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

Abuter, R. and Amorim, A. and Bauböck, M. and Baganoff, F. and Berger, J. P. and Boyce, H. and Bonnet, H. and Brandner, W. and Clénet, Y. and Davies, R. and de Zeeuw, P. T. and Dexter, J. and Dallilar, Y. and Drescher, A. and Eckart, A. and Eisenhauer, F. and Fazio, G. G. and Förster Schreiber, N. M. and Foster, K. and Gammie, C. and Garcia, P. and Gao, F. and Gendron, E. and Genzel, R. and Ghisellini, G. and Gillessen, S. and Gurwell, M. A. and Habibi, M. and Haggard, D. and Hailey, C. and Harrison, F. A. and Haubois, X. and Heißel, G. and Henning, T. and Hippler, S. and Hora, J. L. and Horrobin, M. and Jiménez-Rosales, A. and Jochum, L. and Jocou, L. and Kaufer, A. and Kervella, P. and Lacour, S. and Lapeyrère, V. and Le Bouquin, J.-B. and Léna, P. and Lowrance, P. J. and Lutz, D. and Markoff, S. and Mori, K. and Morris, M. R. and Neilsen, J. and Nowak, M. and Ott, T. and Paumard, T. and Perraut, K. and Perrin, G. and Ponti, G. and Pfuhl, O. and Rabien, S. and Rodríguez-Coira, G. and Shangguan, J. and Shimizu, T. and Scheithauer, S. and Smith, H. A. and Stadler, J. and Stern, D. K. and Straub, O. and Straubmeier, C. and Sturm, E. and Tacconi, L. J. and Vincent, F. and von Fellenberg, S. D. and Waisberg, I. and Widmann, F. and Wieprecht, E. and Wiezorrek, E. and Willner, S. P. and Witzel, G. and Woillez, J. and Yazici, S. and Young, A. and Zhang, S. and Zins, G. (2021) Constraining particle acceleration in Sgr A⋆ with simultaneous GRAVITY, Spitzer, NuSTAR, and Chandra observations. Astronomy and Astrophysics, 654 . Art. No. A22. ISSN 0004-6361. doi:10.1051/0004-6361/202140981.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Amorim, A.0000-0003-0638-2321
Baganoff, F.0000-0003-3852-6545
Berger, J. P.0000-0001-5025-0428
Boyce, H.0000-0002-6530-5783
Brandner, W.0000-0003-1939-6351
Clénet, Y.0000-0002-8382-2020
de Zeeuw, P. T.0000-0003-4175-3474
Dexter, J.0000-0003-3903-0373
Eckart, A.0000-0001-6049-3132
Fazio, G. G.0000-0002-0670-0708
Förster Schreiber, N. M.0000-0003-4264-3381
Gammie, C.0000-0001-7451-8935
Garcia, P.0000-0002-1678-3535
Gao, F.0000-0002-2581-9114
Genzel, R.0000-0002-2767-9653
Ghisellini, G.0000-0001-8949-178X
Gillessen, S.0000-0002-5708-0481
Gurwell, M. A.0000-0003-0685-3621
Haggard, D.0000-0001-6803-2138
Harrison, F. A.0000-0003-2992-8024
Haubois, X.0000-0001-7878-7278
Heißel, G.0000-0003-4965-856X
Henning, T.0000-0002-1493-300X
Hippler, S.0000-0002-3912-6108
Hora, J. L.0000-0002-5599-4650
Horrobin, M.0000-0001-5451-7847
Jiménez-Rosales, A.0000-0002-2662-3754
Jocou, L.0000-0001-9376-1818
Kaufer, A.0000-0002-5270-3686
Kervella, P.0000-0003-0626-1749
Lacour, S.0000-0002-6948-0263
Le Bouquin, J.-B.0000-0002-0493-4674
Léna, P.0000-0002-9091-3051
Lowrance, P. J.0000-0001-8014-0270
Lutz, D.0000-0003-0291-9582
Markoff, S.0000-0001-9564-0876
Mori, K.0000-0002-9709-5389
Morris, M. R.0000-0002-6753-2066
Neilsen, J.0000-0002-8247-786X
Nowak, M.0000-0001-6923-1315
Ott, T.0000-0003-1572-0396
Paumard, T.0000-0003-0655-0452
Perraut, K.0000-0003-3099-757X
Perrin, G.0000-0003-0680-0167
Ponti, G.0000-0003-0293-3608
Rodríguez-Coira, G.0000-0002-4177-6433
Shangguan, J.0000-0002-4569-9009
Shimizu, T.0000-0002-2125-4670
Stadler, J.0000-0001-5888-023X
Stern, D. K.0000-0003-2686-9241
Straub, O.0000-0001-5755-0677
Straubmeier, C.0000-0002-0671-9302
Sturm, E.0000-0002-0018-3666
Tacconi, L. J.0000-0002-1485-9401
Vincent, F.0000-0002-3855-0708
von Fellenberg, S. D.0000-0002-9156-2249
Widmann, F.0000-0002-0327-6585
