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					    Deciphering the Gamma-Ray Sky:
  GeV Astronomy in the Era of GLAST

Vasiliki Pavlidou


                Collaborators:
                Carolyn Brown
                Tonia Venters
                Jennifer Siegal-Gaskins
                Angela Olinto               Brian Fields
                @ The University of Chicago @ University of Illinois
GeV Astronomy: a Play in Two Acts

• Act I: G-rated
   (where it all comes together)

  EGRET, diffuse emission form the Milky Way, blazars,
  extragalactic diffuse emission, GLAST

• Act II: R-rated
   (where it all falls apart)

  GeV excess, unidentified sources,
  the whats whos and how muches of the
  extragalactic diffuse emisison
 Vasiliki Pavlidou   26March2007   Particle Astrophysics Seminar, Fermilab
                                        Act I:

                    The G-rated version of the story




Vasiliki Pavlidou         26March2007            Particle Astrophysics Seminar, Fermilab
There was once a great satellite known as the
Compton Gamma-Ray Observatory, and aboard it lived
EGRET, the GeV gamma-ray telescope.




                                                    EGRET             Credit: NASA
                     CGRO     Credit: NASA
 Vasiliki Pavlidou    26March2007            Particle Astrophysics Seminar, Fermilab
    EGRET observed the sky for almost 10 years,
    and made many important discoveries.




Galactic                                                                          Galactic
poles                                                                             plane



Galactic
center
                The GeV Sky: all EGRET events with Energies > 100 MeV
                Galactic Coordinates
                                                             Credit: S. Digel

      Vasiliki Pavlidou      26March2007       Particle Astrophysics Seminar, Fermilab
EGRET observed diffuse emission from the
Galactic plane.




                                                                           Diffuse
                                                                           emission
                                                                           from the
                                                                           Galactic
                                                                           plane

 It comes from the Galactic through interactions of cosmic rays gas
 This emission is produced plane because this is where all the with
 interstellar gas.
 lives.


 Vasiliki Pavlidou   26March2007        Particle Astrophysics Seminar, Fermilab
  EGRET also observed many point sources.




Some of
the ~ 270
point
sources
resolved
by EGRET

  Of the sources which were identified with some known astrophysical
  high-energy source, the vast majority were found to be blazars.


     Vasiliki Pavlidou   26March2007       Particle Astrophysics Seminar, Fermilab
Blazars are Active Galactic Nuclei with their
jets aligned with our line of sight.




                 Credit: J. Buckley 1998 (Science),
                 illustration: K. Sutliff
 Vasiliki Pavlidou            26March2007             Particle Astrophysics Seminar, Fermilab
Besides the blazars resolved by EGRET, there
are also other blazars too faint to be detected.




         Bright sources detected             wil be seen by GLAST
        Faint blazars unresolved by EGRET as willbe seen by GLAST
         Simulation: S. Digel
        Simulation: S. Digel
These unresolved sources would look to EGRET just like an isotropic,
diffuse emission
  Vasiliki Pavlidou         26March2007   Particle Astrophysics Seminar, Fermilab
EGRET indeed detected isotropic, diffuse
emission




 It looks airtight:
 (a) CR-produced gamma-rays where we expect them
 (b) resolved point sources
 (c) isotropic background from unresolved point sources
 Vasiliki Pavlidou   26March2007       Particle Astrophysics Seminar, Fermilab
 The future: GLAST (Gamma-ray Large Area
 Space Telescope: a bigger, better EGRET) is
 scheduled for launch in fall 2007.
                                              EGRET                      GLAST
                        Peak effective area   1500 cm2                   8000 cm2
                        Field of view         0.5 sr                     2.5 sr
                        Angular resolution    5.8° (100                  3.4° (100
                                              MeV)                       MeV)
                        Source location       15 arcmin                  <0.4arcmin
                        Sensitivity           10-7 cm-2 s-1              310-9 cm-2 s-1
Artist’s impression of GLAST Credit: NASA
   It will again open up the GeV window for observations and will be a
   unique opportunity to follow up on the on the discoveries of EGRET


   Vasiliki Pavlidou    26March2007            Particle Astrophysics Seminar, Fermilab
              And they all lived happily ever after
                                The End

                                    (?)




