School of Cosmic-ray Astrophysics, Erice, July 3, 2004 Thomas K. Gaisser Outstanding problems in...

School of Cosmic-ray Astrophysics, Erice, July 3, 2004

Thomas K. Gaisser

Outstanding problems in Particle Astrophysics

Cosmic-ray propagation

Acceleration

Sources


Thomas K. Gaisser

Spectrometers (A = 1 resolution, good E resolution)

Calorimeters (less good resolution)

Direct measurements

Air showers


Thomas K. Gaisser

A fundamental result

• Excess of Li, Be, B from fragmentation of C, O

• Spallation plus ISM give dwell time of nuclei– Find ~ 3 x 106 yrs– c ~ Mpc >> size of

galactic disk (kpc)– Suggests diffusion in

turbulent ISM plasma– Predictions for -rays,

positrons and antiprotons follow


Thomas K. Gaisser

Diffuse galactic secondaries p + gas 0 antiprotons• e• Hard -spectrum suggests some

contribution from collisions at sources

BESS antiprotons, 1997, ’99, ’00.•Fully consistent with secondary production by collisions in ISM followed by solar modulation varying with solar cycle

Phys.Rev.Lett. 88 (2002) 051101


Thomas K. Gaisser

Electrons & positrons• Primary electrons:

– spectrum steeper than p– energy loss at high E

• e+ as secondaries:– 5-10% fraction– level consistent with p +

gas + + e+

• Bump in charge ratio:– primary e+ ? … or– glitch in e- spectrum?


Thomas K. Gaisser

Energy-dependence of secondary/primary cosmic-ray nuclei

• B/C ~ E-0.6

• Observed spectrum:– (E) = dN/dE ~ K E-2.7

• Interpretation:– Propagation depends on E

– (E) ~ E-0.6

– (E) ~ Q(E) x (E) x (c/4)

• Implication:– Source spectrum Q(E) ~ E-2.1


Thomas K. Gaisser

Energetics of cosmic rays

• Total local energy density: – (4/c) ∫ E(E) dE ~ 10-12 erg/cm3 ~ B2 / 8

• Power needed:(4/c) ∫ E(E) / esc(E) dEgalacticesc ~ 107 E-0.6 yrsPower ~ 10-26 erg/cm3s

• Supernova power:1051 erg per SN~3 SN per century in disk~ 10-25 erg/cm3s

• SN model of galactic CRPower spectrum from shock

acceleration, propagation

Spectral Energy Distribution (linear plot shows most E < 100 GeV) (4/c) E(E) = local differential CR energy density


Thomas K. Gaisser

Solar flare shock acceleration

Coronal mass ejectionCoronal mass ejection 09 Mar 200009 Mar 2000


Thomas K. Gaisser

SOHO/LASCO

CME of 06-Nov 1997


Thomas K. Gaisser

Supernova progenitor

SN ejecta Shocked ISM

Supernova blast wave acceleration

Unshocked ISM

SNR expands into ISM with velocity V~ 104 km/s.Drives forward shock at 4/3 V

Forward shock

u1 ~ 4/3 V

u1 ~ 4/3 V

Particle with E1

E2 = E1

Contact discontinuity, V

TSN ~ 1000 yrs before slowdownEmax ~ Z x 100 TeV


Thomas K. Gaisser

Problems of simplest SNR shock model

• Expected shape of spectrum:– Differential index ~ 2.1 for

diffusive shock acceleration• observed ~ 2.7source ~2.1; ~

0.6 esc(E) ~ E-0.6

• c esc Tdisk ~100 TeV

Isotropy problem

• Emax ~ shock Ze x B x Rshock

Emax ~ Z x 100 TeV with exponential cutoff of each component

– But spectrum continues to higher energy: Emax problem

• Expect p + gas (TeV) for certain SNR– Need nearby target as shown in

picture from Nature (April 02)

– Interpretation uncertain; see• Enomoto et al., Aharonian (Nature);

Reimer et al., astro-ph/0205256

Problem of elusive 0 -rays


Thomas K. Gaisser

Knee

AnkleAnkle

Highest energy cosmic rays• Emax ~ shock Ze x B x Rshock for SNR

Emax ~ Z x 100 TeV• Knee:

– Differential spectral index changes at ~ 3 x 1015eV

– – Some SNR can accelerate protons to

~1015 eV (Berezhko)– How to explain 1017 to >1018 eV ?

