The H.E.S.S. Experiment :
High-Energy Stereoscopic System for
VHE Gamma-Ray Astronomy

PCC/APC

M. Punch,

PCC/APC Collège de France

for the H·E·S·S· Collaboration

3 continents

8 countries

19 institutions

69 physicists

100 engineers and technicians

HESS

High Energy Stereoscopic System

Collaboration

MPI Kernphysik, Heidelberg

Humboldt Univ. Berlin

Ruhr-Univ. Bochum

Univ. Hamburg

Landessternwarte Heidelberg

Univ. Kiel

LLR, Ecole Polytechnique, Palaiseau

PCC, College de France, Paris

LPNHE-Paris, Univ. Paris VI-VII

CEA Saclay

CESR Toulouse

LAOG Grenoble

Observatoire de Paris

GAM, Montpellier

Durham Univ.

Charles Univ., Prag
Yerewan Physics Inst.

Dublin Inst. for Adv. Studies

Univ. Namibia, Windhoek

Univ. Potchefstroom

The HESS collaboration

Next-Generation ACTs (planned/completed)

HESS

MAGIC

CANGAROO

VERITAS

MAGIC

Physics Targets For Gamma-Ray Astronomy

Galactic:
Supernova Remnants (SNR), Pulsars, µQuasars

ExtraGalactic:
Active Galactic Nuclei (AGN)

Exotic:
Gamma-Ray Bursts
Spectral or line emission from WIMP annihilation ...

Test of the hypothesis of SNR as the source of
Galactic Cosmic Rays (or ...)

Understanding of the acceleration mechanisms,	identification of emitting particles,	mechanisms of jet formation

Probing the Infra-Red Background...

Using the Ground-based
Imaging Atmospheric Cherenkov Technique

Correlations with Gamma- and X-ray satellite detectors,	radio and optical telescopes

What ?

Why ?

How ?

HESS Phase-I Essential Characteristics

Four-Telescope network

Sited in Namibia, 23°S, 15°E,
1800 m altitude

Telescope separation: 120 m

Telescope Structures

Mirror dishes: 4 × 107 m2

Diameter: 12 m, Focal length: 15 m

Mirrors

380 × 60cm circular facets

PSF after alignement (r80%)
1.3' / 0.38 mrad on axis

Pointing precision 8”

HESS Phase-I Essential Characteristics

Cameras

960 photomultiplier pixels

Pixels of 0.16° / 2.8 mrad

Wide field of view, 5°

16ns integration window,
fast trigger coincidence

All electronics integrated in-camera

3 cables (1 power , 2 optic fibres)

Fits in 2 m cube

Weight ~ 900 kg

Telescope Threshold ~100 GeV

Electronics Front-End

Large dynamic range, good linearity (up to 1600 ge)

Single photoelectron peak resolution (at 80 d.c.)

Storage of signal in analogue memory
during trigger formation time (~70ns)

On-board data acquisition

Trigger

circuitry in

on-board

crates

&

FIFOs, DAQ system

Analogue signal

processing

Digital signal

processing

Readout

bus

1 GHz
signal

sampling

Active HV

~ 1.5 ns

coinc. time

resolution

Sequence from W. Hofmann

The Atmospheric Cherenkov Technique

Sequence from W. Hofmann

The Atmospheric Cherenkov Technique

Sequence from W. Hofmann

The Atmospheric Cherenkov Technique

ACT: Advantages of Stereoscopy

Shower seen from different angles:
Better discrimination between
gammas and hadronic background

Geometrical determination
of the gamma-ray origin
on the sky

Geometrical determination
of the shower impact
parameter on the ground
 better energy resolution

Triggers from isolated muons are
over half of mono-telescope events
Muon elimination  lowering of threshold

Mono-telescope

events

Stereo events

muons

gamma

hadrons

Stereo: Tells where the shower came from

Geometrical determination of
the source position in the camera

... HESS angular resolution 0.100.06° per photon

CoG

Source

q

Reconstructed Shower Axis

Stereo: Tells where the shower hit

Not to scale

Geometrical determination of the
shower impact point on the ground

 resolution of the energy / impact parameter degeneracy

HESS energy resolution ~15%

Shower origin

Image centre of gravity

Muon Supression with Stereo Trigger

Muons: In single telescope mode, give rings

(as in RICH)  useful for calibration.

Conversion factor ~ 4 %

good channel

damaged

channel

Efficiency of camera over time

Comparison of
individual channels

Plots from
N. Leroy, LLR

Muon Supression with Stereo Trigger

But

Far from telescope muons give events which can mimic gamma-ray images

 difficult background

Muon background eliminated by Stereo !

