Hard Cosmic Ray Sea in the Galactic Center: a consistent - - PowerPoint PPT Presentation

Hard Cosmic Ray Sea in the Galactic Center:

a consistent interpretation of H.E.S.S. and Fermi-LAT 𝛿-ray data

Dario GRASSO (INFN and Università Pisa) in coll. with:

• D. GAGGERO (GRAPPA, Amsterdam)
• A. MARINELLI (INFN, Pisa)
• M. TAOSO (IFT, UAM/CSIC Madrid)
• A. URBANO (CERN)

S. VENTURA (INFN and Università Pisa)

Diffuse Cosmic Rays Shining in the Galactic Center: A Novel Interpretation of H.E.S.S. and Fermi-LAT γ-Ray Data

• D. Gaggero,1,* D. Grasso,2,† A. Marinelli,2,‡ M. Taoso,3,§ and A. Urbano4,∥

1GRAPPA, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, Netherlands 2INFN Pisa and Pisa University, Largo B. Pontecorvo 3, I-56127 Pisa, Italy 3Instituto de Física Teórica (IFT), UAM/CSIC, Cantoblanco, 28049 Madrid, Spain 4CERN, Theoretical Physics Department, 1211 Geneva, Switzerland

(Received 10 February 2017; revised manuscript received 26 April 2017; published 17 July 2017) We present a novel interpretation of the γ-ray diffuse emission measured by Fermi-LAT and H.E.S.S. in the Galactic center (GC) region and the Galactic ridge (GR). In the first part we perform a data-driven analysis based on PASS8 Fermi-LAT data: We extend down to a few GeV the spectra measured by H.E.S.S. and infer the primary cosmic-ray (CR) radial distribution between 0.1 and 3 TeV. In the second part we adopt a CR transport model based on a position-dependent diffusion coefficient. Such behavior reproduces the radial dependence of the CR spectral index recently inferred from the Fermi-LAT

• bservations. We find that the bulk of the GR emission can be naturally explained by the interaction of the

diffuse steady-state Galactic CR sea with the gas present in the central molecular zone. Although we confirm the presence of a residual radial-dependent emission associated with a central source, the relevance

• f the large-scale diffuse component prevents to claim a solid evidence of GC pevatrons.

DOI: 10.1103/PhysRevLett.119.031101

PRL 119, 031101 (2017) P H Y S I C A L R E V I E W L E T T E R S

week ending 21 JULY 2017

(/sites/default/files/thumbnails/image/glap0440.png) July 18, 2017

Gamma-ray Telescopes Reveal a High-energy Trap in Our Galaxy's Center

A combined analysis of data from NASA's Fermi Gamma-ray Space Telescope and the High Energy Stereoscopic System (H.E.S.S.) (https://www.mpi-hd.mpg.de/hfm/HESS/), a ground-based

• bservatory in Namibia, suggests the center of our Milky Way contains a "trap" that concentrates

some of the highest-energy cosmic rays, among the fastest particles in the galaxy. "Our results suggest that most of the cosmic rays populating the innermost region of our galaxy, and especially the most energetic ones, are produced in active regions beyond the galactic center and later slowed there through interactions with gas clouds," said lead author Daniele Gaggero at the University of Amsterdam. "Those interactions produce much of the gamma-ray emission observed by Fermi and H.E.S.S." Cosmic rays are high-energy particles moving through space at almost the speed of light. About 90 percent are protons, with electrons and the nuclei of various atoms making up the rest. In their journey across the galaxy, these electrically charged particles are affected by magnetic fields, which alter their paths and make it impossible to know where they originated. But astronomers can learn about these cosmic rays when they interact with matter and emit gamma rays (https://svs.gsfc.nasa.gov/10690), the highest-energy form of light. Fermi Space Telescope (http:/ /www.nasa.gov/mission_pages/GLAST/main/index.html)

(/)

Search

H.E.S.S. Nature 2006

• The diffuse emission from the CMZ

is harder (𝛥 ≃ 2.3 ) than expected from the hadron interaction of Galactic cosmic rays (CR) if their spectrum is the same as that at the Earth (𝛥 ≃ 2.7 )

• A freshly accelerated (harder)

component was invoked

4

H.E.S.S. Nature 2016 + arXiv 1706.04535

5

250 hours of observation of the 𝛅-ray diffuse emission from 200 GeV to 50 TeV of the central molecular zone (CMZ) region

