GLAST LAT Project SLAC, 15 July The Interstellar Emission of the Milky Way as a Source and a Background for the LAT S. W. Digel Hansen Experimental Physics Laboratory, Stanford Univ. Gamma-ray Large Area Space Telescope GLAST LAT Project SLAC, 15 July Outline Origin & characteristics of the diffuse emission Milky Way as a galaxy Production mechanisms for -rays Large Area Telescope (LAT) on GLAST Diffuse emission as a source (i.e., a topic for study) Modeling the diffuse emission Current problems As a (celestial) background Point sources Extragalactic isotropic emission Current issues in modeling Galactic center Metallicity gradient EGRET (>100 MeV) GLAST LAT Project SLAC, 15 July Milky Way as a Galaxy A large Sbc spiral galaxy with ongoing star formation (Pop II) Makes (and keeps) its own cosmic rays and has a continuing supply of massive stars Massive (~10100 M ) stars (OB-WR) have short lives, tremendous luminosities and winds, and explode at end of life, leaving remnants that can accelerate cosmic rays Idealized diagram of Milky Way - dont take this too literally From in-plane perspective tracing arms is hard Bar is COBE-era realization NGC 1187 (Adam Block/NOAO/AURA/NSF, Sbc M109 (AURA/NOAO/NSF), SBc Powell 8.5 kpc Sun GLAST LAT Project SLAC, 15 July Production mechanisms for high-energy cosmic -rays Bremsstrahlung scattering of CR electrons by nucleons Inverse Compton scattering of low- energy photons by CR electrons 0 decay secondaries from CR proton- nucleon collisions e e Z e e p p 00 EE Bremsstrahlung ee 100 MeV, Phases 1-5 GLAST LAT Project SLAC, 15 July Future Missions AGILE (Astro-rivelatore Gamma a Immagini LEggero) ASI small mission, late 2005 launch, good PSF, large FOV, short deadtime, very limited energy resolution AMS (Alpha Magnetic Spectrometer) International, cosmic-ray experiment for ISS, will have sensitivity to >1 GeV gamma rays, scheduled for 16 th shuttle launch once launches resume GLAST GLAST LAT Project SLAC, 15 July Spectrum Astro GLAST Large Area Telescope (LAT) e+e+ ee Calorimeter Tracker ACD 1.8 m 3000 kg Within its first few weeks, the LAT will double the number of celestial gamma rays ever detected 5-year design life, goal of 10 yearsYears Ang. Res. (100 MeV) Ang. Res. (10 GeV) Eng. Rng. (GeV) A eff (cm 2 sr) # rays EGRET 1991 0.5 0.03 10 6 AGILE 2005 4.70.2 0.03 50 1,500 4 10 6 /yr AMS 2005+? 0.1 0.3 300 1,600 7 10 5 /yr GLAST LAT 2007 3.5 0.1 0.02 ,000 1 10 8 /yr GLAST LAT Project SLAC, 15 July EGRETLAT Point Source Sensitivity (5, >100 MeV) ~5 cm -2 s -1 3 cm -2 s -1 (at high gal. latitude for 1-year sky survey, for photon index of -2) Source Location Determination 15 0.4 (1 radius, flux cm -2 s -1 >100 MeV, 1-year sky survey, high b) Splitting 1 cm -2 s -1 sources 75 6666 Resolving 5 cm -2 s -1 extended sources 90 min (7.5 max) 5555 Derived LAT Capabilities More fine print: E -2 sources, EGRET: 2-week pointed obs. on axis, LAT: 1-year sky survey, flat high- latitude diffuse background For flaring or impulsive sources the relative effective areas (~6x greater for LAT), FOV (>4x greater for LAT), and deadtimes (>3 orders of magnitude shorter for LAT) are relevant as well GLAST LAT Project SLAC, 15 July Interstellar emission: What is it good for? Models permit determination of cosmic-ray intensity/interstellar gas mass across the Milky Way How and why Owing to limited angular resolution & limited photon statistics of gamma-ray telescopes, and bright and structured diffuse emission of the Milky Way a good model of interstellar emission is needed for: Historically, this has meant that source detection is via model fitting, necessarily with a limited number of degrees of freedom. So a good model of the diffuse emission is needed Discrimination between point sources and interstellar emission at low-latitude Get coordinates correct Accurate measurement