Data analysis techniques

Transcription

1 Data analysis techniques

2 How to go from to

3 Cherenkov Telescopes for Gamma-Ray Astrophysics Part II: Astrophysics with Cherenkov Telescopes W. Hofmann MPI für f Kernphysik, Heidelberg H.E.S.S. Telescope System (Photomontage)

4 Imaging atmospheric Cherenkov telescopes Pioneered by the WHIPPLE group Perfectioned in CAT telescope Stereoscopy with HEGRA WHIPPLE 490 PMT camera

5 Catalog of TeV sources Adapted from Weekes, astro-ph/ Source Galactic Crab Nebula PSR Vela SN 1006 RXJ Cassiopeia A Monoceros Centaurus X-3 Extragalactic Markarian 421 Markarian 501 1ES PKS ES H BL Lac 3C66A GRB a Type Plerion Plerion? Plerion? Shell SNR Shell SNR Shell SNR Shell SNR Binary XBL XBL XBL XBL XBL XBL XBL RBL GRB Distance ~ 2 kpc ~ 2 kpc kpc ~ 2 kpc 1 6 kpc ~ 3.5 kpc ~ 1.6 kpc ~ 8 kpc z=0.031 z=0.034 z=0.044 z=0.116 z=0.048 z=0.129 z= z=0.44 Discovery Flux (CU) 1 ~ 0.5 ~ 0.7 ~ 0.5 ~ 0.7 ~ ~ 0.4 up to ~ 10 up to ~ 10 up to ~ 0.6 up to ~ 2 up to ~ 2 ~ 0.1 up to ~ 2 ~ 1.5 Grade A A B B B C C C A A C B B B B C C Group Whipple,... CANGAROO,... CANGAROO CANGAROO CANGAROO HEGRA HEGRA Durham Whipple,... Whipple,... Whipple Durham Tel. Array Whipple,... Crimean Crimean Milagrito A: > 5 σ, confirmed B: > 5 σ, unconfirmed C 5 σ

6 The Crab Nebula with EGRET and HEGRA EGRET on CGRO at ~ 100 MeV HEGRA CT System at ~ 1 TeV 4 degr. 4 degr.

7 Gamma rays are secondary products their emission traces primary particle populations Hadrons π 0 decay Electrons Synchroton radiation Inverse Compton scattering Bremsstrahlung Heavy instable particles (strings, monopoles,...) Problem: to relate the gamma ray flux and the primary spectra, one needs to Know the properties of the production target Deconvolve energy spectra

8 Galactic Sources and the Origin of Cosmic Rays

9 The quest for the sources of cosmic rays Best (only?) candidate: Supernova explosions dn/de ~ E -2.7 ρ E ~ 1 ev/cm 3 Argument 1: Energy balance E CR ~ ρ E V / τ esc ~ erg/s E SN ~ erg/30 y ~ erg/s mostly nuclei; < 1% electrons (above 1 TeV)... need O(10%) efficiency Argument 2: Shock acceleration as mechanism dn/de Shock acc ~ E -2.1 dn/de Obs ~dn/de Acc τ esc (E) where τ esc (E) ~ E -0.6

10 Particle acceleration in Supernova remnants Example: Cassiopeia A about 300 y old size about 5 pc speed of ejecta ~5000 km/s Gamma-ray flux from SNR Drury, Aharonian, Völk 1994 (DAV) F γ (> 1 TeV) ~ 5 θ E 51 n 1 d kpc -2 [Crab] SNR acceleration models give Peak energies O(PeV) Efficiencies θ up to 0.5 θ Efficiency E 51 Energy [10 51 erg] n 1 Density [1/cm 3 ] d kpc Distance [kpc]

11 HEGRA Galactic Plane Survey First survey at TeV energies G. Pühlhofer, H. Lampeitl Limits on TeV flux for over 60 SNR Individual limits O(0.1 1) Crab Combined limit 0.04 Crab, for an average distance of 5.5 kpc DAV: 0.1 Crab for 50% eff.

