PASJ: Publ. Astron. Soc. Japan 51, 161-171 and Plate 6 (1999) Magnetic Reconnection as the Origin of Galactic-Ridge X-Ray Emission Syuniti TANUMA, 1 ' 3 Takaaki YOKOYAMA, 2 Takahiro KUDOH, 3 Ryoji MATSUMOTO, 4 Kazunari SHIBATA, 3 and Kazuo MAKISHIMA 5 1 Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Kongo, Bunkyo-ku, Tokyo 113-0033 E-mail (TANU): tanuma@solar.mtk.nao.ac.jp 2 Nobeyama Radio Observatory, Minamimaki, Minamisaku, Nagano 384-1305 ^National Astronomical Observatory, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 4 Department of Physics, Faculty of Science, Ghiba University, 1-33 Yayoi-cho, Inage-ku, Chiba, Chiba 263-8522 5 Department of Physics, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Received 1998 Novenber 30; accepted 1999 January 4) Abstract We present a scenario for the origin of the hot plasma in our Galaxy as a model of strong X-ray emission [-3-10 keV; L x (2 - 10 keV) ~ 10 38 erg s -1 ], called Galactic Ridge X-ray Emission (GRXE), which has been observed near to the galactic plane. GRXE is thermal emission from a hot component (~ 7 keV) and a cool component (~ 0.8 keV). Observations suggest that the hot component is diffuse, and that it is not escaping away freely. Both what heats the hot component and what confines it in the galactic ridge still remain puzzling, while the cool component is believed to be created by supernovae. We propose a new scenario: the hot component is heated by magnetic reconnection, and confined by a helical magnetic field produced by magnetic reconnection. We solved two-dimensional magnetohydrodynamic equations numerically to study how magnetic reconnection, triggered by a supernova explosion, creates hot plasmas and magnetic islands (helical tubes), and how the magnetic islands confine the hot plasmas in the Galaxy. This is one of the possible mechanisms to trigger reconnection in the Galaxy. We conclude that magnetic reconnection is able to heat the GRXE plasma if the magnetic field is localized in an intense flux tube with Blocal ~ 30 flG. Key words: Galaxies: Milky Way — Galaxies: X-rays — Interstellar: magnetic fields — Magneto- hydrodynamics — Plasmas 1. Introduction is optically thin thermal emission from a hot plasma. The hot plasma has mainly two components: hot (~ 7 keV) Bright diffuse X-ray emission has been observed along anc [ coo i (^ Q.8 keV) components (Kaneda et al. 1997; the galactic plane with various X-ray astronomy satel- Makishima 1994). The hot component of GRXE cannot lites, including HEAO-1 (Worrall et al. 1982), EXOSAT be explained consistently in terms of an assembly of unre- (Warwick et al. 1985), Tenma (Koyama et al. 1986b; solved point X-ray sources emitting thermal X-rays, be- Koyama 1989), Ginga (Koyama et al. 1989; Yamasaki cause of the observed properties of the surface brightness et al. 1997; Yamauchi, Koyama 1993, 1995), ASCA fluctuations and because of the lack of a corresponding (Kaneda et al. 1997; Makishima 1994; Yamauchi et al. population of discrete X-ray sources (Kaneda et al. 1997; 1996), and RXTE (Valinia, Marshall 1998). The emis- Koyama et al. 1986b; Makishima 1994; Yamauchi et al. sion, called Galactic Ridge X-ray Emission (GRXE), is 1995). GRXE is, therefore, likely to be thermal emission distributed along the galactic latitude |6| < l°-2° and f rom a hot plasma distributed diffusely along the Milky the galactic longitude \l\ < 60° (Warwick et al. 1985) Way. The number density of the hot plasma is estimated with a scale height of h ~ 100 pc (Yamauchi, Koyama to be n h - 3 x 10" 3 / h " 1/2 cm" 3 , where / h is the volume- 1993) and a radius of about 8 kpc. The spatially in- fiUing factor o f t h e h o t component ( e . g ., Kaneda et al. tegrated GRXE luminosity in the 2-10 keV range is 1997) L x - (1-2)xlO 38 ergs" 1 (Koyamaet al. 1986b; Warwick The GRXE plasma deviates from ionization equi- et al. 1985; Yamasaki et al. 1997; Yamauchi, Koyama librium? ^ indicated by the center energies of vari- 1993). Koyama et al. (1986b) have revealed that GRXE ous atomic lines; the observed ionization parameter is © Astronomical Society of Japan • Provided by the NASA Astrophysics Data System Downloaded from https://academic.oup.com/pasj/article-abstract/51/1/161/2949146 by guest on 09 July 2020