Mono-telescope

events

Stereo events

HESS-I: A tight installation schedule

Note:
Data taking continued
throughout the installations

 Heterogeneous data-set

Varying thresholds, Complicated analysis

2002

2003

2004

Camera 1

Camera 2

+Upgrade of camera 1

Camera 3

Camera 4

+Upgrade of Cameras 1, 2 & 3

June

February

September

December

Here we are

First design: '96;	Collaboration founded '98;	Groundbreaking '00;
First telescope structure July '01;	Mirrors on 1st telescope September '01

HESS-I: A Heterogenous Data-set

~ 350 Hz

~ 2 kHz

CT 1, 2, 3, 4

(2 out of 4)

Since Dec. 2003

~ 210 Hz

~ 2 kHz

CT 2, 3, 4

(2 out of 3)

Oct. – Nov. 2003

~ 110 Hz

~ 2 kHz

CT 2, 3

(coincidence)

Aug. – Sept. 2003

~ 30 Hz

~ 250 Hz

CT 2 , 3

(independent)

Mar. – July 2003

-

~ 250 Hz

CT 3

July 2002 – Feb. 2003

System rate

Telescope rate

Telescopes

Time

Increased Single-telescope rate		decreased threshold

Increased System rate		increased collection area

The Completed HESS Phase I System

First 4-fold event

December 10, 2003

An overview of the HESS observations

Plus other sources with less observation time ...

Publications underway

Prime Target - Crab Nebula

Our Calibration source: The Crab Nebula

October 2003 (3 tel.)

Live time: 4h1m53s

Mean zenith angle (Z): 47°

Energy Threshold: 325 GeV (at this Z)

Standard Analysis:

Significance: 53.5  (26.6 /hr-0.5)

Excess: 10.8 ± 0.2 .min-1

Signal easily detected,
stable in time

Preliminary

Preliminary

HESS Sensitivity, current standard analysis

Crab:

F(>1 TeV) =
(1.35±0.45) ×10-8 m-2 s-1

At 20º zenith angle, full array,

after selection cuts

Sensitivity::

0.01 Crab in ≈25 hrs

0.05 Crab in ≈1 hr

0.10 Crab in ≈20 min

0.50 Crab in ≈75 sec

1.00 Crab in ≈30 sec

Threshold (trigger, selected):

(105,125) GeV at 0º

(115,145) GeV at 20º

(265,305) GeV at 45º

(785,925) GeV at 60°

with current standard analysis

(threshold = peak in event rate	for Crab-like spectrum)

PKS2155-304

Prototype HBL (high-peaked BL Lac),
peak energy density in X-rays

Redshift z=0.116
 ~4 times more distant than Mrk421/501

Very bright AGN at all wavelengths, few bright emission lines
 previous observing campaigns in '90s (X-ray, optical, radio...)

First “Southern” TeV blazar (Durham collab., Chadwick et al, '99)

Observed by HESS in 2002/2003

Uncorrelated 2D Excess Map, Stereo Data

HESS flux measurements of PKS2155-304

HESS Preliminary

July 2002

October 2002

June-Oct. 2004

• Durham 1996/97

Model from Costamante et al., A&A 2002

Preliminary

45 s, (5.7 s/hr0.5)
1.20 ± 0.03 g/min

hard spectral index ...
... link to IR absorption ?

RXTE / HESS Observation Campaign

HESS Target of Opportunity (ToO) proposal for RXTE
on PKS2155-304, triggered by HESS on 18th October 2003

52 ksec in October, 19 ksec in November, 2-10 ksec/night

Quasi-simultaneous observations, ~14h in October (2/3 tel.), November still under analysis (some technical problems)

XTE light curves
2-10 keV

RXTE / HESS Observation Campaign

Blind-test comparison of HESS/RXTE data

XTE light curves
2-10 keV

per-pointing
(~30 minutes)

HESS light curves
>165 GeV

nightly average

run-by-run
(~30 minutes)

Preliminary

Future on PKS2155-304

More work to be done:

Investigation of short-term
flux variability

Cross-correlation of lightcurves

Spectrum and spectral variability

Source modelling

Comparison to other blazars at
varying z for IR background studies

Future campaign

August 2004

14 nights HESS
(larger zenith-angle range)

230 ksec RXTE

Other telescopes...

XTE light curves
2-10 keV

XTE Photon Index vs. Flux

• RXTE ASM
Useful for trigger,
not for correlation studies
• RXTE ToO (October 2003)
[2 - 10 keV] PCA
90 min orbit with Earth occultation
58 min / orbit on source
• HESS-RXTE campaign August 2004
230 ksec (63,8 h)
90 min orbit with Earth occultation
58 min / orbit on source
• NANCAY radio data

9 or 11 cm + 21 cm

• ROTSE optical data

A “New” source: Mrk 421

First extragalactic VHE source (Whipple, Punch et al., Nature, 1992).

