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

Catia Grimani

University of Urbino and INFN Florence


  • 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

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


Cosmic-ray composition

  • 90% protons

  • 8% helium nuclei

  • 1% electrons

  • 1% heavy nuclei

  • <1% positrons, antiprotons

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)

Cosmic-ray overall


Above a few GeV


Part./(m2 sr s GeV)

o the knee (3x1015 eV)

o 1018 eV

above the ankle (3x1018 eV)


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.

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


    H= 1.23 <HT>

  • Inverse Compton

    Blackbody radiation

    Stellar photons

    (dE/dt)c = 10-16 w E2 GeV/s

    w=0.7 eV/cm3

These interactions imply that …

  • Electrons are less abundant than protons

  • A spectral break is present at the source for electrons only…

Interplanetary electron flux

Origin of electrons

CG et al., to be submitted to CQG


  • 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


Part./(m2 sr s GeV)


About the e- galactic component…

Various authors assume electron spectrum break at the


Moskalenko and Strong:

=2.1 E≤ 10 GeV =2.4 E≥10 GeV

Best agreement to data!


=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

Galactic electron flux estimates

Solar electrons

November 3rd and

September 7th 1973 solar events

Solar electron detection

can be used to

forecast incoming


Posner, 2007

Positron flux observations and calculations

Moskalenko & Strong, 1998

Stephens, 2001a,b

Positron fraction measurements before 1995

Protheroe, 1982

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)


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/


Cheng, Ho & Ruderman, 1986

*Electrons are accelerated in the

outer magnetosphere in vacuum

gaps within a charge separated


*Electrons interact through

syncrotron radiation or inverse

Compton scattering

*e+e- pairs are produced by 


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

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

Solar modulation of cosmic-ray spectra

D. Hathaway and Dikpati M. http://science.nasa.gov/headlines/y2006/10may_lagrange.htm


Positive SOL MAX

Positive SOL MIN

Negative SOL MIN

BESS proton data

Solar Modulation of Galactic Cosmic Rays






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!

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)



Boella G. et al., J. Geophys. Res. 106:355 2001

LEE and AESOP data


Thick dot-dashed lines:

Protheroe, 1982 SLBM

Clem & Evenson, 2004


Positron measurements during the last two solar


CG, A&A, 2007

Secondary calculations by M&S, 1998



PAMELA data…

CG, A&A, 2007 - 550 MV/c


before PAMELA


CG, A&A, 2004

Positron Flux measurement


flux excess

(continuous thick line)


respect to the


component (dot-dashed line

-Moskalenko&Strong, 1998):

same trend

than H&R87

with 1/PB=35 years (dotted line)

CG, ICRC2005

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

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


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

Positron flux

Spectral index

above 20 GeV




Implies 1.9-2.0

at the source (?)

Yuksel, Kistler & Stanev



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,


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

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

Observed gamma-ray pulsar characteristics

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


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


pulsars from Harding&Ramaty

33 ms



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/

Is the proposed scenario consistent with overall pulsar observations?

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)

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

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

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)

Energy losses due to Electromagnetic processes and debris disk


  • 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).

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


Might gravitational waves produced by debris disks be detected?

  • Planetary systems

  • Disk precession

Gravitational energy loss from pulsar planetary systems

LGW = (32/5) G4/c5 M32/a5


 M1M2/(M1+M2)

Planetary disk dimensions > 8 105 km

 < 6.04 10-4 Hz

LGW < 1.24 x 1016 J/s

LISA sensitivity curve

Vocca et al., CQG, 2004

Gravitational wave amplitude from pulsar planetary systems

Signals might lie in the LISA band (r>c/):

ho=-1/r (G2/c4) (4M1M2/a)

  • ho=-1/r (4.59x10-7)

    At 3x10-4 Hz LISA can detect gw with amplitudes larger than 5.07x10-23

    Sources will lie within a light year in 10 years of data taking

Gravitational waves from internal parts of precessing disks (?)

  • I3= 1/2 M (R12 + R22)

  •  = I3/(I1 cos

  • 

  • P=(2G)/(5c5)sin2 (cos2 +16 sin2 

    s (0.01)x

    GW frequencies are similar to those produced by pulsar planetary systems (in the LISA band).

    Decay time are very long!

    d/dt=-1/ 

    1/ =(2G)/(5c5)/I1


Even if the frequencies lie in the LISA band,

ten years of integration would not allow

the detection of planetary systems

beyond one light years. The role of disk

precession in generating gravitatonal waves

must be investigated further.


  • Electrons and positrons are unique tools for cosmic-ray and interstellar medium investigation.

  • Data seem to indicate the model by M&S as the best reproducing e+ ande-data trend.

  • A positron excess is present above a few GeV.

  • The positron excess is compatible with a model of pair production at the polar cap of middle aged pulsars extrapolated from a model of pair production at the polar cap of young pulsars.

  • The hypothesis of fallback debris disks around young pulsars is compatible with positron origin from pulsar polar cap.

Thank you!

ATIC electron data

Supersymmetry and the positron excess in cosmic rays

Kane, Wang & Wells, 2001 hep-ph/0108138

Cirelli, Kadastik, Raidal, Strumia,


Kamionkowski and Turner, 1991

Neutralino annihilation

Cheng et al, 2002

Kaluza-Klein DM annihilation

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