Constraining particle acceleration in Sgr A^⋆ with simultaneous GRAVITY, Spitzer, NuSTAR, and Chandra observations

¹ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
² LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 Place Jules Janssen, 92195 Meudon, France
³ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁴ 1st Institute of Physics, University of Cologne, Zülpicher Strasse 77, 50937 Cologne, Germany
⁵ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁶ Universidade de Lisboa – Faculdade de Ciências, Campo Grande, 1749-016 Lisboa, Portugal
⁷ Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
⁸ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁹ European Southern Observatory, Casilla, 19001 Santiago 19, Chile
¹⁰ Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
¹¹ Departments of Physics and Astronomy, Le Conte Hall, University of California, Berkeley, CA 94720, USA
¹² CENTRA – Centro de Astrofísica e Gravitação, IST, Universidade de Lisboa, 1049-001 Lisboa, Portugal
¹³ INAF-Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, LC, Italy
¹⁴ Department of Astrophysical & Planetary Sciences, JILA, University of Colorado, Duane Physics Bldg., 2000 Colorado Ave, Boulder, CO 80309, USA
¹⁵ Department of Particle Physics & Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
¹⁶ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
¹⁷ Department of Physics, Technical University Munich, James-Franck-Strasse 1, 85748 Garching, Germany
¹⁸ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
²⁰ MIT Kavli Institute for Astrophysics and Space Research, Cambridge, MA 02139, USA
²¹ Department of Physics, McGill University, 3600 University St., Montreal, QC H3A 2T8, Canada
²² McGill Space Institute, 3550 University St., Montreal, QC H3A 2A7, Canada
²³ Spitzer Science Center, California Institute of Technology, Pasadena, CA 91125, USA
²⁴ UCLA Galactic Center Group, Physics and Astronomy Department, University of California, Los Angeles, CA 90024, USA
²⁵ Department of Astronomy, University of Illinois, 1002 West Green Street, Urbana, IL 61801, USA
²⁶ Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1098 XH Amsterdam, The Netherlands
²⁷ Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, Room 1027, New York, NY 10027, USA
²⁸ Bard College Physics Program, 30 Campus Road, Annandale-On-Hudson, NY 12504, USA
²⁹ Villanova University, Department of Physics, Villanova, PA 19085, USA
³⁰ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
³¹ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, MS 169-224, Pasadena, CA 91109, USA

Received: 1 April 2021
Accepted: 1 July 2021

Abstract

We report the time-resolved spectral analysis of a bright near-infrared and moderate X-ray flare of Sgr A^⋆. We obtained light curves in the M, K, and H bands in the mid- and near-infrared and in the 2 − 8 keV and 2 − 70 keV bands in the X-ray. The observed spectral slope in the near-infrared band is νL_ν ∝ ν^{0.5 ± 0.2}; the spectral slope observed in the X-ray band is νL_ν ∝ ν^{−0.7 ± 0.5}. Using a fast numerical implementation of a synchrotron sphere with a constant radius, magnetic field, and electron density (i.e., a one-zone model), we tested various synchrotron and synchrotron self-Compton scenarios. The observed near-infrared brightness and X-ray faintness, together with the observed spectral slopes, pose challenges for all models explored. We rule out a scenario in which the near-infrared emission is synchrotron emission and the X-ray emission is synchrotron self-Compton. Two realizations of the one-zone model can explain the observed flare and its temporal correlation: one-zone model in which the near-infrared and X-ray luminosity are produced by synchrotron self-Compton and a model in which the luminosity stems from a cooled synchrotron spectrum. Both models can describe the mean spectral energy distribution (SED) and temporal evolution similarly well. In order to describe the mean SED, both models require specific values of the maximum Lorentz factor γ_max, which differ by roughly two orders of magnitude. The synchrotron self-Compton model suggests that electrons are accelerated to γ_max ∼ 500, while cooled synchrotron model requires acceleration up to γ_max ∼ 5 × 10⁴. The synchrotron self-Compton scenario requires electron densities of 10¹⁰ cm⁻³ that are much larger than typical ambient densities in the accretion flow. Furthermore, it requires a variation of the particle density that is inconsistent with the average mass-flow rate inferred from polarization measurements and can therefore only be realized in an extraordinary accretion event. In contrast, assuming a source size of 1 R_S, the cooled synchrotron scenario can be realized with densities and magnetic fields comparable with the ambient accretion flow. For both models, the temporal evolution is regulated through the maximum acceleration factor γ_max, implying that sustained particle acceleration is required to explain at least a part of the temporal evolution of the flare.