Fermi: Highlights of GeV Gamma-ray Astronomy

Transcription

1 Fermi: Highlights of GeV Gamma-ray Astronomy Dave Thompson NASA GSFC On behalf of the Fermi Gamma-ray Space Telescope Large Area Telescope Collaboration Neutrino Oscillation Workshop Otranto, Lecce, Italy September, 2010

2 Outline Introduction - Gamma Rays and Neutrinos Distant GeV Gamma-ray Sources Gamma-ray Bursts (GRB) Active Galactic Nuclei (AGN) Local and Starburst galaxies Galactic GeV Gamma-ray Sources Pulsars and Pulsar Wind Nebulae Binary sources Supernova remnants Summary 2

3 Gamma Rays s and Neutrinos High-energy gamma rays are primarily produced by interactions of energetic particles. Typical processes are inelastic nuclear collisions (pion production) and inverse Compton scattering. Gamma rays tracing hadronic processes are of particular interest for neutrino observations. Neutrino observations may be critical to learning the nature of gamma-ray sources. The Universe is mainly transparent to gamma rays with energies less than 20 GeV, so they can probe distant or obscured regions. Potential to identify locations and time variability for neutrinoproducing objects. 3

4 The Fermi Gamma-ray Space Telescope An International Mission Large Area Telescope (LAT) KEY FEATURES Spacecraft Partner: General Dynamics 20 MeV -> >300 GeV 2.4 Steradian field of view Operated in scanning mode, so views the entire sky every 3 hours. Peak effective area ~8000 cm2 Single photon angular resolution 0.8o at 1 GeV, better at higher energies. Source location capability 1-10 arcmin. Energy resolution 10-20% Gamma-ray Burst Monitor (GBM) NaI and BGO Detectors 4 8 kev - 40 MeV

5 The Gamma-ray Sky Seen with Fermi LAT Galactic diffuse emission comes from cosmic-ray interactions with the interstellar medium Sources are seen against a strong diffuse background. E > 1 GeV image. 5

6 Gamma-ray Spectrum at Intermediate Galactic Latitudes These pion-producing interactions imply that there is a diffuse background of neutrinos, too. π Observed Total with Sources Bremsstrahlung Sources Total Diffuse Isotropic Inverse Compton 6

7 1451 Sources 7

8 Extragalactic Gamma-ray Sources 8

9 Gamma-ray Bursts (GRBs( GRBs) ) - the Brightest and Most Distant Sources Seen by Fermi GBM NaI GRB Abdo et al. 2009, Nature 462, 331 Both long (>2 sec) and short (<2 sec) bursts have been seen Collapsar: GBM BGO Rapidly spinning stellar core collapse supernova, with relativistic jets that can produce long GRBs LAT (>1MeV) 31 GeV 0.83 s Constraints on the quantum gravity mass scale (M QG ) by direct measurement of photon arrival times, testing Lorentz Compact Merger: Two neutron invariance violation. stars, or a neutron star and a black hole, merge, producing a jet that gives rise to a short GRB M QG,1 /M Planck > 1.19 Some bursts show highenergy emission afterglow Constraint: lower limit of bulk Lorentz factor: Γ min ~1000 Some bursts have an extra spectral component compared to the standard Band model. These short, bright flashes can be used as tools to probe basic physics, as in the example here. 9

10 Over half the bright sources seen with LAT appear to be associated with Active Galactic Nuclei (AGN) Power comes from material falling toward a supermassive black hole Some of this energy fuels a jet of high-energy particles that travel at nearly the speed of light Fermi LAT sees primarily blazars, for which the jet is pointed toward Earth. 10

11 The Variable Gamma-ray Sky 11

12 Blazar PKS Gamma rays X-rays Ultraviolet Optical Radio The spectral Energy Distribution (SED) of this blazar is complex, requiring multiple components that vary with time. A key result for Fermi and multiwavelength studies: in most cases, simple models for blazars are inadequate. In some models of blazar jets, hadrons transport much of the energy and have neutrino-producing interactions. Seeing neutrinos from a blazar would be the key to verifying hadronic interactions. 12

13 Centaurus A - Radio Galaxy Over ½ of the total >100 MeV observed LAT flux in the lobes E > 200 MeV 22 GHz LAT PSF LAT counts map with background (isotropic and diffuse) and field point sources subtracted WMAP image provided by Nils Odegard (GSFC) Requires TeV electrons in giant relic lobes: accelerated in-situ or efficient energy transport from the center of the galaxy. 13