of extragalactic component GLAST LAT Project SLAC, 15 July Components of an interstellar emission model Although radiative transfer is simple (MW is transparent at these energies), good models are not easy. Main components Interstellar gas (molecular, atomic, and ionized) Molecular gas (H 2 ) in ISM is indirectly measured via CO J = 10 line; even N(H I) can be tricky Distance ambiguity, velocity dispersion, & non-circular velocity field of Milky Way Interstellar radiation field Dependence on position, wavelength, and direction Cosmic rays Spatial and energy distribution Cosmic-ray spectra are measured only within solar system, with low-energy range affected by solar modulation Difficult or impossible to get unique answers for any of the above GLAST LAT Project SLAC, 15 July Aside: Distribution of interstellar gas Neutral interstellar medium 21-cm H I & 2.6-mm CO (standin for H 2 ) Near-far distance ambiguity, rotation curve, optical depth effects, Dame et al. (1987) Hartmann & Burton (1997) (25, 0) W. Keel CO H I G.C. 25 GLAST LAT Project SLAC, 15 July Ionized interstellar medium Most of the ionized gas is extremely diffuse, with essentially no free-free radio continuum emission Detected indirectly via pulsar dispersion measures and fit to a model Cordes & Lazio (2002) 1143 pulsars GLAST LAT Project SLAC, 15 July Overall distribution of interstellar gas Hunter et al. (1997) Pohl & Esposito (1998) Sun Galactic Center Strong & Moskalenko Two studies that started with essentially the same data disagree in many details No unique answer owing to distance ambiguity, choice of rotation curve, streaming motions, radiative transfer, GLAST LAT Project SLAC, 15 July Interstellar radiation field Strong & Moskalenko Interstellar radiation field: from COBE/DIRBE emissivities + detailed stellar model, k from extinction curves, grain albedo Strong, Moskalenko, & Reimer (2000) Wavelength (mm) DIRBE GLAST LAT Project SLAC, 15 July Distribution of cosmic rays Total 0 Inverse Compton Bremsstrahlung Strong, Moskalenko, & Reimer (2004)* Comparison with EGRET Data Cosmic rays are measured directly only in the solar system (obviously) Three approaches for modeling Build in assumption of coupling to gas and derive 3-dim distribution of gas (e.g., Hunter et al. 1997) Fit the cosmic-ray density profile (e.g., Strong & Mattox 1996, and studies of individual clouds) Calculate the density using source distribution and codes for cosmic-ray propagation (e.g., Pohl & Esposito 1998; Strong et al. 2004) All reproduce large-scale structure observed in the diffuse emission differences generally are in contrast and at lower intensities Relative contributions depend on energy & latitude *Galprop, talk by Igor Moskalenko on Tuesday GLAST LAT Project SLAC, 15 July Spectral aspects of models 0 bump in the inner Milky Way (Hunter et al. 1997) Illustrates the overall reasonably good fit with few tunable parameters The GeV excess is significant and has been a subject of intense study 0 shoulder point sources subtracted |l| < 60 |b| < 10 Hunter et al. (1997) Comparison with EGRET Data M 0 /2 Hunter et al. (1997) GLAST LAT Project SLAC, 15 July The Milky Way as a background Perspective on backgrounds for the LAT: Milky Way would be a minor component if it were not truly celestial Charged particles several kHz Albedo -rays (at horizon) ~30 Hz Milky Way ~2 Hz Bright point source ~0.05 Hz Trigger rate GLAST LAT Project SLAC, 15 July Confusion with point sources At low latitudes especially, an inaccurate model will cause spurious detections or at least errors in positions of sources Historical example in Ophiuchus 2CG was a COS-B source, 90% conf. diam. 3 Flux ~10 -6 cm -2 s -1 (>100 MeV) required an extreme enhancement of cosmic rays in in Ophiuchus (~10 4 M , ~100 pc distant) EGRET marginally resolved Oph and found most of the