A facility is described which incorporates many of the best features of existing gamma ray telescopes at a single site. In addition to reducing the flux sensitivity in some energy ranges, it permits the measurements of the energy spectrum over seven decades of energy

Weekes, T. C.

English

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422 OG 9.4-6
AN OBSERVATORY TO STUDY I010 TO I017 EV GAMMA RAYS
Trevor C. Weekes
Whipple Observatory, Harvard-Smithsonian Center
for Astrophysics
Abstract
A facility is described which incorporates many
of the best features of existing gamma ray tele-
scopes at a single site. In addition to reducing
the flux sensitivity in some energy ranges, it per-
mits the measurement of the energy spectrum over
seven decades of energy.
i. Introduction. Ground-based gamma-ray astronomy has
matured from a discipline with a small number of detections
requiring confirmation to one in which there are several
sources that are well-established (Cyg X-3, the Crab,
Her X-l) but which now demand more detailed observations
before the emission mechanisms can be understood. In
at least one source, Cyg X-3, there is some question
concerning the nature of the observed particles. All of
the sources show evidence for time variability. Since many
techniques, whose absolute sensitivities are poorly under-
stood, have been employed and since many of the detections
do not have large statistical significance, it is difficult
to derive an energy spectrum that can be compared with
source models. The energy spectrum in the range 1014 -
1016 eV is particularly interesting since there is the
possibility of detecting the absorption dip caused by pair
production on the 3o K microwave background.
A single facility which combines the best features of
the many techniques used to cover the energy range from
i010 to 1018 eV would eliminate the ambiguity arising from
the use of different techniques at different epochs. Below
we describe in qualitative terms a Very High Energy Gamma
Ray Observatory and then list its advantages for each
decade of the energy range covered.
2. Physical Description. The observatory will consist of a
combination of pointed atmospheric Cherenkov telescopes and
all-sky particle detector arrays. It will be located on a
mountain plateau at an altitude of about 3.0 km. The array
will cover an area of approximately 1.8 x 108 m 2 which must
be reasonably flat for ease of construction and access. A
central control room will house the recording electronics,
the on-line computer, the atomic clocks and the operators.
(a) Atmospheric Cherenkov detectors. This array will
consist of a central large reflector (with aperture in the
range 15-20m) surrounded by six reflectors of 10m aperture
on the corners of a hexagon and at a distance of 75m from
the center (figure i). These atmospheric Cherenkov tele-
scopes will be modeled on the Whipple Observatory 10m
https://ntrs.nasa.gov/search.jsp?R=19850026660 2020-03-20T16:47:02+00:00Z
423 OG 9.4-6
Optical Reflector. Each will be composed of tessellated
segments but the f number will be larger to give a greater
depth of focus. Variable focal length segments will be
used to improve the optical figure and reduce the light
path difference across the reflector. The reflectors will
be protected by domes to preserve the mirror coatings and
also to permit the reflectors to operate as air Cherenkov
muon telescopes under cloudy or brightmoon conditions. Each
reflector will be equipped as an imaging device with an
array of phototubes at its focus. The number and size of
phototubes will be chosen so as to have a pixel diameter of
0.25 o and a field of view of 3.40 (e.g. 169 tubes of
diameter 4 cm). The telescopes will operate in parallel and
will track the position of a suspected source.
The particle detector array will consist of Im2 scin-
tillators with fast timing, arranged in a hexagonal 25m
grid spacing as shown in figure 2. The inner seven detec-
tors will be clusters of five scintillators; the total num-
ber of scintillators will be 65. When any three adjacent
detectors exceed a two particle level, a readout command
will be generated. The read out will include the pulse
size, the relative time of arrival at each detector and the
absolute time.
Figure i. Seven large
reflectors with spacing
of 75 m.
\
Figure 2. Layout of full ....
array. Large circles are
large reflectors; small circle_i ! _iii
%
are scintillators Grid
• _,_,,,-,,,,,,, ;,,_:_,_,,:_,,.,,,,_,
spacing is 25m. :..',':_:!_i':.',,',, ,_i:::I
_ii}''_" ,,¢¢_,,,/_',_¢_¢_,, ,;';;';,_;_',_ _,_,,,_
_:_,,
424 OG 9.4-6
3. Operating Mode. The facility will actually consist of
five separate gamma-ray telescopes, which will operate in
