1. Ionization of H 2 by X-rays in the Central Molecular Zone of the Galactic Center Masahiro Notani and Takeshi Oka Department of Astronomy and Astrophysics, and Department of Chemistry, the Enrico Fermi Institute University of Chicago

2.  Resent studies of the Galactic center (GC) using the infrared spectrum of H 3 + show a high ionization rate of H 2 in the wide regions of Central Molecular Zone (CMZ)  3×10 -15 s -1 (CMZ) * >>  3×10 -17 s -1 (Galactic disk)  The higher rate may originate from cosmic rays due to the high density supernova remnants in the CMZ, but the possible ionization by the abundant intense X-ray sources from 1 keV to 10 keV in the region needs to be estimated (*) T. Oka et al., ApJ. 632, p.882 (2005)

3.  To estimate the ionization rate of H 2 due to X rays based on the large scale ART-P X-ray map of the Galactic center region 1.Obtain the original intensities of the X-ray sources, taking account of the attenuation of the observed X-rays by the foreground gas 2.Calculate the ionization rates of H 2 gas in the CMZ using the corrected X-ray flux X-rays Earth GC CMZ (H 2 ) Interstellar Medium (foreground gas) 8 kpc140 pc

4. X-ray Sources at Galactic Center Sgr A, 1E1743.1-2843, A1742- 294, 1E-74.7-2942, … Luminosities and energy spectra of these sources are observed by various observatories with space and time resolutions M.N. Pavlinskii, S.A. Grebenev and R.A. Syunyaev, Sov. Astron. Lett. 18, p.291 (1992), “ART-P X-ray map of the GC” Luminosity[erg/sec] 19881990 Spring1990 Fall1991 Spring Sgr A*/1E1742.5-28591.66E+361.06E+368.80E+35 A1742-2941.30E+374.48E+365.58E+362.50E+36 1E1740.7-29423.00E+368.28E+365.50E+369.40E+35 GRS1741.9-28531.64E+36 SLX1744-2992.90E+363.76E+361.46E+36 1E1743.1-28432.30E+362.03E+36 1.13E+36 (Total)2.07E+372.10E+371.79E+376.91E+36 8 kpc * tan 1° = 140 pc Central Molecular Zone (CMZ):

5. Observed X-ray spectrum from the GC G. Bélanger et al., ApJ. 636, p.275 (2006), “A persistent high-energy flux from heart of the milky way: INTEGRAL’s View of the GC” Observed spectrum (Power Law)(Black Body) INTEGRALXMM-Newton (  Black Body + Power Law)

6. Effective Cross Section for photoabsorption in the foreground gas / interstellar medium J. Wilms et al., ApJ. 542, p.914 (2000), “On the absorption of X-rays in the interstellar medium” ; R. Morrison and D. McCammon, ApJ. 270, p119 (1983)

7. Interstellar extinction map N H =(6  8)×10 22 cm -2 between the GC and the Earth A&A 495, p.157 (2009), “Interstellar extinction and long-period variable in the Galactic centre”, M. Schultheis et al. Color excess [cm -2 ]

8. Corrected spectrum N H =6×10 22 cm -2 between the GC and the Earth Enhancement of Low-energy X-ray intensity

9. (L = 4.2×10 37 erg/s) X-ray source at GC L = 2.1×10 37 erg/s Central Molecular Zone Electrons Ions Estimation of the ionization rate of H 2 R = 100 pc Interstellar Medium N H =6×10 22 cm -2 Observation at the Earth Ionization rate of H 2 by photoabsorption  = (6.3×10 3 cm -2 s -1 ) (0.38×10 -24 cm 2 ) = 2.4×10 -21 s -1,  3×10 -19 s -1 W(H 2 ) = 36.5 eV for one ion pair creation 8 kpc140 pc

10.  Obtained the original luminosity of X-rays with the energy spectrum above 1 keV  Considered multiple ionization process by an electron emitted from a hydrogen atom due to X-ray photoabsorption. The energy value of one ion pair production (W-value) in the gas phase W(H 2 )=36.5 eV  We obtained  3×10 -19 s -1 from our model

11.  Estimated the ionization rate of H 2 in the CMZ by use of the corrected X-ray spectrum from 1 keV to 25 keV  The X-ray ionization rate of  3×10 -19 s -1 from our model is significantly lower than the recent observation of ionization rate of  3×10 -15 s -1 that is obtained from the infrared spectrum of H 3 +.

13. Simple model Luminosity: L = 4.2×10 37 erg/s = 2.63 × 10 49 eV/s For 5-keV Photons: I x = L/5 keV = 5.25×10 45 photons/s Radius of the CMZ sphere: R = 100 pc = 3.09×10 20 cm Brightness of X rays on the sphere at radius R:  = I x /(2  R 2 ) = (5.25×10 45 )/(1.2×10 42 ) *2= 6.3×10 3 cm -2 s -1 5-keV ionization cross section for H 2 :  ~ 0.38 barn - Chantler’95 Thus, the ionization rate by photoabsorption:  = (6.3×10 3 cm -2 s -1 ) (0.38×10 -24 cm 2 ) = 2.4×10 -21 s -1 (Not for the total number of ions)

14. H/H 2 Photoabsorption process of X-rays in the CMZ Atomic Data and Nucl. Data Tables 54, p181 (1993), “X-ray Interactions: photoabsorption, scattering, transmission and reflection at E=50eV- 30keV, Z=1-92”, B.L. Henke, E.M. Gullikson and J.C. Davis Cross section for Hydrogen Atom Energy dependence of the photoabsorption cross sections Cascade ionization process of beta rays The energy value of one ion pair production (W-value) in the gas phase is given to be 36.5 eV for H 2