Eta Carinae's Thermal X-ray Tail Measured with XMM-Newton and NuSTAR


Kenji Hamaguchi(1,2), Michael F. Corcoran(1,3), Theodore R. Gull(4), Hiromitsu Takahashi(5), Brian Grefenstette(6), Takayuki Yuasa(7), Martin Stuhlinger(8), Christopher M. P. Russell(4,9), Anthony F. J. Moffat(10), Neetika Sharma(2), Thomas I. Madura(1,3), Noel D. Richardson(10), Jose Groh(11), Julian M. Pittard(12), Stan Owocki(13)

1 - CRESST and X-ray Astrophysics Laboratory NASA/GSFC, Greenbelt, MD 20771; 2 - Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250; 3 - Universities Space Research Association, 7178 Columbia Gateway Dr., Columbia, MD 21044; 4 - Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771; 5 - Department of Physical Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan; 6 - Space Radiation Lab, California Institute of Technology, Pasadena, CA 91125; 7 - Nishina Center, RIKEN, 2-1, Hirosawa, Wako, Saitama, Japan, 351-0198, Japan; 8 - European Space Astronomy Centre (ESAC), P.O. Box 78, 28691 Villanueva de la Cañada, Madrid, Spain; 9 - NASA Postdoctoral Program Fellow; 10 - Departement de physique and Centre de Recherche en Astrophysique du Quebec (CRAQ), Universite de Montreal, C.P. 6128; 11 - Geneva Observatory, Geneva University, Chemin des Maillettes 51, CH-1290 Sauverny, Switzerland; 12 - School of Physics and Astronomy, The University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK; 13 - Bartol Research Institute, Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA

The evolved, massive highly eccentric binary system, eta Car, underwent a periastron passage in the summer of 2014. We obtained two coordinated X-ray observations with XMM-Newton and NuSTAR during the elevated X-ray flux state and just before the X-ray minimum flux state around this passage. These NuSTAR observations clearly detected X-ray emission associated with eta Car extending up to ~50 keV for the first time. The NuSTAR spectrum above 10 keV can be fit with the bremsstrahlung tail from a kT~6 keV plasma. This temperature is Delta kT ~2 keV higher than those measured from the iron K emission line complex, if the shocked gas is in collisional ionization equilibrium. This result may suggest that the companion star's pre-shock wind velocity is underestimated. The NuSTAR observation near the X-ray minimum state showed a gradual decline in the X-ray emission by 40% at energies above 5 keV in a day, the largest rate of change of the X-ray flux yet observed in individual eta Car observations. The column density to the hardest emission component, NH~1e24 cm-2, marked one of the highest values ever observed for eta Car, strongly suggesting increased obscuration of the wind-wind colliding X-ray emission by the thick primary stellar wind prior to superior conjunction. Neither observation detected the power-law component in the extremely hard band that INTEGRAL and Suzaku observed prior to 2011. If the non-detection by INTEGRAL is caused by absorption, the power-law source must be small and located very near the WWC apex. Alternatively, it may be that the power-law source is not related to either eta Car or the GeV gamma-ray source.

Reference: ApJ, 817, 23 (2016)
Status: Manuscript has been accepted

Weblink: http://iopscience.iop.org/article/10.3847/0004-637X/817/1/23/meta;jsessionid=76A9B4FEB605C936E1B0430C36DEDDAD.c3.iopscience.cld.iop.org

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Email: Kenji.Hamaguchi@nasa.gov