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Gamma-ray Astronomy: Talking the Talk S. W. Digel SLAC

Gamma-ray Large Area Space Telescope. Gamma-ray Astronomy: Talking the Talk S. W. Digel SLAC. Outline. Coordinate systems Celestial, Galactic, ecliptic Units Flux, intensity, time, distance The sky Solar neighborhood, Milky Way, local group, etc. What you see vs. what we care about

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Gamma-ray Astronomy: Talking the Talk S. W. Digel SLAC

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  1. Gamma-ray Large Area Space Telescope Gamma-ray Astronomy: Talking the Talk S. W. Digel SLAC

  2. Outline • Coordinate systems • Celestial, Galactic, ecliptic • Units • Flux, intensity, time, distance • The sky • Solar neighborhood, Milky Way, local group, etc. • What you see vs. what we care about • Naming sources • Overview of gamma-ray sources

  3. Coordinate systems * Polaris • Celestial coordinates – natural for astronomy from the ground • Right ascension (time) + declination • Denoted RA, Dec or a, d • RA in hours, Dec in degrees • Telephone number in catalogs • N.B.: Epoch must always be specified • Precession period ~26,000 yr [~20 arc sec/yr] • Vernal equinox (i.e., start of spring) is RA = 0 hours • With the RA & Dec you can tell right away whether a given direction on the sky will rise, how high it will reach, and what time of year it will be up at night R. Pogge

  4. M109 (AURA/NOAO/NSF), SBc Coord. systems (2): Galactic coordinates • Galactic coordinates – natural for astronomy if you don’t use a telescope • Even for extragalactic observations, the coordinate system is relevant (for foreground emission/obscuration) • The plane of the Milky Way traces the Galactic Equator • (0,0) is direction to the Galactic center • (180,0) is the anticenter 8.5 kpc Sun Powell

  5. Galactic coordinates (cont) • Galactic longitude is the angle along the Galactic Equator, latitude is angle above or below plane • Denoted l, b [I don’t know why] • Range in degrees is l = 0-360° (or -180° to +180°) and b = -90° to +90° • In older (~30 yrs) literature you will notice lII and bII listed. This was to distinguish between ‘new’ (i.e., correct) and old Galactic coordinates (before radio astronomy cleared up the question of where the Galactic center actually is) • Epoch does not need to be specified • Orbit period ~250 Myr [5 mas/yr]

  6. Coord systems (3): Ecliptic coordinates • Plane of the solar system, illustrated by dust in the plane of the solar system, which is bright at 12 mm • Denotedl, b IRAS

  7. Units • Fluxes are in photons cm-2 s-1. If an energy range is not specified, you can probably assume >100 MeV • Sometimes ‘photons’ is not explicitly written • If you want to convert this to ergs cm-2, then you need to know the spectrum* • Implicitly this is an instantaneous quantity, i.e., applies to a specific time • Intensities (relevant for extended sources) have units of photons cm-2 s-1 sr-1 • For GRBs, flux determinations are problematic because the emission is so impulsive. Instead what is quoted are fluences, ergs cm-2. • You can ask what’s a magnitude? What is a Jansky? EGRET sources ~5 magnitudes, LAT ~ 8 magnitudes * I don’t know why astronomy is stuck in cgi units

  8. Units (2): Dates and distances • JD is Julian Date – number of days since noon on January 1, 4713 BC • MJD – Modified Julian Date = JD – 2,400,000.5 (i.e., number of days since midnight on November 17, 1858 • Today is MJD ~ 53,314 • (Truncated Julian Date TJD = MJD – 40,000) • Distance - Parsec (pc) is the distance at which a star would have an annual parallax of 1” (~3.26 light years)

  9. Local Group • Milky Way and M31 are the dominant galaxies in the local group • Many others are irregular or dwarf spheroidal • Additional members are still being discovered "Teacher's Guide to the Universe by Lindsay M. Clark, MAP Education/Outreach Coordinator" , www.astro.princeton.edu/ ~clark/

  10. Local clusters of galaxies R. Powell

  11. Way beyond the local cluster • Way, way beyond: Romani et al. (astro-ph/0406252) blazar at redshift z = 5.47

  12. The sky we see Aitoff (equal area, allsky) projection

  13. The sky we see (2) • Stars, dust, Galactic equator, local galaxies • Do we care about stars? Basically no • Stars are not gamma-ray sources [OB associations notwithstanding, ref. D. Smith’s talk] • They are good gamma-ray absorbers, but their filling factor is small • Same for most galaxies, too

  14. 3EG catalog (Hartman et al. 1999) EGRET all-sky map • ~1.4 Mg, ~60% interstellar emission from the MW • ~10% are cataloged (3EG) point sources EGRET (>100 MeV)

