A new trigger principle for
pulsar observation at low
energies
E. Oña-Wilhelmi1,2, O.C. de Jager1 & V. Fonseca2 for the
MAGIC collaboration
1
NWU, Potchefstroom, South Africa.
2 UCM, Madrid, Spain.
1
JENAM, Granada, 13-17 September 2004
Outlook
Aim of this technique.
Imaging vs. Pulsar Trigger.
Simulations with MAGIC input parameters.
Results
E = 100 MeV - 20GeV. 7 pulsars in EGRET.
E > 300 GeV. None
Eradio More than 1500 pulsars
Eoptical~5
Ground based Cerenkov Telescopes
Effective threshold energy ~ 30 - 100
GeV
(see V.Fonseca talk)
Gamma satellites give a nice crowded
picture of energies up to 10 GeV.
2
Searching for new γ-ray pulsars
GLAST launch (or start of operations)
Date: FEB 2007
Can we do overlapping work with
GLAST in the 10 GeV range without
having to construct new telescopes?
Spectrum
It is well known that GLAST will be the
best instrument to search for PULSED
emission.
MAGIC
60 GeV
Log(E) MeV
3
Pulsar Trigger
With the standard “Image technique”, 3
or 4 Nearest Neighbour (NN) trigger
reject small events (5 to 30 GeV).
For the “Pulsar Trigger” we reject all
the 3,4 coincident NN ⇒ NO EVENTS
FROM ANY PRESENT TELESCOPE
CONFIGURATION SURVIVES: too
high energy & too much background
Sum analog signal from all pixels in
central 0.5o camera radius.
MAGIC collaboration
4
Pulsar Trigger technique
1) Sum total analog signal in area to
give total charge:Q=ΣRqi (R≤ 0.5º)
2) Trigger signal in 2 - 3 ns if Q > NSB +
7(NSB)1/2
3) Record timestamp and Q only if
event was not seen by the normal NN
(“Hillas mode”)
5 GeV γ-ray shower @ 2 km
ANGULAR DISTANCE FROM CENTER OF
CAMERA (DEGREES)
4) ⇒ Nearest Neighbour or Hillas Mode
serves as an anti coincidence shield
(high background/large energy
events).
Low energy showers
develop high in the
atmosphere
5
Monte Carlo Simulation
Input Parameters:
Altitude ~ 2 Km.
Mirror Area (Reflectivity ~ 90%)= 234 m2
PM quantum efficiency ~ 0.3
Shower Photon Density @ 5 GeV ~ 0.2 ph m-2
Night of the Sky Background (NSB) (300-600 ns): 2 to 4 x 108 ph/cm2/s/sr
1 Hz NSB accidental
trigger in 3 ns
6
Effective Area for 2 to 10 GeV
Select a trigger threshold (TT) of ~ 7
sigma above the NSB to give an
accidental rate 1 Hz.
⇒ TT = <NSB> + 7<NSB>
We exploit the large fluctuations of
low energy γ-rays (where rms ~
mean) to get above the TT
Effective Area:
Aeff = ∫ ε ( E , r ) dA , ε = trigger efficency
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Effective Area for 2 to 10 GeV
8
Background (1): Cosmics
Main contribution comes from protons. Rate ~260 Hz
Rejecting all the 4 NN trigger events: Rate ~ 60 Hz
Low energy events do not
survive the 4NN condition:
⇒ The pulsar trigger
efficiency above 20 GeV
decreases while it holds up
for lower energies.
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Background (2): NSB
Pulsar trigger is highly sensitive
to the LONS level.
Dark region
of the sky
5 GeV Collection Area
versus Night Sky
Background Level for an
accidental trigger rate of
0.1 Hz, which is much
less than the proton rate
of 60 Hz.
Area used
Crab for pulsed
region sensitivity
calculations.
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Combine the low threshold capability derived from
First Cerenkov Telescopes Generation with the noise
rejection capabilities of the Third Generation.
1.9
 E  2
 m
A(E, I dark region) = 69⋅ 
 1GeV 
 E
A( E , I bright region ) = 14 ⋅ 
 1 GeV
2 .4

 m 2

Using EGRET spectra we can infer the expected rate and
observation time.
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Observation time for different
sources detected by EGRET
Pulsar spectra are characterised for a cutoff at a few GeVs
Source
Cutoff (GeV)
Rate (Hz)
Tobs (min)
Crab
30
2.2
5
Vela
8
5.5
<1
Geminga
5
1.7
9
PSR B 1951+32
40
1.1
20
PSR B1706-44
40
2.3
5
3c279
--Good for AGNs
290
<0.112
Conclusions
New trigger method yields large collection areas above 2
GeV (1000 to 4000 m2).
It requires modification in the electronics without
interfering with existing mode of operation.
Sensitive to NSB - becomes unstable if NSB varies too
much during source tracking.
It applies too to AGNs and high redshift Blazar.
Possible application to smaller sized telescopes (<17 m) to
reduce the Energy threshold.
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Thank you!
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