Sabrina Casanova

Max Planck Institut fuer Kernphysik, Heidelberg

TeV Diffuse Galactic Gamma-Ray Emission

Surveys of the sky at TeV energiesMilagro: Northern sky

H.E.S.S. : Southern sky

Outline

1. CR origin and propagation are not well known phenomena

2. Diffuse gamma-ray emission: a unique probe of CR origin and propagation

3. The origin of Galactic Cosmic Rays: searching for the Galactic Pevatron Sources

4. Cosmic Ray Propagation in the Galaxy

• Diffuse gamma rays from molecular clouds close to CR sources

• Large scale TeV diffuse emission

5. On the level of the CR background

6. Summary & Future experimental goals

1. CR origin and propagation are not well known phenomena

The Cosmic Ray Spectrum at Earth

Galactic Origin

Extragalactic Origin

δ = -2.7

δ = -3.0

• Almost featureless spectrum

• Galactic accelerators have to inject particles up to at least PeV energies

• Isotropy up to very high energy: 1 TeV proton in B=1uG, Gyro-Radius = 200AU= 0.001pc

Knee ~ 3 x10 15 eV

Standard Model of Cosmic Rays

observed

observedsourceescape dE

dN Q

2.4- to-2E sourceQAcceleration : DSA in SNRs

Energy dependent Escape0.7- to-0.3E escape

Key Questions in High Energy Astrophysics

The origin of cosmic rays

• What are the cosmic ray accelerators ?

• How high in energy can galactic sources produce particles?

• Are CR accelerators different from electron sources?

• How are cosmic ray sources distributed in the Galaxy ? Is there a nearby (<10's of parsecs) source of cosmic rays ?

The character of propagation of cosmic rays

• How do cosmic rays propagate in the Galaxy ?

• What is the production rate of protons and electrons ?

2. Diffuse Gamma Ray Emission from the Galactic CR population :

a unique probe of CR origin and propagation

Electromagnetic Processes:• Synchrotron Emission

-important for e- energy losses -

– E (Ee/mec2)2 B

• Inverse Compton Scattering

– E f ~ (Ee/mec2)2 E i

• Bremsstrahlung

-more important at GeV energies-

– E ~ 0.5 E e

Hadronic Cascades. Gamma Rays are emitted through decay of neutral pions produced by CRs interacting with the ambient gas. Also secondary electrons and neutrinos are produced.

• p + p ± +o +… e ±+ + +…

-rays from electron and CR Galactic populations

• -rays travel rectilinearly through the Galactic magnetic fields and point back to the production locations

• The highest energy CR particles produce the highest energy -rays.

• Hadronic and leptonic mechanisms produce different energy spectra and -ray morphology. However it is experimentally not easy to detect such differences.

• Gamma ray spectrum and spatial distribution of the diffuse gamma ray emission provide spectra and density of hadrons and leptons in the different regions of the Galaxy

Diffuse TeV -rays from the Galaxy: unique probe of CR origin and propagation

3. The origin of Galactic Cosmic Rays :

searching for the Galactic Pevatron Sources

Are SNRs the Galactic PeVatron sources ?• TeV -rays and shell type

morphology : acceleration of e-

and protons

• There is a significant γ-ray flux (4.8 σ) above 30 TeV. In the hadronic scenario, this implies efficient proton acceleration up to 200 TeV with slope = -2 and energy in CRs of about 1050

erg/s. In the leptonic scenario this implies acceleration up to 100 TeV because of KN losses.

Not yet the knee.

• Correlation TeV/X-ray.

• IC and hadronic scenarios can both explain the -ray emission. We do not have a conclusive proof of CR acceleration.

00 exp

E

EEI

dE

d

Γ= 2.04±0.04

E0 = 17.9 ± 3.3 TeV

00 exp

E

EEI

dE

d

SNR RX J1713-3946 seen in TeVs (H.E.S.S. Collaboration)

Variability of X-rays on year timescales –witnessing particle acceleration in real time

Uchiyama, Aharonian, Tanaka, Maeda, Takahashi, Nature 2007

•flux increase - particle acceleration

• flux decrease - synchrotron cooling

• it requires B-field of order 1 mG in

hot spots and, most likely, 100G

outside. This supports the idea of

amplification of B-field by in strong

nonlinear shocks through non-resonant

streaming instability of charged energetic

particles (Bell, 2000

Zirakashvili, Ptuskin Voelk 2007)

DSA in SNRs accelerates electrons -> DSA is likely to accelerate protons. However :

Quantify the electron vs proton content in SNRs : is VHE emission from SNRs leptonic or hadronic in nature ?

