Particle Astrophysics

Transcription

1 Particle Astrophysics Particle Astrophysics Spring

2 Discovery of cosmic rays! Cosmic rays were discovered in 1912 by Hess! he showed that the intensity of penetrating radiation increased with altitude! therefore not due to natural radioactivity in rocks! Shown to be charged particles by Compton in 1932! flux observed to vary with latitude as expected for charged particles deflected by Earth s magnetic field! East-west asymmetry observed in 1933! showed particles were mainly positively charged (protons & ions) Particle Astrophysics Spring

3 Cosmic Ray Discovery! Physikalische Zeitschrift: The results of these observations seem best explained by a radiation of great penetrating power entering our atmosphere from above. Elevation Rate Ground 12 1km 10 2 km km 15 5 km 27 Victor Franz Hess V. F. Hess. Über Beobachtungen der durchdringenden Strahlung bei sieben Freiballonfahrten. Physikalische Zeitschrift, 13: , November Particle Astrophysics Spring 2015

4 Cosmic Rays!The flux of these charged cosmic rays follows nearly a single power law over 10 decades in energy Direct Measurements in Space Air Showers from the Ground!Single particles have been observed with energies above ev!!there are several kinks in the spectrum where the exponent changes, steepening at the knee and flattening at the ankle.!the source of the highenergy cosmic rays remains elusive.

5 Rigidity! The equation of motion of a particle of charge Ze and rest mass m in a uniform magnetic field B is given by:! It s useful to define a quantity called Rigidity (R)! R=pc/Ze! Ions of equal rigidity will respond in the same way to a given magnetic field! The gyroradius is in a field B is given by r g = R/(Bc)! An iron nucleus with the same rigidity as a proton will have 26 times the energy (Z iron =26) Particle Astrophysics Spring 2015

6 The Charged Particle Spectrum F(E) = KE -" Knee (steepening) ": > -3 Ankle (flattening) ": 3 -> E knee ~ 3 x ev E ankle ~ ev Particle Astrophysics Spring

7 The Solar Wind! Solar Wind effects:! The solar wind shields the solar system from low energy galactic cosmic rays.! Solar activity can result in further suppression of galactic CR (Forbush decrease or an increased amount of high energy particles reaching the ground (ground level enhancement) Particle Astrophysics Spring

8 The Implications of a High Cosmic-Ray Ionization Rate in Diffuse Interstellar Clouds Nick Indriolo, Brian D. Fields, Benjamin J. McCall University of Illinois at Urbana-Champaign November 8, 2008 MWAM 08 1

9 Cosmic Ray Basics Charged particles (e-, e+, p, α, etc.) with high energy ( ev) Galactic cosmic rays are primarily accelerated in supernovae remnants Image credit: NASA/CXC/UMass Amherst/M.D.Stage et al. 2

10 Background Cosmic rays have several impacts on the interstellar medium, all of which produce some observables Ionization: molecules (OH, H 2 O chemistry) CR + H 2 H 2+ + e - + CR H 2+ + H 2 H 3+ + H Spallation: light element isotopes [p, α] + [C, N, O] [ 6 Li, 7 Li, 9 Be, 10 B, 11 B] Nuclear excitation: gamma rays [p, α] + [C, O] [C *, O * ] γ (4.4, 6.13 MeV) 3

11 Motivations Many astrochemical processes depend on ionization Cosmic rays are the primary source of ionization in cold interstellar clouds Low-energy cosmic rays (2-10 MeV) are the most efficient at ionization The cosmic ray spectrum below ~1 GeV cannot be directly measured at Earth 4

12 Example Cosmic Ray Spectra 1 - Herbst, E., & Cuppen, H. M. 2006, PNAS, 103, Spitzer, L., Jr., & Tomasko, M. G. 1968, ApJ, 152, Kneller, J. P., Phillips, J. R., & Walker, T. P. 2003, ApJ, 589, 217 Shading Mori, M. 1997, ApJ, 478, Valle, G., Ferrini, F., Galli, D., & Shore, S. N. 2002, ApJ, 566, Hayakawa, S., Nishimura, S., & Takayanagi, T. 1961, PASJ, 13, Nath, B. B., & Biermann, P. L. 1994, MNRAS, 267, 447 Points AMS Collaboration, et al. 2002, Phys. Rep., 366, 331 5

13 Motivations Recent results from H 3+ give an ionization rate of ζ 2 = s -1 Given a cosmic ray spectrum and cross section, the ionization rate can be calculated theoretically z = ò E high 4p f( E) s ( E) de E low Indriolo, N., Geballe, T. R., Oka, T., & McCall, B. J. 2007, ApJ, 671,

14 Results from Various Spectra Spectrum Propagated Hayakawa et al. Spitzer & Tomasko Nath & Biermann Kneller et al. Valle et al. Herbst & Cuppen Cosmic-Ray Ionization Rate (ζ s -1 ) ζ 2 (diffuse) ζ 2 (dense) Observations 40 a 3 b a Indriolo, N., Geballe, T. R., Oka, T., & McCall, B. J. 2007, ApJ, 671, 1736 b van der Tak, F. F. S., & van Dishoeck, E. F. 2000, A&AL, 358, L79 7

15 Add Flux at Low Energies p-4.3 p-2.0 p0.8 f=0.01 p-2.7 8

16 High Flux Results Cosmic-Ray Ionization Rate (ζ s -1 ) Spectrum Broken Power Law Carrot Observations ζ 2 (diffuse) ζ 2 (dense) This is no surprise, as these spectra were tailored to reproduce the diffuse cloud ionization rate results 9

17 Gamma-Ray Results Diffuse Gamma-Ray Flux from the Central Radian (10-5 s -1 cm -2 rad -1 ) Energy INTEGRAL a Propagated Power Law Carrot 4.44 MeV MeV a Teegarden, B. J., & Watanabe, K. 2006, ApJ, 646,

18 Energy Constraints There are approximately 3±2 supernovae per century, each releasing about erg of mechanical energy The carrot spectrum requires erg per century, while the broken power law requires erg per century Both are well within constraints 13

19 Acceleration Mechanism Carrot spectrum shape does not match acceleration by supernovae remnants Voyager 1 observations at the heliopause show a steep slope at low energies Possible that astropauses are accelerating cosmic rays throughout the Galaxy Fig. 2 - Stone, E. et al. 2005, Science, 309,

20 Conclusions Carrot spectrum explains high ionization rate, and is broadly consistent with various observables p -4.3 power law is inconsistent with acceleration from SNR Perhaps weak shocks in the ISM are responsible for the vast majority of lowenergy cosmic rays 15

21 Acknowledgments Brian Fields The McCall Group 16

22 Cross Sections Bethe, H. 1933, Hdb. d Phys. (Berlin: J. Springer), 24, Pt. 1, 491 Read, S. M., & Viola, V. E. 1984, Atomic Data Nucl. Data, 31, 359 Meneguzzi, M. & Reeves, H. 1975, A&A, 40, 91 17