12 SN 1006 ASCA Cassiopeia A Chandra Tycho Rosat Supernova d [kpc] Flux [Crab Units] Exp. 50% 10% SN CANGAROO ~ RX J CANGAROO Tycho < 0.03 HEGRA ~ Cassiopeia A HEGRA ~ All positive observations remain to be confirmed...

13 SNR 1006 (CANGAROO) Significance map CANGAROO Tanimori et al Tanimori et al Signal observed with different generations of (single) telescopes 3.8 m 7.7 σ 10 m 6.5 σ Signal centered on NE rim of SNR Cannot really map source distribution, but signal appears wider than experimental resolution

14 Interpretation CANGAROO Tanimori et al Tanimori et al Mastichiadis, Stecker 1996 Aharonian, Atoyan SN 1006 accelerates electrons up to energies of ~100 TeV No evidence for (or against) proton acceleration

15 RX J (CANGAROO) CANGAROO Muraishi et al Enemoto et al Rumors about paper subm. to Nature, stating that X-rays and TeV spectra are NOT consistent for electron models α source

16 Tycho SNR (HEGRA) HEGRA Aharonian et al G. Rowell Angular distribution of photon candidates relative to source Very good upper limit of 0.03 Crab Units Cuts into model parameter space of hadronic models

17 Cassiopeia A (HEGRA) HEGRA Aharonian et al Ph.D. G. Pühlhofer Very deep observation 232 h 5 σ signal at 0.03 Crab level C.o.g. of gamma emssion (diameter of SNR only 3-4 arc-min; cannot resolve details)

18 Cassiopeia A spectra Inverse Compton radiation: model synchrotron spectra in a multi-zone model, then predict IC Atoyan et al IC,... π 0 decay Fit of spectral shape: probability for hadronic origin 98% electron origin 15%

19 Problems with injection? Völk 2001 Inefficient injection in the when B fields parallel to shock front? Acceleration only near poles?

20 Monoceros: SNR Interacting with Cloud an EGRET source HEGRA TeV gamma rays ~ 5 σ for total excess F. Lucarelli et al., ICRC 2001

21 Conclusions Cherenkov telescopes have opened up TeV gamma ray astronomy A number of astrophysical objects produce gamma rays with energies well beyond TeV scales (Crab: > 50 TeV, SNRs > 10 TeV, AGNs > 20 TeV) Many of these objects emit as much or more energy at TeV energies than in other wavelength ranges In current objects, data are consistent with a primary electron population (with energies up to and beyond 100 TeV) Shock wave acceleration is strongly supported, but... High energy electrons are much more efficient in producing gammas O( ) at TeV energies hence not obvious if electron sources will also accelerate sufficient cosmic rays Need No proof for nucleon acceleration... just around the corner?

22 Verifying nucleon acceleration Spectral and spatial distribution of synchrotron radiation Inconsistency? in particular at high energy... electrons are hard to accelerate, see LHC vs LEP Spectral and spatial distribution of VHE gamma radiation Need more sensitive instruments, which allow a precise spectral and spatial mapping of sources H.E.S.S., and neutrino observatories...

23 Extragalactic Sources, and the Infrared Background

24 Gamma-Rays from the AGNs Markarian 421 and 501 Mkn 501 Faint galaxy, about 500 Mio light years from Earth In 1997 the strongest source of highest-energy gamma-rays in the sky!

