1. 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

2. Research Background
Resent studies of the Galactic center (GC) using the infrared spectrum of H3+ show a high ionization rate of H2 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 25 keV in the region was estimated (& presented last year)**
We extend the energy region from 1 keV to a few hundred eV by use of theoretical models
(*) T. Oka et al., ApJ. 632, p.882 (2005)
(**) M. Notani et al., ISMS in OHIO (2013)

3. Purpose of this research
To estimate the ionization rate of H2 due to X rays based on the large scale ART-P X-ray map of the Galactic center region
Obtain the original intensities of the X-ray sources, taking into account the attenuation of the observed X-rays by the foreground gas
Calculate the ionization rates of H2 gas in the CMZ using the corrected X-ray flux
X-rays
8 kpc
140 pc GC
CMZ
(H2)
Foreground gas
Earth

4. X-ray Sources near the Galactic Center
Sgr A, 1E , A , 1E , …
Luminosities and energy spectra of these sources are observed by various observatories with space and time resolutions
Luminosity [erg/sec]
Sgr A*
×1036 A
×1036 1E
×1036 1E
×1036
GRS
×1036
SLX
×1036
Total
×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”
𝐶=σ𝝓𝑁

5. Observed X-ray spectrum from the GC
XMM-Newton
INTEGRAL
Observed spectrum
(Black Body + Power Law)
(Power Law)
(Black Body)
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”
𝑪=σ𝜙𝑁

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)
𝐶=𝝈𝜙𝑁

7. Interstellar extinction map
Color excess
(𝐽−𝐾)
𝐸(𝐵−𝑉) 𝐴𝑉 𝑁𝐻
NH=5.8× 𝐸(𝐵−𝑉)
[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
𝐶=σ𝜙𝑵

8. Attenuation Length of X-rays
Assume that the stellar medium
concentrates at the GC region
within ~ 200 pc
Earth
140 pc
CMZ: GC
The X rays from 100 eV to 1 keV are absorbed in the CMZ
The X rays over 4 keV can reach the Earth
𝐶=𝝈𝜙𝑵

9. } Corrected Spectrum NH=6-8×1022 cm-2 between the GC and the Earth
Enhancement of Low-energy X-ray intensity
𝑪=σ𝜙𝑁

10. Model-Calculated Spectra
ADAF: Advection-Dominated Accretion Flow
F. Yuan et al., JPJ. 598, p301(2003)
𝑪=σ𝜙𝑁

11. Estimation of the ionization rate of H2 (1): 1-25 keV
W(H2) = 36.5 eV for one ion pair creation
X-ray source
at GC
Observation
at the Earth
Electrons
140 pc
8 kpc
Ions
Central Molecular Zone
Interstellar Medium
(L = 4.2×1037 erg/s)
R = 100 pc
NH=6×1022 cm-2
L = 2.1×1037 erg/s
Ionization rate of H2 by photoabsorption
Fs = (6.3×103 cm-2s-1) (0.38×10-24 cm2) = 2.4×10-21 s-1 ,
3×10-19 s-1
 = 1𝑘𝑒𝑉 25𝑘𝑒𝑉 F(E)s(E) ( 𝐸 𝑊 )𝑑𝐸 = 2.6×10-19 s-1
𝑪=σ𝜙𝑁

12. Estimation of the ionization rate of H2 (2)
The Ionization rate of H2 by photoabsorption increases when
the energy spectrum extends below 1 keV by use of the ADAF model
Cut-off Energy
ξ [1/sec]
13.5 eV
1.3×10-13
100 eV
1.2×10-15
200 eV
1.3×10-16
200 eV: Upper Limit of ξ (Attenuation Length = ~1 pc)
300 eV
2.0×10-17
550 eV
2.8×10-18
1 keV
2.6×10-19
Magnified from 10 eV to 100 keV
𝑪=σ𝜙𝑁

13. Result of this work
Obtained the oritinal luminosity of X-rays with the energy spectrum from the observation above 1keV as well as the model calculation below 1 keV
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 eV
We obtained 3×10-19 s-1 from our previous work above 1 keV, and a new value of 1.3×10-16 s-1 for 0.2 keV (cut-off) from the ADAF model
<< Infrared spectrum of H3+ : 3×10-15 s-1(CMZ)*

14. 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 H3+ .
Ionization process of H2 in the CMZ by Cosmic-ray excitation