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Diffuse Extragalactic Gamma-ray Background

Diffuse Extragalactic Gamma-ray Background. Julie McEnery NASA/GSFC. Extragalactic diffuse gamma-ray background. Extragalactic diffuse emission encodes unique information about high energy processes in the universe.

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Diffuse Extragalactic Gamma-ray Background

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  1. Diffuse Extragalactic Gamma-ray Background Julie McEnery NASA/GSFC

  2. Extragalactic diffuse gamma-ray background • Extragalactic diffuse emission encodes unique information about high energy processes in the universe. • Gamma-ray range is of particular interest because the universe is transparent to gamma-rays <~20 GeV back to high redshift. Measurement of the gamma-ray diffuse emission at GeV energies can also provide information on the TeV extragalactic emission. • Detailed study of spectrum and spatial fluctuations may provide constraints to different cosmological models. • Two general origins of diffuse emission • Unresolved point sources • Truly diffuse/extended component • The measured background may be a mixture of both, and will probably have different origins in different gamma-ray bands. • Field is brimming with promise • This is another way of saying that there are lots of things that we don’t know, but that we know we would like to learn.

  3. X-ray to -ray multiwavelength spectrum (circa 1997) Sreekumar 1997 Seyfert 1 SN1a Blazars Seyfert 2 Scope of this talk

  4. Medium Energy band • Challenging regime experimentally • Cosmic-rays interacting with the gamma-ray detectors and surrounding material produce a high internal gamma-ray flux due to induced radioactivity. Kappadath et al, 1996 Spectrum consistent with extrapolations of the power-laws from lower (HEAO-1) and higher (EGRET) observations) Earlier observations of “MeV” bump attributed to instrumental backgrounds.

  5. Origin of the MeV diffuse emission • SN1a suggested as dominant contribution at 1 MeV • Lines from decay of 56Ni56Co56Fe, broadened by z dist. • Potentially provides an independent and unique measure of the universal star formation history (Watanabe et al 1999) • Need to know • Cosmological star formation history • Delay between star formation and SN1a Watanabe et al 1999 find that SN1a can provide the bulk of diffuse emission in MeV range. They find constraints for the delay between SF and SN1a in order not to overproduce the gamma-ray background

  6. MeV Background • Reanalysis by Strigari et al 2006, find a much lower contribution to the EGRB from SN1a. If the diffuse emission is not dominated by SN1a, then what is the source? Simple extrapolations of blazar and seyfert spectra can explain the MeV diffuse, but deeper observations of the point sources in the MeV sky would be needed to confirm this.

  7. High Energy Gamma-ray Emission • 20 MeV - 20 GeV • Instrumental background not so important. • Possibility of some contributions from interactions (producing neutral pions) in the material surrounding the detector) • Determination of the extragalactic component is dominated by foreground emission which must be modeled and subtracted • The Galaxy is bright! • Significant number of extragalactic sources have been detected above 20 MeV. • Much better understanding of the potential unresolved point source contribution.

  8. Foregrounds - Galactic diffuse • The EGRET sky is dominated by emission from the Galaxy Gamma-rays produced by interactions of cosmic-rays with interstellar gas and radiation fields.

  9. Determining the extragalactic diffuse (Sreekumar 1998) • Based on the Galactic diffuse model of Hunter et al 1997. Plot model flux vs measured flux for many regions on the sky (but avoiding Galactic plane and bulge). Zero intercept gives isotropic flux level. Flux >100 MeV = 1.45 x105 cm-2s-1sr-1 Power-law spectrum with -2.1 index

  10. Reanalysis by Strong et al 2004 • Using same procedure as Sreekumar 1998, but different Galactic emission model. • The updated model of interstellar emission by Strong et al. (2004) predicts a significantly lower isotropic intensity than the original analysis by the EGRET team (Sreekumar et al. 1998) • Most of the difference is attributable to inverse Compton emission (re-evaluation of ISRF and CR electron distribution) Sreekumar et al. (1998) Strong, Moskalenko, & Reimer (2004) Flux >100 MeV = 1.45 x105 cm-2s-1sr-1 Flux >100 MeV = 1.11 x105 cm-2s-1sr-1

  11. Foregrounds - Our local neighbourhood • More recently, Moskalenko et al (2006), realized that IC emission from Galactic CR electrons upscattering solar photons may be a significant source of foreground emission

  12. IC scattering on Solar photons Intensity of this emission depends on the line of sight to the Sun (brightest at the Sun, dimmest 180 degree away). Estimate that this may make up 5-10% of the isotropic diffuse.

