H.E.S.S. observations of the 2021 periastron passage of PSR B1259-63/LS 2883

¹ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
² Max-Planck-Institut für Kernphysik, PO Box 103980 69029 Heidelberg, Germany
³ Yerevan State University, 1 Alek Manukyan St, Yerevan 0025, Armenia
⁴ Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
⁵ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
⁶ Laboratoire Leprince-Ringuet, École Polytechnique, CNRS, Institut Polytechnique de Paris, 91128 Palaiseau, France
⁷ University of Namibia, Department of Physics, Private Bag 13301, Windhoek 10005, Namibia
⁸ Centre for Space Research, North-West University, Potchefstroom 2520, South Africa
⁹ Deutsches Elektronen-Synchrotron DESY, Platanenallee 6, 15738 Zeuthen, Germany
¹⁰ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, 14476 Potsdam, Germany
¹¹ Université de Paris, CNRS, Astroparticule et Cosmologie, 75013 Paris, France
¹² Department of Physics and Electrical Engineering, Linnaeus University, 351 95 Växjö, Sweden
¹³ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
¹⁴ Laboratoire Univers et Théories, Observatoire de Paris, Université PSL, CNRS, Université Paris Cité, 5 Pl. Jules Janssen, 92190 Meudon, France
¹⁵ Sorbonne Université, CNRS/IN2P3, Laboratoire de Physique Nucléaire et de Hautes Energies, LPNHE, 4 Place Jussieu, 75005 Paris, France
¹⁶ IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
¹⁷ University of Oxford, Department of Physics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹⁸ Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics, Nikolaus-Fiebiger-Str. 2, 91058 Erlangen, Germany
¹⁹ Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
²⁰ Université Savoie Mont Blanc, CNRS, Laboratoire d’Annecy de Physique des Particules – IN2P3, 74000 Annecy, France
²¹ Instytut Fizyki Jdrowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland
²² School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg 2050, South Africa
²³ Laboratoire Univers et Particules de Montpellier, Université Montpellier, CNRS/IN2P3, CC 72, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
²⁴ School of Physical Sciences, University of Adelaide, Adelaide 5005, Australia
²⁵ Aix Marseille Université, CNRS/IN2P3, CPPM, Marseille, France
²⁶ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
²⁷ Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstraße 25, 6020 Innsbruck, Austria
²⁸ Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland
²⁹ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland
³⁰ Department of Physics, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan
³¹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
³² Université Bordeaux, CNRS, LP2I Bordeaux, UMR 5797, 33170 Gradignan, France
³³ Department of Physics, University of the Free State, PO Box 339 Bloemfontein 9300, South Africa
³⁴ GRAPPA, Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
³⁵ Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study (UTIAS), The University of Tokyo, 5-1-5 Kashiwa-no-Ha, Kashiwa, Chiba 277-8583, Japan
³⁶ Department of Physics, Konan University, 8-9-1 Okamoto, Higashinada, Kobe, Hyogo 658-8501, Japan
³⁷ Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany

Received: 14 February 2024
Accepted: 29 April 2024

Abstract

PSR B1259–63/LS 2883 is a gamma-ray binary system that hosts a pulsar in an eccentric orbit, with a 3.4 yr period, around an O9.5Ve star (LS 2883). At orbital phases close to periastron passages, the system radiates bright and variable non-thermal emission, for which the temporal and spectral properties of this emission are, for now, poorly understood. In this regard, very high-energy (VHE) emission is especially useful to study and constrain radiation processes and particle acceleration in the system. We report on an extensive VHE observation campaign conducted with the High Energy Stereoscopic System, comprised of approximately 100 h of data taken over five months, from t_p − 24 days to t_p + 127 days around the system’s 2021 periastron passage (where t_p is the time of periastron). We also present the timing and spectral analyses of the source. The VHE light curve in 2021 is consistent overall with the stacked light curve of all previous observations. Within the light curve, we report a VHE maximum at times coincident with the third X-ray peak first detected in the 2021 X-ray light curve. In the light curve – although sparsely sampled in this time period – we see no VHE enhancement during the second disc crossing. In addition, we see no correspondence to the 2021 GeV flare in the VHE light curve. The VHE spectrum obtained from the analysis of the 2021 dataset is best described by a power law of spectral index Γ = 2.65 ± 0.04_stat ± 0.04_sys, a value consistent with the spectral index obtained from the analysis of data collected with H.E.S.S. during the previous observations of the source. We report spectral variability with a difference of ΔΓ = 0.56 ± 0.18_stat ± 0.10_sys at 95% confidence intervals, between sub-periods of the 2021 dataset. We also detail our investigation into X-ray/TeV and GeV/TeV flux correlations in the 2021 periastron passage. We find a linear correlation between contemporaneous flux values of X-ray and TeV datasets, detected mainly after t_p + 25 days, suggesting a change in the available energy for non-thermal radiation processes. We detect no significant correlation between GeV and TeV flux points, within the uncertainties of the measurements, from ∼t_p − 23 days to ∼t_p + 126 days. This suggests that the GeV and TeV emission originate from different electron populations.