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On cosmic-ray positron origin and the role of circumpulsar debris disks

On cosmic-ray positron origin and the role of circumpulsar debris disks. Catia Grimani University of Urbino and INFN Florence. Contents. Discovery of cosmic rays Characteristics of cosmic rays Electrons and positrons (the lowest mass particles in cosmic rays)

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On cosmic-ray positron origin and the role of circumpulsar debris disks

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  1. On cosmic-ray positron origin and the role of circumpulsar debris disks Catia Grimani University of Urbino and INFN Florence

  2. Contents • Discovery of cosmic rays • Characteristics of cosmic rays • Electrons and positrons (the lowest mass particles in cosmic rays) • Origin of electrons and positrons • Electrons, positrons and pulsar physics

  3. The discovery • 1911-12 cosmic-ray discovery Victor F. Hess • What cosmic rays are made of? • Photons? • No, energetic positive charged particles (protons and ions)! Latitude effect and east-west asymmetry

  4. Cosmic-ray composition • 90% protons • 8% helium nuclei • 1% electrons • 1% heavy nuclei • <1% positrons, antiprotons

  5. Rare particle discovery in cosmic rays • 1932 Positrons (ground) • 1937 Muons (ground) • 1947 Pions (ground) • 1961 Electrons (Galactic cosmic rays) • 1964 Positrons (Galactic cosmic rays) • 1979 Antiprotons (Galactic cosmic rays)

  6. Cosmic-ray overall spectrum Above a few GeV F(E)=AE- Part./(m2 sr s GeV) o the knee (3x1015 eV) o 1018 eV above the ankle (3x1018 eV) 

  7. That special interest in e- and e+… • Electrons and positrons interact with magnetic field and background and stellar photons. • Comparison between proton and electron fluxes (rigidity and velocity propagation processes). • Exotic origin.

  8. Electron energy losses • Ionization (dE/dt)I = 7.6 10-18 n[3 ln(E/mc2)+18.8] GeV/s n=1 atom/cm3 • Bremsstrahlung (dE/dt)B = 8 10-16 n E GeV/s • Synchrotron (dE/dt)s =3.8 10-18 HT2 E2 GeV/s <HT>=3G H= 1.23 <HT> • Inverse Compton Blackbody radiation Stellar photons (dE/dt)c = 10-16 w E2 GeV/s w=0.7 eV/cm3

  9. These interactions imply that … • Electrons are less abundant than protons • A spectral break is present at the source for electrons only…

  10. Interplanetary electron flux Origin of electrons CG et al., to be submitted to CQG Primaries • 1<E<30 MeV Jupiter magnetosphere • 30<E<100 MeV Secondary Galactic origin • E>100 MeV Primary Galactic origin Near Earth Above a few GeV F(E)=AE- Part./(m2 sr s GeV) 

  11. About the e- galactic component… Various authors assume electron spectrum break at the source: Moskalenko and Strong: =2.1 E≤ 10 GeV =2.4 E≥10 GeV Best agreement to data! Stephens: =1.54 E≤ 4.5 GeV =2.54 E≥4.5 GeV Plerion-like input spectrum Above 1 TeV descrete sources (for example nearby SNR- Vela, Monogem, Cygnus Loop - Kobayashi et al., 2004) are expected to produce electrons observed near Earth

  12. Galactic electron flux estimates

  13. Solar electrons November 3rd and September 7th 1973 solar events Solar electron detection can be used to forecast incoming SEPs Posner, 2007

  14. Positron flux observations and calculations Moskalenko & Strong, 1998 Stephens, 2001a,b

  15. Positron fraction measurements before 1995 Protheroe, 1982

  16. Origin of positrons • Secondary particles produced in the interstellar medium as final products of proton interactions. pp    e++   pp    e-+   • But possibly also… Primordial Black Hole Annihilation 56Co decay in Supernova Remnants Supersymmetric particle annihilation  interaction Pulsar magnetosphere (Polar Cap - Outer Gap Models)

  17. POLAR CAP MODEL Goldreich & Julian, 1969 Harding & Ramaty, 1987 • Strong electric fields are • induced by the rotating • neutron star • Electrons are extracted • from the star outer layer • and accelerated • Open field lines originate • at polar caps (rpc= 8 x 102 m) Figure from http://cossc.gsfc.nasa.gov/images/epo/gallery/pulsars/

  18. OUTER GAP MODEL Cheng, Ho & Ruderman, 1986 *Electrons are accelerated in the outer magnetosphere in vacuum gaps within a charge separated plasma *Electrons interact through syncrotron radiation or inverse Compton scattering *e+e- pairs are produced by  interaction Different cut-off energies are predicted by polar cap and outer gap models in the pulsed gamma-ray spectra (GLAST)! Figures from http://cossc.gsfc.nasa.gov/images/epo/gallery/pulsars/ C. Grimani ECRS Florence August 31st - September 3rd 2004

  19. How to distinguish among different hypotheses?It is mandatory to discriminate among various models of secondary e+ - e-calculations…

  20. Solar modulation of cosmic-ray spectra D. Hathaway and Dikpati M. http://science.nasa.gov/headlines/y2006/10may_lagrange.htm

  21. SOLAR POLARITY Positive SOL MAX Positive SOL MIN Negative SOL MIN

  22. BESS proton data

  23. Solar Modulation of Galactic Cosmic Rays J(r,E,t) J(∞,E+) = E2-Eo2 (E+)2-Eo2 J: particle flux r: distance from Sun E: particle total energy t: time Eo= particle mass = particle energy loss from infinity (different for each species) Gleeson and Axford, Ap. J., 154, 1011, 1968 Ok for positive polarity epoch data only!