overlapping energy rangeswith the advantage that cross-
calibration will be straightforward. The ranges covered
are shown in figure 3 and the telescopes are described
below.
(a) Low energy atmospheric Cherenkov telescope; (LEACT)
in this mode the central light detector will be operated in
coincidence with one or more of the outer light detectors at
a very low threshold to detect the minimum energy gamma-ray
showers. Using fast electronics, small pixels and large
mirror areas, it will be possible to achieve an energy
threshold close to i0 GeV. This will have two important
advantages: (a) it will overlap with the upper end of the
coverage available with EGRET on the GRO (b) it opens up a
new, and previously unexplored, region of the electromag-
netic spectrum in which the background contributions from
Cherenkov light deficient proton-initiated air showers will
be small (Turver and Weekes, 1978). The light levels will
be too low for energies below i00 GeV to permit full use to
be made of the imaging technique but the reduced proton
background will compensate for the loss of angular resolu-
tion.
(b) Imaging Telescope (IT); this will operate on the
same principle as the existing telescope at the Whipple
Observatory in the energy range i011 to 1013 eV. However
each shower will be seen by at least three telescopes so
that both the angular resolution and collection area will
be improved: a factor of ten improvement in minimum flux
sensitivity is estimated. Recent Monte Carlo simulations
(Hillas, this conference OG 9.5-3) indicate that, for
showers that are well sampled, it will be possible to
differentiate the primary (electromagnetic or hadronic).
High energy (1013 - 1014 eV) events will saturate the
central pixels but their shapes and energies can be deter-
mined from the outer pixels.
(c) Wide angle Cherenkov detector (WACD); the cen-
troid of the Cherenkov _ght image for a gamma ray initiated
shower of primary energy 1012 eV, with impact parameter 100m
from the detector axis, falls io from the center of the
field. The lateral distribution of Cherenkov light falls
off beyond 100m. However a sensitive detector will have a
large collection area for showers of energy 1013 eV whose
impact parameters are in the range i00-250m; the centroid of
their light distribution will fall in the ring of phototubes
1.5o from the axis. By triggering on signals in the outer
pixels of the adjacent detectors the collection area for air
showers is in the range 1013 to 1015 eV will be _ 105 m2.
This large collection area (a hundred times greater than a
small particle array detector) will compensate for the small
duty cycle (<10%) of the atmospheric Cherenkov detector.
(d) Gas Cherenkov Muon Telescope (GCMT); the small
duty cycle of the atmospheric detectors can be increased by
425 OG 9.4-6
having the reflectors housed in light-tight domes (which
can be opened for night-sky use). Under dark conditions,
the imaging reflectors can easily detect the Cherenkov
light ring from single particles (muons) within the dome.
If the dome is large enough to permit the reflectors to
track sidereally, then sources can be tracked under cloudy
or moonlit conditions (or even during the day if the dome
is sufficiently light-tight). The collection area for
muons is essentially that of the optical collector 75m2
This mode would only be justified if it is shown that the
directional anisotropies detected in the direction of
Cygnus X-3 result from showers with a high component of
muons (Samorski and Stamm (1983); Marshak et al. (1985)).
(e) Particle Air Shower Array (PASA); at mountain
altitude this conventional array of particle detectors will
have an energy threshold close to 1014 eV and a high energy
cut-off dictated by exposure time and flux levels. Using
state-of-the-art fast phototubes and electronics, the angu-
lar resolutions will be _io. There will be considerable
overlap with the atmospheric Cherenkov detectors although
the particle array will act as an all-sky monitor and the
light-detectors will track specific sources.
PHOTON DETECTO_ PARTICLE DETECTO_
IJEACT .. PASA
IT
WACD
GCMT
i0 ii 12 13 14 15 16 17
LOG ENERGY ( eV )
Figure 3. Distribution with energy.
4. D.i_. The facility described above involves no
new technology; rather it requires a duplication and
concentration of existing telescopes at an optimum site.
It also requires an investment on a scale not normally
associated with ground-based cosmic ray experiments.
Marshak, M.L., et al. (1985), preprint.
Samorski, M., Stamm, W., (1983), 18th ICRC. Ii, 244.
Turver, K.E., Weekes, T.C. (1978), Nouvo Cimento. 45B, 99,


An Observatory to Study 10 (10) to 10 (17) EV Gamma Rays

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