• Ankle at ~ 3 x 1018 eV:– Flatter spectrum– Suggestion of change in composition– New population of particles, possibly

extragalactic?• Look for composition signatures of

“knee” and “ankle”

Extragalactic?Extragalactic?

galacticgalactic


Thomas K. Gaisser

B. Peters on the knee and ankle

B. Peters, Nuovo Cimento 22 (1961) 800

< A > increases with E in knee region< A > increases with E in knee region<A> should begin to decrease again<A> should begin to decrease again for E > 30 x Efor E > 30 x Ekneeknee


Thomas K. Gaisser

30

Rigidity-dependence• Acceleration, propagation

– depend on B: rgyro = R/B

– Rigidity, R = E/Ze

– Ec(Z) ~ Z Rc

• rSNR ~ parsec Emax ~ Z * 1015 eV

– 1 < Z < 30 (p to Fe)

• Slope change should occur within factor of 30 in energy

• Characteristic pattern of increasing A with energy


Thomas K. Gaisser

Models of galactic particles, E >> knee• Axford:

– continuity of spectrum over factor 300 of energy implies relation between acceleration mechanisms

– reacceleration by multiple SNR

• Völk:– reacceleration by shocks in galactic

wind (analogous to CIRs in heliosphere)

• Erlykin & Wolfendale:– Local source at knee on top of smooth

galactic spectrum– (bending of “background” could

reflect change in diffusion @ ~1 pc)

• What happens for E > 1017 eV?

Völk & Zirakashvili, 28th ICRC p. 2031

Erlykin & Wolfendale, J Phys G27 (2001) 1005


Thomas K. Gaisser

Lessons from the heliosphere

• ACE energetic particle fluences:

• Smooth spectrum – composed of several distinct

components:• Most shock accelerated

• Many events with different shapes contribute at low energy (< 1 MeV)

• Few events produce ~10 MeV

– Knee ~ Emax of a few events

– Ankle at transition from heliospheric to galactic cosmic rays

R.A. Mewaldt et al., A.I.P. Conf. Proc. 598 (2001) 165


Thomas K. Gaisser

Heliospheric cosmic rays

• ACE--Integrated fluences:– Many events contribute to low-

energy heliospheric cosmic rays;

– fewer as energy increases.

– Highest energy (75 MeV/nuc) is dominated by low-energy galactic cosmic rays, and this component is again smooth

• Beginning of a pattern?

R.A. Mewaldt et al., A.I.P. Conf. Proc. 598 (2001) 165


Thomas K. Gaisser

Speculation on the knee

K-H Kampert et al., astro-ph/0204205

Total

protons

helium

CNOMg…

Fe

1 component: = 2.7, Emax = Z x 30 TeV; or Emax = Z x 1 PeV

3 components


Thomas K. Gaisser

Direct measurements to high energywith calorimeters

RUNJOB: thanks to T. ShibataATIC: thanks to E-S Seo & J. Wefel


Thomas K. Gaisser

Recent Kascade data show increasing fraction of heavy nuclei

Note anomalous He / proton ratio in recent Kascade analyses

K-H Kampert et al., astro-ph/0204205 ICRC 2001 (Hamburg)

M. Roth et al., Proc ICRC 2003 (Tsukuba) vol 1, p 139


Thomas K. Gaisser

Chem. Composition

1 km

2 km

Paper in proof at Astropart. Phys.

AM

AN

DA

(nu

mbe

r o

f m

uons

)

Spase (number of electrons)

Iron

Proton

log(E/PeV)


Thomas K. Gaisser

Rates of contained, coincident eventsArea--solid-angle ~ 1/3 km2sr (including angular dependence of EAS trigger)


Thomas K. Gaisser

Primary composition with IceCube

• N from deep IceCube; Ne from IceTop

• High altitude allows good energy resolution

• Good mass separation from N/Ne

• 1/3 km2 sr (2000 x SPASE-AMANDA)

• Covers sub-PeV to EeV energies


Thomas K. Gaisser

Power needed for knee component

• Integrate to E > 1018 eV assuming – esc ~ 2 x 107 yrs x E-1/3

– Vgalaxy ~ (15 kpc)2 x 200 pc ~ 3 x 1066 cm3

– Total power for “knee” component ~ 2 x 1039 erg/s

• Possible sources– Sources may be nearby

– e.g. -quasar SS433 at 3 kpc has Ljet 1039 erg/s

– Eddington limited accretion ~ 2 x 1038 erg/s


Thomas K. Gaisser

Energy content of extra-galactic component depends on location of transition

• Composition signature: transition back to protons

Uncertainties:• Normalization point:

1018 to 1019.5 usedFactor 10 / decade

• Spectral slope =2.3 for rel. shock =2.0 non-rel.

