Ground Based Gamma Ray Observations

(1)

Follow-Up Questions S. Zimmer

Experimental Techniques in Particle Astrophysics Stockholm, 2010/12/03

Ground Based Gamma Ray

Observations

(2)

Follow-Up Questions from last week



How can gamma-rays be studied using radio detection techniques?



Discuss backgrounds to gamma-ray identification, for example:

pi-zero production in hadronic showers. How are these backgrounds mitigated?



Clarify the detection principle for Milagro.



H.E.S.S. has presented results on the 'all-electron'

(electron+positron) cosmic ray flux. How is the gamma-ray background dealt with?



Bonus question: How does the energy resolution of (e.g.)

H.E.S.S. depend on energy?

(3)

Follow-Up Questions from last week



How can gamma-rays be studied using radio detection techniques?



Discuss backgrounds to gamma-ray identification, for example:

pi-zero production in hadronic showers. How are these backgrounds mitigated?



Clarify the detection principle for Milagro.



H.E.S.S. has presented results on the 'all-electron'

(electron+positron) cosmic ray flux. How is the gamma-ray background dealt with?



Bonus question: How does the energy resolution of (e.g.)

H.E.S.S. depend on energy?

(4)

Detecting Cosmic Rays with Radio



Really rather CR experiment: 10

¹⁶

-10

²⁰

eV (PeV scale)



e+e- deflected in geomagnetic field, synchrotron emission, radio observation (RO)



Normal EAS arrays detect shower only in final stage on ground (not the previous steps)



RO Provides bolometric information about shower anatomy (what exactly happens where at which energy)



In particular: correlation of measured field strength with shower parameters



Eperiment ongoing: LOPES at KASCADE-Grande (future LOFAR at Auger?)



Independent of weather, scalable, cost efficient

Ex-Jobb M.L. Garde, UU 2009

(5)

LOPES



30 short-dipole antennas



East-west orientation to be sensitive to linear east- west polarized component of EAS



Operate in 40-80 MHz range, max. baseline 260m



Trigger: multiple detected signals in KASCADE (~2/min)



Each antenna has absolute reference amplitude calibration inside KASCADE



For UVHE Gamma one can do some CR studies (ideas in development)

W.D. Apel et al, ApJ 32 (2010) 294-303 http://www-ik.fzk.de/KASCADE_home.html

(6)

Follow-Up Questions from last week



How can gamma-rays be studied using radio detection techniques?



Discuss backgrounds to gamma-ray identification, for example:

pi-zero production in hadronic showers. How are these backgrounds mitigated?



Clarify the detection principle for Milagro.



H.E.S.S. has presented results on the 'all-electron'

(electron+positron) cosmic ray flux. How is the gamma-ray background dealt with?



Bonus question: How does the energy resolution of (e.g.)

H.E.S.S. depend on energy?

(7)

Hadronic Background in Gamma Ray Observations



Significant fraction of energy goes into π ⁰ production, π ⁰ →2γ



Claim: two γ-showers that look identical to VHE gamma



Well, not quite...

(8)

The ”Propaganda” Picture



Hadronic shower has always > 1 EM subshower

D. Berge, Diploma Thesis, HU Berlin, 2002

(9)

Background Rejection in-Depth:

Shower reconstruction, TMVA



Hadronic showers irregular in shape



Simultaneous reco-

algorithms, Hillas + Mapping, etc.



Perform detailed MC

studies, assign likelihood based on multi-variate techniques



Not really used in H.E.S.S.

for γ-induced cascades;

stereoscopy & image

analysis gives enough bg rejection

P. Colin et al., ICRC Proceedings 2009

(10)

Follow-Up Questions from last week



How can gamma-rays be studied using radio detection techniques?



Discuss backgrounds to gamma-ray identification, for example:

pi-zero production in hadronic showers. How are these backgrounds mitigated?



Clarify the detection principle for Milagro.



H.E.S.S. has presented results on the 'all-electron'

(electron+positron) cosmic ray flux. How is the gamma-ray background dealt with?



Bonus question: How does the energy resolution of (e.g.)

H.E.S.S. depend on energy?

(11)

Multi Institutional Los Alamos Gamma Ray Observatory



3 layers of detection



1

^st

layer: direction reco (time-of-flight measurements)



2

^nd

layer: looks upwards, sensitive to EM tails and hadronic showers



3

^rd

layer: below opaque barrier, counts muons



Milagro sees Cherenkov Light emitted by:



Electron and positrons from EAS



e+e- pairs from the photons pair-produced in water



Muons traversing through detector



Hadronic cascades



Discriminate CR from γ by looking for

accompanying muons (veto all events that have collected muons)

Gaurang B. Yodh. Space Science Reviews, 75:199

(12)

Follow-Up Questions from last week



How can gamma-rays be studied using radio detection techniques?



Discuss backgrounds to gamma-ray identification, for example:

pi-zero production in hadronic showers. How are these backgrounds mitigated?



Clarify the detection principle for Milagro.



H.E.S.S. has presented results on the 'all-electron'

(electron+positron) cosmic ray flux. How is the gamma-ray background dealt with?



Bonus question: How does the energy resolution of (e.g.)

H.E.S.S. depend on energy?

