Ionization of H 2 by X-rays in the Central Molecular Zone of the Galactic Center Masahiro Notani and...
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Transcript of Ionization of H 2 by X-rays in the Central Molecular Zone of the Galactic Center Masahiro Notani and...
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
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)
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
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):
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)
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]
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]
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
(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
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
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
+ .
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)
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