What is the max proton energy achieved ?

How do CRs escape SNRs and diffuse in the ISM ?

Are SNRs the Galactic PeVatron sources?

– PeV particles are accelerated at the beginning of Sedov phase (~200yrs), when the shock speed is high. PeV particles quickly escape as the shock slows down. Lower energy particles are released later.

– Even if a SNR accelerates particles up to PeV energies, a SNR is a PeVatron for a very short time

– There is still no evidence for the existence of escaping CRs

4. Cosmic Ray Propagation in the GalaxyDiffuse gamma rays from molecular clouds close to CR sources

Gamma Rays from Molecular Clouds close to CR sources

MCs are ideal targets to amplify the emission produced by CRs accelerated by nearby sources (Aharonian & Atoyan, 1996, Aharonian, 2001)

Highest energy protons quickly escape the accelerator and therefore do not significantly contribute to gamma-ray production inside the proton accelerator-PeVatron. Gamma rays from nearby MCs are therefore crucial to probe the highest energy protons .

Cosmic ray flux in MCs close to a SNR: dependence on SNR age and location

Gabici et al, 2009

1 = 500 yr2 = 2000 yr3 = 8000 yr4 = 32000 yr

Power in CRs: ECR = 3 X 1050 erg,

Diffusion coefficient: D = D0 E0.5

GeV versus TeV emission spectra in MC

Gabici et al 2009

M=105 solar masses; R=20pc ;n =120 cm-3 ; B=20G; D=1kpc ;D =1028 cm2 /s

GeVsteady

CR sea TeVt-dependent

SNR CRs

Concave shapeSteep spectrumat GeV energiesand hard at TeV

1 = 500 yr2 = 2000 yr3 = 8000 yr4 = 32000 yr

MWL implications

• GeV-TeV connection • ...and PeV-hard X connection (pp

interactions produce secondary electrons)

Ee = 100 TeV

keVTeV

Ee

GEsyn

2

1003020

B

Multiwavelength emission close to MCsM=105 solar mass; R=20pc ;n=120cm-3 ; B=20G ; d (SNR/MC)=100pc ; D=1kpc

X-RAYS GAMMA RAYSRADIO

t = 500 yrt = 2000 yrt = 8000 yrt = 32000 yrt = ∞ yr

Synchrotron of secondaries and primary e-

The gamma-ray emission is an order of magnitude higher than the emission at other wavelengths. So far unidentified TeV sources (dark sources) could be MCs illuminated by CRs from a nearby SNR.

Gabici et al, 2009

CLOUD

CRs

Assumptions :

• Energy in CRs: ECR = 3 X 1050 erg

• Injection spectrum: f(E) = K E-2

• Diffusion coefficient: D = D0 E0.5

• Source age and location t= 1600 yr, d = 1kpc

gammas

Modeling particle escape from RXJ1713-3946

SNR

CRs

CRs

gammas

gammas

CRs of about 150TeV are escaping from the shell now

Zirakashvili & Aharonian, 2010

Variety of CR spectra close to RXJ1713

c

TeV position dependent

GeVsteady

Casanova et al, 2010

Constraints on diffusion regime

Variety of -ray spectra close to RXJ1713

D = 1026 E0.5 cm2/s at 10 GeV

SNR RXJ1713

SNR RXJ1713

CR sea (Runaway CRs + sea CRs) /sea CRs

6 TeV

The emission from the environment of RX J1713 provides clues concerning the SNR accel history.

Morphological studies of the emission close to CR sources: constraining CR diffusion regime

D0 = 1028 E0.5 cm2/s at 10 GeV D = 1027 E0.5 cm2/s at 10 GeV

Casanova et al, 2010

Molecular Clouds close to the SNR W 28

H.E.S.S. collaboration, F. Aharonian et al., and Y. Moriguchi, Y. Fukui, NANTEN, 2008

W28 : an old SNR and nearby molecular clouds.The VHE/molecular cloud

association could indicate a hadronic origin for HESS J1801-233 and HESS

J1800-240

(Aharonian et al, 2006)

Spectral index2.29 ± 0.07 ± 0.20implies harderCR spectrum than insolar neighborhood Proximity of accelerator and target

