Joint European and National Astronomical Meeting, Granada 2004
MAGIC sensitivity for pulsed emission
from gamma-ray pulsars
R. de los Reyes1 , J.L. Contreras1 and M.V. Fonseca1 for the MAGIC collaboration.
1
Dept. de Física Atómica, Nuclear y Molecular, UCM, Ciudad Universitaria s/n, Madrid, Spain
Abstract
Since their discovery, pulsars have been observed in a wide range of wavelengths. These data led to the development of several
theoretical models for the pulsed and unpulsed. The EGRET detector of the CGRO satellite measured pulsed emission from several
high energy pulsars up to energies around 20 GeV. The energy band between 15 - 300 GeV is at present unexplored. The ground-based
Cherenkov telescope MAGIC (Major Atmospheric Gamma Imaging Cherenkov telescope) located at La Palma, is able to observe below
100 GeV. Here we present (based on MC simulations) the final MAGIC sensitivity for pulsar detection and its dependence on the Earth
magnetic field.
γ-ray detection from pulsars
Pulsar models
•E< 30GeV → pulsed emission of 7 highconfidence pulsars ⇒ Satellites (EGRET).
The two main pulsar models
predict different γ spectra
cutoffs:
•E> 80 GeV → unpulsed component (Crab) ⇒
Cherenkov telescopes.
Sensitivity
Effect of Earth’s magnetic field
•Polar Cap: Ecutoff ~ 10 GeV
•Outer gap: Ecutoff ~ 100 GeV.
Rγ
Sσ = Q
tobs
The significance of an observation (Sσ) depends on the
Rbackg .
gamma and background rate, the observation time and the
quality Q of the analysis (a measure of the signal to background separation
power - includes selection cuts, time
windows, etc.).
Q ~ 33ms
In the case of the low energy photons
of the pulsed emission the Q factor is
dominated by the selected time
window and is about Q≅3 for Crab
(de Oña-Wilhelmi, 2000).
3ms
≅3
In both, the polar cap and the outer gap model, the flux can be expressed as
where K, Γ, E0 and b are free
parameters.
b
⎛
⎞
= K ⋅ E −Γ ⋅ exp⎜⎜ − ⎛⎜ E ⎞⎟ ⎟⎟m − 2 s −1GeV −1
E
0⎠
dE
⎝ ⎝
⎠
dNγ
It deflects the charged particles of the shower which give rise
to the Cherenkov photons collected by the telescope. This
deflection is caused by the component of the Earth’s
magnetic field perpendicular to the particle trajectory.
This effect results in a decrease in the number of collected
photons and therefore the telescope sensitivity.
Knowing the location of the γ-ray observatory, we can get
the value of the local magnetic field (Bx, Bz) and therefore the
value of its perpendicular component as a function of the
pointing direction (θ, ϕ).
B⊥ = Bx2 + Bz2 − (Bx sinθ cosϕ + Bz cosθ )
2
2
We can calculate also the zenith angle at
which the diffusion is minimal .
The right plot shows the
perpendicular component
of B in observer coordinates
(zenith and azimuth),
normalized to the maximum
value of B perpendicular.
The decrease of the MAGIC
effective collection area for
The most important parameters in the flux expression in the energy range 10-200
different directions (North
GeV are the normalization constant K and the spectral cutoff (E0). Given b=2
and South for 15 °, 30 ° and
(superexponential cutoff in the polar cap) and the slope Γ=2.08 and Γ=1.74 as
45 ° ZA) is shown in the plot
determined for Crab and PSR1951 respectively, the plots below show the time
below.
MAGIC would need to get a 5 sigma signal for each particular point in the (E0,
K) plane. The red circles mark the expected values for Crab and PSR1951,
computed from a fit to EGRET data.
Crab (Γ=2.08)
tgΘ min =
Bx
Bz
S
W
E
N
PSR1951+32 (Γ=1.74)
Pulsar
Extrapolating from the EGRET data based on the conservative
assumption of the polar cap model (Nel & de Jager, 1995), data for
the 7 EGRET pulsars are shown in the table. Observation times are Crab
only given for pulsars in the Northern hemisphere.
PSR1951+32
Geminga
PSR1706-44
Conclusions
PSR1055-52
MAGIC covers part of the γ-spectrum of pulsars.
Large effective collection area: possible discovery of new pulsars. Vela
θculm
(° )
E0
(GeV)
GeV)
7
4
11
73
81
74
30
40
5
----
R(Hz)
(θ=45°)
φ=0°
φ=180°
(South) (North)
0.25
0.10
0.30
0.14
0.
0.
0.40
0.20
0.02
0.006
3x10-4
9x10-5
tobs(hour) (Q=1, 5σ)
(Rp=70Hz)
φ=0°
φ=180°
(South)
(North)
30
180
20
100
∞
∞
-------------
References
Observation of pulsed emission may allow to discriminate between the two pulsar models.
Depending on its location and energy threshold, the sensitivity of the detector is also affected by the
Earth magnetic field. It will be important in the location of new gamma observatories (Ethreshold lower). • Nel & de Jager, 1995, APSS.
At La Palma: B is important for primary gammas and electrons (Θ>30° and E≤100 GeV), and negligible •Wiebel-Sooth, B., 1998, PhD thesis.
for protons. This has important implications for pulsars (∆tobs ≅ 20%).
•de Oña-Wilhelmi, 2004, PhD thesis.