Transcript ppt

GLAST LAT Project
Gamma-ray Large
Area Space
Telescope
Galactic Diffuse
Emission for DC2
S. W. Digel
Stanford Linear Accelerator Center
Workshop on Source Catalog and Diffuse Emission, 23-24 May 2005
1
GLAST LAT Project
Outline
•
Motivation: Diffuse gamma-ray emission from the Milky Way is an
important component of the sky model, of course, and also for
likelihood analysis (model fitting) of the LAT gamma rays
EGRET
~60% of EGRET grays were diffuse
emission from the
Milky Way
(~30% isotropic
emission, and
~10% from
detected point
sources)
>100 MeV, Phase 1-5
•
Rest of talk: The current state of the LAT diffuse emission model and
improvements possible for DC2 and Science Tools Checkout 3
Workshop on Source Catalog and Diffuse Emission, 23-24 May 2005
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GLAST LAT Project
Current state of the LAT diffuse emission model
Aside: ‘diffuse emission model’ means the same thing as ‘interstellar emission model’ and
‘Galactic diffuse emission model’. Galaxy with a capital ‘G’ means the Milky Way.
• For observation simulation and likelihood analysis we have the
MapCube source (by Jim Chiang) for specifying a diffuse
emission model in as much detail as anyone could want
• For Science Tools Checkout 2 (March-April this year), we used
a gamma-ray intensity map calculated by GALPROP using the
cosmic-ray distributions of Strong, Moskalenko, & Reimer
(2004)
– This is the latest comprehensive study with GALPROP and
involved optimizing (in some sense) the model for different
regions of the sky
– The MapCube model for Checkout 2 was 720×360×32 (grid
spacing 0.5° in Galactic coordinates and ~30 MeV – 980 GeV
in steps of factors of 1.4 in E)
Workshop on Source Catalog and Diffuse Emission, 23-24 May 2005
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GLAST LAT Project
Current state (2)
• Example Checkout 2 GALPROP intensity map for 1.2 GeV
From http://confluence.slac.stanford.edu/display/DC2/Interstellar+Emission+Modeling
• Many of the DC2 goals related to the diffuse emission model
are evident here
Workshop on Source Catalog and Diffuse Emission, 23-24 May 2005
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GLAST LAT Project
Current state (3)
•
•
•
Gamma-ray intensity maps from GALPROP have not been used for
before for quantitative analyses like source detection (to my
knowledge)
– Details that were not particularly important for studies of cosmicray propagation or diffuse emission on large angular scales now
become important
Within 10-15° longitude of the Galactic center and anticenter, the
model is essentially interpolated.
– At low latitudes in particular this is because the distribution of gas
along these lines of sight is not well known
– Hard edges for Taurus and Ophiuchus clouds are artifacts of this
interpolation, too
Some of the large-scale, low-level striping is probably due to
calibration artifacts of the old H I surveys used (see later); the rest
may be due to what (IC model? line-of-sight integration effects?)
– Calibration of absolute intensity is more important than small
scale stripes
Workshop on Source Catalog and Diffuse Emission, 23-24 May 2005
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GLAST LAT Project
What’s new (or newly available)
• The best-yet composite H I survey of the sky has been released
(although it is not yet available on Vizier)
Leiden/Argentine/Bonn (Kalberla et al. 2005)
• Other developments for inputs – new CO surveys, evidence for
‘dark’ molecular gas, a new model for the ISRF, and new g-ray
production functions (see http://wwwglast.slac.stanford.edu/sciencetools/workshops/May05DiffuseCat/ )
Workshop on Source Catalog and Diffuse Emission, 23-24 May 2005
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GLAST LAT Project
What’s new (2)
• For current work on GALPROP, please see the presentation by
Igor Moskalenko at the Catalogs and Diffuse Emission
workshop last month at SLAC
– http://wwwglast.slac.stanford.edu/sciencetools/workshops/May05Diffu
seCat/talks/GALPROP_Moskalenko.pdf
Workshop on Source Catalog and Diffuse Emission, 23-24 May 2005
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GLAST LAT Project
Issues to be resolved for DC2
• Can we resolve kinematic distance ambiguities sufficiently well
to make it worth doing?
• How will we incorporate the ‘dark’ molecular gas, how will we
deal with cold atomic hydrogen (seen in self absorption) and
determining N(H I) in general, and how significant are the dark
and cold components relative to the ISM traced by CO and H I
emission.
Grenier, Casandjian, &
Terrier (2004)
• Modeling the interstellar medium toward the Galactic center
and anticenter
Workshop on Source Catalog and Diffuse Emission, 23-24 May 2005
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GLAST LAT Project
Issues to be resolved for DC2 (2)
• What angular resolution is appropriate for the diffuse emission
model? Do we need a full MapCube representation or just a 2dimensional map × one spectrum?
Average spectra
for 10° ranges of
Galactic latitude
-10-0° and 0-10°
Spectral variations in
current GALPROP
model are not very great
Artifact (reached
end of IC model, I
think)
From http://confluence.slac.stanford.edu/display/DC2/Interstellar+Emission+Modeling
Workshop on Source Catalog and Diffuse Emission, 23-24 May 2005
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GLAST LAT Project
Issues (3)
• Yes, as Julie has pointed out, we don’t necessarily want the
model provided for DC2 analyses to be perfect
– Analysis of the diffuse emission per se is not a formal goal
of DC2, as far as I know, but we know that our model won’t
be perfect, even after tuning
Workshop on Source Catalog and Diffuse Emission, 23-24 May 2005
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GLAST LAT Project
Who does what?
• The interstellar emission model is a deliverable from the LAT
team and the Diffuse Emissions science working group has
been chartered to develop it (surprise)
• If analysis of the Galactic diffuse emission itself is not also a
goal of DC2, then the Diffuse Emissions group can be asked to
provide both the sky model version (secret) and the distributed
model (used for analyses)
Workshop on Source Catalog and Diffuse Emission, 23-24 May 2005
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