Vasiliki Pavlidou     26March2007         Particle Astrophysics Seminar, Fermilab
                                        Act II:

                    The R-rated version of the story




Vasiliki Pavlidou         26March2007             Particle Astrophysics Seminar, Fermilab
Diffuse emission from the MW: a closer look

                                              Ingredients for building a MW
                                              diffuse emission model:

                               EGRET data Locally measured CR
                                        •
        GeV Excess                      spectrum
                                  Model • Observed distribution of
                                            interstellar matter + light
                                        • Theoretical/experimental
                                        cross
                                            sections for relevant
                                        interactions

                                              Add them all up, calculate
                                              energy spectrum of gamma-ray
   Credit: Strong, Moskalenko & Reimer 2004
 Vasiliki Pavlidou       26March2007          intensity
                                                 Particle Astrophysics Seminar, Fermilab
What causes the GeV excess?

 1. The CR spectrum elsewhere in the Galaxy is not
    the same as the CR spectrum in the Solar System
     (Strong, Moskalenko, Reimer, Mori -
     but, why? and needs to tweak electron, proton spectra
     separately to fully fix problem)

2.   Point Sources: pulsars, gas clouds, SNRs, bright stars (Büsching, Pohl,
     Schlickeiser, Strong, Mori -
     but GeV excess persists in all directions and sources live mostly in the plane)

3.   Obsolete proton-proton cross section (Kamae, Abe, Koi -
     but also need to tweak proton spectrum to fully account for the problem)

4.   Annihilating dark matter [de Boer -
     but resulting MW dark matter halo shape counter-intuitive and would make too many anti-
     protons (Bergstrom et al 2006) and what about the TeV excess (Prodanovic et al 2006)? ]

     Vasiliki Pavlidou           26March2007                 Particle Astrophysics Seminar, Fermilab
Conclusion: we don’t really understand how to
model the GeV emission from the Galaxy
                     But, if we don’t understand how to model the
                     Galaxy, how can we subtract it from the sky map
                     to obtain the extragalactic diffuse background?




  With very large uncertainties.

  Dar & De Rujula (2001) and
  Keshet, Waxman & Loeb
  (2004) would prefer NO
  extragalactic background
                                                Credit: Strong et al. 2004
                                                Credit: Strong et al 2004
 Vasiliki Pavlidou          26March2007      Particle Astrophysics Seminar, Fermilab
 Conclusion: we don’t really know how many
 EGRET photons come from the extragalactic
 diffuse background.
•However: certain source classes are guaranteed to make some
contribution to the extragalactic diffuse background - e.g. blazars.

•Contribution of these sources must place a lower limit to
extragalactic diffuse background intensity!

Complications:
•Blazar contribution to the extragalactic background extensively
studied, [e.g. Padovani et al. 1993; Stecker et al. 1993; Salamon & Stecker 1994; Chiang et al.
1995; Stecker & Salamon 1996; Chiang & Mukherjee 1998; Mukherjee & Chiang 1999; Mücke &
Pohl 2000; Dermer 2006; Giommi et al. 2006; Narumoto & Totani 2006]
but no consensus on result
•Estimates of unresolved blazar flux vary by ~ 2 orders of magnitude

    Vasiliki Pavlidou         26March2007              Particle Astrophysics Seminar, Fermilab
Conclusion: we don’t really know how much
blazars contribute to the extragalactic diffuse
gamma-ray background.

Therefore the extragalactic diffuse emission is
very poorly understood.

Do we at least understand the nature of the
resolved point sources?

• Of the 270 EGRET point sources:
  - 93 are blazars or “maybe blazars”
  - 170 were originally unidentified
  - most of those are still unidentified - see http://GeVSky.org
  Vasiliki Pavlidou   26March2007     Particle Astrophysics Seminar, Fermilab
Conclusion: we don’t really understand the
EGRET observations at all.
Summary of Puzzles:
• EGRET detected diffuse emission from the Galactic plane,
  but we don’t really understand how it is produced.
• EGRET resolved point sources, and we don’t know what most
  of those are.
• EGRET detected some isotropic diffuse emission, but we
  don’t know how much exactly, and we don’t know what its
  spectrum should look like.
• We don’t know what the origin of the isotropic diffuse
  emission is.



