VHE Gamma Ray Astronomyan overview

John LearnedUniversity of HawaiiP711/A736 12 April 2005

Origins of the Field

• Early ideas and attempts by Russians, English, particularly J.V.Jelly.

• CfA’s Trevor Weekes in 1970’s, Mt. Hopkins first dedicated (10m) telescope.

• Key idea from Micheal Hillas…. How to distinguish image of Gamma Shower from predominant proton (and nucleon) showers.

• Marginalized, almost no support, and largely considered a failure through 80’s.

• Blossoms in 90’s with first TeV sources (mainly the Crab Nebula), but really taking off now.

TeV Gammas

• Lower energies (say <GeV) are observed with direct counting, few m2. Must be from balloons at few gm/cm2 or better satellites (or on moon).

• Energies ~MeV can be nuclear. Higher E’s involve particle acceleration, peculiarly common in the universe. Must be there, since CRs known to exist, but what flux and what origin?

• If seeking TeV energies, need areas in the range of >10,000 m2. Use showers.

• Was soon (80’s) known that there were no huge point sources in cosmic rays. Protons stirred by gal mag field, so should not point. So, no huge gamma fluxes, re P’s, and thus need means to sort out gamma showers.

Aside: our UH role• No GR or any point sources detected.• Weekes at Mt. Hopkins, pursued imaging and

we were involved a bit (Gorham thesis, etc. JGL built calibrator, Stenger did data analysis). Not making much progress. Imaging a flop.

• Mid-80’s, we built GR telescope on Haleakala, with Wis, Purdue.

• Bet on wrong horse: simulations showed handle on GR showers by fast timing. Did not work.

• Finally, Weekes gets enough pixels, fast electronics and good calibrations…. Finds Crab.

Methods of Detection• Cost: counters ~>$1000/m2, if need ~105 m2, then

~$100M.• Detect showers: at ground their size is about 105 m2 (less

in high mountains).• Use Cherenkov light in atmosphere. Too low an energy

for air fluorescence (EeV). Sort showers by image Mt. Hopkins, VERITAS, Cangaroo, HESS, Magic, and

around 6 others).• Use dense counter array, only water affordable

(MILAGRO, HAWC). • Use RPCs (Tibet).• New technology ACT, ASHRA in Hawaii.

CGRO/EGRET

• Apr 1991 – Jun 2000• 30 MeV – 30 GeV 67%=5.85(100 MeV/E)0.534

Following slides from Mori, of ICRR and Cangaroo collab

EGRET Allsky Map

Diffuse gamma-ray spectrum

• Flatter than expected (E -2.75): why? Flatter proton/electron spectrum??⇒

S. Hunter, Heidelberg WS, 2000

EGRET Nishimura et al.

JACEE

Whipple0

Brems

IC

————————————————

uniform

Third EGRET catalog

R.C. Hartman et al., ApJS, 1999

EGRET point source summary

Pulsars 5

AGN (mostly blazars) 6627 (marginal)

Radio galaxy (Cen A) 1 (marginal)

Unidentified(Some may be SNRs)

170

Large Magellanic Cloud 1

Solar flare 1

Total 271

R.C. Hartman et al., ApJS, 1999

Pulsars

GeV Thompson, Heidelberg WS, 2000

Radio Princeton catalog (706 pulsars), 1995

(GeV candidates: 1046-58, 0656+14, J0218+4232)

MeV only

Gamma-ray pulsar light curves

GLAST proposal

BL Lac’s and EGRET AGNs

TeV Whipple, HEGRA, CAT, 7TA, Durham

RED EGRET 3rd catalog AGNs

Green Padovani & Giommi MN 1995

Gamma-ray blazars

• Mostly FSRQ and BL Lac’s

Lin et al. ApJ 1999Mukherjee et al. ApJ 1997

H(igh freq. peaked) BL X(-ray selected) BL

L(ow-freq. peaked) BL R(adio-selected) BL

Multiwavelength spectrum of AGNs

• Double-peaked structure= synchrotron + inverse Compton

Kubo et al. ApJ 1998

PKS0528+134 (z=2.1, FSRQ)

Mrk 421 (z=0.03, XBL)

Kataoka, Ph.D 2000

↑

νsync

↑

νIC

=γ2νsync

EGRET unidentified sources

• Low vs High latitude

• Persistent vs Variable

• Geminga-like pulsars?

• SNRs?• OB

associations?• Gould belt?

I. Grenier, GeV-TeV WS, 1999

EGRET unIDs and SNRs

GeV Esposito et al. ApJ 461, 1996

TeV CANGAROO

RED EGRET 3rd catalog unID

Green D.A. Green’s catalog

TeV HEGRA

Extragalactic diffuse gamma-rays

• Single power-law E –2.100.03 (30 MeV-100 GeV)

• Unresolved point sources (ex. Blazars etc.)?Upscattered CMB?

