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Ionization of H2 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 InstituteUniversity of Chicago
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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 10 keV in the region needs to be estimated
Research Background
(*) T. Oka et al., ApJ. 632, p.882 (2005)
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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
Purpose of this research
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 H2 gas in the CMZ using the corrected X-ray flux
X-rays
EarthGC CMZ(H2)
Interstellar Medium(foreground gas)
8 kpc140 pc
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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]1988 1990 Spring 1990 Fall 1991 Spring
Sgr A*/1E1742.5-2859 1.66E+36 1.06E+36 8.80E+35A1742-294 1.30E+37 4.48E+36 5.58E+36 2.50E+361E1740.7-2942 3.00E+36 8.28E+36 5.50E+36 9.40E+35GRS1741.9-2853 1.64E+36SLX1744-299 2.90E+36 3.76E+36 1.46E+361E1743.1-2843 2.30E+36 2.03E+36 2.03E+36 1.13E+36
(Total) 2.07E+37 2.10E+37 1.79E+37 6.91E+36
8 kpc * tan 1° = 140 pcCentral Molecular Zone (CMZ):
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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)
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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)
0.02 0.2 20.0E+00
1.0E+02
2.0E+02
3.0E+02
4.0E+02
5.0E+02
6.0E+02
7.0E+02HHe+(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)
E [keV]
σ(E)
*E3
[bar
n· k
eV3]
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Interstellar extinction map
NH=(68)×1022 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.
NH
Color excess
𝐸 (𝐵−𝑉 )
( 𝐽−𝐾 )
𝐴𝑉 𝑁𝐻
[cm-2]
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Corrected spectrum
0.16 1.6 16 1601E-07
1E-06
1E-05
1E-04
1E-03
1E-02
Observed by Belanger2006
Corrected Data (This work)
E [keV]
Inte
nsit
y(E)
[Cou
nt/s
ec/k
eV]
NH=6×1022 cm-2
between the GCand the Earth
Enhancement of Low-energy X-ray intensity
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(L = 4.2×1037 erg/s)
X-ray sourceat GC
L = 2.1×1037 erg/s
Central Molecular Zone
Electrons
Ions
Estimation of the ionization rate of H2
R = 100 pc
Interstellar Medium
NH=6×1022 cm-2
Observationat the Earth
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
= = 2.6×10-19 s-1
W(H2) = 36.5 eV for one ion pair creation
8 kpc140 pc
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Result of this work
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(H2)=36.5 eV
We obtained 3×10-19 s-1 from our model
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Conclusion
Estimated the ionization rate of H2 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 H3
+ .
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Simple model
• Luminosity: L = 4.2×1037 erg/s = 2.63×1049 eV/s
• For 5-keV Photons: Ix = L/5 keV = 5.25×1045 photons/s
• Radius of the CMZ sphere: R = 100 pc = 3.09×1020 cm
• Brightness of X rays on the sphere at radius R:
F = Ix/(2pR2) = (5.25×1045)/(1.2×1042) *2= 6.3×103 cm-2 s-1
• 5-keV ionization cross section for H2:
s ~ 0.38 barn - Chantler’95
• Thus, the ionization rate by photoabsorption:
Fs = (6.3×103 cm-2s-1) (0.38×10-24 cm2) = 2.4×10-21 s-1
(Not for the total number of ions)
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H/H2Photoabsorption 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 H2