  Vasiliki Pavlidou   26March2007   Particle Astrophysics Seminar, Fermilab
How can progress be made?

 • Ask the right questions
     identify potentially important effects

 • Global approach
     problems interconnected

 • Model-independent techniques
     too detailed models won’t necessarily improve understanding

 • Explicitly account for uncertainties
     measurement uncertainties high and unequal


 Vasiliki Pavlidou   26March2007       Particle Astrophysics Seminar, Fermilab
Unidentified Sources: how can they hurt you?

• If members of an already known class (e.g. blazars):
    – affect normalization of bright-end of class luminosity
      function
    – affect contribution of class to the diffuse background

• If members of an unknown Galactic class:
    – unresolved members contribute to Galactic diffuse emission
       GeV excess?
    – counterparts in external galaxies enhance emission of hosts

• If members of an unknown extragalactic class:
    – unresolved members contribute to extragalactic background
      - may even be dominant component!
 Vasiliki Pavlidou   26March2007      Particle Astrophysics Seminar, Fermilab
Are most EGRET unIDed sources Galactic or
extragalactic?
• Can isotropy help?




• Is there another test?



  Vasiliki Pavlidou   26March2007   Particle Astrophysics Seminar, Fermilab
Are most EGRET unIDed sources
Galactic? A new approach
• If EGRET unIDed sources represent a Galactic
  population, galaxies similar to the MW must have
  similar such populations

• In this scenario:
   if we place all unIDs at distance of M31,
   their summed flux should not exceed the
   M31 gamma-ray flux



 Vasiliki Pavlidou   26March2007   Particle Astrophysics Seminar, Fermilab
Are most EGRET unIDed sources
Galactic? using M31 to test
• For this exercise to work:
    – need unIDed source luminosities.
    – but EGRET only measured fluxes & angular positions
    – so we need way to assign distances

• Simple first tests:
    – place all sources at same distance from Earth
    – place all sources at same distance from Galactic Center
    – assume all sources have similar luminosities

• More sophisticated technique:
    – perform Monte Carlo simulations according to mass
      distribution in Galaxy
 Vasiliki Pavlidou   26March2007     Particle Astrophysics Seminar, Fermilab
 Monte-Carlo Method of Assigning Distances

Assume unidentified source population
follows MW mass distribution.
For each source:
   –   probability of source found within
        distance r  integrated mass out to r

One realization of the model:
   –    assign a distance to each unidentified source
   –    calculate total luminosity for each realization

Hypothesis testing:
   –   generate many realizations of the model
   –   Calculate how probable it is to get a total luminosity less than
       the M31 upper limit

     Vasiliki Pavlidou   26March2007         Particle Astrophysics Seminar, Fermilab
Case I: Could most EGRET unIDed
sources be a Galactic halo population?




   Vasiliki Pavlidou   26March2007   Particle Astrophysics Seminar, Fermilab
Case II: Could most EGRET unIDed
sources be a Galactic disk+bulge population?




   Vasiliki Pavlidou   26March2007   Particle Astrophysics Seminar, Fermilab
Conclusion: very likely that EGRET unIDs
contain a very significant extragalactic
component

• What would then be the contribution of their unresolved
  counterparts to the extragalactic diffuse emission?

• Caution! Question involves daunting uncertainties even for
  blazars with 67 well-studied members and deep AGN surveys
  – how can we ever hope to have meaningful result for
  unidentified sources? (no redshifts / no luminosities / no idea
  about their cosmic evolution)

• Instead: try to assess whether it is likely for the unID
  component to be important/dominant

  Vasiliki Pavlidou   26March2007      Particle Astrophysics Seminar, Fermilab
Assessing the unIDed contribution to the
extragalactic background:
I know one thing, that I know nothing

•   An empirical model for collective emission from
    unresolved unIDs
•   Seek to answer 2 questions:
    1. Numbers/Fluxes:
       How plausible is that unresolved unIDs, if extragalactic,
       have significant contribution to gamma-ray background?
    2. Spectral indices:
       Would collective unresolved emission from unIDs be
       spectrally consistent with the gamma-ray background?