14 Galaxies Dominated by Cosmic-Ray Interactions π o Large Magellanic Cloud NGC 253 Starburst Galaxies Small Magellanic Cloud Spectrum Small Magellanic Cloud GeV gamma rays in these galaxies come primarily from the interactions of cosmic ray hadrons and electrons with interstellar matter and photon fields. M82 14

15 Galactic Gamma-ray Sources 15

16 The Pulsing γ-ray Sky Over 60 gamma-ray pulsars are now known. Pulses at 1/10 th true rate 16

17 Pulsar Wind Nebulae - Powerful Particle Accelerators Crab Nebula Spectral Energy Distribution. Red points are Fermi LAT data, showing transition from synchrotron to Compton components. Vela Pulsar E>800 MeV Test Statistic (significance) map. PWN Vela X E>2 GeV counts map. PWN HESS J E>10 GeV counts map. PWN MSH

18 Cygnus X-3 - Binary System Hour Period Gamma rays X-rays Neutron star or black hole binary system, accelerating particles to high energies. The system remains largely a mystery. 18

19 Cygnus X-3 - Binary System Hour Period LAT LAT Fermi LAT only sees gamma-ray emission when Cyg X-3 has strong low-energy X-ray flux and weaker high-energy X-ray flux. 19

20 Supernova Remnants (SNR) - Spatially Resolved Strong evidence for cosmic ray production in SNR. Note: LAT does not resolve Cas A 20

21 Fermi Observations of SNR Supernova remnant W44 - spatially resolved GeV front-converting events, deconvolved image. Green contours are from Spitzer IRAC, shocked H 2 Black cross is PSR B , not seen as a pulsed source. Note: leptonic models are not excluded. Neutrinos could confirm the acceleration of hadrons by SNR

22 Fermi Observations of SNR Supernova remnant W51C - spatially resolved GeV front-converting events, deconvolved image. Magenta contours are region of shocked CO Black diamond is possible PWN CXO J White ellipse is the outer boundary of W51. LAT PSF 22 22

23 A Surprise - A Gamma-ray Nova In early March, the LAT skywatchers found a new, flaring gamma-ray source in the Cygnus region. To our surprise, we learned that an optical flare of the symbiotic system V407 Cyg (red giant/white dwarf binary) had occurred at about the same time. 23

24 A Surprise - A Gamma-ray Nova The energy spectrum of the nova is plausibly produced by accelerated protons from the expanding shell of the nova colliding with the wind from the red giant star. 24

25 What Next for Fermi? As we start the third year of operations, we have only scratched the surface of what the Fermi Gamma-ray Space Telescope can do. The gamma-ray sky is changing every day, so there is always something new to learn about the extreme Universe. Beyond pulsars, blazars, X-ray binaries, SNR, starburst galaxies and gamma-ray bursts, other sources remain mysteries. Nearly 40% of the sources in the First LAT Catalog do not seem to have obvious counterparts at other wavelengths. Multiwavelength/multimessenger studies, including neutrino observations, will be critical for learning the nature of such sources. 25

26 Light dark-matter particles produce 511 kev (low-energy) gamma rays Dark-matter particles annihilate with one another, leading to gamma rays WIMP dark-matter particles (neutralinos) produce 30 MeV to 10 GeV (medium-energy) gamma rays Heavy dark-matter particles produce 300 to 600 GeV (high-energy) gamma rays Illustrations by Gregg Dinderman/Sky & Telescope 26

27 Dark Matter Searches Upper limits on Dark Matter annihilation cross section. Dwarf Spheroidal Galaxies are known to be dominated by Dark Matter. 27

28 Summary Gamma rays seen with the Fermi Gamma-ray Space Telescope are revealing sites of particle acceleration and interaction, ranging from distant Gamma-ray bursts and Active Galactic Nuclei to sources in our Galaxy. Gamma-ray bursts, flares from Active Galactic Nuclei, Starburst Galaxies, Supernova Remnants, and Novae are all good candidates for astrophysical neutrino sources. All the Fermi gamma-ray data are public. The Fermi Science Support Center, at is the access center for these data. Join the fun! 28