HESS observations of the diffuse emission from the GC

Molecular Clouds close to the SNR IC 443•seen by MAGIC & VERITAS

• 12CO emission line intensity by Dame et al, 2001 (cyan line)

•contours of 20 cm VLA radio data from Condon et al. (1998) (green)

•1720 MHz OH maser indication for a shock in a high matter density environment (black point)

•Xray contours from Rosat (purple) (Asaoka & Aschenbach 1994)

•Gamma –ray contours from EGR-ET (Hartman et al. 1999) (black)

•gamma-ray excess coincides with cloud and masers

•soft spectrum: Γ = 3.1 ± 0.3

[Albert et al. ApJ 664L 87A 2007

4. Cosmic Ray Propagation in the Galaxy Large scale TeV diffuse emission

Milagro analysis of Galactic TeV diffuse emission

Flux profiles ← Source subtraction + offset/base

GALPROP Model hadronicleptonictotal

(Abdo et al, 2008)

Galactic Latitude Profile

The narrow distributions of Milagro data points agrees better with a dominant hadronic component.

Below HorizonCygnus Region

GALPROP (optimized)

Sources SubtractedAbdo et al, 2008

2.7x

GP

1.5x

GP

Longitude Profile |b|<2°

Galactic Longitude Flux Profile

There is an excess of diffuse TeV gamma rays from the Galactic plane• Additional unresolved sources (Casanova & Dingus, 2008)?• Cosmic-ray acceleration sites? Most significant discrepancy

between data and model predictions in the Cygnus region.

Diffuse Emission from the Cygnus Region

Cygnus Region: Matter Density Contours overlaying Milagro Obs.

Strong & Moskalenko

GALPROP model of Cygnus Region

standard

optimized

Inverse Compton

Pin

bremsstrahlung

Abdo et al, 2007

Pevatrons in the Cygnus Region ?

100 pc

MGRO J2031+41

The extension of the entire Cygnus region is about 300 pc, and thus a single accelerator might influence strongly the entire region

In fact only one or at most a few strong young accelerators in the Cygnus region are needed to explain the excess emission measured by Milagro with respect to GALPROP

5. On the level of the cosmic ray background

Average CR flux (“CR sea”) is produced by the effective mixture of CRs from individual sources diffusing in the Galaxy for times of 107 years

CR at Earth is representative of the average CR flux ?

Observational test : MCs as cosmic ray barometers• Assuming CR sea = CR at Earth• An observed gamma-ray flux from MC less than

predicted would imply a CR density less than that observed at Earth

On the level of the cosmic ray background

The level of the CR background

100 GeV

only one or at most few massive MCs

many low mass MCs

sr-1 c

m-2 s

-1 T

eV-1

Casanova et al, 2010

CLOUDS

CR sea

gammas

gammas

Flu

x >

1

Ge

vLocalisation of the peak emission along the line of sight

DE

NS

ITY

MA

SS

Casanova et al, 2010

CR= MH2 /dH22

The emission from MCs constrains the CR flux in specific regions of the Galaxy

Mass and integral flux as a function of the distance for the region 345.3<l<346 and 1<b<1.7

Casanova et al, 2010

85% of the emission for the region 345.3<l<346 and 1<b<1.7 is produced between 0.5 and 3 kpc

The CR origin and propagation are still uncertain. TeV diffuse gamma-ray emission is a unique probe

of CR origin and propagation TeV diffuse emission has been observed from MCs

close to CR sources by HESS,MAGIC and Veritas and on a larger scale by Milagro

MCs close to CR sources are ideal targets to amplify the emission produced by CRs penetrating the cloud and to produce very specific observable features in the gamma-ray emission

Diffuse gamma-ray from MCs far away from known CR sources can help constraining the level of the CR sea.

Summary

Future experimental goals

Detect large (>>10G) magnetic field : x-ray telescopes such as Chandra and Astro-H

Observe -ray at hundreds TeV : CTA, AGIS, HAWC

Observed X-rays produced by secondary electrons

Observe neutrinos : Icecube and KM3NET

Image the spectrum and spatial distribution of the Galactic diffuse emission

Compare the source -ray images, spectra, and variability with those from other wavelengths (for instance with radio)

Detect many sources of many different classes : population studies with CTA, AGIS, HAWC

Thanks for the invitation !