Sep. 13, JPS meeting

Transcription

1 Recent Results on Cosmic-Rays by Fermi-LAT Sep. 13, JPS meeting Tsunefumi Mizuno (Hiroshima Univ.) On behalf of the Fermi-LAT collaboration 1

2 Outline Introduction Direct measurement of CRs CRs in the Milky Way/external galaxies 2

3 Cosmic-Rays (CRs) at the Earth Discovered by Hess (1912), Nobel Prize (1936) Majority protons, 0.1-1% contribution from e - /e + N/dE (GeV cm -2 sr -1 s -1 ) protons Galactic (E<E knee) ), ExtraGalactic (E>E knee ) electrons U CR ~1 ev cm -3, comparable to positrons U B and U photon Origin and propagation of CRs is one of main topics of modern astrophysics. E 2 d antiprotons Galactic Extra Galactic E knee 3

4 Cosmic-Rays (CRs) at the Earth Discovered by Hess (1912), Nobel Prize (1936) Majority protons, 0.1-1% contribution from e - /e + N/dE (GeV cm -2 sr -1 s -1 ) protons Galactic (E<E knee) ), ExtraGalactic (E>E knee ) electrons U CR ~1 ev cm -3, comparable to positrons U B and U photon Origin and propagation of CRs is one of main topics of modern astrophysics. E 2 d antiprotons Galactic Extra Galactic E knee 4

5 CR Propagation in Milky Way CRs propagate for ~10 7 years before escaping to intergalactic space During the propagation they produce EM radiations Direct measurements show the spectrum averaged over time (~10 Myr) and space (~kpc) Indirect measurements through EM radiation provide a snapshot of CRs in distant locations. sun High energy CR electrons suffer rapid energy loss, hence may probe a few nearby sources. 5

6 CR Measurements SNR, Pulsar, etc. 6

7 CR Measurements SNR, Pulsar, etc. Fermi (2008-) 7

8 Fermi Gamma-ray Space Telescope Launched in 2008 Large Area Telescope (LAT) and Gamma-ray Burst Monitor (GBM) LAT as a GeV Gamma-ray Telescope 20 MeV - >300 GeV, 8000 cm 2 Aeff (>1GeV), ~2.4 sr FOV Sky survey to probe Galactic CRs LAT as a CRE detector Imaging calorimeter + ACD/TKR Exposure factor > 10 8 m 2 sr s Precise measurement of CRE spectrum Atwood et al., ApJ (2009) 8

9 Part I: Direct Measurement of CRs 9

10 CRE by Fermi-LAT (2009) High energy CREs may probe nearby sources An initial study used CREs collected for 6 month 4.5M above 20 GeV, >400 events in highest energy bin flat and relatively hard (~E -3 ) spectrum Pure diffusive models with proper choice of params, or models with additional e - /e + sources fit data well Fermi Data (2009) Ackermann et al., PRL (2009) 10

11 CRE by Fermi-LAT (2010) CREs collected for 12 month 8M above 7 GeV, >1000 events in highest energy bin Careful examinations of systematic uncertainty incl. cross-check with events with long path in CAL (>=13X 0 ) Noticeable deviation from single PL 20 GeV Fermi Data (2010) pre-fermi model Ackermann et al., accepted by Phys. Rev. D 11

12 CRE by Fermi-LAT (2010) Noticeable deviation from single PL Additional e - /e + sources can provide a good fit to Fermi CRE and PAMELA e + /(e - + e + ) Nature still in question. Astrophysical (pulsar), exotic (DM) or others Example of an additional component Ackermann et al., accepted by Phys. Rev. D 12

13 CRE Anisotropy Fermi offers an opportunity to search for possible CRE anisotropies (large statistics) Local CR sources, propagation environment No-anisotropy map Construct no anisotropy map from flight data shuffling and direct integration Then search for anisotropies with different energy thresholds (60 GeV min.) and on different angular scales (10 o -90 o ) Direct bin-to-bin comparison or spherical harmonic analysis Flight data sky map Significance ifi map No evidence of anisotropy above 60 GeV Ackermann et al., accepted by Phys. Rev. D 13