  13. Sources of extragalactic gamma-rays between 20 MeV - 20 GeV • Guaranteed contributors to the extragalactic diffuse • AGN • Directly from MeV-GeV emitting AGN (Bignami 1979) • Cascade radiation from TeV AGN (Stecker and Salamon 1996, Coppi and Aharonian 1997) • Cosmic-ray interactions • Galaxies (Pavlidou and Fields 2002, Thompson et al 2006, Stecker 2006)

  14. = EGRET blazars seen sometimes = EGRET blazars seen always = EGRET unidentified high-latitude variables EGRET Blazars • Most populous known gamma-ray source class, clearly will form a substantial fraction of the extragalactic gammas. The question is how much? • Highly variable • Most blazars only seen during flares • Poorly determined luminosity function • Biases in source identification/cross correlation with surveys at other wavelengths. • Most of the EGRET AGN were identified by finding radio counterparts and would remain unidentified if the counterpart under the flux limit of radio surveys.

  15. Approaches • Gamma-ray luminosity function • Chiang et al. 1995, Chiang and Mukherjee 1998 • Scaled luminosity function (assume that the gamma-ray flux is correlated with the flux in some other waveband) • Padovani et al 1993, Stecker et al 1993, 1996, Stecker and Salamon 1994, Narumoto and Totani 2006; Scaling SED - Giommi et al 2006. • AGN unification scheme and emission model • Muecke and Pohl 2000

  16. Blazar Contributions • Gamma-ray luminosity functions • <25% blazar contribution • Scaling luminosity functions • Stecker et al 1996 - scaled radio luminosity function • 100% blazar contribution to EGRB • Narumoto and Totani 2006 • PLE - 50-55% blazar contribution • LDDE - 20-50% blazar contribution • AGN unification and emission model • 20-85% radio loud AGN contribution to EGRB

  17. Cascade radiation from TeV sources • However, the attenuated HE photons do not disappear! • They cascade down to GeV energies and become part of the EGRB - expect the EGRB spectrum to turn up at 10-20 GeV. • Measurements of the GeV diffuse emission provides information on the cosmological distribution of TeV gamma-ray sources • The spectral shape of the EGRB between 10 GeV and 300 GeV will be determined by the redshift distribution of VHE sources and the flux and spectrum of the optical-IR background radiation. Extragalactic background light (EBL): Photons with E>~10 GeV are attenuated by the diffuse UV-optical-IR EBL. Measurement of attenuation of high energy spectra as a function of redshift will provide information on the UV-optical-IR EBL. (see talk by Paolo Coppi, poster by Luis Reyes)

  18. Galaxies • Gamma-ray emission due to comic-ray interactions with diffuse gas. • Key assumption is that cosmic-ray flux is proportional to the supernova rate and thus the massive star formation rate (quantified observationally by the cosmic star formation rate) • Also assume that high mass end of IMF is universal, cosmic-ray spectral shape is universal and cosmic-ray escape properties are the same as the Milkyway. Pavlidou and Fields 2002 predict that ~1/3 of the EGRB at 1 GeV could be produced by galaxies Predict a distinct spectral shape.

  19. Other proposed sources • Structure formation in the intergalactic medium • Electrons accelerated at shock waves induced by gravity during formation of large scale structure scatter CMB photons to gamma-ray energies (e.g. Loeb and Waxman 2000, Gabici and Blasi 2002): 10%-75% of the EGRB • Hadronic interactions in Galaxy clusters (e.g. Ensslin et al, 1997) • Phase of baryon-antibaryon annihilation (Stecker et al 1971, Gao et al 1990, Dolgov and Silk 1993) • Evaporation of primordial black holes (Hawking 1974, Maki, Mitsui and Orito 1996) • Annihilation of dark matter particles (e.g. Ando and Komatsu 2006, Ullio 2002, Elsaesser and Mannheim 2005, Oda et al 2006, Jungman et al 1996) • Kpc scale jets in FR1 radio galaxies (Stawarz et al, 2006) • Magnetic fields must be less than 10 G on average as otherwise the extragalactic gamma-ray background would be overproduced.