  15. Gamma-ray sky • Milky Way is bright with diffuse emission • Indications are that the Galactic sources that we care about (i.e., can detect) are relatively nearby on the scale of the Milky Way • Concentration toward the equator is evident • Typical luminosity ~ (1–15) × 1035 erg s-1 (isotropic) for an EGRET Galactic point source (characteristic distance 1–6 kpc), Mukherjee et al. (1995) • The bright pulsars are within a few 100 pc, i.e., not detectable by the LAT if they were as distant as the G.C. • Some extragalactic sources have large fluxes • They are quite variable – see later • GRBs are briefly more intense than the entire rest of the sky • EGRET also saw a solar flare, and the moon Thompson et al.

  16. Astronomical catalogs • The idea is to label sources so you can refer to them • No uniform standards, although standards are being imposed • Historically, naming was just sequential, e.g., HD12345, W49 • Now the convention is to use the ‘telephone number’, with appropriate level of precision, along with a designator for the origin; catalogs that undergo revisions also have a version number; the J indicates the epoch – hence, 3EG J1835+5918 • One exception is transient sources • E.g., GRBs, for which the name is the date (not Y2K compliant) of the burst, e.g., GRB030328 • SNR, which are numbered by the year of discovery, with a letter (or letters) to indicate sequence, e.g., SNR 1998bw Henry Draper Gart Westerhout

  17. Naming sources (2) • One-of-a-kind sources can get unique names • For extended sources, they can be descriptive, or too cute • Becklin-Neugebauer Object • Baade’s Window, Lockman’s Hole • Rabbit, Mouse, and the Galactic Center Snake

  18. Identified Gamma-Ray Sources 237 ms • Pulsars (7-10 known from EGRET) • Rotating magnetized neutron stars • They do this: • Blazars (~70+ known as g-ray sources) • Active galaxies (accreting masive black holes with jets toward Earth) • They do this: Geminga Sreekumar PKS 1622-297 Mattox et al. (1997)

  19. Some plausible (or not impossible) new source classes • Millisecond pulsars, binary pulsars (microquasars), pulsar wind nebulae (plerions) • Supernova remnants, SNOBs, OB/Wolf-Rayet star associations • Galaxy clusters • Starburst (ultraluminous infrared) galaxies

  20. Summary • A little jargon can go a long way • The conventions and nomenclature shouldn’t be a hurdle • Gamma-ray astronomy is not stellar astrophysics LAT Sim. 1-yr EGRET Phases 1-5

  21. Backup slides follow

  22. e– e+ GLAST Large Area Telescope (LAT) • Within its first few weeks, the LAT will double the number of celestial gamma rays ever detected • 5-year design life, goal of 10 years Spectrum Astro 1.8 m Tracker ACD 3000 kg Calorimeter

  23. Galactic g-rays in perspective • Diffuse luminosity is miniscule relative to optical and IR (and small relative to blazars) • Particle acceleration and g-ray production mechanisms don’t have to be very efficient • Diffuse luminosity >> total luminosity of Galactic point-sources • Typical luminosity ~ (1–15) × 1035 erg s-1 (isotropic) for an EGRET Galactic point source (characteristic distance 1–6 kpc), Mukherjee et al. (1995) Zombeck, M. V. 1990, Handbook of Astronomy and Astrophysics, Second Edition (Cambridge, UK: Cambridge University Press).

  24. History of g-ray astronomy of the Milky Way • 1948-1952, relevant cosmic production mechanisms of g-rays understood (Feenberg & Primakoff, Hutchinson, Hayakawa) • 1951, 21-cm line of H I (Ewen & Purcell, Muller & Oort) • 1949-1960s, upper limits on cosmic fluxes from balloon, suborbital, and satellite experiments • 1960s-1970s, theoretical development from advances in particle physics (Stecker, Ginsburg,...) • 1967-1968, OSO-3 detection of Milky Way as an extended g-ray source - Kraushaar et al. (1972) • 1975-1982, COS-B maps of the plane, study of CRs and ISM • 1991-2000, EGRET all-sky maps, same plus resolution of spurious sources, better statistics

  25. Brief History of Detectors • 1967-1968, OSO-3 detected Milky Way as an extended g-ray source, 621 g-rays • 1972-1973, SAS-2, ~8,000 celestial g-rays • 1975-1982, COS-B, orbit resulted in a large and variable background of charged particles, ~200,000 g-rays. • 1991-2000, EGRET, large effective area, good PSF, long mission life, excellent background rejection, and >1.4 × 106g-rays COS-B SAS-2 OSO-3 SAS-2 COS-B EGRET EGRET

  26. 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 16th shuttle launch once launches resume • GLAST…

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