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