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Molecular clouds and gamma rays
Yasuo FukuiNagoya University H. Sano, S. Yosiike, T. Fukuda, H. MatsumotoT. Inoue, S. Inutsuka, R. Yamazaki, T. Tanaka, F. Aharonian, G, Rowell, M. Tavani, A. Guliani,N.McClure-Griffiths+
March 2-4, 2013Physics with GAMMA 400, Trieste
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Interstellar Medium ISM• Molecular clouds: dense neutral gas H2 (2.6mm CO) - density 10^3 cm-3 or higher, Tk=10-20K• Atomic clouds: dense atomic gas HI (21cm HI) - density 1-100 cm-3, Ts=30-100KGamma-rays produced by 1) Hadronic scenario: This has been verified in 2012-13 - cosmic ray CR proton - ISM proton reaction, neutral pions decay into gamma rays2) Leptonic scenario - CR electrons, Inverse Compton (IC) process, CMB etc. Gamma-rays (0.1GeV-100TeV) observed by HESS, MAGIC,
VERITAS, Fermi, AGILE and CTA[2016-]
Molecular clouds and gamma-raysThe origin of the cosmic rays is SNR?- Yes.
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Hadronic Cosmic-ray Interactions
p
p (Inter stellar medium)
p
(Cosmic Microwave Background)
e+e–
Compton scattering
(CMB)
π0
π± → µ± →e±
(CMB)
Target ISM protonsNo real test yet for the targets
By H. Tajima 2006
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CO transitions to probe molecular hydrogen
• Hydrogen molecules are not observable in radio. Too high energy levels. Only in absorption.
• Carbon monoxide CO and others can be observed in rotational transitions.
• Sub-mm transitions from generally higher excited states ratio between J and J’ gives density/temperature.
Left: NANTEN 12CO(1-0) image (beam size : 2.7’) of the W 28 region for VLSR=0 to 10 km/s with VHE γ ray significance contours overlaid (green) -levels 4,5,6σ. The radio boundary of W 28, the 68% and 95% location contours of GRO J1801- 2320 and the location of the HII region W 28A2 (white stars) are indicated. Right: NANTEN 12CO(1-0) image for VLSR=10 to 20 km/s. (Aharonian et al. 2007)
TeV γ vs. CO(J=1-0)
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SNR G347.3-0.5 (RX J1713.7-3946)- Shell-like structure: similar with X-rays- No significant variation of spectrum index across the regions- spatial correlation with surrounding molecular gas
Aharonian et al. 2005
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Ellison et al. 2010 paper
• Uniform model, to account for gamma rays we need high proton density like 10 cm-3
• Then we have too strong thermal X rays, contradicting no thermal X rays (Suzaku)
• This is not a problem if the ISM is highly clumpy; cavity (0.1 cm-3) and dense clumps (1000 cm-3)
• Cavity is the site of particle acceleration and clumps are the target
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Shock propagation into dense gas
n : density of clump n0 : ambient density (=1 cm-3)
10^4 cm-3, t 〜 1000yrs Penetrating Depth = 0.03 pc €
Vsh ~ 3000 km s−1 n n0
Sano et al. 2010
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SNR RXJ1713 summary Gamma-rays corresponds well with interstellar H nuclei, CO+HI, allowing detailed identification of target protons in a density range from 100 to 103 cm-
3. The gas is highly clumpy. Wp ~ 1048erg for 100cm-3: gamma rays ~ Wp x ISM
• Hadronic origin is consistent with the spatial correspondence
• Careful analysis of dense atomic and molecular gas, HI and CO, yields total ISM protons
• Shock-cloud interaction causes gas turbulence and strong B field up to mG (e.g., Uchiyama+ 2007)
RX J0852: CO distribution (interact with the SNR)
image: CO(1-0) I.I. (Vlsr: 24-33 km/s)contours: X-ray (1-5 keV)
CO vs. X-rays
good spatial corresp-ondence between the CO and X-rays
Interacting with the SNR
RX J0852: HI distribution (interact with the SNR)
Image: HI I. I. (Vlsr: 28-34 km/s)contours: X-ray (1-5 keV)
HI vs. X-rays
HI wind bubble at same velocity in CO
ISM cavity created by the progenitor
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TeV gamma-ray SNR RX J0852 ISM Proton Column Density Distributions
Fukui et al. 2013, in prep.
Color: HI+2H2
Contour : TeV g rays
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TeV gamma-ray SNR RX J0852ISM Proton and TeV gamma-ray Distributions
gamma raysISM protonsgood correspondence
ISM protonsas targets for cosmic ray protons
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Gamma-ray spectrum of RXJ1713Abdo et al. 2011
The hard spectrum is not unique to the leptonic scenario
The hard spectrum is explained by energy dependent penetrationof CR protons into dense molecular gas.
Hadronic g-ray spectrum for dense cloud cores Gabici 2007, Zirakasivili Aharonian 2010, Inoue, Inutsuka, Yamazaki, Fukui 2012
accelerated particles in the cavity diffuse into clouds and pions are created
diffusion length :
mass of the cloud penetrated by CR particles ( number of gamma rays)∝
Dense cloud cores [ r ∝ r - 2 ]
Photon spectrum : N(E) ∝ M(E) E -p ∝ E 1/2-p = E -1.5 for p = 2
The same with the leptonic spectrum by IC Hadronic scenario is consistent with the Fermi spectrum
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ISM/gamma-ray comparison
• RXJ1713.7-3946 -Dense molecular rich/clumpy M(H2)=104Mo, M(HI)=104Mo
-Hard gamma-ray spectrum • RXJ0852.0-4622 -Diffuse atomic rich/uniform M(H2)=103Mo, M(HI)=104Mo
-Soft gamma-ray spectrum
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Summary• ISM plays a key role in production of the gamma-rays and X-rays. ISM target protons showing good spatial correspondence with gamma rays
• In RX J1713 and RX J0852, gamma rays are hadronic, CR proton acceleration has been verified -Their total CR proton energy is 10^48 erg• Dense clumps amplify B field via shock-cloud interaction, leading to enhanced X-rays X-ray photon index becomes small toward dense clumps. This may suggest additional acceleration in turbulent magnetic field
• Middle-aged SNRs W44 etc.• GAMMA400/CTA will provide future steps forward