MAGIC — Major Atmospheric Gamma Imaging Cherenkov Telescopes

MAGIC Performance

All aspects of the MAGIC performance (including tabulated energy-dependent performance numbers) are described in detail in Aleksić et al., Astroparticle Physics, Volume 72, 1 January 2016, Pages 76-94. The performance shown here corresponds to data taken in 2013.

Integral sensitivity

We define the sensitivity as the integrated flux of a source above a given energy for which the number of excess events (\(N_{\mathrm{excess}}\)) divided by the square root of the number of background events (\(N_{\mathrm{bkg}}\)) is equal to 5 after 50 hours of effective observation time. In addition, we require that \(N_{\mathrm{excess}} > 10\) and \(N_{\mathrm{excess}} > 0.05\,N_{\mathrm{bkg}}\).

The corresponding energy threshold is computed as the peak of the true-energy distribution of a Monte Carlo sample with a Crab-like spectrum. Integral sensitivity computed according to different definitions and tabulated numbers are available in Aleksić et al. (2016).

All the curves are computed using samples of Crab Nebula data. For example, the sensitivity computed according to the Li & Ma significance with five background regions is (0.67 ± 0.04)% C.U. above 290 GeV and (1.45 ± 0.02)% C.U. above 100 GeV for 50 hours of observations.

Integral sensitivity of the MAGIC telescopes as a function of energy threshold.
Evolution of the integral sensitivity of the MAGIC telescopes. Gray circles: sensitivity of the MAGIC-I single telescope with the Siegen (light gray, long dashed) and MUX readouts (dark gray, short dashed). Black triangles: stereo before the upgrade. Squares: stereo after the upgrade, for zenith angles below 30° (red, filled) and 30-45° (blue, empty). Figure reproduced from Aleksić et al. (2016). The added pink point shows the integral sensitivity above 55 GeV obtained with the PKS 1441+25 detection sample.

Differential sensitivity

Differential sensitivity of MAGIC: the flux of a source that gives a \(5\sigma\) signal after 50 hours of observations, calculated according to the Li & Ma formula assuming three background regions, in the estimated-energy bin \(E_1\)-\(E_2\). Low-zenith-angle observations (below 30°). Conversion to absolute flux units is done using the Crab Nebula spectrum from Aleksić et al. (2016, APh, 72, 76). Points above 63 GeV are obtained with Crab Nebula data (Aleksić et al. 2016, APh, 72, 76); the 40-63 GeV point is obtained with PKS 1441+25 data (Ahnen et al. 2015, ApJ, 815, 23). You can download the data in tabular format from this file.

Differential sensitivity of the MAGIC telescopes in absolute flux units.
Differential sensitivity of the MAGIC telescopes in Crab units.

Sensitivity at approximately 50 GeV

Even while 50 GeV is usually below the analysis threshold of the MAGIC telescopes (for a standard stereo trigger) for a Crab-like spectrum, it is still possible to perform scientific observations with the MAGIC telescopes at those energies. Due to unfolding corrections for the energy resolution and bias, it is also possible to reconstruct reliable spectra at energies not far below the energy threshold.

Moreover, for steep-spectrum sources (distant AGNs, GRBs, and pulsars), the peak energy shifts to lower values. For example, a strong \(25\sigma\) detection of the \(z = 0.939\) flat-spectrum radio quasar PKS 1441+25 corresponds to a sensitivity of 5% Crab flux above 55 GeV for 50 hours of observations.

Angular resolution

We define the angular resolution as the sigma parameter of a two-dimensional Gaussian function fitted to the gamma-ray excess distributions. This corresponds to approximately a 39% containment radius of gamma rays.

Angular resolution of the MAGIC telescopes as a function of energy.
Angular resolution of the MAGIC telescopes for data (points) and Monte Carlo simulations (lines). Red points: low-zenith-angle sample; blue points: medium-zenith-angle sample. For comparison, the pre-upgrade low-zenith-angle angular resolution is shown as gray points. Figure reproduced from Aleksić et al. (2016).