14 CRE Anisotropy (cont.) No evidence of anisotropies above 60 GeV and 10 o -90 o Upper limit for the dipole anisotropy: 0.5-5% 5% This limit is comparable to the value expected for a single nearby source dominating HE spectrum. will improve as more data are collected Example: Single astrophysical source + (almost) homogeneous Galactic CRE CRE spectrum at Earth Dipole Anisotropy Fermi HESS Fermi (3σ UL) Monogem Vela ea Monogem Vela Ackermann et al., accepted by Phys. Rev. D 14

15 Summary (Up to Now) Fermi-LAT can study CRs, directly (inclusive electron spectrum) and indirectly (γ-ray obs). Fermi has published precise CRE spectrum up to 1 TeV allows quantitative discussion of additional sources No evidence of anisotropies in the arrival direction above 60 GeV Upper limits are already interesting in terms of modeling 15

16 Part II: CRs in the Milky Way and external galaxies 16

17 Diffuse Gamma-Rays to Probe CRs CRs produce EM radiations during propagation LAT all-sky map ~90% of gamma-rays are diffuse emission, mostly produced via interaction of CR protons with the ISM gas GeV γ-rays are a powerful probe to study CRs incl. those in local group galaxies and nearby starburst galaxies. Planck microwave map (ISM gas) 17

18 CRs close to the Solar System 1) Intermediate lat. region: local ISM gas. Study CRs near Sun Abdo et al., ApJ 703, 1249 (2009) Abdo et al., PRL 103, (2009) 18

19 CRs close to the Solar System 1) Intermediate lat. region: local ISM gas. Study CRs near Sun 10 o <= b <=20 o Data agree with the model based on the LIS CR protons directly measured ~ local CR pool π 0 isotropic i bremss IC Abdo et al., ApJ 703, 1249 (2009) Abdo et al., PRL 103, (2009) 19

20 CR Distribution in Milky Way II quad. III quad. 2) Obs. of the outer Galaxy provides an accurate measurement of CRs beyond solar circle Abdo et al., ApJ 710, 133 (2010) Ackermann et al., ApJ submitted 20

21 The Gradient of CR Densities Emissivity = gamma-ray emission rate per H-atom gives an estimate of CR densities ( CR densi ity) Sun Preliminary II quad. III quad. Uncertainties dominated by HI optical depth (often overlooked in the past) Hint of CR density variation near Sun No significant CR gradient Abdo et al., ApJ 710, 133 (2010) Ackermann et al., ApJ submitted 21

22 The Gradient of CR Densities (cont.) CR densities beyond 11 kpc are greater than expected even if we take account of systematic uncertainty. Large CR halo Flat CR source distribution Preliminary i II quad. III quad. model from SNR distribution and a standard 4 kpc CR halo Abdo et al., ApJ 710, 133 (2010) Ackermann et al., ApJ submitted 22

23 The Large Scale Diffuse Emission The large scale diffuse analysis (Gal. plane) in progress. Already reproduces data well. Will provide CR p/e - distribution in the whole MW. π 0 Preliminary Inner Galaxy ( l <30 o, b <5 o ) Fermi Data Catalog sources IC bremss isotripic 23

24 CRs in nearby galaxies Study CR density distribution, correlation with SF activity M82 Fermi, VERITAS LMC Fermi N H =10 21 H cm -2 NGC253 Fermi, HESS 24

25 Local Group Galaxies Fermi, HESS and VERITAS reported detection of γ-rays from LMC, M82 and NGC 253. In addition, SMC Abdo et al., arxiv: First detection in gamma-rays CR density < 15% of local MW value Not a clear correlation between massive stars, neutral gas, pulsars or SNRs Hα contours Pulsars(*), SNRs(.) M31 Preliminary residuals after BG model subtraction and IRIS 100 um contours (convolved with LAT PSF) First detection in gamma-rays 25

26 Gamma-ray Luminosity vs SFR Correlation between gamma-ray luminosity and SFR over wide range in galaxy properties Details of relationship not yet understood Preliminary --- case of α=1.0 Lγ γ ~ SFR α with α=1.4 +/- 0.3 (fit with Local Group only) 26

27 Summary Fermi-LAT can study CRs directly and indirectly. Updated CRE spectrum and (non-) anisotropy provide a good constraint on additional e - /e + component. Larger CR densities in the outer Galaxy than expected Large CR halo and/or flat source distribution Detection of SMC and M31. Correlation found btw. gamma-ray luminosity and star-formation rate. Details yet to be understood. Continued gamma-ray and CRE observation provides further insight into CR production and transport. 27

