The Galactic diffuse gamma ray emission in the energy range 30 TeV 3 PeV

Transcription

1 The Galactic diffuse gamma ray emission in the energy range 30 TeV 3 PeV Mount Rainier by Will Christiansen Silvia Vernetto & Paolo Lipari 35th ICRC July Busan - South Korea

2 Gamma ray astronomy at E > 30 TeV Point source sensitivity Galactic astronomy: 1) Point-like sources 2) Diffuse fluxes with detectors with large FOV (HAWC, LHAASO, HiSCORE ): HAWC CTA-North CTA-South LHAASO g-rays from c.r. interactions g-rays associated to the ICECUBE neutrinos? Absorption of gamma rays in the Galaxy by pair production with target radiation fields

3 Attenuation of the gamma ray flux by pair production g + g e + e - Cross section Gamma ray energy threshold: E γ = 2me2 ε (1 cosθ) Maximum cross section for: E TeV ε ev = 1.02 (1 cosθ) x = s 4me = E γε (1 cosθ) 2 4me 2 E = gamma ray energy ε = target photon energy q = angle between photons Flux attenuation: F = F 0 exp ( τ E, x )

4 Absorption of gamma rays in the Galaxy Radiation fields in the Galaxy: Photon number density vs. energy Extragalactic components uniform and isotropic CMB DUST CMB PeV g-rays EBL (negligible absorption) Galactic components anisotropic and of increasing intensity towards the Galactic center Dust IR 100 TeV g-rays Starlight 1 TeV g-rays Infrared data COBE IRAS EBL STARLIGHT

5 Survival probability vs. gamma ray energy Absorption for 3 source positions IR absorption CMB absorption Tables with absorption coefficients are available at E g from 1 TeV to 100 PeV Different gamma ray arrival directions Source distance up to 30 kpc from GC Absorption of very high energy gamma rays in the Milky Way, Phys.Rev. D 94, , 2016

6 High energy Galactic diffuse gamma ray emissions Standard diffuse flux produced by cosmic ray interactions in the Galactic disk Possible diffuse gamma rays models inspired by ICECUBE results 1) Dark matter decay 2) Extended c.r. halo 3) Fermi Bubbles

7 Galactic diffuse gamma ray flux Galactic diffuse g-rays are produced by cosmic ray (nuclei & electrons) interactions with the interstellar matter and radiation: p 0 decay Inverse Compton Bremsstrahlung Fermi LAT E g > 1 GeV Image Credit: NASA/DOE/Fermi LAT Collaboration

8 Galactic diffuse gamma ray flux data from the Northern emisphere b < 5 Cosmic rays all particle flux 10-4???

9 Conventional diffuse emission model Construction of a model based on the extrapolation of Fermi measurements: Assumption 1: most of the diffuse gamma ray emission is generated by the hadronic mechanism [p 0 decay dominant channel of production] Assumption 2: the cosmic ray spectral shape is equal in all points in the Galaxy Model for hadronic interactions based on Sibyll The space distribution of the emission is inferred from the Fermi angular distribution for E > 10 GeV

10 A simple phenomenological model to describe the Galactic diffuse gamma ray emission Gamma ray flux ( GeV) Emission spatial distribution Exponential model q g R, Z R 0 = 3.9 kpc = C exp ( R R 0 - Z Z 0 ) Z 0 = 0.27 kpc Fermi data Exponential model Gaussian model Gal. latitude distribution determined by fitting the Fermi angular distribution for E > 10 GeV Z Gal. longitude distribution R Fermi data Exponential model Gaussian model

11 Absorption of the Galactic diffuse flux The flux attenuation depends on the direction of gamma rays Survival probability for E = 100 TeV P

12 Expected Galactic diffuse gamma ray flux Grey band: expected gamma ray flux in the region lat < 5 long = year LHAASO 5 sigma sensitivity (approximate) Unabsorbed flux

13 Gamma rays & IceCube neutrinos Fermi diffuse gamma rays Angle integrated flux HESE neutrinos Neutrino flux (from upgoing muons) Are neutrinos Galactic or extragalactic? Icecube neutrino angular distribution is consistent with isotropy Neutrino emission is usually accompanied by a gamma ray emission of similar intensity and spectral shape. Extragalactic gamma rays would be completely absorbed. But if a significant fraction of neutrinos is Galactic, the associated gamma ray flux can be observed.

