NASA HQ July 2, 2004 Approaching the Knee -- Balloon-Borne Observations of Cosmic Ray Composition...

NASA HQ July 2, 2004

Approaching the Knee -- Balloon-Borne Observations of Cosmic Ray Composition

Michael Cherry

Louisiana State University

Physics @ the End of the Galactic Cosmic Ray Spectrum

Aspen, April 26, 2005


Current and Recent Balloon Instrumentsto measure high energy (> 1 TeV) cosmic ray composition

• JACEESeries of emulsion experiments, 1979 – 199411 balloon flights, cumulative exposure 644 m2 hrs @ ~ 3.5 – 5.5 g/cm2

Zenith angle acceptance out to tan θ ~ 72-79° ~80 m2 sr days exposureHighest energy proton event ~ 800 TeV

• RUNJOB Series of emulsion experiments, 1995 – 199910 balloon flights, cumulative exposure 575 m2 hrs @ ~ 9.0 – 10.7 g/cm2

Highest energy proton event seen at E > 1 PeV• ATIC

Silicon matrix-scintillator-BGO calorimeter2 balloon flights, 2000-2003, 31 days exposure

~ 7 m2 sr days exposure3rd LDB Antarctic flight scheduled for December 2005

• CREAMCombined Scintillator, Si Charge Detector, W-scintillator calorimeter, TRDOne balloon flight, 2004-2005, 41 days

~ 12 m2 sr days exposureGoal is to fly multiple 100 day (ULDB) flights to build up exposure


• TRACERScintillator-Cherenkov-TRD for 8 ≤ Z ≤ 26Two flights, 1999-2004

~ 40 m2 sr days exposure

• TIGERScintillator-Cherenkov-fiber hodoscope to measure Z ≥ 30 Three flights, 1997-2004, 50+ days exposure

~ 4 m2 sr days exposureOriginally planned as first ULDB instrument; future flights planned

• CAKENuclear track detectors (CR-39, Lexan) to measure 6 ≤ Z ≤ 74One flight, 1999, 22 hours exposure

~ 0.9 – 1.8 m2 sr days exposure @ 3 – 3.5 g/cm2 Planning to fly larger version on ULDB


Standard Model of Cosmic Ray Acceleration • Supernova shock waves may accelerate cosmic rays via first order

Fermi process– Model predicts an upper energy limit Emax ~ Z x 1014 eV

composition growing heavier with increasing energy

ATIC Energy Range


Standard picture of cosmic ray acceleration in expanding supernova shocks

But models need to include jets, stellar winds, additional classes of sources in addition to standard supernovae


Transient gamma ray sources:

CGRO and Air Cerenkov telescopes have shown that high energy sky is constantly variable

Variability on scales from the Sun to AGNs

There are lots of sources other than supernovae in our Galaxy --

• Black Hole Candidates• Galactic Microquasars – superluminal jets• Exhibit transient behavior on the time scale of minutes to

months• Observed from radio waves to gamma-rays


BATSE earth occultation measurements – Ling and Wheaton (2004)


GRO J1655-40 - Light Curve of Phase 2-6 (Case et al. 2004)


GRS 1915+105 Spectrum

• • Emission seen out to ~1 MeV• Spectrum consistent with broken power law with break energy ~300 keV• Hard tail component more prominent in some shorter time intervals

Sum of high state of Flare 1 and Flare 2 (274 days)

TJD 9084-9180 (Flare 1 decline – 81 days)


Is it really true that composition gets heavier approaching the knee as low-Z components successively cut off in supernova accelerator?

ATIC Energy Range

If cosmic ray accelerator is a complex composite of young stars, supernovae, pulsars, jets, shocks in star forming regions, etc., then a distinct cutoff at Z x 1014 eV for each component becomes a sum over an unknown distribution of magnetic field strengths, acceleration region sizes, shock strengths, stellar masses, etc.

“Multiple source” model could lead to much slower variation of composition with energy – and higher Emax values


Goals of high energy balloon-borne composition studies:• Measure p-He spectra to as high an energy as possible (>

100 TeV) – JACEE, RUNJOB, ATIC, CREAM• Measure nuclear composition (secondary-to-primary ratios

and elemental composition up to Fe) to as high an energy as possible – TRACER, ATIC, CREAM

Balloon measurements will NOT get over the knee!

If there is a proton cutoff very near 100 TeV, they may be able to see it, and can provide a direct particle-by-particle measurement (with individual element resolution) which will serve as an “anchor” for the indirect air shower measurements at higher energy.


