NASA HQ July 2, 2004 Approaching the Knee -- Balloon-Borne Observations of Cosmic Ray Composition Michael Cherry Louisiana State University the End of the Galactic Cosmic Ray Spectrum Aspen, April 26, 2005

NASA HQ July 2, 2004 Current and Recent Balloon Instruments to measure high energy (> 1 TeV) cosmic ray composition JACEE Series of emulsion experiments, 1979 – balloon flights, cumulative exposure 644 m 2 ~ 3.5 – 5.5 g/cm 2 Zenith angle acceptance out to tan θ ~ 72-79°  ~80 m 2 sr days exposure Highest energy proton event ~ 800 TeV RUNJOB Series of emulsion experiments, 1995 – balloon flights, cumulative exposure 575 m 2 ~ 9.0 – 10.7 g/cm 2 Highest energy proton event seen at E > 1 PeV ATIC Silicon matrix-scintillator-BGO calorimeter 2 balloon flights, , 31 days exposure  ~ 7 m 2 sr days exposure 3 rd LDB Antarctic flight scheduled for December 2005 CREAM Combined Scintillator, Si Charge Detector, W-scintillator calorimeter, TRD One balloon flight, , 41 days  ~ 12 m 2 sr days exposure Goal is to fly multiple 100 day (ULDB) flights to build up exposure

NASA HQ July 2, 2004 TRACER Scintillator-Cherenkov-TRD for 8 ≤ Z ≤ 26 Two flights,  ~ 40 m 2 sr days exposure TIGER Scintillator-Cherenkov-fiber hodoscope to measure Z ≥ 30 Three flights, , 50+ days exposure  ~ 4 m 2 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 m 2 sr days 3 – 3.5 g/cm 2 Planning to fly larger version on ULDB

NASA HQ July 2, 2004 Standard Model of Cosmic Ray Acceleration Supernova shock waves may accelerate cosmic rays via first order Fermi process –Model predicts an upper energy limit E max ~ Z x eV  composition growing heavier with increasing energy ATIC Energy Range

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

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

NASA HQ July 2, 2004 GRS 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 (Flare 1 decline – 81 days)

NASA HQ July 2, 2004 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 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 E max values

NASA HQ July 2, 2004 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 JACEE and RUNJOB emulsion payloads both had the advantage of being simple, large area (~1 m 2 ) packages that could be flown multiple times JACEE Balloon Flights: Cumulative Flight Launch Launch AltitudeDuration Units Exposure Date Site (g/cm 2 ) (hrs) (cm x cm) (m 2 - hrs) JACEE 0 5/79 Sanriku, Japan (40 x 50)6 JACEE 1 9/79 Palestine, TX (40 x 50)26 JACEE 2 10/80 Palestine, TX (40 x 50)50 JACEE 3 6/82 Greenville, SC (50 x 50)59 JACEE 4 9/83 Palestine, TX (40 x 50)107 JACEE 5 10/84 Palestine, TX (40 x 50)119 JACEE 6 5/86 Palestine, TX (40 x 50)143 JACEE 7 1/87 Alice Springs, (40 x 50)233 Australia JACEE 8 2/88 Alice Springs (40 x 50)305 JACEE 9 10/90 Ft. Sumner, NM (40 x 50)340 JACEE 10 12/90 McMurdo, (30 x 40)389 Antarctica JACEE 11 12/93 McMurdo (40 x 50) * JACEE 12 1/94 McMurdo (40 x 50)644 JACEE 13 12/94 McMurdo (40 x 50)1016 JACEE 14 12/95 McMurdo (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 JACEE 1-12 analysis based on 656 proton events above 6 TeV and 414 helium above 2 TeV/nucleon.

NASA HQ July 2, 2004 JACEE integral spectra seemed to show difference in spectral slope at 2  level  H = 1.80 ± 0.04  He = /-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 ATIC – a fully electronic detector (Si matrix + BGO calorimeter) J.H. Adams 2, H.S. Ahn 3, G.L. Bashindzhagyan 4, K.E. Batkov 4, J. Chang 6,7, M. Christl 2, A.R. Fazely 5, O. Ganel 3, R.M. Gunasingha 5, T.G. Guzik 1, J. Isbert 1, K.C. Kim 3, E.N. Kouznetsov 4, M.I. Panasyuk 4, A.D. Panov 4, W.K.H. Schmidt 6, E.S. Seo 3, N.V. Sokolskaya 4, J.P. Wefel 1, J. Wu 3, V.I. Zatsepin 4 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

NASA HQ July 2, 2004 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 m 2 sr  Multiple flights needed to obtain necessary exposure  ATIC-1 during – 14 days exposure  ATIC-2 during – 17 days exposure  ATIC-3 anticipated for 2005

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

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

NASA HQ July 2, 2004 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 V. S. Ptuskin et al. astro-ph/ at low energies (reacceleration process)

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

NASA HQ July 2, 2004  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/cm 2 ), is needed. Preliminary charge histograms for E > 50 GeV from the ATIC-2 flight Preliminary Results from ATIC-1 and ATIC-2 CO NeMgSi S Fe SCa

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

NASA HQ July 2, 2004 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?

NASA HQ July 2, 2004 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 (E 1 = E max … E n = E min )

NASA HQ July 2, 2004 In any interval  E i = E i-1 – E i, the expected # of events is i = dN/dE i  E i G i where G i = acceptance factor (m 2 sr days TeV). Poisson probability of seeing one event when are expected is P i (n=1) = e -. Evaluate likelihood of spectrum from product of probabilities ln L = ln  i P i =  i ln i – N events where N events = total # of events Assume a broken power law dN/dE = a E -  / [ 1 + (E/E o )  ]

NASA HQ July 2, 2004 Plot L for low energy index  -  and high energy index  = Choose a and  to maximize L. Plot maximized L vs break energy E o. 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 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 E o must satisfy N (> E o ) ~ (1 – 3) √N events (Cherry, 1999) For JACEE, this corresponds to E o ~ 40 – 90 TeV with exposure ~ 84 m 2 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 How much exposure factor is required to look for a break at 500 TeV? Assume spectrum steepens by  at E o. Required exposure factor G ~ G JACEE (E o /40 TeV) 2(  +  -1) ~ 7 x 10 4 G JACEE for  = 2.7,  = 0.5, E o = 500 TeV ~ 5600 m 2 sr for 1000 days Apply this analysis to CREAM, assuming day flights: How high in energy will CREAM be able to detect a break? Eo ~ 40 TeV (0.3 m 2 sr x 1000 days / 84 m 2 sr days) 1/2(  +  -1) ~ 53 TeV (Recall ~150 m 2 sr days cumulative exposure for RUNJOB + JACEE compared to 12 m 2 sr days for CREAM after its recent very successful 41 day initial flight)

NASA HQ July 2, 2004 How many events can one expect to see on a long duration (100 day) balloon flight? H-He: Assume calorimeter geometry factor 0.9 m 2 sr B-Fe: Assume TRD geometry factor 6 m 2 sr Assume a proton spectral index  = 2.75, B spectral index = 3.1, spectral index for all other nuclei  = Assume no break. E (TeV)HHeBCSiFe 0.3 – 13.2 E 62.3 E 61.6 E 41.7 E 51.5 E 53.6 E 5 1 – 33.8 E 53.1 E 51.5 E 32.3 E 41.9 E 44.8 E 4 3 – E 45.2 E E 33.2 E 38.0 E 3 10 – E 37.7 E E 3 > > > 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 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.