Non-thermal X-rays from colliding wind shock acceleration in the massive binary Eta Carinae
Cosmic-ray acceleration has been a long-standing mystery and, despite more than a century of study, we still do not have a complete census of acceleration mechanisms. The collision of strong stellar winds in massive binary systems creates powerful shocks that have been expected to produce high-energy cosmic rays through Fermi acceleration at the shock interface. The accelerated particles should collide with stellar photons or ambient material, producing non-thermal emission observable in X-rays and γ-rays. The supermassive binary star Eta Carinae (η Car) drives the strongest colliding wind shock in the solar neighbourhood. Observations with non-focusing high-energy observatories indicate a high-energy source near η Car, but have been unable to conclusively identify η Car as the source because of their relatively poor angular resolution. Here we present direct focussing observations of the non-thermal source in the extremely hard X-ray band, which is found to be spatially coincident with the star within several arc-seconds. These observations show that the source of non-thermal X-rays varies with the orbital phase of the binary, and that the photon index of the emission is similar to that derived through analysis of the γ-ray spectrum. This is conclusive evidence that the high-energy emission indeed originates from non-thermal particles accelerated at colliding wind shocks.
Additional Information© 2018 Macmillan Publishers Limited, part of Springer Nature. Published: 02 July 2018. This research has made use of data obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC), provided by NASA's Goddard Space Flight Center. This research has made use of NASA's Astrophysics Data System Bibliographic Services. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center. We appreciate M. Yukita, K. Madsen and M. Stuhlinger on helping resolve the NuSTAR and XMM-Newton data analysis. K.H. is supported by the Chandra grant GO4–15019A, GO7–18012A, the XMM-Newton grant NNX15AK62G, NNX16AN87G, NNX17AE67G, NNX17AE68G, and the ADAP grant NNX15AM96G. C.M.P.R. acknowledges initial support from Chandra Theory grant TM7-18003Z used in combination with an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by Universities Space Research Association under contract with NASA, and current support from FONDECYT grant 3170870. A.F.J.M. is supported by NSERC (Canada) and FQRNT (Quebec). Author Contributions: K.H. and M.F.C. led the project, from proposing and planning observations, analysing the data to composing the manuscript. J.M.P. constructed a theoretical model that explains the variation of the non-thermal component. N.S. performed initial analysis of the NuSTAR data in 2015. H.T analysed and discussed Fermi data of η Car. C.M.P.R. performed theoretical simulations of η Car's thermal X-ray emission. B.W.G. and D.R.W. discussed NuSTAR data analysis, especially the background characteristics. T.R.G. worked for the observation planning. T.R.G., N.D.R., T.I.M. and A.F.J.M. discussed the wind property of η Car. All authors reviewed the manuscript and discussed the work. The authors declare no competing interests. Data availability: The raw data of the NuSTAR and XMM-Newton observations are available from the NASA HEASARC archive at https://heasarc.gsfc.nasa.gov.
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Supplemental Material - 41550_2018_505_MOESM1_ESM.pdf