ESTIMATED SOFT XRAY SPECTRUM AND IONIZATION OF MOLECULAR

ESTIMATED SOFT X-RAY SPECTRUM AND IONIZATION OF MOLECULAR HYDROGEN 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

Research Background • 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 ke. V to 25 ke. V in the region was estimated (& presented last year)** • We extend the energy region from 1 ke. V to a few hundred e. V by use of theoretical models (*) T. Oka et al. , Ap. J. 632, p. 882 (2005) (**) M. Notani et al. , ISMS in OHIO (2013)

Purpose of this research • 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 into account 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 8 kpc 140 pc GC CMZ (H 2) Foreground gas Earth

X-ray Sources near the Galactic Center • • Sgr A, 1 E 1743. 1 -2843, A 1742294, 1 E-74. 7 -2942, … Luminosities and energy spectra of these sources are observed by various observatories with space and time resolutions Luminosity [erg/sec] Sgr A* 0. 9 -1. 7× 1036 A 1742 -294 2. 5 -4. 5× 1036 1 E 1740. 7 -2942 0. 9 -8. 3× 1036 1 E 1743. 1 -2843 1. 1 -2. 3× 1036 GRS 1741. 9 -2853 1. 6 -1. 7× 1036 SLX 1744 -299 1. 5 -2. 3× 1036 Total 0. 7 -2. 1× 1037 Central Molecular Zone (CMZ): 8 kpc * tan 1° = ~ 140 pc 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”

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

σ(E)*E 3 [barn· ke. V 3] Effective Cross Section for photoabsorption in the foreground gas / interstellar medium H He+(H) C+(He+H) N+(C+He+H) O+(…H) Ne+(…H) Mg+(…H) Al+(…H) Si+(…H) S+(…H) Ar+(…H) Ca+(…H) Cr+(…H) Fe+(…H) Ni+(…H) 7, 0 E+02 6, 0 E+02 5, 0 E+02 4, 0 E+02 3, 0 E+02 2, 0 E+02 1, 0 E+02 0, 0 E+00 0, 02 0, 2 2 E [ke. V] J. Wilms et al. , Ap. J. 542, p. 914 (2000), “On the absorption of X-rays in the interstellar medium” ; R. Morrison and D. Mc. Cammon, Ap. J. 270, p 119 (1983)

Interstellar extinction map Color excess [cm-2] A&A 495, p. 157 (2009), “Interstellar extinction and long-period variable in the Galactic centre”, M. Schultheis et al. NH=(6 8)× 1022 cm-2 between the GC and the Earth

Attenuation Length of X-rays • Assume that the stellar medium concentrates at the GC region within ~ 200 pc 100000 L_(1/e) [pc] 10000 1000 Earth 100 10 CMZ: 1 140 pc 0, 1 0, 01 0, 1 1 E[Ke. V] 10 100 GC • The X rays from 100 e. V to 1 ke. V are absorbed in the CMZ • The X rays over 4 ke. V can reach the Earth

Corrected Spectrum Intensity(E) [Count/cm 2/sec/ke. V] 1 E-01 Observed by Belanger 2006 Corrected Data (This work, N=6 E+22) Corrected Data (This work, N=8 E+22) 1 E-02 1 E-03 1 E-04 1 E-05 1 E-06 1 E-07 1 E [ke. V] 100 Enhancement of Low-energy X-ray intensity } NH=6 -8× 1022 cm-2 between the GC and the Earth

Model-Calculated Spectra ADAF Model*200 This work (N=7 E+22) Observation (ART-P) Blackbody(1 ke. V) Thermal Brems (1 ke. V) f. E [photons/s/cm 2/ke. V] 1 E+02 1 E+01 1 E+00 1 E-01 1 E-02 1 E-03 1 E-04 1 E-05 1 E-06 1 E-07 0, 01 0, 1 1 hν [ke. V] 10 ADAF: Advection-Dominated Accretion Flow F. Yuan et al. , JPJ. 598, p 301(2003)

Estimation of the ionization rate of H 2 (1): 1 -25 ke. V X-ray source at GC W(H 2) = 36. 5 e. V for one ion pair creation Electrons 140 pc 8 kpc Observation at the Earth Ions Central Molecular Zone (L = 4. 2× 1037 erg/s) R = 100 pc Interstellar Medium NH=6× 1022 cm-2 L = 2. 1× 1037 erg/s Ionization rate of H 2 by photoabsorption Fs = (6. 3× 103 cm-2 s-1) (0. 38× 10 -24 cm 2) = 2. 4× 10 -21 s-1 , 3× 10 -19 s-1

Estimation of the ionization rate of H 2 (2) Cut-off Energy 13. 5 e. V 100 e. V 200 e. V 300 e. V 550 e. V 1 ke. V ξ [1/sec] 1. 3× 10 -13 1. 2× 10 -15 1. 3× 10 -16 2. 0× 10 -17 2. 8× 10 -18 2. 6× 10 -19 Reaction Rate, ξ [1/sec] The Ionization rate of H 2 by photoabsorption increases when the energy spectrum extends below 1 ke. V by use of the ADAF model E [ke. V] Magnified from 10 e. V to 100 ke. V

Result of this work Obtained the oritinal luminosity of X-rays with the energy spectrum from the observation above 1 ke. V as well as the model calculation below 1 ke. V � Considered multiple ionization process by an electron emitted from H due to X-ray photoabsorbtion. The energy value of one ion pair production (W-value): 36. 5 e. V � We obtained 3× 10 -19 s-1 from our previous work above 1 ke. V, and a new value of 1. 3× 10 -16 s-1 for 0. 2 ke. V (cut-off) from the ADAF model � << Infrared spectrum of H 3+ : 3× 10 -15 s-1(CMZ)*

Conclusion The X-ray ionization rate of 1. 3× 10 -16 s-1 from our calculation is still lower than the recent observation of ionization rate of 3× 10 -15 s-1 that is obtained from the infrared spectrum of H 3+. � Ionization process of H 2 in the CMZ by Cosmic-ray excitation �