.…”

“…For experiments exposed to NSB, this means extending the measurements into nights with a higher moon fraction or into twilight. Such an approach is already investigated and followed by some PMTbased experiments like MAGIC [2] and VERITAS [3] where γray observations are performed also under bright moonlight up to three times the NSB of a dark extragalactic field (as in case of VERITAS [4]) increasing the duty cycle by a factor of ∼2 (as in case of MAGIC [5]). A similar idea has been proposed for the fluorescence detector (FD) of the Pierre Auger Observatory [6] where the duty cycle could be increased by about 50%, from a current observation period of 15% to 21% (reduc-tion due to bad weather, power cuts, and malfunctions already included) [7].…”

On the possibilities of high-energy neutrino production in the jets of microquasar SS433 in light of new observational data

“…VHE gamma-ray observations have been long performed with different instruments. A joint search by HESS+MAGIC has led to very stringent constraints for gamma-rays in the energy range E γ ∼ (200 − 5000) GeV emitted from SS433 [12], while VER-ITAS has also produced updated upper limits [13]. At higher energies (E γ ∼ 25 TeV), VHE photons were recenlty detected by HAWC, and the emission was found to be produced at the terminal regions of jets, at distances ∼ 10 19−20 cm from the central BH [14].…”