Status and Issues for the LAT Interstellar Emission Model

1. 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 (http://www-glast.slac.stanford.edu/ScienceTools/LATDiffuse/meetings) and exchanged e-mail; 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. Cygnus Scutum Norma Carina Crux Sagittarius 3 kpc arm 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 Seven 3EG sources

8. Galactic Center CO (J = 1–0) vs. C18O (J = 1–0) Dahmen et al. (1998) Bitran (1987) Future (2) 3EG 1746-2851 95% 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