MAGIC

Javier Rico

The MAGIC telescopes explore the most violent phenomena of the Universe and search for dark matter through the detection of gamma rays in the 50 GeV – 50 TeV energy range, with good spectral and spatial resolutions. MAGIC is currently in a period of steady astronomical observations and intense scientific exploitation.

Introduction

IFAE is one of the leading institutes of the MAGIC Collaboration. We are the second group by size and one of the most active ones. We have high-level roles in the management of the Collaboration, notably O. Blanch is the Deputy Spokesperson since 2014 and M. Martínez the Chair of the Time Allocation Committee since 2017. We are responsible for the maintenance of the receiver boards and the DAQ cooling system, and we built and operate the MAGIC Data Center. During 2017, we have continued our R&D activities for future instrumentation in Gamma-ray Astronomy, mainly regarding a new concept for photo-detectors dubbed the light-trap, and a new low-energy trigger system named topo-trigger.

During 2017, the main focus of MAGIC has been the scientific exploitation of the instrument. IFAE has led 6 of the 13 MAGIC papers published during this period

During 2017, the main focus of MAGIC has been the scientific exploitation of the instrument. IFAE members were principal authors in 6 of the 13 MAGIC papers published during this period: D. Guberman led the observations and analysis resulting in the measurement of the spectral break in the young supernova remnant Cassiopeia A at an energy of ~3.5 TeV (see more details below). This was possible partially thanks to the fact that observations under the Moon illumination were possible using the UV-pass filters developed at IFAE (see 2016 report), and whose characterization was also published in a technical paper led by Guberman. A. Fernández-Barral led 3 papers published during 2017, reporting the results of observations with MAGIC of the micro-quasars Cygnus X-1 and V404 Cygni, and the type Ia Supernova SN 2014J, respectively. Finally, J. E. Ward led the analysis of the MAGIC multi-year observational campaign of the Galactic center, aiming at studying the passage of a gas cloud in close proximity to the central super-massive black hole.

Figure 1: The MAGIC telescopes at the Observatorio del Roque de los Muchachos, in the Canary island of La Palma, observing the gamma-ray sky since 2004 (single-telescope mode) and 2009 (stereoscopic system)

Cassiopeia A: a prime candidate to explain the cosmic-ray sea that runs short of energy

Thanks to MAGIC observations, now we know that Cassiopeia A is accelerating cosmic rays, although to a rather low energy of a few tera-electronvolt.

Cosmic rays are sub-atomics particles that reach Earth from all directions, up to energies higher than anything achievable in laboratories on Earth. Cosmic rays of Galactic origin in particular, reach at least a few Peta-electronvolts ($PeV = 10^{15} eV$). It is widely believed that the bulk of the Galactic cosmic rays are accelerated in supernova remnants (SNRs). However, no observational evidence of the presence of particles of PeV energies in SNRs has yet been found. The young historical SNR Cassiopeia A appears as one of the best candidates to study acceleration processes. It has historically been considered a prime candidate to explain the origin of Galactic cosmic rays up to PeV energies.

Figure 2: Gamma-ray energy spectrum from the direction of Cassiopeia A, as measured by the MAGIC telescopes (black dots) and Fermi-LAT (blue squares). The red solid line shows the result of fitting the MAGIC spectrum with a power-law with exponential cutoff at an energy of 3.5 TeV. These findings provide a crucial insight into the acceleration processes in one of the most prominent non-thermal objects in our Galaxy.

Cassiopeia A is a famous SNR, the product of a gigantic explosion of a massive star that happened about 350 years ago. Discovered by radio observations 50 years ago, now we know that its emitted radiation spans from radio to high-energy gamma rays. It is also one of the few SNRs for which the birth date and the type of supernova (type IIb, the result of a core collapse supernova explosion) are known. The precise knowledge of its nature makes Cassiopeia A one of the most interesting and investigated objects in the sky, and in particular the study of its connection with the cosmic rays.

The very-high-energy part of the spectrum of Cassiopeia A results from the cosmic rays (either electrons or protons) interacting within the remnant. Until now, this range of energy could not be measured with sufficient precision to pinpoint its origin. Sensitive observations above 1 TeV were required but achieving them was daunting. The MAGIC Collaboration has finally succeeded in performing those observations. More than 160 hours of data were recorded between December 2014 and October 2016, revealing that Cassiopeia A is an accelerator of massive particles, mostly hydrogen nuclei (protons). However, even though those particles are 100 times more energetics than the ones we can reach in artificial accelerators such the one in CERN, their energy is not high enough to fully explain spectrum of the cosmic ray sea that fills our Galaxy.

Cassiopeia A was considered the perfect object to be a PeVatron, that is, an accelerator of cosmic rays up to PeV energies: it is young, bright, with a shock expanding a great velocity and with very large magnetic fields that can accelerate cosmic rays up to at least, conservatively, 100 or 200 TeV. But contrary to what we expected, in Cassiopeia A the particle energies do not reach more than a few tens of TeV. At these energies, the radiation suddenly drops and the emission stops abruptly (see Figure 2): either the remnant cannot accelerate the particles to higher energies, which challenges our knowledge of shock acceleration, or maybe, the fastest ones escaped quickly the shock, leaving only the slowest ones for us to observe.

Thanks to MAGIC observations, now we know that Cassiopeia A is accelerating cosmic rays, although to a rather low energy of a few TeV. Some of the observed gamma rays could be caused by accelerated electrons, rather than cosmic rays, but that would not affect the conclusion that acceleration in Cassiopeia A falls short of the maximum energies of the Galactic cosmic ray spectrum.

A detailed study of the observed spectral cut-off shape is crucial to understand the reason behind this low acceleration efficiency, because it displays different characteristics if due to escape of cosmic rays, to the maximum acceleration energy, or some other mechanism. Observations with the future Cherenkov Telescope Array, with a superior angular resolution and sensitivity, will allow a detailed spectroscopic investigation on the cut-off regime, providing new insights on the acceleration processes in Cassiopeia A and SNRs in general.