25 a strongly variable source Mkn 421 Whipple Whipple data M. Catanese, T. Weekes, astro-ph/

26 Energy spectrum of gamma rays Shape (almost) independent of intensity

27 Interpretation of Blazar sources Black hole of solar masses Relativistic jet Small Blob accelerating particles Size (c T) δ

28 Acceleration of of charged particles in in the the blob blob Electrons? Protons? Spectrum & cutoff? Size of blob? Variability of injection rate? of spectrum?... Targets for for photon production within the the blob blob IC: on synchrotron photons? Disk photons? Background photons? Proton interactions with moving clouds?... Use time dependence of flux and spectral shape and correlation with other wavelengths (X-rays) Absorption within the the blob blob Optically thick source? If not, need to limit energy density in source volume! Could generate cutoff... Pair creation on background photons limits path; could generate cutoff... Photon density not well known! Absorption in in intergalactic space Energy resolution of 10-30% may smear cutoff Syst. error in scale may shift cutoff Measurement of of flux flux and and spectrum

29 Acceleration of of charged particles in in the the source Spectrum? Electrons, protons,...? CAT data Piron et al kev TeV Looks like electrons!

30 Origin of variability in synchrotron self-compton (SSC) models e X-ray γ-ray Electron injection increased by x 3; at 1, 2, τ Maximum energy of electrons increased by x 5 Magnetic field increased by x 3 A. Mastichiadis, J. Kirk astro-ph/

31 Correlation between TeV gammas and X-rays HEGRA vs. RXTE Mkn 501 H. Krawczynski et al. astro-ph/ Expect TeV ~ (X-ray) 2 for SSC if electron spectrum does not change in the static limit

32 Correlation between X-ray flux and spectral shape L. Maraschi et al., astro-ph/ , astro-ph/ , Harder X-ray spectrum for higher flux: either electron spectrum or B field or source boost δ change in addition to electron flux; Data constrain mainly δ/b

33 Correlation between VHE flux and spectral shape: Mkn 421 Long-term F. Piron, CAT astro-ph/ Flux syst. error Hardness ratio Short-term D. Horns, HEGRA

34 Fits using SSC models F. Tavecchio, L. Maraschi, astro-ph/ R (10 16 cm) B (G) δ Index High Low Flux γ break x Values for R, B, γ break differ by more than one order of magnitude between the two fits! F. Tavecchio et al. ApJ 554 (2001) 725

35 Targets for for photon production within the the source 3C 279 There is more than SSC: SSC + ECD (Direct disk radiation) + ECC (Reprocessed disk rad.) Hartmann et al., 2001

36 Absorption within the the source Source size ~ δ t Pair creation optical depth ~F opt t 1 δ 6 E γ minimum doppler factor ~8.5 for Mkn 501

37 VHE gamma e + Background photon Absorption in in intergalactic space Largest cross section near threshold, 4 E VHE E Photon ~ (2 m e c 2 ) 2 e -

38 Energy ε of background photon near peak of X-section (ev) Redshift Mkn 421, 501 Density of target photons x ε 2 Optical depth (Redshift z) (Funk et al, 98) Energy of γ-ray

39 G. Hauser, E. Dwek astro-ph/ Density of target photons x ε 2

40 Recent Whipple and HEGRA results on Mkn 421 F. Krennrich et al. ApJ 560 (2001) L45 HEGRA Whipple Mkn 421 HEGRA Whipple Mkn 501 Mkn 501 Mkn 421 Energy cutoff Mkn 421 Mkn 501 Whipple 4.3 ± 0.3 TeV 4.6 ± 0.8 TeV HEGRA 4.0 ± 0.4 TeV 6.2 ± 0.4 TeV Syst. errors ~ 30%

41 Spectral cutoff and the IR/O background Origin of the cut-off could be Interaction with IR/O background Cutoff in electron spectra in source KN regime of IC process Optically thick source depends only on z source different for each source Approaches in literature 1. Assume power law source spectrum; neglect other effects determine IR/O density 2. Fit electron spectrum, B, δ from X-rays; predict source γ spectrum; assume that source is transparent determine IR/O density 3. Assume IR/O density, determine source spectrum from observed spectrum consistency check: reasonable source spectrum? (constant spectral index or steepening with energy) ok ok not ok

