A&A 643, L14 (2020) Astronomy https://doi.org/10.1051/0004-6361/202039131 & c ESO 2020 Astrophysics LETTER TO THE EDITOR Detection of the Geminga pulsar with MAGIC hints at a power-law tail emission beyond 15 GeV MAGIC Collaboration: V. A. Acciari1,2, S. Ansoldi3, L. A. Antonelli4, A. Arbet Engels5, K. Asano6, D. Baack7, A. Babic´8, A. Baquero9, U. Barres de Almeida10, J. A. Barrio9, J. Becerra González1,2, W. Bednarek11, L. Bellizzi12, E. Bernardini13, M. Bernardos14, A. Berti15, J. Besenrieder16, W. Bhattacharyya13, C. Bigongiari4, A. Biland5, O. Blanch17, G. Bonnoli12, Ž. Bošnjak8, G. Busetto18, R. Carosi17, G. Ceribella16;?, M. Cerruti20, Y. Chai16, A. Chilingarian21, S. Cikota8, S. M. Colak17, E. Colombo1,2, J. L. Contreras9, J. Cortina14, S. Covino4, G. D’Amico16, V. D’Elia4, P. Da Vela19,36, F. Dazzi4, A. De Angelis18, B. De Lotto3, M. Delfino17,37, J. Delgado17,37, C. Delgado Mendez14, D. Depaoli15, T. Di Girolamo15, F. Di Pierro15, L. Di Venere15, E. Do Souto Espiñeira17, D. Dominis Prester22, A. Donini3, D. Dorner23, M. Doro18, D. Elsaesser7, V. Fallah Ramazani24, A. Fattorini7, G. Ferrara4, L. Foffano18, M. V. Fonseca9, L. Font25, C. Fruck16, S. Fukami6, R. J. García López1,2, M. Garczarczyk13, S. Gasparyan26, M. Gaug25, N. Giglietto15, F. Giordano15, P. Gliwny11, N. Godinovic´27, J. G. Green4, D. Green16, D. Hadasch6, A. Hahn16, L. Heckmann16, J. Herrera1,2, J. Hoang9, D. Hrupec28, M. Hütten16, T. Inada6, S. Inoue29, K. Ishio16, Y. Iwamura6, J. Jormanainen24, L. Jouvin17, Y. Kajiwara30, M. Karjalainen1,2, D. Kerszberg17, Y. Kobayashi6, H. Kubo30, J. Kushida31, A. Lamastra4, D. Lelas27, F. Leone4, E. Lindfors24, S. Lombardi4, F. Longo3,38, R. López-Coto18, M. López-Moya9;?, A. López-Oramas1,2, S. Loporchio15, B. Machado de Oliveira Fraga10, C. Maggio25, P. Majumdar32, M. Makariev33, M. Mallamaci18, G. Maneva33, M. Manganaro22, K. Mannheim23, L. Maraschi4, M. Mariotti18, M. Martínez17, D. Mazin6,16, S. Mender7, S. Micanovi´ c´22, D. Miceli3, T. Miener9, M. Minev33, J. M. Miranda12, R. Mirzoyan16, E. Molina20, A. Moralejo17, D. Morcuende9, V. Moreno25, E. Moretti17, P. Munar-Adrover25, V. Neustroev34, C. Nigro17, K. Nilsson24, D. Ninci17, K. Nishijima31, K. Noda6, S. Nozaki30, Y. Ohtani6, T. Oka30, J. Otero-Santos1,2, M. Palatiello3, D. Paneque16, R. Paoletti12, J. M. Paredes20, L. Pavletic´22, P. Peñil9, C. Perennes18, M. Persic3,39, P. G. Prada Moroni19, E. Prandini18, C. Priyadarshi17, I. Puljak27, W. Rhode7, M. Ribó20, J. Rico17, C. Righi4, A. Rugliancich19, L. Saha9, N. Sahakyan26, T. Saito6, S. Sakurai6, K. Satalecka13, F. G. Saturni4, B. Schleicher23, K. Schmidt7, T. Schweizer16;?, J. Sitarek11, I. Šnidaric´35, D. Sobczynska11, A. Spolon18, A. Stamerra4, D. Strom16, M. Strzys6, Y. Suda16, T. Suric´35, M. Takahashi6, F. Tavecchio4, P. Temnikov33, T. Terzic´22, M. Teshima16,6, N. Torres-Albà20, L. Tosti15, S. Truzzi12, A. Tutone4, J. van Scherpenberg16, G. Vanzo1,2, M. Vazquez Acosta1,2, S. Ventura12, V. Verguilov33, C. F. Vigorito15, V. Vitale15, I. Vovk6, M. Will16, D. Zaric´27, K. Hirotani40;?, and P. M. Saz Parkinson41,42 (Affiliations can be found after the references) Received 9 August 2020 / Accepted 18 September 2020 ABSTRACT We report the detection of pulsed gamma-ray emission from the Geminga pulsar (PSR J0633+1746) between 15 GeV and 75 GeV. This is the first time a middle-aged pulsar has been detected up to these energies. Observations were carried out with the MAGIC telescopes between 2017 and 2019 using the low-energy threshold Sum-Trigger-II system. After quality selection cuts, ∼80 h of observational data were used for this analysis. To compare with the emission at lower energies below the sensitivity range of MAGIC, 11 years of Fermi-LATdata above 100 MeV were also analysed. From the two pulses per rotation seen by Fermi-LAT, only the second one, P2, is detected in the MAGIC energy range, with a significance of 6:3σ. The spectrum measured by MAGIC is well-represented by a simple power law of spectral index Γ = 5:62 ± 0:54, which smoothly extends the Fermi-LATspectrum. A joint fit to MAGIC and Fermi-LATdata rules out the existence of a sub-exponential cut-off in the combined energy range at the 3:6σ significance level. The power-law tail emission detected by MAGIC is interpreted as the transition from curvature radiation to Inverse Compton Scattering of particles accelerated in the northern outer gap. Key words. gamma rays: stars – pulsars: general – pulsars: individual: PSR J0633+1746 – pulsars: individual: Geminga ? Corresponding authors; e-mail: [email protected] Article published by EDP Sciences L14, page 1 of6 A&A 643, L14 (2020) 1. Introduction Rubidium oscillator, which provides an absolute time stamp pre- cision of 200 ns. Geminga (PSR J0633+1746) is an archetype of the radio- The detection of the