Transcript ppt - Caltech Astronomy

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
CGRO
Vasiliki Pavlidou
Credit: NASA
Credit: NASA
26March2007
Particle Astrophysics Seminar, Fermilab
EGRET observed the sky for almost 10 years,
and made many important discoveries.
Galactic
poles
Galactic
center
Galactic
plane
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
This
It
comes
emission
from the
is produced
Galactic through
plane because
interactions
this isof
where
cosmic
allrays
the gas
with
lives.
interstellar
gas.
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.
Faint
Bright
blazars
sources
unresolved
detected by EGRET as will
wil be
beseen
seenby
byGLAST
GLAST
Simulation:
Simulation:S.S.Digel
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
MeV)
3.4° (100
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:
GeV Excess
EGRET data• Locally measured CR
spectrum
Model • Observed distribution of
interstellar matter + light
• Theoretical/experimental
cross
sections for relevant
interactions
Credit: Strong, Moskalenko & Reimer 2004
Vasiliki Pavlidou
26March2007
Add them all up, calculate
energy spectrum of gamma-ray
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,
3.
Obsolete proton-proton cross section (Kamae, Abe, Koi -
4.
Annihilating dark matter [de Boer -
Schlickeiser, Strong, Mori but GeV excess persists in all directions and sources live mostly in the plane)
but also need to tweak proton spectrum to fully account for the problem)
but resulting MW dark matter halo shape counter-intuitive and would make too many antiprotons (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
Strong et
et al
al.2004
2004
Credit:
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
at least at low energies?
Power-law fit
to range of fully resolved fluxes
– is the extrapolation
extreme and unlikely or
moderate and plausible?
Cutoff so that EGRB
is not exceeded
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
can never
hope
to adequately
understand
• If mostWe
unidentified
sources
Galactic,
most probable
total luminosity ~
the origin
of the diffuse
emission
50% of upper limit
of M31 gamma-ray
luminosity.
without at least some understanding
• For high-latitude, likely extragalactic, unidentified sources:
of the nature
offlux
unidentified
a moderate extrapolation
of the
distribution sources
for ~ 1 order of
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 energyreleasing 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
Vasiliki Pavlidou
26March2007
Credit: de Boer et al 2005
Particle Astrophysics Seminar, Fermilab