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Sabrina Casanova Max Planck Institut fuer Kernphysik, Heidelberg

TeV Diffuse Galactic Gamma-Ray Emission. Sabrina Casanova Max Planck Institut fuer Kernphysik, Heidelberg. Surveys of the sky at TeV energies. Milagro: Northern sky. H.E.S.S. : Southern sky. Outline. CR origin and propagation are not well known phenomena

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Sabrina Casanova Max Planck Institut fuer Kernphysik, Heidelberg

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  1. TeV Diffuse Galactic Gamma-Ray Emission Sabrina Casanova Max Planck Institut fuer Kernphysik, Heidelberg

  2. Surveys of the sky at TeV energies Milagro: Northern sky H.E.S.S. : Southern sky

  3. Outline • CR origin and propagation are not well known phenomena • Diffuse gamma-ray emission: a unique probe of CR origin and propagation • The origin of Galactic Cosmic Rays: searching for the Galactic Pevatron Sources • Cosmic Ray Propagation in the Galaxy • Diffuse gamma rays from molecular clouds close to CR sources • Large scale TeV diffuse emission • On the level of the CR background • Summary & Future experimental goals

  4. 1. CR origin and propagation are not well known phenomena

  5. The Cosmic Ray Spectrum at Earth • Almost featureless spectrum • Galactic accelerators have to inject particles up to at leastPeV energies • Isotropy up to very high energy:1 TeV proton in B=1uG, Gyro-Radius = 200AU= 0.001pc δ = -2.7 Galactic Origin Knee ~ 3 x10 15 eV δ = -3.0 Extragalactic Origin

  6. Standard Model of Cosmic Rays Acceleration : DSA in SNRs Energy dependent Escape

  7. 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 ?

  8. 2. Diffuse Gamma Ray Emission from the Galactic CR population :a unique probe of CR origin and propagation

  9. g-rays from electron and CR Galactic populations • Electromagnetic Processes: • Synchrotron Emission -important for e- energy losses - • E g a (Ee/mec2)2 B • Inverse Compton Scattering • E f ~ (Ee/mec2)2 E i • Bremsstrahlung -more important at GeV energies- • E g ~ 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 -> p± +po +…-> e ±+ n + g +…

  10. Diffuse TeV g-rays from the Galaxy: unique probe of CR origin and propagation • g-rays travel rectilinearly through the Galactic magnetic fields and point back to the production locations • The highest energy CR particles produce the highest energy g-rays. • Hadronic and leptonic mechanisms produce different energy spectra and g-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

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

  12. Are SNRs the Galactic PeVatron sources ? SNR RX J1713-3946 seen in TeVs (H.E.S.S. Collaboration) • 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. Γ= 2.04±0.04 E0 = 17.9 ± 3.3 TeV

  13. Variability of X-rays on year timescales –witnessing particle acceleration in real time • flux increase - particle acceleration • flux decrease - synchrotron cooling • it requires B-field of order 1 mG in • hot spots and, most likely, 100G • 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) Uchiyama, Aharonian, Tanaka, Maeda, Takahashi, Nature 2007

  14. Are SNRs the Galactic PeVatron sources? 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 ? • 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

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

  16. 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 .

  17. Cosmic ray flux in MCs close to a SNR: dependence on SNR age and location Gabici et al, 2009 1 = 500 yr 2 = 2000 yr 3 = 8000 yr 4 = 32000 yr Power in CRs: ECR = 3 X 1050 erg, Diffusion coefficient: D = D0 E0.5

  18. GeV versus TeV emission spectra in MC TeV t-dependent SNR CRs GeV steady CR sea 1 = 500 yr 2 = 2000 yr 3 = 8000 yr 4 = 32000 yr Concave shape Steep spectrum at GeV energies and hard at TeV M=105 solar masses; R=20pc ;n =120 cm-3 ; B=20mG; D=1kpc ;D =1028 cm2 /s Gabici et al 2009

  19. MWL implications • GeV-TeV connection • ...and PeV-hard X connection (pp interactions produce secondary electrons) Ee = 100 TeV

  20. Multiwavelength emission close to MCs M=105 solar mass; R=20pc ;n=120cm-3 ; B=20G ; d (SNR/MC)=100pc ; D=1kpc Gabici et al, 2009 Synchrotron of secondaries and primary e- t = 500 yr t = 2000 yr t = 8000 yr t = 32000 yr t = ∞ yr GAMMA RAYS RADIO X-RAYS 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.

  21. Modeling particle escape from RXJ1713-3946 Zirakashvili & Aharonian, 2010 CLOUD CRs CRs SNR gammas CRs gammas gammas 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 CRs of about 150 TeV are escaping from the shell now

  22. Variety of CR spectra close to RXJ1713 TeV position dependent GeV steady c

  23. Variety of g-ray spectra close to RXJ1713 Constraints on diffusion regime Casanova et al, 2010

  24. The emission from the environment of RX J1713 provides clues concerning the SNR accel history. D = 1026 E0.5 cm2/s at 10 GeV 6 TeV SNR RXJ1713 SNR RXJ1713 CR sea (Runaway CRs + sea CRs) /sea CRs

  25. Morphological studies of the emission close to CR sources: constraining CR diffusion regime D = 1027 E0.5 cm2/s at 10 GeV D0 = 1028 E0.5 cm2/s at 10 GeV Casanova et al, 2010

  26. 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

  27. HESS observations of the diffuse emission from the GC Spectral index 2.29 ± 0.07 ± 0.20 implies harder CR spectrum than in solar neighborhood  Proximity of accelerator and target (Aharonian et al, 2006)

  28. 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

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

  30. Milagro analysis of Galactic TeV diffuse emission Flux profiles ← Source subtraction + offset/base

  31. Galactic Latitude Profile GALPROP Model hadronic leptonic total (Abdo et al, 2008) The narrow distributions of Milagro data points agrees better with a dominant hadronic component.

  32. Abdo et al, 2008 Sources Subtracted 1.5x GP 2.7x GP GALPROP (optimized) Below Horizon Cygnus Region Galactic Longitude Flux Profile Longitude Profile |b|<2° 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.

  33. Strong & Moskalenko GALPROP model of Cygnus Region Pin standard optimized bremsstrahlung Inverse Compton Diffuse Emission from the Cygnus Region Cygnus Region: Matter Density Contours overlaying Milagro Obs. Abdo et al, 2007

  34. Pevatrons in the Cygnus Region ? MGRO J2031+41 100 pc 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

  35. 5. On the level of the cosmic ray background

  36. 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

  37. The level of the CR background only one or at most few massive MCs gammas sr-1 cm-2 s-1 TeV-1 CLOUDS CR sea gammas many low mass MCs 100 GeV Casanova et al, 2010

  38. Casanova et al, 2010 Localisation of the peak emission along the line of sight MASS DENSITY Flux >1 Gev

  39. The emission from MCs constrains the CR flux in specific regions of the Galaxy Casanova et al, 2010 FCR= Fg MH2 /dH22 Mass and integral flux as a function of the distance for the region 345.3<l<346 and 1<b<1.7 85% of the emission for the region 345.3<l<346 and 1<b<1.7is produced between 0.5 and 3 kpc

  40. Summary • 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.

  41. Future experimental goals • Detect large (>>10G) magnetic field : x-ray telescopes such as Chandra and Astro-H • Observe g-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 g-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

  42. Thanks for the invitation !

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