Internal Irradiation of the Sgr B2 Molecular CloudCasey Law

Northwestern University, USA

A reanalysis of archived X-ray and radio observations to understand the cause of fluorescent iron line emission in Sgr B2.

Collaborators: F. Yusef-Zadeh, M. Fromerth, and F. Melia

Outline: 1) Fluorescent iron emission in the GC 2) X-ray observations of diffuse and compact sources 3) Is a Sgr A* flare needed?

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- Sgr B2 shows strong fluorescent iron emission at 6.4 keV (Koyama et al. 1996).

- Line emission can be explained by Sgr A* flare 106 times its current luminosity.- Sgr A* flare ended in the past 300 years and lasted dozens of years.

Sgr B2 Fluorescent Iron and Sgr A*

Could there be an alternate explanation?

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(Left) 20cm radio continuum and (Right) Chandra smoothed fluorescent iron line flux of Sgr B on the same scale.

Other Fluorescent Sources in the GC

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G0.13 – 0.13- EW ~ 0.9 keV- GC absorption, no iron absorption edge- brightest emission at edges, near NRFs

Sgr C - EW ~ 0.5, 1.5 keV - GC absorption - one source near NRF

Arches Cluster - EW ~ 0.8 keV - GC absorption - X-ray luminous cluster

6.4 keV with molecular gas contours

Radio gray with X-ray continuum contours

X-ray continuum in color and contours

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Morphology of Line Emission

- “Tilemap” method fits spectra and maps spectral parameters.

- Significant fluorescent emission throughout Sgr B.

(Left) Sgr B fluorescent iron flux according to tilemap and (Right) adaptive smoothing.

Spectral Modeling of Diffuse X-ray Emission

Continuum Properties:- Highly absorbed: N

H ~ 4-6 x 1023 cm-2

- Continuum can be modeled as 1) power law: ~ 0.6 2) power law + thermal bremsstrahlung: kT ~ 1 keV, ~ 0, with similar 2-10 keV fluxes

Line Properties:

- Iron Kα line Equivalent Width ~ 1.5 keV

- Iron Kα luminosity: 1.5e34 ergs s-1 Fluorescence likely caused by irradiation. Hard spectrum required, but thermal not

- Strong iron edge at 7.1 keV excluded.

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Sgr B2 diffuse X-ray spectrum

Embedded Compact X-Ray Sources

X-rays with radio continuum contours from Takagi et al. (2002)

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Source 13: - Thermal bremss with kT ~ 1 keV - N

H ~ 4 x 1023 cm-2

L(2-10 keV) ~ 5 x 1033 ergs s-1

X-ray emission fills faint radio-continuum shell.

Source 10: - Power law with ~ 0-1 - N

H ~ 2-4 x 1023 cm-2

- 6.5 keV line with EW ~ 2-3 keV

L(2-10 keV) ~ 5 x 1032 ergs s-1

“Warm” fluorescent iron.

Is a Sgr A* Flare Needed? Morphology

(Above) Fluorescent iron with HII regions, masers and hot cores. (Below) CH

3CN from de Vicente et al. (1997)

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or does shapeshow intrinsicstructure?

Does the shape require external irradiation?

Fluorescent iron with molecular linecontours from Murakami et al. (2001)

Can Internal Sources Cause Fluorescence?

Required hard X-ray flux: I8

req = 7x1033 (4T) (6.6x10-5/∞

Fe) ergs s-1 keV-1

(Sunyaev & Churazov 1998)

Observed sources:For =4 and

T=0.25 ==> I

8obs = 0.005 I

8req

In total, the two X-ray point sources can explain at least 0.5-1% of fluorescent emission.Scaling by radio continuum ==> all 50 UCHII regions can explain 5-10% of emission.Consistent with wind/ISM shocks, where 1% of wind luminosity ==> ~1 keV gas: I

8ws = 7x1032 (M/2x10-6 M◦ yr-1) (v∞/2800 km s -1)2 (N

UCHII/50) (Smith et al. 2005)

What more might be expected from internal sources?- Colliding wind binaries? I

8cwb ~ 1032-34 ergs s-1 keV-1 (Portegies-Zwart et al. 2002)

- Scaling diffuse X-ray continuum by radio diffuse-to-compact flux ratio: L

2-10 keV ~ 7x1034 ergs s-1 must be hidden by cloud.

The Sgr B2 molecular cloud can easily hide these sources from detection with NH ~ 1024-25 cm-2.

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Conclusions

1) Some, but not necessarily all, irradiation flux is nonthermal.

2) Sgr B2 fluorescent morphology seems to follow intrinsic gas conditions.

3) Observed X-ray sources cause 0.5-1% of Sgr B2 fluorescence.

4) Colliding-wind binaries and wind-ISM shocks can account for significant amounts of fluorescence and may remain undetected.

Possible test: Check for morphological variability in fluorescent emission.

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- Sgr B2 shows strong fluorescent iron emission at 6.4 keV (Koyama et al. 1996).

- Line emission can be explained by Sgr A* flare 106 times its current luminosity.- Sgr A* flare ended in the past 300 years and may have lasted >70 years.

Sgr B2 Fluorescent Iron and Sgr A*

Could there be an alternate explanation?

5th APC – High Energy Phenomena in the GC, Paris

(Left) Adaptively smoothed and (Right) “Tilemap” of fluorescent iron line flux.

Can Internal Sources Cause Fluorescence?

Required hard X-ray flux: I8

req = 7x1033 4T 6.6x10-5/∞

Fe ergs s-1 keV-1

(Sunyaev & Churazov 1998)

For =4 and T=0.25 ==> I

8obs = 0.005 I

8req

In total, the two X-ray point sources can explain at least 0.5-1% of fluorescent emission.

What more might be expected from internal sources?

- Colliding wind binaries? I8

cwb ~ 1032-34 ergs s-1 keV-1 (Portegies-Zwart et al.

2002)

- Scale by radio continuum ==> all UCHII regions have I8

uchii = 5-10x1032 ergs s-1 keV-1 (5-10%)

- Young stellar wind shocks? 1% of wind luminosity ==> ~1 keV gas: (Smith et al.

2005) I

8ws = 7x1032 (M/2x10-6 M⊙ yr-1) (v∞/2800 km s -1)2 (N

UCHII/50)

- Early-type stars? I8

es ~ 5x1030 ergs s-1 keV-1 (1 Ori C, Schulz et al.

2003)

The Sgr B2 molecular cloud can easily hide these fluxes from detection with NH ~ 1024-25 cm-2.5th APC – High Energy Phenomena in the GC, Paris