• Emin ~ mp (shock)2


Thomas K. Gaisser

Power needed for extragalactic cosmic rays assuming transition at 1019 eV

• Energy density in UHECR, CR ~ 2 x 10 erg/cm3

– Such an estimate requires extrapolation of UHECR to low energy CR = (4/c) E(E) dE = (4/c){E2(E)}E=1019eV x ln{Emax/Emin}

– This gives CR ~ 2 x 10 erg/cm3 for differential index = 2, (E) ~ E-2

• Power required ~ CR/1010 yr ~ 1.3 x 1037 erg/Mpc3/s– Estimates depend on cosmology and extragalactic magnetic fields:– 3 x 10-3 galaxies/Mpc3 5 x 1039 erg/s/Galaxy– 3 x 10-6 clusters/Mpc3 4 x 1042 erg/s/Galaxy Cluster– 10-7 AGN/Mpc3 1044 erg/s/AGN– ~1000 GRB/yr 3 x 1052 erg/GRB

• Assume E-2 spectrum. Then signal ~ 10 to 100/km2yr

– ~20% have E>50 TeV (greater than atmospheric background)


Thomas K. Gaisser

GRB model

• Assume E-2 spectrum at source, normalize @ 1019.5

• 1045 erg/Mpc3/yr• ~ 1053 erg/GRB• Evolution like star-formation

rate• GZK losses included• Galactic extragalactic

transition ~ 1019 eV

Bahcall & Waxman, hep-ph/0206217Waxman, astro-ph/0210638


Thomas K. Gaisser

Berezinsky et al. AGN

• Assuming a cosmological distribution of sources with:– dN/dE ~ E-2, E < 1018 eV

– dN/dE ~ E, 1018< E < 1021

– = 2.7 (no evolution)

– = 2.5 (with evolution)

• Need L0 ~ 3 ×1046 erg/Mpc3 yr

• They interpret dip at 1019 as– p + 2.7p + e+ + e-

Berezinsky, Gazizov, Grigorieva astro-ph/0210095


Thomas K. Gaisser

Composition with air showers• Cascade of nucleus

– mass A, total energy E0 – X = depth in atmosphere along shower axis– N(X) ~ A exp(X/), number of subshowers– EN ~ E0 / N(X), energy/subshower at X– Shower maximum when EN = Ecritical

– N(Xmax) ~ E0 / Ecritical

– Xmax ~ ln { (E0/A) / Ecritical }– Most particles are electrons/positrons

• from -decay a distinct component– decay vs interaction depends on depth– N ~ (A/E)*(E0/AE)0.78 ~ A0.22

• Showers past max at ground (except UHE) large fluctuations poor resolution for E, A– Situation improves at high energy and/or high

altitude– Fluorescence detection > 1017 eV

Schematic view of air shower detection: ground array and Fly’s Eye


Thomas K. Gaisser

Change of composition at the ankle? Original Fly’s Eye (1993): transition coincides with ankle

G. Archbold, P. Sokolsky, et al.,Proc. 28th ICRC, Tsukuba, 2003

HiRes new composition result: transition occurs before ankle


Thomas K. Gaisser

Composition from density of muonsρµ(600) vs. E0 (Akeno, AGASA)

From heavy

toward protons


Thomas K. Gaisser

Hi-Res stereo fluorescence detector in Utah

UHE shower detectors

AGASA (Akeno, Japan) 100 km2 ground array

Sketch of ground array with fluorescence detector – Auger Project realizes this concept


Thomas K. Gaisser

Measuring the energy of UHECR

• Ground array samples shower front– Well-defined acceptance– Simulation relates observed

ground parameter to energy

• Fluorescence technique tracks shower profile– Track-length integral gives

calorimetric measure of energy– Xmax sensitive to primary mass:

