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Neutrons (and Neutrinos) from the Galactic Center

Neutrons (and Neutrinos) from the Galactic Center. The AGASA/SUGAR EHE cosmic ray anisotropies and TeV gamma rays Roland Crocker Harvard-Smithsonian CfA Aspen, 2005. References: (i) ApJ 622, 892 (2005) (ii) ApJL 622, L37 (2005). Collaborators: Marco Fatuzzo , Xavier University

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Neutrons (and Neutrinos) from the Galactic Center

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  1. Neutrons (and Neutrinos) from the Galactic Center The AGASA/SUGAR EHE cosmic ray anisotropies and TeV gamma rays Roland Crocker Harvard-Smithsonian CfA Aspen, 2005

  2. References: (i) ApJ 622, 892 (2005) (ii) ApJL 622, L37 (2005) Collaborators: Marco Fatuzzo, Xavier University Randy Jokipii and Fulvio Melia, University of Arizona Ray Volkas, University of Melbourne

  3. Introduction • AGASA and SUGAR see an overabundance of cosmic rays towards the Galactic Center (GC) at extremely high energies (EHE), ~1018 eV • This anisotropy is observed to “turn on” and then “turn off” at well-defined energies • A natural explanation of this phenomenon is that it is due to neutrons generated at the GC: the Lorentz-boosted decay length of the neutron becomes equal to the distance to the GC at precisely the energy at which the anisotropy turns on – i.e., a neutron at 1018 eV experiences a quarter of an hour in its propagation from the GC • By relating gamma-ray observations of the GC with the CR anisotropy data we have investigated – and found plausible - a conventional astrophysical explanation for the production of these putative EHE neutrons: that they are produced in proton-proton collisions at the supernova remnant Sgr A East located near the GC • This research very tentatively identifies a definite Galactic object as a strong source of cosmic rays between the knee and the ankle

  4. Evidence for EHE CR Anisotropy • AGASA – Japanese, giant air-shower array – finds anisotropy (~25 % over-abundance) at the 4 σ level towards the GC over a 20 degree diameter circle for 17.9 < log[ E/eV ] < 18.3 Data taken over a 20 year period and amount to 200 000 >1017 eV CR showers – Hayashida et al. 1999. • Re-analysis of SUGAR data (1968-1979) also uncovers point source near GC (4000 events within a priori restricted energy range) – Bellido et al. 2001. • New AGASA data strengthens case for anisotropy: get 4.5 σ result for 18.0 < log[ E/eV ] < 18.4 • AGASA also sees enhancement towards Cygnus (3 σ effect) and a deficit towards the Galactic anti-center (3.7 σ effect) • Fly’s Eye: Galactic Plane enhancement at 3.2 σ (Bird et al. 1999) for 17.3 < log[ E/eV ] < 18.5 • BUT HiRes: data consistent with isotropic source model for log[ E/eV ] > 18.5 (Stokes et al. 2004) - does not exclude the AGASA GC anisotropy result

  5. AGASA CR Sky Galactic Plane Hayashida et al. 1999 Instrument horizon

  6. Issues • AGASA cannot see GC directly (below instrument horizon) • SUGAR can see GC but their point source off-set from GC by 7.5 +/- 3.0 degrees • SUGAR “source” in field of view of AGASA – should be seen by AGASA but is not • No particularly compelling astrophysical object in direction of SUGAR source (though close to Galactic Plane) • We proceed under the hypothesis that the SUGAR directional determination is in error

  7. Modeling of GC CR Propagation • Bossa et al. J.Phys.G. 29, 1409 (2003): successfully reproduce broad features of observed anisotropies with assumed, GC point-source of n’s governed by spectral index of 2.2 • Model combined signal from neutrons and neutron-decay protons (latter’s paths bent in Galactic B fields forming a “halo” around the point source) – n’s and p’s indistinguishable at these energies. • Anisotropy becomes increasingly point-like with increasing energy (until all remain neutrons after propagation) • Neat explanation of disappearance of AGASA anisotropy at certain energy: get point source below horizon • AGASA and SUGAR fluxes consistent with each other in n decay picture given AGASA is seeing the halo p’s and SUGAR the n point source

  8. EHE Neutron Production • Three basic HE n production scenarios: • Dissociation (on target photons or p’s) of heavy ions into component p’s and n’s • Charge exchange in p-γ : p γ→ n X • Charge exchange in p-p: p p → n X • In the above X is mostly pions (10’s thereof in p p, significantly fewer in p γ)