Willner, S. P.0000-0002-9895-5758
Witzel, G.0000-0003-2618-797X
Woillez, J.0000-0002-2958-4738
Zhang, S.0000-0002-2967-790X
Zins, G.0000-0001-8862-7702
Additional Information:© GRAVITY Collaboration 2021. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Received 1 April 2021; Accepted 1 July 2021; Published online 05 October 2021. GRAVITY is developed in a Collaboration by the Max Planck Institute for extraterrestrial Physics, LESIA of Observatoire de Paris/Université PSL/CNRS/Sorbonne Université/Université de Paris and IPAG of Université Grenoble Alpes/CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the CENTRA – Centro de Astrofisica e Gravitação, and the European Southern Observatory. SvF thanks Giulia Focchi for her contribution to the H-band acquisition camera pipeline. SvF, and FW acknowledge support by the Max Planck International Research School. GP is supported by the H2020 ERC Consolidator Grant Hot Milk under grant agreement Nr. 865637. A.A. and P.G. were supported by Fundação para a Ciência e a Tecnologia, with grants reference UIDB/00099/2020 and SFRH/BSAB/142940/2018. This work is based in part on observations made with the Spitzer Space Telescope, which was operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA. The scientific results reported in this article are based in part on observations made by the Chandra X-ray Observatory. This work is based in part on observations made with NuSTAR, which is operated by NASA/JPL-Caltech. GGF, JLH, HAS, and SPW acknowledge support for this work from the NASA ADAP program under NASA grant 80NSSC18K0416.
Group:Astronomy Department, Space Radiation Laboratory, NuSTAR
Funding AgencyGrant Number
International Max Planck Research School (IMPRS) for Astronomy and AstrophysicsUNSPECIFIED
European Research Council (ERC)865637
Fundação para a Ciência e a Tecnologia (FCT)UIDB/00099/2020
Fundação para a Ciência e a Tecnologia (FCT)SFRH/BSAB/142940/2018
Subject Keywords:Galaxy: center – accretion, accretion disks – black hole physics
Record Number:CaltechAUTHORS:20211011-220006826
Persistent URL:
Official Citation:Constraining particle acceleration in Sgr A⋆ with simultaneous GRAVITY, Spitzer, NuSTAR, and Chandra observations. GRAVITY Collaboration, R. Abuter, A. Amorim, M. Bauböck, F. Baganoff, J. P. Berger, H. Boyce, H. Bonnet, W. Brandner, Y. Clénet, R. Davies, P. T. de Zeeuw, J. Dexter, Y. Dallilar, A. Drescher, A. Eckart, F. Eisenhauer, G. G. Fazio, N. M. Förster Schreiber, K. Foster, C. Gammie, P. Garcia, F. Gao, E. Gendron, R. Genzel, G. Ghisellini, S. Gillessen, M. A. Gurwell, M. Habibi, D. Haggard, C. Hailey, F. A. Harrison, X. Haubois, G. Heißel, T. Henning, S. Hippler, J. L. Hora, M. Horrobin, A. Jiménez-Rosales, L. Jochum, L. Jocou, A. Kaufer, P. Kervella, S. Lacour, V. Lapeyrère, J.-B. Le Bouquin, P. Léna, P. J. Lowrance, D. Lutz, S. Markoff, K. Mori, M. R. Morris, J. Neilsen, M. Nowak, T. Ott, T. Paumard, K. Perraut, G. Perrin, G. Ponti, O. Pfuhl, S. Rabien, G. Rodríguez-Coira, J. Shangguan, T. Shimizu, S. Scheithauer, H. A. Smith, J. Stadler, D. K. Stern, O. Straub, C. Straubmeier, E. Sturm, L. J. Tacconi, F. Vincent, S. D. von Fellenberg, I. Waisberg, F. Widmann, E. Wieprecht, E. Wiezorrek, S. P. Willner, G. Witzel, J. Woillez, S. Yazici, A. Young, S. Zhang and G. Zins. A&A, 654 (2021) A22; DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:111362
Deposited By: Tony Diaz
Deposited On:11 Oct 2021 22:29
Last Modified:03 Jan 2022 23:05

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