Xmax ~ ln(E0/A)protons penetrate more than

heavier nuclei


Thomas K. Gaisser

Complementarity

• Ground array– Assigning energies

• Measure a ground parameter (e.g. (600) )

• Compare to simulation

• Depends on model of hadronic interactions

– Determining spectrum• aperture set by physical

boundary of array

• correct for attenuation of oblique showers

• Fluorescence detector– Assigning energies

• Infer S(X) from signals (depends on atmosphere)

• Fit shower profile, S(X)

• Integrate track-length: 2.19 eV/g/cm2 ∫ S(X) dX

• Model-independent

– Determining spectrum• energy-dependent aperture

• must be simulated


Thomas K. Gaisser

AGASA 2 x 1020 eV event


Thomas K. Gaisser

Biggest event

• Comparison to – Proton showers– Iron showers– showers

• Horizontal EAS– only muons survive– Haverah Park:

/p<40%, E>1019eV

– AGASA: similar limit

• Limit on showers constrains TD models

Fly’s Eye, Ap. J. 441 (1995) 295


Thomas K. Gaisser

The “Hillas Plot” (1984)

• Emax ~ shock (ZeB) R

• Plot shows B, R to reach 1020 eV

• Two more candidates since 1984

GRB jets

Magnetars

•Active Galaxies,Gamma-ray Bursts favored


Thomas K. Gaisser

Highest energy cosmic rays• GZK cutoff?

– Expected from p + 2.7N + for cosmological sources

Attenuation length in microwave backgroundPlot from HiRes, astro-ph/0208301Plot from HiRes, astro-ph/0208301


Thomas K. Gaisser

Compare exposures: HiRes, AGASA

1700 km1700 km22 sr yr sr yrAGASAAGASA

• HiRes: ~ 10HiRes: ~ 1044 km km22srsr x 0.05 efficiencyx 0.05 efficiency x few yearsx few years ~2000 km~2000 km22 sr yr @ 10 sr yr @ 102020 eV eV

•AGASA: 180 kmAGASA: 180 km22srsr x 0.90 efficiencyx 0.90 efficiency x 10 yearsx 10 years ~1700 km~1700 km22 sr yr sr yr


Thomas K. Gaisser

Akeno-AGASA / HiRes: comparison of what is measured

As measured


Thomas K. Gaisser

Models of UHECR• Bottom up (acceleration)

– Jets of AGN• External

• Internal (PIC models)

– GRB fireballs

– Accretion shocks in galaxy

clusters – Galaxy mergers

– Young SNR

– Magnetars

• Observed showers either protons (or nuclei)

• Top-down (exotic)– Radiation from topological

defects

– Decays of massive relic particles in Galactic halo

– Resonant neutrino interactions on relic ’s (Z-burst)

• Large fraction of showers (especially if local origin)

( Incomplete list )

If no cutoff, require a significant contributionfrom nearby sources. Local overdensity ofgalaxies is insufficient if UHECR sourcedistribution follows distribution of galaxies.Violation of Lorentz invariance a way out?


Thomas K. Gaisser

Active Galaxies: JetsActive Galaxies: Jets

VLA image of Cygnus A

Radio Galaxy 3C296 (AUI, NRAO). --Jets extend beyond host galaxy.

Drawing of AGN core


Thomas K. Gaisser

AGN Mulitwavelength observations

• SSC, EC, PIC models– 1st peak from electron

synchrotron radiation

– 2nd peak model-dependent; predict flux if PIC

– Interpretation complex:• Sources variable

• Locations of peaks depend on source-- factor of >100 range of peak energy

• New detectors (GLAST, HESS, MAGIC, VERITAS) will greatly expand number, variety of sources

Example of Mrk421 with new (preliminary) result from STACEE ~100 GeV

Rad

io

mm

IRUV

X-rayGeV (Egret)

TeV


Thomas K. Gaisser

Questions

• Is B/C ~ E-0.3 at high energy?

• Are all antiprotons & positrons secondary?

• SN accelerate CR?• Knee?• Where is transition from

galactic to extragalactic?• Emax (Emin) of cosmic

accelerators?