flux to be from a blazar, with accurate position [& variability] 2CG Dame et al. (2001) Hunter et al. (1994) PKS ! CO (115 GHz) EGRET (>100 MeV) Swanenburg et al. (1981) Diffuse Emission in Ophiuchus GLAST LAT Project SLAC, 15 July Point Sources in the Milky Way A large fraction of the point sources detected by EGRET is within the Milky Way Known or plausible source classes: rotation-powered pulsars, millisecond pulsars, binary pulsars, plerions, microquasars, SNR, OB/WR associations What could be under the diffuse emission as observed by EGRET? Getting the positions right is essential to multiwavelength astronomy/source identification Blazar Pulsar LMC Solar flare Unidentified 3EG catalog (Hartman et al. 1999) Maddalena et al. (1986) >300 MeV >1 GeV >100 MeV Digel et al. (1999) Orion example GLAST LAT Project SLAC, 15 July Isotropic background The updated model of interstellar emission by Strong et al. (2004) predicts a significantly lower isotropic intensity than the original analysis by the EGRET team (Sreekumar et al. 1998) Most of the difference is attributable to inverse Compton emission (re- evaluation of ISRF and CR electron distribution) Note that the result is implicitly dependent on the sensitivity of the gamma-ray telescope Sreekumar et al. (1998) Strong, Moskalenko, & Reimer (2004) Spectrum of the Isotropic Emission* * for EGRET GLAST LAT Project SLAC, 15 July Isotropic background (2) Note that the result is implicitly dependent on the sensitivity of the -ray telescope The result is clearly limited by statistics at the high-energy end, where the effective area of EGRET rolled off. In the >10 GeV range, A eff (LAT) will be >15 A eff (EGRET), or more relevantly A eff (LAT) will be ~70 A eff (EGRET) in this energy range GLAST LAT Project SLAC, 15 July Recent work and open issues Special regions of the sky Cygnus in particular, and other spiral arm tangents source crowding, need detailed ISRF of OB associations? Galactic center loss of CO as a mass tracer Metallicity gradient in Milky Way Abundances of C and O relative to H are not uniform across the Galaxy Relation between N(H 2 ) and W CO should depend on abundances; although the relationship is complicated, N(H 2 )/W CO is certainly inversely related to metallicity Sagittarius Norma Scutum CygnusCarina 3 kpc arm Crux GLAST LAT Project SLAC, 15 July Galactic Center (3EG J17462851) Hooper & Dingus (2002) Yusef-Zadeh (2002) 20 cm radio continuum Sgr A* CS (2-1) line 3EG source confidence region Sgr A East >5 GeV -rays Recent re-analyses of EGRET data Source not coincident with the Galactic center itself Variable, too, although systematics are significant (Nolan et al. 2003) Many plausible candidates exist for the point source and its nature is of intense interest Many complications affect modeling the diffuse emision of this region Hooper & Dingus used the same diffuse emission model as EGRET team (Hunter et al. 1997) Complications go beyond the loss of kinematic distance resolution Arches cluster (~150 O stars) GLAST LAT Project SLAC, 15 July Galactic center (cont.) Failure of CO as a tracer of molecular hydrogen in the Galactic center region Molecular clouds there have extremely broad velocity dispersions, and this makes the clouds much brighter in the (optically thick) CO line Bitran (1987) CO (J = 10) C 18 O (J = 10) Dahmen et al. (1998) Galactic Center GLAST LAT Project SLAC, 15 July Metallicity and -ray models Inferred Distribution of CR Sources Lorimer (2004) from pulsars Gradient used in earlier work Case & Battacharya (1998) Inferred Variation of N(H 2 )/W CO Metallicity gradient may be as steep as kpc -1 in log Z Z dependence of N(H 2 )/W CO may be as steep as Z -1 or Z -2.5 This gradient could explain the long-standing discrepancy between the cosmic-ray gradient inferred from modeling and that expected from