(13)

H.E.S.S. All-electron measurement &

Discrimination of Gamma Ray BG



electrons/ gamma-ray showers very similar (electrons start one step later)



e+e- should be isotropically distributed → Gamma- free sample (exclude GC, potential sources)



Remaining background due to diffuse extraglactic Gamma



Low fluxes expected (model of extragalactic diffuse)



Experimental discrimination through radiation length

X

_max

(14)

H.E.S.S. All-Electron Measurement



X

_max

½ radiation length higher for electrons



Still: contamination of maximal 50%

K . E gb er s, LP H E s em in ar 2 01 0/0 6/ 21

(15)

H.E.S.S. All-Electron Measurement - Results



More difficult bg from CR; use high/low energy analysis



Proof-of-concept: first ground-based measurement



Analysis range 350 GeV – 5 TeV

F. A. Aharonian et al., A&A, 508 (2) 561 (2009)

(16)

Follow-Up Questions from last week



How can gamma-rays be studied using radio detection techniques?



Discuss backgrounds to gamma-ray identification, for example:

pi-zero production in hadronic showers. How are these backgrounds mitigated?



Clarify the detection principle for Milagro.



H.E.S.S. has presented results on the 'all-electron'

(electron+positron) cosmic ray flux. How is the gamma-ray background dealt with?



Bonus question: How does the energy resolution of (e.g.)

H.E.S.S. depend on energy?

(17)

Energy Resolution



Here Magic-1/2 comparison



Calibration based on look-up tables (E↔ pixel

brightness)



Use TMVA to extract resolution from MC

simulations (okay for EM but not for Had)

P. Colin et al., ICRC Proceedings 2009

(18)

References



Colin et al. Performance of the MAGIC telescopes in stereoscopic mode, Proc. 30

^th

ICRC 2009



Aharonian et al. Probing the ATIC peak in the cosmic-ray electron spectrum with H.E.S.S., A&A 508



Apel et al., Lateral distribution of the radio signal in

extensive air showers measured with LOPES, ApJ 32 (2010)



Ground Based Gamma Ray Observations

Follow-Up Questions S. Zimmer

Experimental Techniques in Particle Astrophysics Stockholm, 2010/12/03

Ground Based Gamma Ray

Observations

Follow-Up Questions from last week

How can gamma-rays be studied using radio detection techniques?

Discuss backgrounds to gamma-ray identification, for example:

pi-zero production in hadronic showers. How are these backgrounds mitigated?

Clarify the detection principle for Milagro.

H.E.S.S. has presented results on the 'all-electron'

(electron+positron) cosmic ray flux. How is the gamma-ray background dealt with?

Bonus question: How does the energy resolution of (e.g.)

H.E.S.S. depend on energy?

Follow-Up Questions from last week

How can gamma-rays be studied using radio detection techniques?

Discuss backgrounds to gamma-ray identification, for example:

pi-zero production in hadronic showers. How are these backgrounds mitigated?

Clarify the detection principle for Milagro.

H.E.S.S. has presented results on the 'all-electron'

(electron+positron) cosmic ray flux. How is the gamma-ray background dealt with?

Bonus question: How does the energy resolution of (e.g.)

H.E.S.S. depend on energy?

Detecting Cosmic Rays with Radio

Really rather CR experiment: 10

-10

eV (PeV scale)

e+e- deflected in geomagnetic field, synchrotron emission, radio observation (RO)

Normal EAS arrays detect shower only in final stage on ground (not the previous steps)

RO Provides bolometric information about shower anatomy (what exactly happens where at which energy)

In particular: correlation of measured field strength with shower parameters

Eperiment ongoing: LOPES at KASCADE-Grande (future LOFAR at Auger?)

Independent of weather, scalable, cost efficient

LOPES

30 short-dipole antennas

East-west orientation to be sensitive to linear east- west polarized component of EAS

Operate in 40-80 MHz range, max. baseline 260m

Trigger: multiple detected signals in KASCADE (~2/min)

Each antenna has absolute reference amplitude calibration inside KASCADE

For UVHE Gamma one can do some CR studies (ideas in development)

Follow-Up Questions from last week

How can gamma-rays be studied using radio detection techniques?

Discuss backgrounds to gamma-ray identification, for example:

pi-zero production in hadronic showers. How are these backgrounds mitigated?

Clarify the detection principle for Milagro.

H.E.S.S. has presented results on the 'all-electron'

(electron+positron) cosmic ray flux. How is the gamma-ray background dealt with?

Bonus question: How does the energy resolution of (e.g.)

H.E.S.S. depend on energy?

Hadronic Background in Gamma Ray Observations

Significant fraction of energy goes into π 0 production, π 0 →2γ

Claim: two γ-showers that look identical to VHE gamma

Well, not quite...

The ”Propaganda” Picture

Hadronic shower has always > 1 EM subshower

Background Rejection in-Depth:

Shower reconstruction, TMVA

Hadronic showers irregular in shape

Simultaneous reco-

algorithms, Hillas + Mapping, etc.

Perform detailed MC

studies, assign likelihood based on multi-variate techniques

Not really used in H.E.S.S.

for γ-induced cascades;

stereoscopy & image

analysis gives enough bg rejection

Follow-Up Questions from last week

How can gamma-rays be studied using radio detection techniques?

Discuss backgrounds to gamma-ray identification, for example:

pi-zero production in hadronic showers. How are these backgrounds mitigated?

Clarify the detection principle for Milagro.

H.E.S.S. has presented results on the 'all-electron'

(electron+positron) cosmic ray flux. How is the gamma-ray background dealt with?

Bonus question: How does the energy resolution of (e.g.)

H.E.S.S. depend on energy?

Multi Institutional Los Alamos Gamma Ray Observatory

3 layers of detection

1

layer: direction reco (time-of-flight measurements)

2

Significant fraction of energy goes into π ⁰ production, π ⁰ →2γ