Future directions in Ground-Based Gamma-Ray Astronomy

Simon Swordy - TeV Particle Astro II, UW Madison, 2006

Future of ground-based gamma-rays,

postulate:

"Where there's a will there's a way.."

Discuss..

Some History........

The Crab in early x-rays from a rocket flight....

Also....

Balloon "sky survey"......

Catalog of objects, mostly not there...

What happened next?

Balloon/ x-rays >20keV

Then Now

still awaiting NuSTARNASA/Explorer

Rocket/ x-rays<10keV

etc....

WHY did <10keV do so much better?

The technology of x-ray mirrors as focusing optics could be used <10keV,(now also possible >20keV, hence NuSTAR)

Low energy x-ray detectors could be built from silicon -> CCDs

Low energy single photon resolution became sub arcsec

The energy window ~20-100keV is only being more fully explored recentlybecause modern detector technology in SWIFT has angularresolution ~17arcmin. (Coded mask and CdZnTe).

Ground-based gamma-ray astronomy will not be able get much better than ~5arcmin (for single gamma), so several objects will always seem close to point-like (e.g. Cass A, Tycho, Crab..)

It cannot compete with optical, radio, soft x-ray in the detailed morphologyof sources..... but it can provide a clear outline of the extreme non-thermal pieces of our Galaxy and beyond.

So.....

So what "ways" are there and where might they go?

Air Cerenkov Future Particle Arrays Future

Energy Thr

(GeV)

~100 <50 ~2000 <200

FOV

(sq deg.)

~12 ~100? ~5000 ~5000

Livetime ~8% 10%? 95% >95%

-ray ang. res. (deg.)

0.1 0.05? 1 <0.4

Collection

Area (m2)

105 106 104 105

-ray energy res.

~20% 15% ~75% 40%

hadron rejection

>99.9% >99.95% ~90% ~90%

"Easy" ways to go..

Make 'em bigger (increase to an array size of sqkm)Make 'em higher (go up a bigger mountain)

"Tricky" ways to go...

Lower energy threshold (going up a mountain helps, high QE devices help)Increase FOV for air cherenkov (some optical limits to this)

Seemingly impossible stuff...

Get better single photon angular resolutionIncrease live-time for air cherenkov

Dis

tan

ce F

rom

Cen

ter

Of

Arr

ay [

m]

Distance From Center Of Array [m]

Array1. 217 telescopes 2. 8 hexagonal rings + 13. 80m separation

Telescope and Detector1. ø10m equivalent2. QE = 0.25 (Bialkali)3. 15º field of view

Facts and Figures1. Outer radius: 640m2. Single cell area: 5543m2 3. Total area: 1.06km2

Some examples:S. Fegan, V. Vassiliev, UCLA "HE-ASTRO" concept

Fie

ld o

f vi

ew [

π s

r] Field

of view

[deg

]

Current IACTAsNarrow field of view<0.01 km2 @ 40 GeV0.05-0.1 km2 @ 100 GeV0.2-0.3 km2 @ 10 TeVSquare KM ArrayContinuum of modesTrade area for solid angleParallel modeNarrow field of view1 km2 @ 40 GeV2 km2 @ 100 GeV4-5 km2 @ 10 TeV

“Fly’s Eye” modeWide field of view0.02-0.03 km2 @ 40 GeV0.1-0.2 km2 @ 100 GeV3-4 km2 @ 10 TeV

Observation Modes

Collecting Area [km2]

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

HAWC or miniHAWC? (300m versus 150m baseline)

New Info…

Milagro group + collaborators

few 1000 m

High-energy section~0.05% area coverage

Eth ~ 1-2 TeV

250 m

Medium-energy section~1% area coverage

Eth ~ 50-100 GeV

70 m

Low-energy section~10% area coverage

Eth ~ 10-20 GeV

Hofmann: Array layout: 2-3 Zones

FoV increasingto 8-10 degr.

in outer sections

CTA - European Initiative (HESS+MAGIC)

Not to scale !

Option:Mix of telescope types

Sensitivity on Crab:

Whipple 5/√hrMilagro ~8/√yr (wide angle)

VERITAS-4, etc 23/√hr

HAWC 7/√hr (wide angle)

HE-ASTRO 23 /√hr (wide angle)HE-ASTRO 166 /√hr (sees Crab in 3s!)

Whipple, HEGRA, CANGAROOII,

Milagrito

HESS, MAGIC VERITAS, CANGAROOIII,

Milagro

HESS2, MAGIC2 VERITAS2, CANGAROOIII+?,

MiniHAWC

CTA, HE-ASTRO,HAWC, +….

Ground Gamma-Ray Timeline

Some Ways Forward:

• In principle, collection area can be increased ad infinitum. The collection area of present ACTs is defined by the light pool size. The detector becomes larger than the light pool above ~105m2. Future ACT arrays head toward >1km2

• Higher altitude sites help ACTs and ground arrays, probably >3000m (presently ~2000m).

• Coverage of full sky is highly desirable -> north and south facilities.

• Given expected world-wide resources (<$500M?) this will probably be a limit -> two observatories

• All-sky monitoring capability at <0.1 Crab level seems essential. Possibly with a co-located HAWC-type detector, or with a single HE-ASTRO-type detector, or maybe something new.

• The interested science community will probably grow significantly - we need to get our world-wide act together