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

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 Instruments to measure high energy (> 1 TeV) cosmic ray composition.

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

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  1. 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 NASA HQ July 2, 2004

  2. Current and Recent Balloon Instrumentsto measure high energy (> 1 TeV) cosmic ray composition • JACEE Series of emulsion experiments, 1979 – 1994 11 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 exposure Highest energy proton event ~ 800 TeV • RUNJOB Series of emulsion experiments, 1995 – 1999 10 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 calorimeter 2 balloon flights, 2000-2003, 31 days exposure  ~ 7 m2 sr days exposure 3rd LDB Antarctic flight scheduled for December 2005 • CREAM Combined Scintillator, Si Charge Detector, W-scintillator calorimeter, TRD One balloon flight, 2004-2005, 41 days  ~ 12 m2 sr days exposure Goal is to fly multiple 100 day (ULDB) flights to build up exposure NASA HQ July 2, 2004

  3. TRACER Scintillator-Cherenkov-TRD for 8 ≤ Z ≤ 26 Two flights, 1999-2004  ~ 40 m2 sr days exposure • TIGER Scintillator-Cherenkov-fiber hodoscope to measure Z ≥ 30 Three flights, 1997-2004, 50+ days exposure  ~ 4 m2 sr days exposure Originally planned as first ULDB instrument; future flights planned • CAKE Nuclear track detectors (CR-39, Lexan) to measure 6 ≤ Z ≤ 74 One 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 NASA HQ July 2, 2004

  4. 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 NASA HQ July 2, 2004

  5. 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 NASA HQ July 2, 2004

  6. 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 NASA HQ July 2, 2004

  7. BATSE earth occultation measurements – Ling and Wheaton (2004) NASA HQ July 2, 2004

  8. GRO J1655-40 - Light Curve of Phase 2-6 (Case et al. 2004) NASA HQ July 2, 2004

  9. GRS 1915+105 Spectrum Sum of high state of Flare 1 and Flare 2 (274 days) TJD 9084-9180 (Flare 1 decline – 81 days) • 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 NASA HQ July 2, 2004

  10. 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. Is it really true that composition gets heavier approaching the knee as low-Z components successively cut off in supernova accelerator? ATIC Energy Range “Multiple source” model could lead to much slower variation of composition with energy – and higher Emax values NASA HQ July 2, 2004

  11. 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. NASA HQ July 2, 2004

  12. 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: Cumulative Flight 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) 6 JACEE 1 9/79 Palestine, TX 3.7 25.2 4 (40 x 50) 26 JACEE 2 10/80 Palestine, TX 4.0 29.6 4 (40 x 50) 50 JACEE 3 6/82 Greenville, SC 5.0 39.0 1 (50 x 50) 59 JACEE 4 9/83 Palestine, TX 5.0 59.5 4 (40 x 50) 107 JACEE 5 10/84 Palestine, TX 5.0 15.0 4 (40 x 50) 119 JACEE 6 5/86 Palestine, TX 4.0 30.0 4 (40 x 50) 143 JACEE 7 1/87 Alice Springs, 5.5 150.0 3 (40 x 50) 233 Australia JACEE 8 2/88 Alice Springs 5.0 120.0 3 (40 x 50) 305 JACEE 9 10/90 Ft. Sumner, NM 4.0 44.0 4 (40 x 50) 340 JACEE 10 12/90 McMurdo, 3.5 204.0 2 (30 x 40) 389 Antarctica JACEE 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) 644 JACEE 13 12/94 McMurdo 5.0 310.0 6 (40 x 50) 1016 JACEE 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. NASA HQ July 2, 2004

  13. JACEE 1-12 analysis based on 656 proton events above 6 TeV and 414 helium above 2 TeV/nucleon. NASA HQ July 2, 2004

  14. JACEE integral spectra seemed to show difference in spectral slope at 2 s level gH = 1.80 ± 0.04 gHe = 1.68 + 0.04/-0.06 (Asakimori et al, 1998) “Waviness in integral spectra” – an artifact of meager statistics or an indication of systematic errors in combining data from multiple flights? NASA HQ July 2, 2004

  15. 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 • Louisiana State University • Marshall Space Flight Center • University of Maryland • Skobeltsyn Institute of Nuclear Physics • Southern University • Max Plank Inst. for Solar System Research • Purple Mountain Observatory NASA HQ July 2, 2004

  16. 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 NASA HQ July 2, 2004

  17. All particle spectrum: ATIC, emulsion, and EAS data RUNJOB JACEE CASA-BLANCA Tibet KASKADE TUNKA ATIC-2 NASA HQ July 2, 2004

  18. Charge resolution in the p-He group EBGO > 50 GeV EBGO > 500 GeV EBGO > 5 TeV NASA HQ July 2, 2004

  19. Testing of models with the ATIC-2 spectra of protons and Helium AMS CAPICE98 ATIC-2 Diffusion model (Kolmogorov spectrum of fluctuations) at high energies at low energies (reacceleration process) V. S. Ptuskin et al. astro-ph/0301420 NASA HQ July 2, 2004

  20. Energy spectra for H and He NASA HQ July 2, 2004

  21. Preliminary Results from ATIC-1 and ATIC-2 • 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. Ne Mg Si Fe C O Preliminary charge histograms for E > 50 GeV from the ATIC-2 flight S S Ca NASA HQ July 2, 2004

  22. Energy spectra of abundant nuclei Mg C O/10 Si/10 Fe/100 Ne/100 HEAO-3-C2 ATIC-2 CRN NASA HQ July 2, 2004

  23. ATIC also is able to identify CR electrons • 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? e NASA HQ July 2, 2004

  24. 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) NASA HQ July 2, 2004

  25. In any interval dEi = Ei-1 – Ei , the expected # of events is <n>i = dN/dEi dEi 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 Pi Pi = Si ln <n>i – Nevents where Nevents = total # of events Assume a broken power law dN/dE = a E- g / [ 1 + (E/Eo)d ] NASA HQ July 2, 2004

  26. Plot L for low energy index g = 2.6 - 2.7 and high energy index g + d = 3.0 - 3.3 . Choose a and g 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. NASA HQ July 2, 2004

  27. 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. NASA HQ July 2, 2004

  28. How much exposure factor is required to look for a break at 500 TeV? Assume spectrum steepens by d at Eo. Required exposure factor G ~ GJACEE (Eo/40 TeV)2(g+d-1) ~ 7 x 104 GJACEE for g = 2.7, d = 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(g+d-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) NASA HQ July 2, 2004

  29. 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 sr B-Fe: Assume TRD geometry factor 6 m2 srAssume a proton spectral index g = 2.75, B spectral index = 3.1, spectral index for all other nuclei g = 2.65. Assume no break. 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. NASA HQ July 2, 2004

  30. 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

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