JACEE and RUNJOB emulsion payloads both had the advantage of being simple, large area (~1 m2) packages

that could be flown multiple times

JACEE Balloon Flights:

CumulativeFlight Launch Launch Altitude Duration Units Exposure Date Site (g/cm2) (hrs) (cm x cm) (m2-hrs)JACEE 0 5/79 Sanriku, Japan 8.0 29.0 1 (40 x 50) 6JACEE 1 9/79 Palestine, TX 3.7 25.2 4 (40 x 50) 26JACEE 2 10/80 Palestine, TX 4.0 29.6 4 (40 x 50) 50JACEE 3 6/82 Greenville, SC 5.0 39.0 1 (50 x 50) 59JACEE 4 9/83 Palestine, TX 5.0 59.5 4 (40 x 50) 107JACEE 5 10/84 Palestine, TX 5.0 15.0 4 (40 x 50) 119JACEE 6 5/86 Palestine, TX 4.0 30.0 4 (40 x 50) 143JACEE 7 1/87 Alice Springs, 5.5 150.0 3 (40 x 50) 233 AustraliaJACEE 8 2/88 Alice Springs 5.0 120.0 3 (40 x 50) 305JACEE 9 10/90 Ft. Sumner, NM 4.0 44.0 4 (40 x 50) 340JACEE 10 12/90 McMurdo, 3.5 204.0 2 (30 x 40) 389 AntarcticaJACEE 11 12/93 McMurdo 4.5 217.0 6 (40 x 50) *JACEE 12 1/94 McMurdo 5.0 212.0 6 (40 x 50) 644JACEE 13 12/94 McMurdo 5.0 310.0 6 (40 x 50) 1016JACEE 14 12/95 McMurdo 5.0 350.0 6 (40 x 50) 1436

* JACEE 11 was lost in the ocean due to a malfunction at cutdown after a nine day flight.


JACEE 1-12 analysis based on 656 proton events above 6 TeV

and

414 helium above 2 TeV/nucleon.


JACEE integral spectra seemed to show difference in spectral slope at 2 level

H = 1.80 ± 0.04He = 1.68 + 0.04/-0.06

(Asakimori et al, 1998)

“Waviness in integral spectra” – an artifact of meager statistics or an indication of systematicerrors in combining datafrom multiple flights?


ATIC – a fully electronic detector (Si matrix + BGO calorimeter)

J.H. Adams2, H.S. Ahn3, G.L. Bashindzhagyan4,

K.E. Batkov4, J. Chang6,7, M. Christl2, A.R. Fazely5,

O. Ganel3 , R.M. Gunasingha5, T.G. Guzik1, J. Isbert1,

K.C. Kim3, E.N. Kouznetsov4, M.I. Panasyuk4, A.D. Panov4,

W.K.H. Schmidt6, E.S. Seo3, N.V. Sokolskaya4, J.P. Wefel1,

J. Wu3, V.I. Zatsepin4

1. Louisiana State University

2. Marshall Space Flight Center

3. University of Maryland

4. Skobeltsyn Institute of Nuclear Physics

5. Southern University

6. Max Plank Inst. for Solar System Research

7. Purple Mountain Observatory


ATIC Program Summary Investigate relationship between Supernova

Remnant (SNR) Shocks and high energy galactic cosmic rays (GCR)

Are SNR the “cosmic accelerators” for GCR

Measure GCR Hydrogen to Nickel from 50 GeV to ~100 TeV total energy

Determine spectral differences between elements

18 r.l. deep BGO calorimeter (22 for ATIC-3), 0.24 m2 sr

Multiple flights needed to obtain necessary exposure ATIC-1 during 2000-2001 – 14 days exposure ATIC-2 during 2002-2003 – 17 days exposure ATIC-3 anticipated for 2005


All particle spectrum: ATIC, emulsion, and EAS data

RUNJOB

JACEE

CASA-BLANCA

TibetKASKADETUNKA

ATIC-2


Charge resolution in the p-He group

EBGO > 50 GeV EBGO > 500 GeV EBGO > 5 TeV


Testing of models with the ATIC-2 spectra of protons and Helium

AMS

CAPICE98

ATIC-2

Diffusion model (Kolmogorov spectrum offluctuations)

at high energies

V. S. Ptuskin et al.astro-ph/0301420

at low energies(reacceleration process)


Energy spectra for H and He


Fill gap between low energy AMS and high energy JACEE with accurate measurements

Preliminary indication that H and He spectral indices are very similar Measurements of Iron group show flattening of spectrum Have measured GCR electrons up to about 2 TeV At the highest energies, the heavy ion spectra show deviations, which

might suggest that a modified Leaky Box Model, including a constant residual pathlength (0.13 g/cm2), is needed.