P. Sreekumnar et al., ApJ 1998

E –2.100.03

Whipple1968

Imaging Cherenkov technique

Image parameters

●

D.J. Fegan, J.Phys.G, 1997(Simulation)

Example of image cut analysis

• Hadron rejection power ～ 100

M. Punch et al., Nature, 1992 T. Yoshikoshi et al., ApJ, 1997

CANGAROO (Vela)

Whipple

TeV catalog 2000

Classification Object Group Remark

Grade A

(>5σ,multiple)

Crab

PSR1706-44

Mrk 421

Mrk 501

Many

CANGAROO, Durham

Many

Many

Plerion

Plerion

AGN (BL Lac)

AGN (BL Lac)

Grade B

(>5σ)

SN1006

Vela

RXJ1713.7-3946

PKS2155-304

1ES1959+650

BL Lac

CANGAROO

CANGAROO

CANGAROO

Durham

Utah7TA

Crimea

SNR

Plerion

SNR

AGN (BL Lac)

AGN (BL Lac)

AGN (BL Lac)

Grade C

(strong but with somequalifications)

Cas A

Cen X-3

1ES2344+514

3C66A

Geminga

B1509-58

HEGRA CT

Durham

Whipple

Crimea

Crimea

CANGAROO

SNR

X-ray binary

AGN (BL Lac)

AGN (z=0.44)

Pulsar

Plerion

T.C. Weekes, Heidelberg WS, 2000

TeV sky 2000

TeV observations of AGNs

Krennrich, astro-ph/0101120

(Detection of 1ES1426+428 (z=0.13) is claimed by Whipple but not published)

AGN: Mrk 421 variability

• Time scale < a few hours• Correlation with X-ray flux

Takahashi et al. ApJ 542, 2000Gaidos et al., Nature, 383, 1996

AGN: Mrk 421 spectrum

• Synchrotron+ inverse Comptonmodel workswell

e⇒ origin• Proton model

still possible

Takahashi et al. ApJ 542, 2000

One-zone SSC model

δ=14, B=0.14G

synchrotron

inverse Compton

z=0.031

AGN: TeV gamma-ray absorption by IR background

Protheroe et al. astro-ph/0005349

IR BackgroundMean free path for e+e- pair production

AGN: Mrk 501 spectrum

Protheroe et al. astro-ph/0005349

Aharonian et al. A&Ap 349, 1999

CrisisCrisis?↓?↓

z=0.033

Sensitivity of various detectors

(2000)

MILAGRO PrincipleHAWC: High Altitude Water Cherenkov

• 200m x 200m water Cherenkov detector• Two layers of 8” PMTs on a 2.7 meter grid

– Top layer under 1.5m water (trigger & angle)– Bottom layer under 6m water (energy & particle ID)– ~11,000 PMTs total (5,000 top and 5000 bottom)– Trigger: >50 PMTs in top layer

• Two altitudes investigated– 4500 m (~Tibet, China)– 5200 m (Atacama desert Chile)

6 meters

e

200 meters

Following Milagro/HAWC slides from Gus Sinnis, LANL

Event Reconstruction

Angular resolution ~0.75 degrees

Gam

mas

Pro

tons

Background Rejection Bottom Layer30 GeV 70 GeV 230 GeV

20 GeV 70 GeV 270 GeV

Background Rejection

Uniformity ParameternTop/cxPE > 4.3

Reject 70% of protons

Accept 87% of gammas

1.6x improvement in sensitivity

Gammas

Protons

D.C. Sensitivity: Galactic Sources

• Crab Spectrum: dN/dE = 3.2x10-7 E-2.49

– Milagro 0.002 (0.001) Hz raw (cut) rate– HAWC 0.220 (0.19) Hz raw (cut) rate– Whipple 0.025 Hz– Veritas 0.5 (.12) Hz raw (cut) rate

• Background rate 80 (24) Hz raw (cut)• 4 /sqrt(day) raw data• 6 /sqrt(day) cut data

– 120 /sqrt(year)

• 40 mCrab sensitivity (all sky) in one year– Whipple: 140 mCrab per source– VERITAS: 7 mCrab per source (15 sources/year)

Excess Coincident with EGRET source 3EG J0520+2556

Source Reported twice beforeby Milagro:

1) APS Meeting: April 2002Reported as a Hot Spot. ALarger than optimal bin size was used in that initial survey.

2) Location of one of the topexcesses in our published pointsource All Sky search.

Crab3EG J0520+2556

5.5 detection at (79.8o, 42o) using binsize= 2.9o

Distribution of Excess in the Cygnus Region:

Gaussian Weighted Excess

l=80

l=85

l=75

b=0b=-5

b=+52 regions of excess give riseto the observed signal.

Cyg OB2 field

Existing Arrays

Milagro

Dense sampling

Moderate altitude (2650m)

Background rejection

Tibet Array

Sparse sampling

High altitude (4300m)

No background rejection

EAS Arrays• Provide synoptic view of the sky • See an entire hemisphere every day• Large fov & high duty cycle

– Gamma ray bursts– Transient astrophysics– Extended objects– New sources

• Excellent complement to GLAST– With >1000 sources need an all-sky instrument in VHE

• Current EAS arrays lack sensitivity to complement GLAST• What can be done?

