1. Status and issues for the LAT interstellar emission model
Gamma-ray Large Area Space Telescope
Status and issues for the LAT interstellar emission model
S. W. Digel
Stanford Linear Accelerator Center

2. Outline Mandate – deliverable from the LAT team (see previous talk)
Why – because we (ultimately) need one
What has been happening
Plan for DC2 – i.e., a concept for discussion
Future work

3. Why a model is needed
Limited angular resolution, limited g-ray statistics, and relatively bright, structured interstellar emission
Errors in model translate to false detections or at least bad source positions
At higher |b|, a good model is required for study of the extragalactic (isotropic) component
~60% of EGRET g-rays were diffuse emission from the Milky Way
(~30% isotropic emission, and ~10% from detected point sources)
EGRET
>100 MeV, Phase 1-5

4. Current status
Over the summer we managed to have one VRVS meeting ( and exchanged ; we have a mailing list that you can subscribe to
ISRF: T. Porter (LSU) has done extensive work on this and is willing to contribute to the effort for the model
Describing the ingredients and methods for deriving the ISRF as a function of position, energy, and direction would be a talk in itself
Basically inputs are detailed stellar models, models of dust distribution and dust properties (for absorption/re-radiation and scattering, and scattering again,…)
He is expecting to be able to provide a first version (in r, z) with full wavelength dependence within the next month or two
Interstellar medium: Investigations by J.-M. Casandjian and I. Grenier and S. Hunter (this session)
Cosmic-ray production and propagation: I. Moskalenko, A. Strong, & Reimer – see Igor’s talk later in this session
Gamma-ray production functions: T. Kamae’s calculation of diffractive dissociation contributions for proton-nucleon interactions

5. Current concept for constructing the model
From the science tools side
A FITS image in flat projection is as good as anything else
The gridding scale relates to smallest structure that we’d hope to resolve with the LAT
Development concept for DC2 (for discussion):
Make updates to the inputs to GALPROP including a more detailed ISM distribution, and Porter’s new ISRF
Make some near-term improvements to GALPROP itself
Possibly including use of more detailed gas distributions in the cosmic-ray propagation, certainly incorporating improved pion production cross sections (Blattnig et al.) and Kamae’s diffractive dissociation effects
Use GALPROP to calculate CR distributions, and to calculate g-ray intensities.
It already has the capability of writing FITS maps for a grid of energy ranges, and to write the components separately for the different emission mechanisms

6. Concept (2) For the DC2 science tools
Use one of these models generated through GALPROP
Investigate whether we can improve on GMULT/GBIAS approach for adjustable parameters
Answering the question depends on having data to compare against, of course
Ideally, of course, we wouldn’t need GMULT; GBIAS is probably unavoidable

7. Future (post-DC2)
Analysis: M. Pohl is advocating that we also produce detailed estimates of systematic uncertainties in the model in the analysis
If the systematic uncertainties could be quantified then the likelihood function (likelihood of the data given the model) could be ‘marginalized’ over the uncertainties, at least in principle
Supporting observations: Special regions of the sky or special molecular lines; as we heard yesterday this is in principle part of the Multiwavelength Observation plan for the collaboration
Tangent directions of spiral arms and GC
Sagittarius
Norma
Scutum
Cygnus
Carina
3 kpc arm
Crux
Seven 3EG sources

8. Future (2) Galactic Center CO (J = 1–0) vs. C18O (J = 1–0)
Bitran (1987)
CO (J = 1–0)
C18O (J = 1–0)
vs.
Dahmen et al. (1998)
Galactic Center
3EG % error circle (Hartman et al. 1999)

9. Future (3)
High-latitude clouds - small but detectable as LAT point sources are being found in an unbiased intermediate-latitude survey (Dame et al.)
Dame, Hartmann & Thaddeus (2001)
Dame & Thaddeus (2004)

10. Another note: Science with the interstellar emission model
The deliverable model is related to, but distinct from the scientific study of the diffuse emission
The model needs to be verified against EGRET data and certainly against LAT flight data.
Verify means adjusted as needed
Also will need to iterate with source detection
These adjustments should be telling us something about, e.g., gas or cosmic-ray distributions and this information will be fed back into the model
Free parameters in the likelihood analysis can help in optimizing the model

11. Backup slides follow

12. From EGRET to LAT Interstellar Emission Model
Bertsch et al. (1993), Hunter et al. (1997), & Sreekumar et al. (1998)
Adequate for now – used both for observation simulation and likelihood analysis; for DC1 it was literally perfect
Shortcomings that we’d like to fix include
Inadequate IC (ISRF) model – well-known ‘GeV excess’ and the ‘Dixon halo’ of observed intensity vs. model; more recent data (radio, WMAP) and models are available
NB: Halo is dark blue
Dixon et al. (1998)
>1 GeV
Hunter et al. (1997) EGRET spectrum of the inner Galaxy

13. From EGRET to LAT (2)
Better surveys of the interstellar medium are available
Higher-resolution, better coverage CO and much better calibrated H I
Dame, Hartmann & Thaddeus (2001)
Dame et al. (2004)
Dame & Thaddeus (2004)

14. From EGRET to LAT (3)
Better surveys of the interstellar medium are available (cont)
LAT-like resolution all-sky sub-mm and IR surveys (WMAP, COBE, DIRBE, IRAS)
WMAP 93 GHz