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Title:Estimated Soft X-ray Spectrum And Ionization Of Molecular Hydrogen In The Central Molecular Zone Of The Galactic Center
Author(s):Notani, Masahiro
Contributor(s):Oka, Takeshi
Abstract:From observed high H$_3^+$ column densities in the Central Molecular Zone (CMZ), a region with a radius of $\sim$150~pc at the center of our Milky Way Galaxy, H$_2$ ionization rates of $\zeta$~$\sim$~3~$\times$~10$^{-15}$~s$^{-1}$ have been reported.\footnote{Oka, T., Geballe, T. R., Goto, M., Usuda, T., and McCall, B. J. 2005, ApJ, \textbf{632} 882}\footnote{Geballe, T. R., and Oka, T. 2010, ApJ, \textbf{709} L70.} This ionization rate which is higher than those in dense clouds and diffuse clouds in the Galactic disk by $\sim$100 and $\sim$10, respectively, have been ascribed to high flux of cosmic rays due to the high density of supernova remnants in the region. We are studying the ionization rate due to X-rays intensely observed in the CMZ as a possible competing process. Last year we reported the estimated ionization rate due to observable X-rays with energy 1 - 10 keV as negligible compared to the observed $\zeta$. \vspace{0.1 in} However, just like cosmic ray ionization is dominated by low energy ($E\leq$~100~MeV) cosmic rays that are not directly observable because of deflection by solar magnetic field, the X-ray ionization is dominated by soft X-rays ($E\leq$~1~keV) that are not observable due to optical depth of the foreground gas. Our task therefore resembles those by Hayakawa et al. (1961)\footnote{Hayakawa,S.,Nishimura, S., Takayanagi, K. 1961, PASJ, \textbf{18} 184} and Spitzer and Tomasko (1968)\footnote{Spitzer,L. T., Tomasko, M. G. 1968, ApJ, \textbf{152} 971} who estimated the cosmic ray ionization rate $\zeta$ based on high energy ($>$~1 GeV) cosmic ray observations. \vspace{0.1 in} We use theoretical X-ray spectrum and interpolate the observed X-rays at 4 keV, which are observable nearly un-attenuated from the CMZ, to the low energy region. Two theoretical spectra are presented, one due to Bremsstrahlung with variable temperature and proper cut off and the other the advection-dominated accretion flow (ADAF) model.\footnote{Yuan, F., Quataert, E., Narayan, R. 2003, JPJ, \textbf{598} 301.}. Discussion of the calculations and the results will be presented.
Issue Date:2014-06-16
Publisher:International Symposium on Molecular Spectroscopy
Citation Info:Notani, M.; Oka, T. ESTIMATED SOFT X-RAY SPECTRUM AND IONIZATION OF MOLECULAR HYDROGEN IN THE CENTRAL MOLECULAR ZONE OF THE GALACTIC CENTER. Proceedings of the International Symposium on Molecular Spectroscopy, Urbana, IL, June 16-21, 2014. DOI: 10.15278/isms.2014.MF05
Rights Information:Copyright 2014 by the authors. Licensed under a Creative Commons Attribution 4.0 International License.
Date Available in IDEALS:2014-09-17

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