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A powerful flare from Sgr A* confirms the synchrotron nature of the X-ray emission

Ponti, G. and George, E. and Scaringi, S. and Zhang, S. and Jin, C. and Dexter, J. and Terrier, R. and Clavel, M. and Degenaar, N. and Eisenhauer, F. and Genzel, R. and Gillessen, S. and Goldwurm, A. and Habibi, M. and Haggard, D. and Hailey, C. and Harrison, F. and Merloni, A. and Mori, K. and Nandra, K. and Ott, T. and Pfuhl, O. and Plewa, P. M. and Waisberg, I. (2017) A powerful flare from Sgr A* confirms the synchrotron nature of the X-ray emission. Monthly Notices of the Royal Astronomical Society, 468 (2). pp. 2447-2468. ISSN 0035-8711. http://resolver.caltech.edu/CaltechAUTHORS:20170314-130058938

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Abstract

We present the first fully simultaneous fits to the near-infrared (NIR) and X-ray spectral slope (and its evolution) during a very bright flare from Sgr A*, the supermassive black hole at the Milky Way's centre. Our study arises from ambitious multiwavelength monitoring campaigns with XMM–Newton, NuSTAR and SINFONI. The average multiwavelength spectrum is well reproduced by a broken power law with Γ_(NIR) = 1.7 ± 0.1 and Γ_X = 2.27 ± 0.12. The difference in spectral slopes (ΔΓ = 0.57 ± 0.09) strongly supports synchrotron emission with a cooling break. The flare starts first in the NIR with a flat and bright NIR spectrum, while X-ray radiation is detected only after about 10^3 s, when a very steep X-ray spectrum (ΔΓ = 1.8 ± 0.4) is observed. These measurements are consistent with synchrotron emission with a cooling break and they suggest that the high-energy cut-off in the electron distribution (γ_(max)) induces an initial cut-off in the optical–UV band that evolves slowly into the X-ray band. The temporal and spectral evolution observed in all bright X-ray flares are also in line with a slow evolution of γ_(max). We also observe hints for a variation of the cooling break that might be induced by an evolution of the magnetic field (from B ∼ 30 ± 8 G to B ∼ 4.8 ± 1.7 G at the X-ray peak). Such drop of the magnetic field at the flare peak would be expected if the acceleration mechanism is tapping energy from the magnetic field, such as in magnetic reconnection. We conclude that synchrotron emission with a cooling break is a viable process for Sgr A*'s flaring emission.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1093/mnras/stx596DOIArticle
https://academic.oup.com/mnras/article-lookup/doi/10.1093/mnras/stx596PublisherArticle
http://arxiv.org/abs/1703.03410arXivDiscussion Paper
ORCID:
AuthorORCID
Harrison, F.0000-0003-2992-8024
Additional Information:© 2017 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2017 March 8. Received 2017 March 8; in original form 2016 October 12. Published: 10 March 2017. The authors wish to thank Jan-Uwe Ness, Ignacio de la Calle, Karl Foster and the rest of the XMM–Newton and NuSTAR scheduling teams for the enormous support that made this multiwavelength campaign possible, as well as the referee for the careful reading of the paper. GP thanks Mark R. Morris, Gabriele Ghisellini, Lorenzo Sironi, Hendrik J. van Eerten, Michi Bauböck and Francesco Coti-Zelati, for useful discussion. RT and AG acknowledge support from CNES. This research has made use both of data obtained with XMM–Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA, and on data obtained from the Chandra Data Archive. The GC XMM–Newton monitoring project is supported by the Bundesministerium für Wirtschaft und Technologie/Deutsches Zentrum für Luft- und Raumfahrt (BMWI/DLR, FKZ 50 OR 1408 and FKZ 50 OR 1604) and the Max Planck Society.
Group:NuSTAR, Space Radiation Laboratory
Funders:
Funding AgencyGrant Number
Centre National d'Études Spatiales (CNES)UNSPECIFIED
European Space Agency (ESA)UNSPECIFIED
NASAUNSPECIFIED
Bundesministerium für Wirtschaft und Technologie (BMWi)FKZ 50 OR 1408
Bundesministerium für Wirtschaft und Technologie (BMWi)FKZ 50 OR 1604
Max Planck SocietyUNSPECIFIED
Subject Keywords:black hole physics – methods: data analysis – Galaxy: centre
Record Number:CaltechAUTHORS:20170314-130058938
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20170314-130058938
Official Citation:G. Ponti, E. George, S. Scaringi, S. Zhang, C. Jin, J. Dexter, R. Terrier, M. Clavel, N. Degenaar, F. Eisenhauer, R. Genzel, S. Gillessen, A. Goldwurm, M. Habibi, D. Haggard, C. Hailey, F. Harrison, A. Merloni, K. Mori, K. Nandra, T. Ott, O. Pfuhl, P. M. Plewa, I. Waisberg; A powerful flare from Sgr A* confirms the synchrotron nature of the X-ray emission, Monthly Notices of the Royal Astronomical Society, Volume 468, Issue 2, 21 June 2017, Pages 2447–2468, https://doi.org/10.1093/mnras/stx596
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:75099
Collection:CaltechAUTHORS
Deposited By: Joy Painter
Deposited On:15 Mar 2017 18:38
Last Modified:28 Sep 2017 22:29

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