the SN distribution Ref. Strong et al. (astro- ph/ ) N(H 2 )/W CO GLAST LAT Project SLAC, 15 July The diffuse emission of the Milky Way is bright and pervasive In high-energy -rays most of the luminosity of the MW is diffuse Owing to the limited area and angular resolution of the LAT, a good model of the diffuse emission will be required for accurate characterization of point sources It relates to the density of cosmic rays and the distribution of interstellar gas and radiation, which are themselves of interest to astronomers Models have been used since COS-B; refinements are being investigated for the LAT to maximize the scientific return Summary EGRET Phases 1-5 LAT Sim. 1-yr GLAST LAT Project SLAC, 15 July Backup slides follow GLAST LAT Project SLAC, 15 July History of -ray astronomy of the Milky Way , relevant cosmic production mechanisms of -rays understood (Feenberg & Primakoff, Hutchinson, Hayakawa) 1951, 21-cm line of H I (Ewen & Purcell, Muller & Oort) s, upper limits on cosmic fluxes from balloon, suborbital, and satellite experiments 1960s-1970s, theoretical development from advances in particle physics (Stecker, Ginsburg,...) , OSO-3 detection of Milky Way as an extended -ray source - Kraushaar et al. (1972) , COS-B maps of the plane, study of CRs and ISM , EGRET all-sky maps, same plus resolution of spurious sources, better statistics GLAST LAT Project SLAC, 15 July GeV excess Evident in Hunter et al. (1997) spectrum of the inner Galaxy Not obviously a problem with calibration or with calculated production functions (Mori 1997) The excess is clearly associated with the inter-stellar medium; the isotropic spec-trum does not harden (Sreekumar et al. 1998) Is it a local phenomenon? Digel et al. (1996 & 2001) GLAST LAT Project SLAC, 15 July Inverse Compton emission - ISRF - recent progress from COBE/DIRBE emissivities + detailed stellar model, k from extinction curves, grain albedo Galactic Diffuse Radiation Strong & Moskalenko Inverse Compton - ray longitude distributions for 3 SNR/century MeV GeV TeV Application: Simulated SNR sources of cosmic rays GLAST LAT Project SLAC, 15 July Cosmic rays Measured locally Corrected for solar modulation Menn et al. (2000) Electrons Protons Barwick et al. (1998) GLAST LAT Project SLAC, 15 July Do we understand the -ray sky? OSO-III (>50 MeV) EGRET (>100 MeV) Simulated LAT (>100 MeV, 1 yr) Simulated LAT (>1 GeV, 1 yr) GLAST LAT Project SLAC, 15 July Design of the LAT for -ray detection e+e+ ee Tracker 18 XY tracking planes with interleaved W conversion foils. Single-sided silicon strip detectors (228 m pitch). Measure the photon direction; gamma ID. Calorimeter 1536 CsI(Tl) crystals in 8 layers; PIN photodiode readouts. Image the shower to measure the photon energy. Anticoincidence Detector 89 plastic scintillator tiles. Reject background of charged cosmic rays; segmentation limits self- veto at high energy. Calorimeter Tracker ACD Electronics System Includes flexible, robust hardware trigger and software filters. ~800 k channels, 600 W GLAST LAT Project SLAC, 15 July Brief History of Detectors , OSO-3 detected Milky Way as an extended - ray source, 621 -rays , SAS-2, ~8,000 celestial -rays , COS-B, orbit resulted in a large and variable background of charged particles, ~200,000 -rays , EGRET, large effective area, good PSF, long mission life, excellent background rejection, and >1.4 10 6 -rays SAS-2 COS-B EGRET OSO-3 SAS-2 COS-B EGRET GLAST LAT Project SLAC, 15 July Nature of the LAT Data Events are readouts of TKR hits, TOT, ACD tiles, and CAL crystal energy depositions, along with time, position, and orientation of the LAT Intense charged particle background & limited bandwidth for telemetry data are extremely filtered ~3 kHz trigger rate ~300 Hz filtered event rate in telemetry ~13 Gbyte/day raw data ~2 10 5 -rays/day T. Usher (SLAC)