– Need low threshold (GLAST overlap) < 100 GeV– High sensitivity

EAS Arrays in the GLAST Era

Effect of EBL on Distant Sources

z = 0.03z = 0.1z = 0.2

z = 0.3

z = 0.0

Detection principle:Stereoscopic

imagingof Cherenkov light from air-showers

• Large collection area• Multiple views of the shower

– improved direction– improved energy– improved rejection of

background (cosmic rays!)

Crab Nebula

Preliminary3-Telescope data (2003)

54 , (27 /hr0.5)10.8 +/- 0.2 /minute

@ 45 degree zenith angle

Official detections by H.E.S.S. so far…

• Crab Nebula (2003, 3 Tel.) - 54 sigma

• PKS 2155 (2003, 2 Tel.) - 45 sigma

• Mrk 421 (2004, 4 Tel.) - 71 sigma

• PSR B1259 (2004, 4 Tel.) - 8 sigma

• RX J1713 (2003, 2 Tel.) - 20 sigma

• Sagittarius A* (2003. 2 Tel.) - 11 sigma

Very confident detections – all but Mrk 421 and PSR B1259 were confirmed independantly in datasets from two hardware configurations

The Galactic Centre -rays detected by CANGAROO and Whipple but:

• Very complex region - lots of potential sources of -rays– Sagittarius A* - supermassive black hole - curvature

radiation of accelerated protons?– Several SNR, including Sag-A East, 'standard' CR

acceleration?– Dark matter annihilation?

• To resolve the ambiguity we need– precise spectrum– well determined position

Sagittarius A*• H.E.S.S. 2003

– 2 telescopes, 16 hours

– Ethresh

= 160 / 250

GeV(2 data sets)

– 11 significance

Good source localisation

Hard energy spectrum

-ray candidates (hard cuts)

Sagittarius A* - Source Location

Chandra GC surveyNASA/UMass/D.Wang et al.

CANGAROO (80%)

Whipple(95%)

H.E.S.S.

Contours from Hooper et al. 2004

Point-like emission from Sgr A* direction

H.E.S.S.

ChandraF. Banagoff et al.

95%

68%

Sgr A EastChandra & Radio NASA/G.Garmire (PSU)F.Baganoff (MIT)Yusef-Zadeh (NWU)

Sgr-A East not ruled out

H.E.S.S.limit on rmssource size

Sagittarius A* - SpectrumDM annihilation: ?Curvature radiation: ?SNR Shocks: ?Shocks in winds: ?

Galactic Centre – Dark Matter

• Neutralino annihilation?– Use DarkSusy– Expect two lines and continuum

• Power law index - 2.2 - 2.4• Cut off at roughly m / 3

• We see no lines and no cut off – exponential cut off is limited to < 4 TeV

Which implies m > 12 TeV

RX J1713

• H.E.S.S. smoothed gamma-candidate map after image size cuts (> 800 GeV) - no background subtraction or acceptance correction

– Only two telescopes– 18 hours – 20 sigma

• c.f. ASCA (1-3 keV)

Flux = 70% of Crab

THE MAGIC TELESCOPEE. LORENZ, for the MAGIC COLLABORATION

COLLABORATION:IFAE BARCELONA,UA BARCELONA,U. BERLIN, UC-DAVIS,U.LODZ, UC MADRID,MPI MUNICHINFN PADOVA, U. POTCHEFSTROOM, INFN SIENA, TUORLA OBSERVATORY, INFN UDINE, U. WUERZBURG,YEREVAN PHYSICS INST. , ETH ZURICH. 3 CANDIDATES, IN TOTAL 120 MEMBERS

17 mtr

MUON ARC IMAGES

• THE MAGIC TRIGGER SUPPRESSES FULL MUON RINGS

(EX. CLOSE TO THRESHOLD)

• LIGHT YIELD FROM MUON ARCS AGREES WITHIN 10%

WITH OTHER METHODS (F-FACTOR ANALYSIS OF LIGHT

PULSER SIGNAL)

• FROM MC SIMULATION: RESIDUAL MUON BG ONLY A

FRACTION OF HADRONIC BG.

• IMPORTANT: MUONS DO NOT PEAK AT SMALL ALPHA

AND DO NOT FAKE A SOURCE

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

MC SIMULATION OF MUON IMPACTON GAMMA ANALYSIS AFTER STANDARD CUTS

GAMMAS

HADRONIC BACKGROUND

CONTRIBUTION FROM MUONS

ECO - 1000

STUDY OF A 1000 m2 CHERENKOV TELESCOPE

34 m

17 m

Summary

• VHE Gamma Ray Astronomy blossoming

• An interesting future with new telescopes and techniques

• Moving rapidly from explorations to regular observational astronomy.

• UH may have a role in ASHRA all sky observations.