  20. Where are we now? • SAS-2 demonstrated that an isotropic background exists • EGRET • More accurate/detailed spectrum • Extended spectral measurements to ~100 GeV • demonstrated that point sources (blazars) likely to form a substantial fraction of the background. • Still don’t know how much of the EGRB could be truly diffuse. • Large uncertainties in all the proposed sources of the extragalactic background.

  21. Large Area Telescope (LAT) GLAST Burst Monitor (GBM) GLAST - The next step forward Two Instruments: Large Area Telescope (LAT) PI: P. Michelson (Stanford University) 20 MeV - 300 GeV 2.5 sr FoV GLAST Burst Monitor (GBM) PI: C. Meegan (NASA/MSFC) 8 keV – 30 MeV 9 sr FoV Launch: late 2007 Lifetime: 5 years (req) 10 years (goal)

  22. Why should you care about GLAST? • Huge improvement over previous missions in this waveband • Greatly improved sensitivity, angular resolution and increased energy range. Surveys entire sky every 3 hours. • EGRET made many ground breaking discoveries, but left many tantalising questions for GLAST to address. • Highest energy photons from GRB/Energetics • AGN populations • New source classes likely to emerge: Galaxy clusters, ULIRGs etc

  23. GLAST and the EGRB • Dig deeper into the blazar luminosity function • Be able to do this for FSRQs and BL Lacs separately • The LAT will observe all regions of the sky for 30 mins every 3 hours. All blazars will be continuously monitored - understand duty cycle • Directly detect other source classes to determine contributions - eg galaxy clusters • Resolve out more of the point sources -- limit on the diffuse component will drop. • The accuracy with which we measure the spatial and spectral properties of the diffuse will increase. • Interesting things may become detectable • Need to take great care in subtracting foregrounds and instrumental background. • GLAST will provide measurements of the EGRB up to several hundred GeV.

  24. GLAST Blazars - how many? • Blazar luminosity function and its evolution can be constrained from the number count of GLAST blazars. The number of blazars detectable by GLAST is 3000 - LDDE 5250 - PLE 10000 - SS96 Narumoto and Totani 2006

  25. GLAST Blazars -how much of the background will be resolved? Thick lines - best fit models; thin lines - models that can explain 100% of the EGRB. The resolvable fraction of the EGRB by GLAST blazars is: 20-26% - LDDE 33-42% - PLE Narumoto and Totani 2006

  26. Galaxies and starburst galaxies • To really understand the contribution of ordinary galaxies to the extragalactic diffuse gamma-ray emission we need to detect individual galaxies. Thompson et al 2006 GLAST is likely to detect several local galaxies and some starburst galaxies: SMC, LMC, M31, NGC 253, M82 - Better understanding of the universality of the galactic diffuse cosmic-ray emission spectrum. - As more of the diffuse emission is resolved (by detection of blazars), the EGRB spectrum should exhibit a deviation from a power-law at ~1 GeV.

  27. GLAST at high energies • GLAST has excellent background rejection, large effective area and wide field of view and will provide well measured diffuse spectrum up to several hundred GeV. • Point source contributions above ~100 GeV will be determined by ground based gamma-ray telescopes (CANGAROO, H.E.S.S., MAGIC and VERITAS). Ullio et al 2002 Measure the shape of the EBL from 10 GeV to 300 GeV - information about the cosmological distribution of extragalactic high energy gamma-ray sources and UV-optical-IR radiation fields. Increasing the energy range and resolving point sources (decreasing limits on truly diffuse component) opens up window for new discoveries.

  28. Summary • The current measurements of the extragalactic gamma-ray background set interesting limits on a wide variety of high energy emission processes in the Universe. • The contribution to the extragalactic background by unresolved known classes of point sources is poorly understood. • Observations by GLAST and ground-based TeV telescopes within the next 2 years will: • improve knowledge of the contribution from known classes of gamma-ray emission. • Improved sensitivity and energy reach will greatly improve the “discovery” potential for finding unique spatial or spectral signatures of something new.

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