  24. Solar polarity effect on GCR p and He @ solar minimum Negative Polarity -40% @ 100 MeV(/n) -30%@ 200 MeV(/n) -25%@ 1 GeV(/n) -a few % up to 4 GeV(/n) p He Boella G. et al., J. Geophys. Res. 106:355 2001

  25. LEE and AESOP data A>0 Thick dot-dashed lines: Protheroe, 1982 SLBM Clem & Evenson, 2004 A<0

  26. Positron measurements during the last two solar cycles CG, A&A, 2007 Secondary calculations by M&S, 1998 A>0 A<0

  27. PAMELA data… CG, A&A, 2007 - 550 MV/c Best-fit before PAMELA 0.064+/-0.003 CG, A&A, 2004

  28. Positron Flux measurement e+ flux excess (continuous thick line) with respect to the secondary component (dot-dashed line -Moskalenko&Strong, 1998): same trend than H&R87 with 1/PB=35 years (dotted line) CG, ICRC2005

  29. Positron Flux from Young Pulsar Polar Caps Harding & Ramaty, 1987 Maximum pulsar age for e+ production: 104 years 1/PB=30 years Crab and Vela pulsar parameters Spectral index above 20 GeV:  Rate of positron emission per pulsar Le+(E)B12P-1.7 E-2.2 s-1 GeV-1 Measurements before 1995 1/PB=60 years CG, Ap&SS, 241, 295, 1996

  30. Positron fraction data after 1995 and calculation uncertainties Harding & Ramaty, 1987 Top region corresponds to the secondary component + H&R with a 1/PB of 30 yrs Dashed region corresponds to the secondary component + H&R with a 1/PB of 200 yrs BEST FIT: 1/PB=200+/-100 years Bottom region corresponds to the secondary component + H&R with a 1/PB of 250 yrs CG, A&A, 418, 649, 2004

  31. Positron flux Spectral index above 20 GeV PAMELA data points:  Implies 1.9-2.0 at the source (?)

  32. Yuksel, Kistler & Stanev astro-ph/0810.2784V2

  33. PULSAR BIRTHRATE ESTIMATES LMT-1985: Lyne, Manchester & Taylor, 1985 L-1993: Lorimer, 1993 H-1999: Hansen, 1999 R-2001: Regimbeau, 2001 CET-1999: Cappellaro, Evans & Turatto, 1999 35.7 years Fucher-Giguère and Kaspi, astro-ph/0512585

  34. However… middle aged pulsars are favoured over young ones in producing positrons reaching the interstellar medium as an increasing fraction of them lies outside their host remnants as a function of age. 0.0625 % of pulsars have an age ranging between 0 and 104 years Arzoumanian, astro-ph/0106159

  35. What it was proposed: • Positrons and electrons observed near Earth are generated by Geminga e B0656+14 • Positrons and electrons fluxes are generated by galactic middle aged pulsars

  36. Observed gamma-ray pulsar characteristics

  37. 3.8  1012 G H&R87 Radio pulsar observed magnetic field distribution Observed gamma-ray pulsar magnetic field (3.92  1.97)  1012 G Figure from Gonthier et al., 2002

  38. Gamma-ray Pulsars from e+ measurements 200-300 ms Radio pulsar observed period distribution Average observed gamma-ray pulsar period 121  29 ms Figure from Gonthier et al., 2002 Gamma-ray pulsars from Harding&Ramaty 33 ms

  39. ELECTRON FLUx

  40. Different cut-off energies are predicted by polar cap and outer gap models in the pulsed gamma-ray spectra (GLAST)! Figure from http://cossc.gsfc.nasa.gov/images/epo/gallery/pulsars/

  41. Is the proposed scenario consistent with overall pulsar observations?

  42. Pulsar energy loss processesand braking indeces n= -( d2/dt2 )/ (ddt)2 • Electromagnetic (n=3) • Gravitational (n=5) • Supernova fallback debris disk friction (n<3)

  43. Observed young pulsar braking indeces Pulsar n • J1846-0258 2.65 • B0531+21 2.51 • B1509-58 2.839 • J1119-6127 2.91 • B0540-69 2.140 • B0833-45 1.4

  44. Pulsar gravitational wave energy losses… • …cannot be the only answer however electromagnetic AND gravitational wave energy losses can explain observed braking indeces (LIGO shows Crab loses at most 6% of energy in gws; Abbott et al., 2008). • Debris disks lead to braking indeces compatible with observations

  45. Fallback Debris disks • It was suggested that protoplanetary disks might form around pulsars from remnant fallback material • A debris disk has been observed around the young pulsar 4U 0142+61 (brightest known 8.7 s AXP)

  46. Energy losses due to Electromagnetic processes and debris disk MP&H01

  47. At most 12%-29% is lost by a young pulsar such as Crab because of a debris disk surrounding the pulsar • This leads to a wrong estimate of pulsar dP/dt due to em processes and therefore to wrong estimates of pulsar magnetic fields between 6% and 16% (B2 prop P dP/dt) and age. Positron flux calculations are affected similarly (Le+ prop. B). • … however, present positron measurements are still consistent with this scenario within uncertainties (a factor of two on the magnetic field).

  48. An exercise:estimate of gravitational wave emission from pulsar+debris disk systems • Disk dimensions (theory): 2000 - 200000 km • Disk dimensions (observed): 2.02x106- 6.75x106 km • Disk mass: 10Earth mass = 5.97 1025 kg • Pulsar mass = 2.8 1030 kg Assumptions:

  49. Might gravitational waves produced by debris disks be detected? • Planetary systems • Disk precession

  50. Gravitational energy loss from pulsar planetary systems LGW = (32/5) G4/c5 M32/a5 M=M1+M2  M1M2/(M1+M2) Planetary disk dimensions > 8 105 km  < 6.04 10-4 Hz LGW < 1.24 x 1016 J/s

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