H.E.S.S. Nature 2016 The PeVatron scenario

359.0 359.5 00.0 00.5 01.0 Galactic longitude (degrees) –00.6 –00.4 –00.2 +00.0 +00.2 +00.4 +00.6 Galactic latitude (degrees) Sgr A*

a

359.5 00.0 Galactic longitude (degrees) –00.4 –00.2 +00.0 +00.2 +00.4 Sgr A*

b

–1.4 –0.5 1.9 7.8 23.0 61.7 160.0

Figure 1 VHE -ray image of the Galactic Centre region. molecular gas, as traced by its CS line emission30. Black star, location of

X 10

• Diffuse emission traces gas (from CO,

CS emission), strong losses ➡ hadronic emission

• the diffuse emission around J1745-290

(positionally compatible with SgrA* ) extends up to ~ 50 TeV ➡ CR protons up to ~ PeV

• J1745-290 is suggested be the source of

the CR producing the diffuse emission that requires (too ?) strong attenuation

6

Same spectra in the ridge ( | l | < 1° , | b | < 0.3° ), d < 150 pc ΓHESS17 = 2.28 ± 0.03stat ± 0.2sys and in the “pacman” 0.15° < 𝜄 < 0.45° , 22 < d < 67 pc ΓHESS16 = 2.32 ± 0.05stat ± 0.11sys

H.E.S.S. Nature 2016 + arXiv 1706.04535

7 pacman ridge

Same spectra in the ridge ( | l | < 1° , | b | < 0.3° ), d < 150 pc ΓHESS17 = 2.28 ± 0.03stat ± 0.2sys and in the “pacman” 0.15° < 𝜄 < 0.45° , 22 < d < 67 pc ΓHESS16 = 2.32 ± 0.05stat ± 0.11sys

H.E.S.S. Nature 2016 + arXiv 1706.04535

8 pacman ridge

It is suggested that both are originated by a freshly accelerated CR population at the GC

We propose a new interpretation accounting for two recently observed features which do not fit the conventional scenario

• 1. the CR hardening found by PAMELA,

AMS and CREAM @ 300 GeV/ nucleon

• 2. the CR proton spectral index radial

gradient found in the Fermi-LAT data

ADDING NEW PIECES TO THE PUZZLE

9

Fermi-LAT coll. 2016

• L. Tibaldo

Interstellar gamma-ray emission

proton spectral index

• 3.1
• 3
• 2.9
• 2.8
• 2.7
• 2.6
• 2.5
• 2.4
• 2.3

(c)

Normalized star formation rate Galactocentric radius (kpc) 5 10 15 20 25 30

proton spectral index

CR hardening @ 300 GeV/n CREAM coll. ApJ Lett. 2010 PAMELA coll. SCIENCE 2011 AMS-02 coll. PLR 2015 CALET coll. , P. Marrocchesi this conference [CRD145]

10

If the effect is present in the whole Galaxy - as expected if due to CR propagation (see e.g. Blasi et al. 2012, 2015) - it should affect the diffuse 𝝳-ray emission spectrum:

Thoudam & Hoerandel 2013

AMS-02

CR spectral index radial gradient

11

Fermi-LAT coll. 2016

• L. Tibaldo

Interstellar gamma-ray emission

proton spectral index

• 3.1
• 3
• 2.9
• 2.8
• 2.7
• 2.6
• 2.5
• 2.4
• 2.3

(c)

Normalized star formation rate Galactocentric radius (kpc) 5 10 15 20 25 30

proton spectral index

(b)

GALPROP Fermi LAT collab. ApJ 750 2012 3A DRAGON Gaggero+ PhRvD 91 2015 083012

Yang, Aharonian & Evoli 2016 Yang, this conference [GA015]

inferred from 𝛅-ray diffuse emission longitude dependence along the Galactic plane for E > 10 GeV

CR spectral index radial gradient

Gaggero, Urbano, Valli & Ullio PRD 2015

12

The model reproduces CR spectra and the diffuse 𝛅-ray spectrum on the whole sky including the inner Galactic plane where conventional models underestimate the flux

KRA𝛿 model conventional model

The model assumes a radial dependent diffusion coefficient with so that

D(E) = D0 (E/E0) - δ(R)

δ(R) = A R + B

𝛥 (R) = 𝛥 source + δ(R)

Fermi-LAT coll. 2016

The first step above the TeV: solution of the Milagro anomaly Gaggero, D.G., A. Marinelli, Urbano, Valli ApJ L 2015

13

Incorporate the CR spectral hardening in the KRA𝛿 model (assuming it is present in the

whole Galaxy). This automatically reproduces the Milagro flux @ 15 TeV which was 4σ larger than the GALPROP prediction testable by HAWC (work in progress) and CTA !