• Wefel(3), Müller(5), Ptuskin (6)

• Müller (11)

• Ptuskin (4)• Hörandel (7)• Teshima (7)

• Ostrowski (3, 11)


Thomas K. Gaisser

More questions

• Do AGN or GRB accelerate (U)HERCR?

• Are AGN or GRB (or something else) sources?

• What are sources of super-GZK particles?

• Are there super-GZK particles?

• Stanev (11)

• Migneco(6), Sulak(10), Stanev(3)

• Teshima(9), Kuzmin(9)

• Klages (12)


Thomas K. Gaisser

Cosmic-ray antiprotons

secondarysecondary

primaryprimary

Secondary antiproton spectrum expected at Earth from Secondary antiproton spectrum expected at Earth from cosmic-ray interactions in the ISM during propagation as cosmic-ray interactions in the ISM during propagation as compared to a “primary” source of antiprotonscompared to a “primary” source of antiprotons(TKG & E.H. Levy, 1974)(TKG & E.H. Levy, 1974)

Kinematic peak at 2 GeVKinematic peak at 2 GeV characteristic ofcharacteristic of p p p p p p p p p p p p--


Thomas K. Gaisser

Shock acceleration

• First order (diffusive) shock acceleration– “Fermi acceleration”--originally 2nd order– Power-law spectrum: dN/dE ~ E

• = 2 for strong shock (large Mach number)

– E = E at each shock crossing: • dE/dt = E / Tcycle • with Tcycle ~ rL/ c shock ~ ( E / Ze B) / c shock • dE/dt = (Ze B) c shock • Emax ~ (Ze B) (c shock T), after a time T• shock


Thomas K. Gaisser

Uncertainty from spectral index• The most promising accelerators involve relativistic shocks:

– AGN: ~ 30 GRB: ~300 ( = bulk Lorentz factor)– Achterberg: Relativistic shocks have spectral index ~ 2.2 - 2.3– steeper spectral index more power required– Recalculate energy density in UHECR: CR = (4/c) E(E) dE ~ (1019) x f() x {1/Emin}-2

CR ~ 100 times = 2 case for ~ 2.3 and Emin = mp ~ 1 GeV

• Problem for these models???– Vietri: Emin ~ 2 for relativistic shocks

– Reduces extra power factor to ~10 for AGN, ~ 3 for GRB– 10-7 AGN/Mpc3 1044 erg/s/AGN 1045 erg/s/AGN – ~1000 GRB/yr 3 x 1052 erg/GRB 1053 erg/GRB

• Neutrino signal enhanced somewhat, but steeper spectrum


Thomas K. Gaisser

Large fluctuations in the knee region are worse at sea level

Linear plot: green = e+/e-; blue = Log plot: fluctuations bad at sea level

10 proton showers at 1 PeV


Thomas K. Gaisser

Example: Fluctuations in N, Ne

at two depths


Thomas K. Gaisser

Compare HiRes (mono) & AGASA

• Exposure (>103 km2 yr sr):– comparable @ 1020 eV– HiRes < AGASA at

lower energy

• Number events >1020

– HiRes (mono): 2– AGASA: 11

• Shift E down 20% so spectra agree then

• 5 AGASA events > 1020

– Need more statistics and stereo results

Fluorescence detector Ground array

(astro-ph/0209422)(astro-ph/0209422)


Thomas K. Gaisser

Depth of Maximum

Combined HiRes/MIA hybrid plus new HiRes result suggest normalization of extra-galactic component at relatively low energy of 1018 eV.


Thomas K. Gaisser

E/TeV

Blazar spectra at high energy

• Mrk 421 & Mrk 501, – Cutoffs

• Intrinsic?

• Effect of propagation?

– Variable sources• Low intensity – softer

spectrum

• Interpretation under debate

– Need more observations of more sources at various redshifts

HEGRA plots from Aharonian et al. astro-ph/0205499. DifferentEcut of 421 and 501 suggest cutoffs are intrinsic.

Comparable analysis of Whipple extends to lower energy. Seeing comparable cutoffs, they suggest effect is due to propagation. Krennrich et al., Ap.J. 560 (2002) L45

both at z ~ .03

igh

ow

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School of Cosmic-ray Astrophysics, Erice, July 3, 2004 Thomas K. Gaisser Outstanding problems in...

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