42 The TeV-gamma-ray crisis (?) R. Protheroe, H. Meyer, astro-ph/

43 Solutions Violation of Lorenz invariance modifies photon propagation (Coleman & Glashow 1999,...) Photon condensates fake high-energy photons (Harwit et al. 1999) Measure height of shower maximum, ~ log(e γ )

44 Reanalysis of HEGRA data with improved energy resolution: consistent, but last point lower IR crisis goes away, if systematic errors on VHE data and errors on IR data are taken into account Aharonian et al., A&A 366 (2001) 62 O. de Jager, F. Stecker, astro-ph/ Measurement of of flux flux and and spectrum

45 H XBL at z=0.129 Synchrotron extends to > 100 kev Detected by VERITAS D. Horan et al ICRC 2001 HEGRA source location Also seen by HEGRA F. Aharonian et al., ICRC 2001 see also CAT Source clearly identified

46 Spectrum & Infrared Background HEGRA spectrum consistent with E -2.6 also consistent with strongly absorbed spectrum...

47 AGNs Time variability in VHE data requires compact source and significant boost (~10) SSC models describe data reasonable well, but other (e.g. hadronic) models are not completely excluded AGN spectra deviate from power laws, with cutoffs around a few TeV for Mkn 421 and 501 (with z ~ 0.03) Data are (within statistical and systematic errors) consistent with IR absorption using typical IR spectra; no obvious crisis or need to invoke exotic phenomena More precise data (multiwavelength spectra, time variability) are needed to fix model details More sources are needed, at different z, luminosity, etc Many new results expected for the next years... with O(100) sources

48 Physics with non-imaging Cherenkov instruments STACEE CELESTE secondary reflector and PMTs Sample light distribution on the ground, rather than angular distribution

49 CELESTE was designed to reach a very low energy threshold without a large expenditure in time and resources by exploiting the mirrors of an existing structure... (M. De Naurois et al., astro-ph/ ; see also CELESTE Proposal, 1996) CELESTE STACEE GRAAL Instrument Location No. of heliostats Mirror area (sqm) No. of PMTs F.o.V. mrad Threshold (GeV ) CELESTE Pyrenees, France ~10 60 ( 53) ( 2700) STACEE Albuquerque, US ~ ( 64) ( 2400) GRAAL Almeria, Spain SOLAR 2 Barstow, US ~ 10?

50 Problem of current experiments: very limited field of view γ p

51 CELESTE detection rates Crab: 2.1 σ / h 3.4 σ / h with double pointing Rate ~ 4 /min Quality fact. ~ 1.5 (homogeneity)

52 Detection of Crab Nebula GRAAL CELESTE CELESTE M. De Naurois et al., astro-ph/ STACEE S. Oser et al., astro-ph/ GRAAL F. Arqueros et al., astro-ph/

53 hopefully soon detailed AGN data

54 The Near Future

55 H.E.S.S. in Namibia 1 st Tel. ready in Spring nd late in rd and 4 th in 2003

56

57 MAGIC at La Palma 1 st light in 2002

58 CANGAROO III in Australia 1 st Tel. operational, 2 nd in Spring rd in th in 2004

59 Ideas for the Far Future Skip topic

60 Higher sites Images still ok at 10 GeV Possible location: F. Aharonian et al., Astropart. Phys. 15 (2001) m telescopes at 5 km provide sub 10 GeV threshold and very high rate

61 Better vision: novel photon detectors ε ~ 22% ε ~ 15% ε ~ 62%

62 Wide-angle systems with Fresnel lenses D.J. Lamb (OWL) Potential problems Size lens diameter is about 2 x effective aperture for most designs Technical issues concerning lens construction (stability, glare, stray light,...) Number of pixels O(10 5 ) for 60 degr. fov

63 Rosat X-Ray (kev) Ginga GLAST Number of sources Uhuru Einstein COS-B CGRO Gamma (GeV) 10 SAS-2 Cherenkov telescopes Gamma (TeV) Year