H.E.S.S. + Fermi-LAT Gaggero, D.G., A. Marinelli, Taoso & Urbano, PRL 2017 + S. Ventura

14 10−2 10−1 100 101

E [TeV ]

10−7 10−6 10−5 10−4 10−3

E2 dΦ/dEγ/dΩ [GeVcm−2s−1sr−1]

Galactic Ridge

Hard diffusion

Conventional diffusion

Comparison with HESS 2017

Gamma model Base model Fermi Data PASS8 HESS Data 2017 Best Fit HESS+Fermi Gamma model Base model Fermi Data PASS8 HESS Data 2017 Best Fit HESS+Fermi

PASS8 Fermi-LAT 470 weeks of data extracted with the v10r0p5 Fermi tool. Point sources from the 3FGL catalogue subtracted.

| l | < 1° , | b | < 0.3°

H.E.S.S. + Fermi-LAT Gaggero, D.G., A. Marinelli, Taoso & Urbano, PRL 2017 + S. Ventura

15 10−2 10−1 100 101

E [TeV ]

10−7 10−6 10−5 10−4 10−3

E2 dΦ/dEγ/dΩ [GeVcm−2s−1sr−1]

Galactic Ridge

Hard diffusion

Conventional diffusion

Comparison with HESS 2017

Gamma model Base model Fermi Data PASS8 HESS Data 2017 Best Fit HESS+Fermi Gamma model Base model Fermi Data PASS8 HESS Data 2017 Best Fit HESS+Fermi

| l | < 1° , | b | < 0.3°

Power-law best fit of both data sets Ridge: 𝚾(1 TeV) = (1.19 ± 0.04) × 10^(-8) (TeV cm^2 s sr)^(-1) Γ = - 2.41 ± 0.02 𝚾(1 TeV) = (1.33 ± 0.06) × 10^(-8) (TeV cm^2 s sr)^(-1) Γ = - 2.45 ± 0.02 Pacman:

The effect of the new CR sea at the GC

Gaggero, D.G., A. Marinelli,

Taoso & Urbano, PRL 21 Jul. 2017 + S. Ventura

16

| l | < 1° , | b | < 0.3°

10−2 10−1 100 101

E [TeV ]

10−7 10−6 10−5 10−4 10−3

E2 dΦ/dEγ/dΩ [GeVcm−2s−1sr−1]

Galactic Ridge

Hard diffusion

Conventional diffusion

Comparison with HESS 2017

Gamma model Base model Fermi Data PASS8 HESS Data 2017 Best Fit HESS+Fermi Gamma model Base model Fermi Data PASS8 HESS Data 2017 Best Fit HESS+Fermi

Degeneracy between poorly known gas and CR source densities at the GC We use the [Ferriere 2007] 3-D gas model choosing a normalisation (XCO ) such to match the gas column density maps adopted by HESS

CR energy density for 0.1 < E < 3 TeV

17

10−2 10−1 100 101

E [TeV ]

10−7 10−6 10−5 10−4 10−3

E2 dΦ/dEγ/dΩ [GeVcm−2s−1sr−1]

Galactic Ridge

Hard diffusion

Conventional diffusion

Comparison with HESS 2017

Gamma model Base model Fermi Data PASS8 HESS Data 2017 Best Fit HESS+Fermi Gamma model Base model Fermi Data PASS8 HESS Data 2017 Best Fit HESS+Fermi

10−2 10−1 100 101

E [TeV ]

10−7 10−6 10−5 10−4 10−3

E2 dΦ/dEγ/dΩ [GeVcm−2s−1sr−1]

Pacman region

Hard diffusion

Conventional diffusion

Comparison with HESS

Gamma model Base model Fermi Data PASS8 HESS Data 2016 Best Fit HESS+Fermi Gamma model Base model Fermi Data PASS8 HESS Data 2016 Best Fit HESS+Fermi

No evidence a local component taking over at high energy ! Very good agreement with large scale model of the diffuse emission based on Fermi data

The effect of the new CR sea at the GC

Gaggero, D.G., A. Marinelli,

Taoso & Urbano, PRL 21 Jul. 2017 + S. Ventura

Future perspectives

18

CTA may observe more external clouds where the PeVatron scenario predicts a lower CR density than that expected in our scenario

19

• CR advect/diffuse in self-generated

Alfvén-waves below/above ∼ 50 GeV ⇒ harder CR spectrum if advection dominates

• the effect is larger in the inner Galaxy

(diffusion takes over at larger energies) This mechanism however should be absent for E > 100 GeV this seems to be at odd with Fermi data and Milagro and HESS anomalies

advection dominated diffusion dominated

diffusion coefficient

A possible theoretical interpretation for the radial hardening ( I )

