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Diffuse Emission and Unidentified Sources

0. Diffuse Emission and Unidentified Sources. Plane of the Milky Way (diffuse emission). 3C279 (quasar). Geminga (pulsar). Crab (SNR). PSR 1951+32 (pulsar). EGRET, E > 100 MeV. PKS 0528+134 (quasar). 3C454.3 (quasar). Vela (pulsar).

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Diffuse Emission and Unidentified Sources

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  1. 0 Diffuse Emission and Unidentified Sources Plane of the Milky Way (diffuse emission) 3C279 (quasar) Geminga (pulsar) Crab (SNR) PSR 1951+32 (pulsar) EGRET, E > 100 MeV PKS 0528+134 (quasar) 3C454.3 (quasar) Vela (pulsar) More than half of all gamma-ray sources are still unidentified!

  2. Diffuse g-Ray Emission Dominant diffuse g-ray production mechanisms: Interactions of cosmic-rays (highly relativistic particles in space) with the interstellar medium (ISM) and/or the interstellar radiation field. • pcr pISM→ p p + p0 ; p0→ 2 g • Bremsstrahlung of cosmic-ray electrons • Compton scattering of cosmic-ray electrons off the interstellar radiation field (infrared/optical light from stars) But also: Sum of the contributions from many faint, individual, unresolved sources

  3. Cosmic Rays Charged high-energy particles (electrons, protons, heavier nuclei), but also photons (g-rays) in space F(E) ~ E-2.7 Galactic origin (supernovae) ~ 1015 eV Energies: MeV - >1020 eV (ultra-high energy cosmic rays = UHECRs) F(E) ~ E-3.1 Extragalactic origin (AGN, GRBs?)

  4. Ultra-High-Energy Cosmic Rays The Greisen-Zatsepin-Kuzmin cutoff The Universe is pervaded with a thermal “afterglow” of the Big Band: The Cosmic Microwave Background (CMB) radiation: Blackbody at T ≈ 2.7 K. UHECR nuclei with energies E > 3x1019 eV interact with the CMB: (pg→ pp0 or pg → np+) • Protons with E > 3x1019 eVfrom cosmological distances lose much of their energy on their way to us • UHECRs at E > 3x1019 eV must come from within ~ 100 Mpc. The resulting cutoff (“GZK cutoff”) in the UHECR spectrum has recently been measured by the Auger collaboration.

  5. Spectrum of Diffuse g-Ray Emission p0 decay Bremsstrahlung Compton scattering Unresolved extragalactic sources

  6. The Structure of the Milky Way 75,000 light years Disk Most gas and dust is concentrated in the nuclear bulge and the spiral arms. Nuclear Bulge Sun Halo Open Clusters (newly born stars) Globular Clusters (old stars)

  7. The Structure of the Milky Way Distribution of dust Distribution of stars and neutral hydrogen Sun Bar Ring

  8. 0 The Problem of Identifying g-ray Sources EGRET error contours Pulsar Black Hole X-Ray Binary What’s the source of the g-ray emission? Need more information (broadband spectrum; variability)

  9. Unidentified g-Ray Sources (UIDs) Out of 270 sources in the EGRET catalog (sources of > 100 MeV g-rays), 170 are unidentified! Also, about two dozen TeV g-ray sources (detected by HESS, MAGIC) are unidentified. Almost all within Galactic latitude |l| < 30o => Almost certainly of Galactic origin

  10. The Nature of UIDs Unidentified sources show a variety of different properties: They are certainly not one homogeneous source class. • Possible identifications: • Background AGN (→ Variability!) • Supernova remnants (→ Non-variable, extended) • Pulsars (→ Pulsed emission; hard spectrum) • Pulsar wind nebulae (→ non-variable, extended) • X-ray/g-ray binaries (→ periodic [orbital] variability) • O/B Associations (young, very massive stars with strong stellar winds) (→ non-variable, extended)

  11. High-Latitude Sources Comparison of spectral and variability properties Complete EGRET catalog High-Lat. UIDs AGN Pulsars Number of Sources Spectral Index Variability Index

  12. High-Latitude Sources Most low-latitude sources are non-variable. Several previously unidentified TeV sources could be identified with pulsar wind nebula; most remain unidentified. Similar flux – number diagrams as AGN Number of Sources Flux (> 100 MeV)

  13. Examples:1) The UID 3EG J1837-0423 • Located only 1o off the Galactic plane • High peak g-ray flux for only 3.5 days • Never detected before or afterwards • Gamma-ray spectrum with photon index -2.1 Strong variability; hard photon spectrum → Background blazar? Should be detectable in radio/infrared for normal blazar properties

  14. 2) HESS J1303-631 • TeV g-ray source 0.5o north of a known g-ray pulsar • Less variable than the pulsar. HESS J1303-631 PSR B1259-63

  15. Possible Counterparts HESS J1303-631 Radio and X-ray sources near HESS J1303-631 None of the nearby pulsars is powerful enough to power the TeV source HESS J1303-631 Related to a powerful stellar wind (WR124)?

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