  9. Identifying the Neutron Source Process • Dissociation may operate, but no attendant GeV-TeV photon signal (cannot normalize) • p-gamma: not effective until Ep > 1018 eV (when NIR photons above threshold) and, even then, GC has too small an ambient density of photon targets • p-p best bet: charge exchange can occur for Ep>GeV and many target p’s…associated pions decay to produce a concomitant gamma-ray signal

  10. Multiplicity and Inelasticity of Charge Exchange • From experiments conducted at CERN (ISR) and Fermilab in 70’s: (i) Occurs in ~40 % p-p interactions, i.e., (leading) neutron multiplicity of 0.4 (ii) Neutron gets about 25 % of incoming proton’s energy, i.e., leading neutron elasticity is 0.25 • Data on multiplicity and elasticity are at much lower E_CMS energies than we require (60 GeV rather than 60 TeV) – cf. p-γ interaction… • →Require modeling. This indicates that these fractions are approximately energy-independent

  11. p-p Cross-section • p-p cross-section at relevant energies (s~60 TeV) extracted from cosmic ray data • scaling-violating but slow growth of the total p-p cross-section [from ~40 mb to 120 mb over the energy span of concern]

  12. Spectrum-weighted moments • SWMs relate emissivity in daughter particles (n’s or γ’s) to the number [/s/cm3/eV] of interacting parent particles [p’s] where Feynman scaling holds • For power-law parent distribution daughter particles have same distribution with same spectral index (in scaling regime) • Have also allowed for: • growth of the total p-p cross-section • exponential cut-off in parent p distribution

  13. Gamma Rays: Independent Evidence for p-p Interactions at GC • GC gamma ray signal seen at both ~GeV (EGRET) and ~TeV energies (Whipple, CANGAROO, HESS) • This signal convincingly ascribed to neutral pion decay from p-p interactions • Gamma ray signal supports existence of population of HE, shock-accelerated p’s

  14. Predicting the Neutron Flux from the Gamma-ray Signal (i) normalization supplied by the (p-p) gamma-ray signal + (ii) expectation from shock acceleration theory + (iii) particle physics [spectrum-weighted moments with slow cross-section growth, constant multiplicity and inelasticity] = predict the expected (p-p) neutron flux at EHE …provided the parent p population extends to ~1019 eV

  15. Gamma-ray and n Signals Compatible… • With a spectral index (in agreement with that supplied by observation and in good agreement with expectation from acceleration at a strong shock) of 2.2-2.3 the (GeV) EGRET gamma-ray signal predicts the right neutron flux – at 9 orders of magnitude higher in energy • The (TeV) HESS signal with a spectral index of 2.0 is also compatible with the n signal

  16. Chi-squared Fitting to Combined Data: EGRET Best-fit spec. ind. = 2.23 chi^2/dof = 0.33/(4-2) = 0.17

  17. Chi-squared Fitting to Combined Data: HESS Spec. ind. = 2.0 (set) chi^2/dof = 5/(11-1) = 0.5

  18. Considerations • How might p’s be accelerated to such high energies? • Total power requirements • HESS and EGRET apparently incompatible: EGRET “predicts” much larger ~TeV (20x) flux than seen by HESS • CANGAROO and HESS incompatible • Whipple and HESS incompatible • (No evidence for variability from any one instrument, only between instruments) Re-analysis reveals compatible

  19. Total Power • EGRET – EHECR normalization: ~4 1038 erg/s in interacting p’s → 1.3 1050 erg over 104 years • HESS – EHECR normalization: ~4 1036 erg/s in interacting p’s → 1.3 1048 erg over 104 years

  20. TeV GC Observations: position determinations by various instruments • Position of TeV GC source overlaid on 21 cm radio map • Bright spot in HESS confidence region is Sgr A* • Sgr A East is ring like feature to East of Sgr A* • The HESS source is inside the CANGAROO field of view (Horns, astro-ph/0408192) Gal East ~25 pc Gal West

  21. TeV and GeV GC Fluxes (various instruments)Figure and HESS data from Aharonian et al. 2004 CANGAROO HESS Now revised downwards