Nearby: z=0.03, “Northern source” (dec. 38°12'31.8")

Source culminates at Zenith angle > 60° at HESS latitude

High threshold

Very large collection area, so
possibility to determine spectrum at highest energies

Strong activity seen by ASM aboard RXTE

Historical level of 110 mCrab in mid-April !!

Decreasing to 20-40 mCrab late April

April Campaign of observations also with
Whipple, Bordeaux radiotelescopes, others...

HESS Observations of Mrk 421

All Mrk 421 data taken with 4 telescopes

Average zenith angle 62°

Jan 2004, low-state

6  in 2.12h,
(vs. ~10 sigma/hr0.5 for Crab, @ Z=62°)

April 2004 active state !!

Excess of 8167 gammas in 11.5h

95.7 	(28.16 /hr0.5) !!!

~ 11.8 /min

1-2 Crab flux level (@ Z=62°)

Plots by A.Lemière
PCC Paris

Preliminary

Preliminary

Excess of 8167 gammas

ON	11891/1=11891

OFF	25055/6.73= 3724

excess	8167

Mrk 421 Time Variability in April

HESS Mrk 421 flux goes up to roughly ~3 Crab at maximum

Whipple first results reported at 3 Crab in April also

Bordeaux
radiotelescopes
show high-level,
but stable emission

Plot by M.deNaurois
LPNHE Paris

Campaign should provide much
information on this source
up to the highest energies

SN1006

Shell-type SuperNova Remnant

Observed by HESS in 2003 (14 h)

No signal seen by HESS

Signal has been claimed by CANGAROO

HESS Preliminary

CANGAROO
(Tanimori et al, 2001)
(1996: 5.3 ; 1997: 7.7)

SN1006

Shell-type SuperNova Remnant

Observed by HESS in 2003 (14 h)

No signal seen by HESS

Signal has been claimed by CANGAROO

0.5o

HESS Preliminary

CANGAROO
(Tanimori et al, 2001)
(1996: 5.3 ; 1997: 7.7)

SN1006: NE RIM (“Hot-Spot”)

HESS preliminary

Nuclear -ray
emission
 (NH)2 !
Parameters
favoured
from other
wave-bands:
D = 2.2 kpc
NH  0.05-
0.2 cm-3
Broad-band synchrotron + X-ray structure:
Bint=120 -
150 µG

SN1006 : Whole Remnant

HESS preliminary

Nuclear -ray
emission
 (NH)2 !
Parameters
favoured
from other
wave-bands:
D = 2.2 kpc
NH  0.05-
0.2 cm-3
Broad-band synchrotron + X-ray structure:
Bint=120 -
150 µG

Other Galactic Sources

PSR 1706-44
SuperNova Remnant
Upper limit...
See preceding talk from W. Hofmann

PSR B1259-63
Binary system with pulsar in eccentric orbit about Be star
Detection by HESS near to periastron
See following talk from M. Beilicke

Etc...

Conclusions

HESS-I installed and now functioning at full sensitivity

Most sensitive detector worldwide: 0.01 Crab in 25 h

Galactic Sources:

Crab, PSR 1259-53 detected

Upper limits on SN1006, PSR1706-44

Other sources, articles in preparation (publication embargo)

AGNs:

PKS 2155-304	Multi-wavelength campaigns

Mkn 421	Very large zenith angle observations

Many exciting results, on various sources ... papers to follow...

Conclusions

HESS-I installed and now functioning at full sensitivity

Most sensitive detector worldwide: 0.01 Crab in 25 h

Galactic Sources:

Crab, PSR 1259-53 detected

Upper limits on SN1006, PSR1706-44

Other sources, articles in preparation (publication embargo)

AGNs:

PKS 2155-304	Multi-wavelength campaigns

Mkn 421	Very large zenith angle observations

Many exciting results, on various sources ... papers to follow...

SkyMap: Due to the very huge signal the alpha factor is significantly overestimated resulting in underestimated excess/significances in the skymaps. Camera acceptance correction not yet applied skymap

run-by-run lightcurve: The gamma rate (= excess events / lifetime) is corrected for variations in triggerrate (weather, zenith angle). Note: The data quality has not yet been checked for individual runs (clouds, techn. problems, etc.) and the data is not yet calibrated, so the indication for very short term variations (from run to run) should not yet been interpreted. The gamma rate for the Crab at similar zenith angles (z=62 deg) is ~3 gammas/min (same analysis and cuts), so the Mkn421 flux goes up to roughly ~3 Crab at MJD 53114.

2004-05-14T10:35:06