Recchia, Blasi & Morlino, 2016

A possible theoretical interpretation for the radial hardening (II) Cerri, Gaggero, Vittino, Evoli & DG, to be submitted. 2017

• A. Vittino [ CRD045 this conference ]

20

Dij (x,ρ) = [ D|| (x,ρ) - D⊥(x,ρ) ] bi bj + D⊥(x,ρ) δij Anisotropic CR diffusion in the presence of a poloidal magnetic field component

based on Jansson & Farrar 2012

D|| and D⊥ are expected to have different rigidity dependence ( Blasi, De Marco, Stanev 2007 and Snodin et al. 2012 ) found D|| ∝ ρ1/3 D|| ∝ ρ1/2

for Kolmogorov turbulence.

We incorporated this behaviour in the DRAGON 2 code ( Evoli, Gaggero, Vittino,, Di Bernardo, Ligorini, Di Mauro, Ullio, DG , JCAP 2017) allowing for anisotropic diffusion

2 4 6 8 10 12 14

R [kpc ]

2.0 2.2 2.4 2.6 2.8 3.0 3.2

Proton spectral index

Mainly parallel CR escape Mainly perpendicular CR escape

Fermi-LAT analysis Gaggero+2015 analysis ⊥ = 0.5, ✏D = 0.01 ⊥ = 0.6, ✏D = 0.01 ⊥ = 0.7, ✏D = 0.01 ⊥ = 0.5, ✏D = 0.01 ⊥ = 0.6, ✏D = 0.01 ⊥ = 0.7, ✏D = 0.01

CONCLUSIONS

21

• H.E.S.S. and Fermi-LAT data are consistent showing the presence of a single CR

component in the CMZ region with index ～ 2.4

• The large scale, steady-state, CR Galactic population can nicely account for that

emission if computed with a model accounting for the radial gradient of the CR spectral index found in the Fermi data (if it extends beyond 100 GeV) and the CR spectral hardening found by Pamela, CREAM, AMS (if it is present in the whole Galaxy)

• CTA may confirm the scenario we proposed observing the emission from molecular

clouds at distances > 200 pc from the GC

• The neutrino emission from the Galactic ridge is expected to be significantly

enhanced under those conditions and be detectable by IceCube (see C. Haak [NU013] ) and KM3NeT (see R. Coniglione [NU034] and A. Marinelli talks [NU 136] at this conference)

• Those results strongly motivate to go beyond conventional modelling of CR

propagation in the Galaxy

BACKUP SLIDES

Sgr B

23

0.4° < l < 0.9° , - 0.3° < b < 0.2°

10−2 10−1 100 101

E [TeV ]

10−7 10−6 10−5 10−4 10−3

E2 dΦ/dEγ/dΩ [GeVcm−2s−1sr−1]

Sagitarius B region

Hard diffusion

Conventional diffusion

Models + HESS + Fermi-LAT for Sagittarius B

Base Model Gamma Model Fermi Data PASS8 HESS Data SgrB Best Fit HESS+Fermi Base Model Gamma Model Fermi Data PASS8 HESS Data SgrB Best Fit HESS+Fermi

The spectrum in the ridge with/without CR spectral hardening @ 300 GeV/n

24

| l | < 1° , | b | < 0.3°

This seems to require that the hardening found in the proton and He spectrum by Pamela and AMS is present in the whole Galactic disk !

10−2 10−1 100 101

E [TeV ]

10−7 10−6 10−5 10−4 10−3

E2 dΦ/dEγ/dΩ [GeVcm−2s−1sr−1]

Galactic Ridge

Hard diffusion

Comparison with HESS 2017

Gamma model Fermi Data PASS8 HESS Data 2017 Best Fit HESS+Fermi Gamma model Fermi Data PASS8 HESS Data 2017 Best Fit HESS+Fermi

Gamma model without hardening Gamma model with hardening

“ without hardening

The CR energy density radial profile

Implications for neutrino astronomy Gaggero, D.G., A. Marinelli, Urbano,

Valli ApJ L 2015

ANTARES coll. , Phys. Lett. B, 2016

ANTARES coll. + D. Gaggero, D.G. arXiV:1705.00497

26

• On the whole sky the diffuse flux due to the Galaxy is

8 % (4 % for conventional models) of that measured by IceCube

• In the inner Galactic plane however the gain is

much larger ➡

• A neutrino telescope in the North hemisphere is

more suited to detect the Galactic component. IceCube coll. is using our templates to look for this Galactic component. ANTARES present upper limit is at 1.25 times our most optimistic prediction. Observable by KM3NeT (work in progress ) !