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)

week ending P H Y S I C A L R E V I E W L E T T E R S PRL 119, 031101 (2017) 21 JULY 2017 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. Urbano 4, ∥ 1 GRAPPA, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, Netherlands 2 INFN Pisa and Pisa University, Largo B. Pontecorvo 3, I-56127 Pisa, Italy 3 Instituto de Física Teórica (IFT), UAM/CSIC, Cantoblanco, 28049 Madrid, Spain 4 CERN, 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 observations. 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 of the large-scale diffuse component prevents to claim a solid evidence of GC pevatrons. DOI: 10.1103/PhysRevLett.119.031101

(/) Search Fermi Space Telescope (http:/ /www.nasa.gov/mission_pages/GLAST/main/index.html) (/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 observatory 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 a ff ected 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.

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 250 hours of observation of the 𝛅 -ray diffuse emission from 200 GeV to 50 TeV of the central molecular zone (CMZ) region 5

160.0 +00.6 H.E.S.S. Nature 2016 a b +00.4 61.7 +00.4 Galactic latitude (degrees) +00.2 +00.2 23.0 The PeVatron scenario +00.0 +00.0 7.8 Sgr A* Sgr A* –00.2 –00.2 1.9 –00.4 –00.4 –0.5 –00.6 –1.4 01.0 00.5 00.0 359.5 359.0 00.0 359.5 Galactic longitude (degrees) Galactic longitude (degrees) molecular gas, as traced by its CS line emission 30 . Black star, location of • Diffuse emission traces gas (from CO, Figure 1 VHE -ray image of the Galactic Centre region. CS emission), strong losses X 10 ➡ 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

H.E.S.S. Nature 2016 + arXiv 1706.04535 Same spectra in the ridge ( | l | < 1° , | b | < 0.3° ), d < 150 pc Γ HESS17 = 2.28 ± 0.03 stat ± 0.2 sys and in the “ pacman” 0.15° < 𝜄 < 0.45° , 22 < d < 67 pc pacman ridge Γ HESS16 = 2.32 ± 0.05 stat ± 0.11 sys 7

H.E.S.S. Nature 2016 + arXiv 1706.04535 Same spectra in the ridge ( | l | < 1° , | b | < 0.3° ), d < 150 pc Γ HESS17 = 2.28 ± 0.03 stat ± 0.2 sys and in the “ pacman” 0.15° < 𝜄 < 0.45° , 22 < d < 67 pc pacman ridge Γ HESS16 = 2.32 ± 0.05 stat ± 0.11 sys It is suggested that both are originated by a freshly accelerated CR population at the GC 8

ADDING NEW PIECES TO THE PUZZLE 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 Fermi-LAT coll. 2016 -2.3 proton spectral index (c) -2.4 proton spectral index -2.5 2. the CR proton spectral index radial -2.6 -2.7 gradient found in the Fermi-LAT -2.8 -2.9 -3 data -3.1 L. Tibaldo Interstellar gamma-ray emission Normalized star formation rate 0 5 10 15 20 25 30 Galactocentric radius (kpc) 9

AMS-02 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] 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 10

CR spectral index radial gradient Fermi-LAT coll. 2016 -2.3 proton spectral index GALPROP (b) (c) -2.4 Fermi LAT collab. ApJ 750 2012 3A proton spectral index -2.5 DRAGON -2.6 Gaggero+ PhRvD 91 2015 083012 -2.7 -2.8 -2.9 -3 -3.1 L. Tibaldo Interstellar gamma-ray emission Normalized star formation rate 0 5 10 15 20 25 30 Galactocentric radius (kpc) Yang, Aharonian & Evoli 2016 Yang, this conference [GA015] inferred from 𝛅 -ray diffuse emission longitude dependence along the Galactic plane for E > 10 GeV 11

CR spectral index radial gradient Fermi-LAT coll. 2016 Gaggero, Urbano, Valli & Ullio PRD 2015 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 conventional model KRA 𝛿 model The model assumes a radial dependent diffusion coefficient D(E) = D 0 (E/E 0 ) - δ (R) with δ (R) = A R + B so that 𝛥 (R) = 𝛥 source + δ (R) 12

The first step above the TeV: solution of the Milagro anomaly Gaggero, D.G., A. Marinelli, Urbano, Valli ApJ L 2015 Incorporate the CR spectral hardening in the KRA 𝛿 model (assuming it is present in the whole Galaxy). This automatically reproduces the Milagro flux @ 15 TeV which was 4 σ larger than the GALPROP prediction testable by HAWC (work in progress) and CTA ! 13

H.E.S.S. + Fermi-LAT Gaggero, D.G., A. Marinelli, Taoso & Urbano, PRL 2017 | l | < 1° , | b | < 0.3° + S. Ventura Comparison with HESS 2017 10 − 3 Gamma model Gamma model Galactic Ridge Base model Base model Fermi Data PASS8 Fermi Data PASS8 HESS Data 2017 HESS Data 2017 E 2 d Φ /dE γ /d Ω [GeVcm − 2 s − 1 sr − 1 ] Best Fit HESS+Fermi Best Fit HESS+Fermi 10 − 4 Hard di ff usion Conventional di ff usion 10 − 5 PASS8 Fermi-LAT 470 weeks of 10 − 6 data extracted with the v10r0p5 Fermi tool. Point sources from the 3FGL catalogue subtracted. 10 − 7 10 − 2 10 − 1 10 0 10 1 E [ TeV ] 14

H.E.S.S. + Fermi-LAT Gaggero, D.G., A. Marinelli, Taoso & Urbano, PRL 2017 + S. Ventura | l | < 1° , | b | < 0.3° Comparison with HESS 2017 10 − 3 Gamma model Gamma model Galactic Ridge Base model Base model Fermi Data PASS8 Fermi Data PASS8 HESS Data 2017 HESS Data 2017 E 2 d Φ /dE γ /d Ω [GeVcm − 2 s − 1 sr − 1 ] Best Fit HESS+Fermi Best Fit HESS+Fermi 10 − 4 Power-law best fit of both data sets Ridge: Hard di ff usion Conventional di ff usion 𝚾 (1 TeV) = (1.19 ± 0.04) × 10^(-8) 10 − 5 (TeV cm^2 s sr)^(-1) Γ = - 2.41 ± 0.02 Pacman: 𝚾 (1 TeV) = (1.33 ± 0.06) × 10^(-8) 10 − 6 (TeV cm^2 s sr)^(-1) Γ = - 2.45 ± 0.02 10 − 7 10 − 2 10 − 1 10 0 10 1 E [ TeV ] 15