    Vasiliki Pavlidou   26March2007     Particle Astrophysics Seminar, Fermilab
     Which resolved sources?

1.   Exclude likely Galactic sources based on location on the sky




2.   Try analysis with 3 different samples (from “generous” to “conservative”)
     to test sensitivity of results
     a) all sources with no 3EG ID + all “maybe blazars”
     b) all sources with no 3EG ID
     c) all sources with neither 3EG ID nor later proposed possible ID
       Vasiliki Pavlidou   26March2007        Particle Astrophysics Seminar, Fermilab
UnIDs and the extragalactic diffuse background:
are there enough?
Use cumulative flux distribution of resolved objects
and extrapolate to lower fluxes
– how far into the low-flux regime
  should it be extrapolated
  so that contribution is dominant     Cutoff so that EGRB
  at least at low energies?              is not exceeded

            Power-law fit
   to range of fully resolved fluxes
– is the extrapolation
  extreme and unlikely or
  moderate and plausible?
  UnIDs and the extragalactic diffuse background:
  does the spectrum work?
• Energy spectrum of
  cumulative emission
  depends on spectra of
  individual sources

• Critical input: spectral
  index distribution.
   – Assume spectral index
     distribution of unresolved
     objects same as that of
     resolved objects
   – BUT - need to account for
     individual measurement
     errors! likelihood
     approach
Spectrum of Extragalactic background from UnIDs

                             Sreekumar et al (1998) EGRB fit




                                         SMR (2004) EGRB

                                                 unresolved unID emission
         SMR (2004) systematics




  Strong, Moskalenko & Reimer (2004)
EGRET Unidentified Sources - Conclusions

• M31 constraint excludes with high significance the possibility that
  unIDs are a Galactic halo population
• Galactic disk+bulge population allowed by M31 constraint, although
  sky distribution uncomfortable

• If mostWe can never hope to adequately understand
         unidentified sources Galactic, most probable total luminosity ~
                the of M31 of the diffuse emission
  50% of upper limit origingamma-ray luminosity.
                      without at least some understanding
• For high-latitude, likely extragalactic, unidentified sources:
               of the nature of unidentified for ~ 1 order of
  a moderate extrapolation of the flux distribution sources
  magnitude implies unID contribution to extragalactic background
  dominant at low energies
• Spectrum of unresolved unID emission in excellent agreement with
  observed extragalactic diffuse background at energies where possibly
  dominant, within systematics elsewhere.
  Vasiliki Pavlidou        26March2007    Particle Astrophysics Seminar, Fermilab
The next steps - GLAST!

• Whatever the nature of the EGRET unIDs, GLAST
  will resolve many more
    – if unresolved unIDs currently responsible for
      considerable fraction of extragalactic background:
        associated reduction of GLAST isotropic background
    – If unresolved unIDs currently responsible for
      considerable fraction of Galactic diffuse emission:
        associated reduction of GLAST diffuse MW
        (GeV excess affected?)

• GLAST will detect M31, all M31-based constraints
  stronger.

 Vasiliki Pavlidou     26March2007   Particle Astrophysics Seminar, Fermilab
The next steps - theory

• Unresolved unIDs a potentially significant contribution to
  diffuse emission worth pursuing more detailed models.

• Nature unknown, but there are only so many energy-
  releasing mechanisms sufficient to generate high-energy
  emitters
     – What if they follow AGN evolution?
     – What if they follow evolution of cosmic structure?
     – What if they follow cosmic star formation?

• Depending on nature, GLAST will detect different numbers
   population identification with no need of identification of
  individual members

  Vasiliki Pavlidou    26March2007         Particle Astrophysics Seminar, Fermilab
                         To be continued…
                    in a preprint server near you




Vasiliki Pavlidou   26March2007         Particle Astrophysics Seminar, Fermilab
Vasiliki Pavlidou   26March2007   Particle Astrophysics Seminar, Fermilab
GeV Excess Explained



                                     Credit: Strong et al. 2004




 Credit: Kamae et a. 2005
                                                                    Credit: de Boer et al 2005
   Vasiliki Pavlidou        26March2007              Particle Astrophysics Seminar, Fermilab

				
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posted:12/19/2011
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