GLAST LAT Project

GLAST-for-lunch, 4 November 2004 1

Gamma-ray Astronomy: Gamma-ray Astronomy: Talking the TalkTalking the Talk

S. W. DigelSLAC

Gamma-ray Large Gamma-ray Large Area Space Area Space TelescopeTelescope

GLAST LAT Project

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OutlineOutline

• 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

GLAST LAT Project

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Coordinate systemsCoordinate systems

• Celestial coordinates – natural for astronomy from the ground

• Right ascension (time) + declination

– Denoted RA, Dec or – 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

* Polaris

GLAST LAT Project

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Coord. systems (2): Galactic coordinatesCoord. 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

M109 (AURA/NOAO/NSF), SBc

Powell

8.5

kpc

Sun

GLAST LAT Project

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Galactic coordinates (cont)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]

GLAST LAT Project

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Coord systems (3): Ecliptic coordinatesCoord systems (3): Ecliptic coordinates

• Plane of the solar system, illustrated by dust in the plane of the solar system, which is bright at 12 m

• Denoted

IRAS

GLAST LAT Project

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UnitsUnits

• 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

GLAST LAT Project

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Units (2): Dates and distancesUnits (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)

GLAST LAT Project

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Local GroupLocal 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

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GLAST LAT Project

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Local clusters of galaxiesLocal clusters of galaxies

R. Powell

GLAST LAT Project

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Way beyond the local clusterWay beyond the local cluster

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

GLAST LAT Project

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The sky we seeThe sky we see

Aitoff (equal area, allsky) projection

GLAST LAT Project

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The sky we see (2)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

GLAST LAT Project

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EGRET all-sky mapEGRET all-sky map

• ~1.4 M, ~60% interstellar emission from the MW

• ~10% are cataloged (3EG) point sources 3EG catalog (Hartman et al. 1999)

EGRET(>100 MeV)

GLAST LAT Project

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Gamma-ray skyGamma-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.

GLAST LAT Project

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Astronomical catalogsAstronomical 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

GLAST LAT Project

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Naming sources (2)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

GLAST LAT Project

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Identified Gamma-Ray SourcesIdentified Gamma-Ray Sources

• Pulsars (7-10 known from EGRET)– Rotating magnetized neutron stars– They do this:

• Blazars (~70+ known as -ray sources)– Active galaxies (accreting masive black holes with jets

toward Earth)– They do this:

Geminga

Sreekumar

PKS 1622-297

Mattox et al. (1997)

237 ms

GLAST LAT Project

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Some plausible (or not impossible) new source Some plausible (or not impossible) new source classesclasses

• Millisecond pulsars, binary pulsars (microquasars), pulsar wind nebulae (plerions)

• Supernova remnants, SNOBs, OB/Wolf-Rayet star associations• Galaxy clusters• Starburst (ultraluminous infrared) galaxies

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• A little jargon can go a long way• The conventions and nomenclature shouldn’t be a hurdle• Gamma-ray astronomy is not stellar astrophysics

SummarySummary

EGRETPhases 1-5

LATSim. 1-yr

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Backup slides followBackup slides follow

GLAST LAT Project

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Sp

ect

rum

Ast

ro

GLAST Large Area Telescope (LAT)GLAST Large Area Telescope (LAT)

e+ e–

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 years

YearsAng. Res.(100 MeV)

Ang. Res. (10 GeV)

Eng. Rng. (GeV)

Aeff Ω

(cm2 sr)# rays

EGRET 1991–00 5.8° 0.5° 0.03–10 750 1.4 × 106

AGILE 2005– 4.7° 0.2° 0.03–50 1,500 4 × 106/yr

AMS 2005+?– – 0.1° 0.3–300 1,600 7 × 105/yr

GLAST LAT 2007– 3.5° 0.1° 0.02–300 25,000 1 × 108/yr

GLAST LAT Project

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Galactic Galactic -rays in perspective-rays in perspective

• Diffuse luminosity is miniscule relative to optical and IR (and small relative to blazars)

– Particle acceleration and -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)

BandBandLuminosityLuminosity(10(103838 erg s erg s-1-1,,aka 2.5 × 10aka 2.5 × 1044 L L☼☼))

RadioRadio 33

IRIR 3 × 103 × 1033

OpticalOptical 3 × 103 × 1055!!

X-rayX-ray 10-10010-100

>100 MeV>100 MeV 4040

Zombeck, M. V. 1990, Handbook of Astronomy and Astrophysics, Second Edition (Cambridge, UK: Cambridge University Press).

GLAST LAT Project

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History of History of -ray astronomy of the Milky Way-ray astronomy of the Milky Way

• 1948-1952, relevant cosmic production mechanisms of -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 -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

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Brief History of DetectorsBrief History of Detectors

• 1967-1968, OSO-3 detected Milky Way as an extended -ray source, 621 -rays

• 1972-1973, SAS-2, ~8,000 celestial -rays

• 1975-1982, COS-B, orbit resulted in a large and variable background of charged particles, ~200,000 -rays.

• 1991-2000, EGRET, large effective area, good PSF, long mission life, excellent background rejection, and >1.4 × 106 -rays

SAS-2

COS-B

EGRET

OSO-3

SAS-2COS-B

EGRET

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Future MissionsFuture 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…