  22. Possible resolutions of EGRET-HESS “disagreement” 1. Energy-independent mis-calibration of one or more instruments? Possibility – certainlybig problems in observing noisy regions like the GC at ~TeV energies - but EGRET predicts 20 x HESS flux at ~TeV 2. Pair production on NIR-optical (~1 eV) photons attenuating TeV+ photons (cf. some X-ray binaries)? Can get effectively energy-independent attenuation of photons, but photon column density too small given low emission of GC (cf. other galactic nuclei) 3. Two effective sources? Definitely plausible • evidence that EGRET and HESS sources angularly-separated – Hooper and Dingus (astro-ph/0212509) find EGRET source center of gravity ~12 arcminutes from the GC, whereas the HESS source lies within 1 arcminute of the GC. Pohl (astro-ph/0412603) confirms offset • in SNR population flux level and energy cut-off observed to go in opposite directions (ambient particle density) but • does not explain very similar spectral indices in Fit 1 • requires 7 orders magnitude difference in maximum energies at two sites (EGRET source must cut-off not to pollute HESS signal)

  23. Plausible Sources of EHE n’s: 1. Sgr A* • Sgr A* - shock(s) in the accretion disk surrounding the supermassive (~3 M solar mass) BH at the GC (@ 40-120 R_Schwz) Obvious choice, but • previous modeling (Markoff et al. 1999) determined max E_p to be 4x1017 eV (given maximum shock size and magnetic field)– i.e., too low to produce n signal • synchrotron emissivity of secondary leptons in inferred 10 G+ fields near central BH would far exceed Sgr A*’s observed radio flux • lack of variability seen in ~GeV and ~TeV data tells against compact source • inefficient – most p’s escape → tough energy demand

  24. Plausible Sources of EHE n’s: 2. Sgr A East • Sgr A East: SNR located close (shell located from 10 pc to within 1 pc) to GC • Good evidence for association with EGRET GC source (3EG J1746-2851) → gamma-ray luminosity 2 orders of magnitude larger than other EGRET-detected SNRs…Why? …near GC, find unusually-high • magnetic field strengths of O[mG] • ambient particle densities of ≥ 104 /cc (105 - 106 /cc molecular clouds)

  25. Unusually High Maximum p Energies at Sgr A East • With 4mG field Sgr A East shock can accelerate particles to 1019(R/10pc) Z eV in a perpendicular shock configuration (Jokipii 1982 & ApJ 1987) • p-p cooling-limited p energy is ~1021 eV • Time-limited p energy is ~1020 eV (given 10 000 year age)

  26. Sgr A East and the Two Source Model • Natural fit: Sgr A East shell subtends regions of widely varying particle density and magnetic field strength (and orientation) • EGRET source: 0.1 mG field to accelerate p’s and create secondary leptons which then gyrate to produce observed radio emission (6 and 20 cm) – self-consistent • HESS source: 4 mG field required to produce radio signal – good match with field required to produce required1019 eV p’s for n production (Fatuzzo and Melia 2004) and also good match with direct polarimeter measurements of GC field (Chuss et al. 2003)

  27. Extension: GC Neutrino Flux • Substantial neutrino flux from GC pion, muon decay which can be normalized to gamma-ray and neutron signals (β-decay anti-nue’s insignificant) • Signal should be visible in future Northern Hemisphere km3 neutrino detectors (within 1.5 years for HESS normalization case) in numu-induced muons • Signal should also be visible to IceCube detector (within 1.6 years for HESS normalization case) in down-going nue and nutau-induced showers • HE neutrinos → can also detect through Glashow process: anti-nue + e- → W-in IceCube

  28. Summary • From the observed anisotropy in the EHE CR spectrum, there appear to be n’s coming from the GC • These n’s can be explained as arising from charge exchange in p-p interactions • We have shown that conventional astrophysics can explain the n’s (and, therefore, the anisotropy): shock acceleration in the unusual SNR Sgr A East can produce the required population of extremely energetic p’s provided a perpendicular shock geometry is realized • The GC gamma-ray signals are fully compatible with – in fact, predict – the EHE n flux required to explain the EHE CR anisotropy, but they are not (simply) compatible with each other

  29. The Future • Auger – first Southern Hemisphere extensive air shower array since SUGAR. Promises massive increase in EHE CR data down to ~1018 eV. Should see strong, GC point source of CRs and halo. Will either confirm or rule-out our scenario. • GLAST – should establish definitively which GC object is supplying the ~GeV gamma-rays seen by EGRET. • Need continued monitoring of GC with air Cerenkov telescopes at ~TeV energies with Whipple,HESS, and CANGAROO…results from Whipple upgrade (VERITAS) eagerly awaited.

  30. Continuing Work • What about the p’s that get away? Could the GC actually be a dominant CR source in a restricted energy regime above the knee? • How to explain why the accelerated p population at the HESS source extends up to ~1019 eV, whereas the EGRET p population only extends to ~ 1011 eV ? – importance of diffusion? • Contrasting nu signal from our scenario with that due to DM decay at GC

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