0. 14th ISCRA, Erice July 2004
Observation of VHE gamma rays from the remnant of SN1006 with HEGRA CT1
SN1006 seen by CHANDRA
Vincenzo Vitale for the HEGRA-CT1Munich Group

1. Outline Cosmic Rays and Super-Nova Remnants
TeV gamma rays emission from SNRs
The remnant of SN1006
Imaging Atmospheric Cherenkov Telescopes
Data analysis ( observations)
Large zenith angle observations
Off-axis and Extended source observations
Calibrations
Results

2. Shock SNR Gamma rays G.Cosmic Rays
VHE electrons + photons  gamma rays via Inverse Compton
(synchrotron radiation)
VHE nucleus + target (Inter- Stellar Matter)  π 0  
Gamma rays

3. The remnant of SN1006 One of the brightest SN (type Ia) Age ~ 1Ky
Distance /- 0.3
Radius pc (~ 30 arcmin)
Shock speed Km/sec
From Roque D.L.Muchachos -> not
more than 19 degrees above horizon
CT1 obs. : 220h, 345K deg ZA

4. Non-thermal emissions
ASCA NE SW
CANGAROO
Significance map
5.2
7.5
ROSAT
NE and SW parts of the rim are non-thermal X rays emitters (ASCA, ROSAT)
1-20 TeV gamma rays from NE, reported by Cangaroo

5. Models Berezhko et al. 2002 Naito et al.1999
Theories exploit both Inverse Compton (electrons) and neutral Pion (nuclei) for the TeV gamma rays emission
Evidence of acceleration of electrons up to TeV
Acceleration of nuclei not excluded

6. Some New Observations XMM Newton Chandra Magnetic Field SN 1006
CANGAROO significance map 6
Chandra
Magnetic Field
SN 1006
Berezhko et al 2003 ~ 100 μG
Bamba et al ~ μG

7. Imaging Air Cherenkov Telescope
Energy, flux, type and direction of primary particle from shower image
Gammas and hadrons shower differ for image features and arrival direction
Information extracted from images (image parameters)
Monte Carlo simulations allow gamma/hadrons separation and calibration
Standard candle (Crab Nebula)
Gamma
1 gamma/ 300 nuclei
Shara 2001

8. Analysis technique
Different shape ==> different distributions of the image parameters
Selection cuts on the image parameters

9. Large zenith angle observations
Higher maximum detectable energy
Increase of eff. collection area
Transformation of images: dimensions reduction, images closer to field of view center
Increasing of energy threshold (factor ~25 from 0 to 70deg ZA)
Reduction of gamma/h separation power
Difficulties for MC simulations

10. Background rejection (hadrons)
Exp.Hadrons
MC gamma
Study of the dependence of the image parameters on the zenith angle
Selection cuts with zenith angle dependence

11. Off-axis and Extended source observations
HEGRA CT1
signal from source candidate is searched in the F.O.V.
False Source Method : point- like source analysis repeated on a regular grid (according to angular resolution)
Alternative method : the incoming direction of a shower lies on its axis and has distance from COG computable using MC
Point-like source excess 2d distribution
For HEGRA CT1 radial sigma ~ 0.12 deg =7.2’

12. Mrk 501 Off-axis
160 20
Excess sky map
Analysis tested with a strong signal (Mrk501, 1997) off-axis,
in order to simulated the SN1006 observation with exp. data.
A significant bump at small Alpha (signal)
Good estimate of the residual background

13. Effective collection area
106 m2
PI=-1
PI=-3 50
TeV
Differential trigger rate
(Source differential flux= A*E-PI A= cm-2 sec -1, PI from -1 to -3 )
Effective collection area

14. Calibration(I): Impact Parameter
Impact Parameter as linear comb.of parameters *f(Z.A.)
Free parameters fixed with an optimization routine and MC

15. Calibration (II): Energy
Energy calculated as E=L.C.(Size,Imp.Par.)*g(Z.A.)
Free parameters fixed with an optimization routine and MC

16. Stars as position reference
Sky map
Map of anode currents

17. Results: Sky Maps An excess is found at NE part of the rim
Beta Lup
Kappa Can
SN1006 SHELL
Beta Lup
Kappa Can
SN1006 SHELL
An excess is found at NE part of the rim

18. Results: signal distribution
Distribution of the excess
100
-50
103 +/- 17 excess events over a background of 225 ev.

19. Results: Alpha parameter distributions
Beta Lup
Kappa Can
A signal is present in the Alpha parameter distribution

20. Results: Integral and differential fluxes
Photon
Index
Energy
threshold
TeV
Flux
10-13 cm-2 s-1
Flux error
-1.0
22+-2
2.40
0.39
-1.5
2.15
0.36
-2.0
18+-2
2.35
-2.5
2.21
-3.0
1.72
0.29

21. Conclusions (I)
A large zenith angle observation (first time at >70 deg) has been successfully performed.
Evidence for an excess of multi-TeV γ rays has been found.
The excess sky position is consistent with NE part of the
SN1006 remnant
Use of bright stars as position reference (0.1deg precision)
The statistical significance of the excess is 5.1 standard deviations after trials (5.6 before trials).

22. Conclusions (II) Energy threshold is ~(18 +/- 2) TeV.
The Integral Flux from the NE part of the rim is
(2.35 +/- 0.4stat +/- 0.7sys ) *10-13 ph cm-2 sec-1, phot. ind. = -2.0
The Diff. Flux (without 30% systematic error) is
(0.30+/-0.15 ) *10-11(E/TeV) –( ) ph cm-2 sec- 1 TeV-1
The flux upper limits for SW cap and shell-center are:
0.83 and 0.66 *10-13 ph cm-2 sec -1 (95% C.L., P.I.=-2.0)

23. Conclusions (III)
experiment
Flux at 1TeV
10-11 cm-2 sec-1 TeV-1
Photon index
CANGAROO
1-20 TeV
CT1
18-45(60) TeV
CT1 results are obtained above an energy threshold of 18TeV, the 1 TeV flux is extrapolated for comparison
The maximum flux variation is in the order of (–6%)*year-1