Preliminary charge histograms for E > 50 GeV from the ATIC-2 flight

Preliminary Results from ATIC-1 and ATIC-2

C O Ne Mg Si

S

Fe

S Ca


Energy spectra of abundant nuclei

C

O/10

Ne/100

Mg

Si/10

Fe/100

HEAO-3-C2

CRN

ATIC-2


ATIC also is able to identify CR electrons

e

• High energy electrons provides addition information about the GCR source• Possible bump at 600 – 800 GeV seen by both Kobayashi and ATIC may be

a source signature?


Statistical analysis of a spectral break –Where is the bend in the cosmic ray proton spectrum?

Use Poisson-weighted maximum likelihood approach applied to integral spectrum

Events are ordered by

decreasing energy

(E1 = Emax … En = Emin)


In any interval Ei = Ei-1 – Ei , the expected # of events is

<n>i = dN/dEi Ei Gi

where Gi = acceptance factor (m2 sr days TeV).

Poisson probability of seeing one event when <ni> are expected is

Pi (n=1) = <ni> e-<ni> .

Evaluate likelihood of spectrum from product of probabilities

ln L = ln i Pi = i ln <n>i – Nevents

where Nevents = total # of events

Assume a broken power law

dN/dE = a E- / [ 1 + (E/Eo) ]


Plot L for low energy index - and high energy index = 3.0 - 3.3 .

Choose a and to maximize L. Plot maximized L vs break

energy Eo.

Maximized likelihood peaks near 100 TeV Governed by ~ 20 protons above 100 TeV.

Excess above “no break” value at 0.1% level

At the level of 0.1% in ln L, it is equally likely that there is or is not a break in the spectrum. 10 100 1000

-622

-620

-618

-616

-614

= 0.4

= 0.5

= 0.6

= 0.7

No breakln

Lik

elih

ood

E0 (TeV)


Statistical uncertainty in ln L ~ ± 25

Much larger than difference between curves with and without break

If a break occurs and is significant, minimum # of events above break energy Eo must satisfy

N (> Eo) ~ (1 – 3) √Nevents (Cherry, 1999)

For JACEE, this corresponds to Eo ~ 40 – 90 TeV with

exposure ~ 84 m2 sr days. This is the maximum energy at which JACEE can make a statistically meaningful statement about the possible existence of a break.


How much exposure factor is required to look for a break at 500 TeV?

Assume spectrum steepens by at Eo.

Required exposure factorG ~ GJACEE (Eo/40 TeV)2(+-1)

~ 7 x 104 GJACEE for = 2.7, = 0.5, Eo = 500 TeV ~ 5600 m2 sr for 1000 days

Apply this analysis to CREAM, assuming 10 100-day flights:How high in energy will CREAM be able to detect a break?

Eo ~ 40 TeV (0.3 m2 sr x 1000 days / 84 m2 sr days)1/2(+-1) ~ 53 TeV

(Recall ~150 m2 sr days cumulative exposure for RUNJOB + JACEE compared to 12 m2 sr days for CREAM after its recent very successful 41 day initial flight)


How many events can one expect to see on a long duration (100 day) balloon flight?

H-He: Assume calorimeter geometry factor 0.9 m2 srB-Fe: Assume TRD geometry factor 6 m2 sr

Assume a proton spectral index = 2.75, B spectral index = 3.1, spectral index for all other nuclei = 2.65. Assume no break.

E (TeV) H He B C Si Fe

0.3 – 1 3.2 E 6 2.3 E 6 1.6 E 4 1.7 E 5 1.5 E 5 3.6 E 5

1 – 3 3.8 E 5 3.1 E 5 1.5 E 3 2.3 E 4 1.9 E 4 4.8 E 4

3 – 10 5.7 E 4 5.2 E 4 180 3.8 E 3 3.2 E 3 8.0 E 3

10 – 50 7.5 E 3 7.7 E 3 18 550 470 1.2 E 3

>100 140 190 3 20 160 410

> 300 21 30 4 4 67

> 1000 3 4 9

One needs to emphasize either p-He (calorimeter), B/C (TRD), or C-Fe (TRD)All three experiments need LARGE detectors and LONG exposure times.


Pessimistic conclusion:

Unless one is going to return to flights of

large emulsion detector arrays for multiple 100-day flights,

then:

high energy cosmic ray composition measurements in the atmosphere have gone about as far as they are going to go.

Unless a large space detector is flown, understanding the knee may have to come from ground-based detectors from now on.

NASA HQ July 2, 2004 Approaching the Knee -- Balloon-Borne Observations of Cosmic Ray Composition...

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