14 Possible Galactic gamma ray emissions associated to ICECUBE neutrinos 1 - Dark Matter model (Esmaili & Serpico, 2015) Neutrinos are produced in the decay of a heavy DM particle Space distribution of emission points (spherical simmetry around GC ) r r = r 0 r r c 1+ r r c 2 r c = 20 kpc Angular distribution of the emission points

15 Possible Galactic gamma ray emissions associated to ICECUBE neutrinos 2 Extended halo model (Taylor, Gabici & Aharonian, 2014) Neutrinos are produced by c.r. interactions in an extended halo Space distribution of emission points (spherical simmetry around GC ) r r = exp ( r2 ) 2r 2 0 r 0 = 57 kpc < r 2 > = 100 kpc Angular distribution of the emission points

16 Possible Galactic gamma ray emissions associated to ICECUBE neutrinos 3 Fermi Bubbles (Lunardini et al., 2014) Neutrinos are produced in the Fermi Bubbles Space distribution of emission points 2 spheres of radius R = 3.9 kpc centered at x = y = 0, z = ±5.5 kpc r (r) = 1 / sqrt (1 r 2 / R 2 ) r = distance from the sphere center Angular distribution of the emission points

17 All models Distance distribution & gamma ray average survival probability Distance distribution of g-ray flux (before absorption) Average survival probability of gamma rays Energy (TeV)

18 1) Dark Matter decay 2) Extended c.r. halo HESE neutrino flux (isotropic ) Unabsorbed gamma rays DM gamma rays + absorbtion Large halo gamma rays + absorbtion CASA MIA, 1997 KASCADE, ICRC 2003

19 3) Gamma rays from the Fermi Bubbles Fermi gamma rays HAWC gamma ray U.L. No neutrino excess is observed from the Fermi Bubbles HESE neutrino flux (isotropic ) 1 y LHAASO sensitivity (approximate) Hypotetical gamma ray flux Absorbed gamma ray flux

20 Conclusions The gamma ray sky at energies > 30 TeV will be explored for the first time by a new generation of high sensitivity gamma ray detectors. The study of diffuse fluxes with wide fields of view instruments will be also of great importance for the understanding of high energy processes in the Galaxy. Gamma Ray absorption in the Galactic radiation fields must be carefully considered, but does not preclude the study. Telescopes with an effective area of 1 km 2 and good rejection of c.r. background should be able to measure the diffuse gamma ray flux generated by cosmic rays interactions in the Galaxy disk. If the ICECUBE astrophysical neutrinos have a significant galactic component, the associated gamma ray emission should be detectable in the next future.

21 Backup slides

22 Infrared radiation model vs. data Energy density at 100 mm 100 mm Latitude distribution Infrared spectrum GAL. CENTER Longitude distribution 100 mm SUN

23 Infrared radiation anisotropy

24 Mean free path

25 Survival probability: models comparison Solid lines: our model Dotted lines: Moskalenko et al. Solid lines: our model Dotted lines: Esmaili & Serpico

26 Diffuse Galactic gamma ray flux data FERMI 100 MeV 100 GeV All sky HESS E > 250 GeV l = -75 to 60 b < 2 (2014) ARGO-YBJ TeV l = 25 to 100 b < 5 (2015) MILAGRO > 3.5 TeV l = 40 to 100 b < 5 (2005) 15 TeV l = 30 to 85 b <10 (2008) Above 100 TeV only upper limits: BASJE, EASTOP, UMC... The lowest are: CASA-MIA TeV l = 50 to 200 b < 2,5,10 (1996)