28 Backup Slides 28

29 Alternative Event Selection Test possible systematic effect related to the energy resolution Events with long gp path (13 X 0 min, 16 X 0 ave.) in the instrument and contained in a single calorimeter module Energy dispersion much narrower and more symmetric, energy resolution better than 5% (1σ) up to 1 GTeV. Acceptance reduced to 5% of the standard one Ackermann et al., accepted by Phys. Rev. D 29

30 Alternative Event Selection Test possible systematic effect related to the energy resolution Two spectra are consistent within systematic errors Long path selection only optimized for energy resolution More challenging in terms of systematics (small sample) Not necessarily more accurate E res (1σ)~10% (@600GeV) E res (1σ)<5% (up to 1TeV) Ackermann et al., accepted by Phys. Rev. D 30

31 Low-Energy Extension Rigidity cutoff depends on the detector geomagnetic position ~7 GeV is the minimum energy accessible by Fermi orbit Data are divided in 10 independent McIlwain L bins Use bin of low cutoff to reconstruct low-energy spectrum Ackermann et al., accepted by Phys. Rev. D 31

32 Low-Energy Extension Rigidity cutoff depends on the detector geomagnetic position ~7 GeV is the minimum energy accessible by Fermi orbit Data are divided in 10 independent McIlwain L bins Use bin of low cutoff to reconstruct low-energy spectrum Ackermann et al., accepted by Phys. Rev. D 32

33 Significance Skymap A pre-trial significance map produced by a bin to bin comparison Because of the large number of trials (from ~100 trials at 90 o up to ~ 5000 at 10 o ) all the observed fluctuation is insignificant 10 o integ. radius 30 o integ. radius Ackermann et al., accepted by Phys. Rev. D 60 o integ. radius 90 o integ. radius 33

34 Bin to Bin Comparison Curves: Correspondence btw. a pre- and post-trials significance Markers: highest significance for different min. energy and radius All results are post-trials insignificant 90 o 60 o 45 o 30 o 10 o Ackermann et al., accepted by Phys. Rev. D 34

35 CRs close to the Solar System Probe nearby CRs through obs. of mid-latitude region gamma-rays HI column density Agree with model from LIS CR protons directly measured ~ local CR pool Emissivity = γ-ray emission rate per H-atom gives an estimate t of CR Spectrum (Ep~10Eγ) Abdo et al., ApJ 703, 1249 (2009) Abdo et al., PRL 103, (2009) 200 o <l<260 o 22 o < b <60 o protons (π 0 -decay) electrons (bremsstrahlung) 100 MeV 1 GeV 10 GeV 35

36 The Outer Galaxy Galactic Rotation->Doppler shift of Gas lines No ambiguity: velocity->distance Local arm (Orion Spur), Perseus arm and Outer arm Gal. Center Sun II quqd. III quad. 36

37 Why Outer Galaxy? 37

38 The Gradient of CR Densities Emissivity = gamma-ray emission rate per H-atom gives an estimate of CR densities Gould belt local spur Preliminary Perseus arm local spur Interarm region II quad. III quad. Perseus arm Abdo et al., ApJ 710, 133 (2010) Ackermann et al., ApJ submitted 38

39 EM and CR Spectrum of MW Muti-frequency Spectrum of Milky Way (GALPROP model based on Fermi results) CR e - /e + energy input ~ output via sync. (1/3) and IC (2/3) dust stellar Input CRs: protons conversion efficiency up to ~80% Helium (in case of large halo) X/γ via CR intearction Strong et al., accepted by ApJL sync radio IR opt UV microwave π 0 -decay IC Brems. X-rays γ-rays Pri. e- Sec. e+ Sec. e- 39

40 Spatially Resolved LMC Gamma-ray Emissivity map Contours: N(H) column density Pulsars (+) WR start (*) SNRs ( ) Supergiant shells (circles) CR density correlated with massive start-forming regions 40

41 Spatially Resolved SMC Not a clear spatial correlation with massive stars, neutral gas, pulsars or SNRs Significant contribution from pulsars? Average CR density <15% of local MW value Hα contours Pulsars(*) ), SNRs(.) 41