Geminga pulsar with MAGIC was pos- quiet gamma-ray pulsar population (Bignami & Caraveo 1996; sible thanks to the implementation of the Sum-Trigger-II sys- Caraveo 2014). First detected by SAS-2 and COS-B (Fichtel tem. The standard MAGIC trigger requires that the signals of et al. 1975; Hermsen et al. 1977; Bignami & Caraveo 1992) three neighbouring camera pixels exceed a preset threshold of as a bright gamma-ray source with no counterpart at any other ∼4:0 photo electrons (phe.). In the Sum-Trigger-II, the pixels are wavelength and subsequently associated with an X-ray source grouped into hexagonal-shape cells of 19 pixels each. The ana- (Bignami et al. 1983), it was ultimately identified as a pulsar logue sum of all pixel signals within any given cell is compared by ROSAT and EGRET (Halpern & Holt 1992; Bertsch et al. against a discriminator threshold of ∼18 phe. Integrating the sig- ' 1992). It has a period of P 237 ms and a characteristic nal from a large area leads to a better signal-to-noise ratio (S/N) ∼ age of 300 ky. Two independent measurements of the distance for very low energy showers. To counteract the effect of noise reported 157+59 pc (Caraveo et al. 1996) and 250+120 pc (Faherty −34 −62 after-pulses, which are typical for photo-multiplier tubes, the et al. 2007), respectively. This makes Geminga one of the closest individual pixel amplitudes are clipped when exceeding 8:5 phe. known pulsars. The trigger geometry, thresholds, and clipping values were opti- The Fermi-LATdetector measured the pulsed gamma-ray mised by Monte Carlo (MC) simulations with the aim of min- spectrum of Geminga using one year of data and found that imizing the trigger threshold (Dazzi et al., in prep.). Compared it can be described by a power law with an exponential cut- to the standard trigger, the trigger energy threshold of the Sum- ± off at 2:46 0:04 GeV (Abdo et al. 2010). The increase of Trigger-II is about 50% lower. For a spectral index of −5, which Fermi-LATstatistics in the following years favoured a softer is similar to that of Geminga as reported here, the peak of the sub-exponential cut-off (Abdo et al. 2013; Ahnen et al. 2016). gamma energy distribution (threshold) is approximately 15 GeV. Subsequent ground-based observations by the Imaging Atmo- The MAGIC data were processed with the Magic Standard spheric Cherenkov Telescopes (IACT) VERITAS (Aliu et al. Analysis Software (MARS, Zanin et al. 2013). To improve the 2015) and MAGIC (Ahnen et al. 2016) could not detect any sig- analysis performance close to the Sum-Trigger-II energy thresh- nificant emission above 100 GeV and 50 GeV, respectively. A old, we developed a new algorithm in which the calibration and ∼ 2 deg steady halo around Geminga was first detected by the the image cleaning are performed in an iterative procedure. The MILAGRO experiment (Abdo et al. 2009), and later reported by rest of the higher level analysis followed the standard pipeline the HAWC (Abeysekara et al. 2017) and Fermi-LAT(Manconi described in Aleksic´ et al.(2016b). This comprises the recon- et al. 2019) collaborations at energies above 5 TeV and 8 GeV, struction of the energy and direction of the incoming gamma respectively. rays and the suppression of the hadronic background. Boosted In this paper, we report the detection of pulsed gamma-ray decision trees and look-up tables were built for these purposes, emission from the Geminga pulsar by the MAGIC telescopes. using gamma-ray simulated shower events following the trajec- This makes Geminga the first middle-aged pulsar detected by tory of Geminga in the sky and background events from dedi- IACTs and the third pulsar detected by these type of telescopes cated observations. after the Crab (Aliu et al. 2008) and Vela (Abdalla et al. 2018). For the timing analysis, the pulsar rotational phases of the This detection had become possible thanks to the use of the new events were computed using the Tempo2 package (Hobbs et al. low-energy trigger system, dubbed Sum-Trigger-II (Dazzi et al., 2006). An ephemeris for Geminga covering the MAGIC obser- in prep.; García et al. 2014), designed to improve the perfor- vations was obtained from the analysis of Fermi-LATdata (Kerr mance of the telescopes in the sub-100 GeV energy range. et al. 2015). In Sect.2 we present the MAGIC observations and the tech- nical innovations that were imperative for this detection. The analysis of Fermi-LATdata is described in Sect.3. The resulting 3. Fermi-LAT data and analysis MAGIC and Fermi-LATlight curves and spectra are presented in Sect.4 and discussed in Sect.5. Finally, we compare our To characterise the Geminga emission at energies lower than observations with the predictions of the pulsar outer gap model those accessible to MAGIC, we analysed 10:6 years (from MJD (Cheng et al.