MAGIC Telescopes

IFAE is one of the leading institutes of the MAGIC Collaboration. We have high-level roles in the management of the Collaboration, e.g. O. Blanch is the spokesperson, and J. Rico is the Analysis and Publications coordinator. In addition, two of the four physics working groups were co-led through 2020 by IFAE members (E. Moretti and D. Kerszberg). The MAGIC group at IFAE is responsible for the maintenance of part of the data acquisition system (receiver boards and cooling system), and of significant parts of the data analysis software. We also built and operate the official MAGIC Data Center at the Port d’Informació Científica.

Besides the maintenance and operation tasks, the activities of the group in 2020 focused on the scientific exploitation of the instrument. The lockdown imposed by the COVID-19 pandemic resulted in a loss of three months of observations, but the scientific production stayed at a high level, with 15 MAGIC articles published in peer-reviewed journals. Among the newly published results, we highlight: i) the observation of the Geminga pulsar, detected for the first time with an IACT, showing evidence of a power-law spectral tail beyond 15 GeV; and ii) the search for possible violations of Lorentz invariance carried out through the analysis of our observations of the gamma-ray burst GRB190114C. IFAE post-doctoral researcher D. Kerszberg was corresponding author of the latter publication (PRL 125). It must also be highlighted that during 2020 MAGIC announced another clear detection of a gamma-ray burst, GRB201216C (ATEL #14277), which, at z $\simeq 1.1$, is probably the most distant VHE source ever detected.

The observation of GRB190114C in the afterglow phase by MAGIC represents a unique opportunity to test the possible dependence of the speed of light in vacuo with the energy of photons. The combination of a large flux of photons (reaching 100 times the flux from the brightest steady source, the Crab Nebula) of energies up to ~ 1 TeV, with the fast variability, and a cosmological distance (z=0.42), provides an ideal laboratory to put Lorentz invariance to the test. These time-of-flight measurements are arguably the most promising phenomenological tests of quantum theories of gravity. Despite being observed in challenging conditions (with the moon above the horizon, and the target at low elevation), MAGIC recorded around two thousand VHE photons from GRB190114C with energies above 200 GeV. Before this remarkable event, the highest-energy photon ever detected (by the Fermi-LAT space-borne gamma-ray telescope) from a GRB had an energy of just 95 GeV. On the downside, the VHE signal recorded with the MAGIC telescopes from $T_0$ + 62 s, decayed monotonically with time, lacking significant features that could be directly used as “timestamps” in the measurement of the potential energy-dependent delay. We used instead a maximum likelihood approach, making two different assumptions on the intrinsic shape of the afterglow time evolution. No significant deviation from the null hypothesis (i.e., same c for all photons) was observed, but we obtained competitive limits on the energy scale $E_{QG}$ of the speed of light modification (see Table 1). The constraints are the best reported so far from IACTs for the sub-luminal linear scenario, i.e. $c^{\prime} = c (1 - E/E_{QG,1})$.