The detection of the Crab Nebula via the Cherenkov imaging technique places TeV astronomy on a secure observational footing. The motivation for TeV observations, a discussion of the atmospheric Cherenkov technique, the experimental details of the Crab Nebula detection, and its scientific implications are presented. The present dilemma of VHE/UHE astronomy is that the Crab appears to be the only source whose showers match theoretical expectations. The situation will be clarified as improved ground-based detectors come on-line with sensitivities matching those of the GRO (Gamma Ray Observatory) instruments

Lamb, R. C.

English

NASA Technical Reports Server

N90-23306
TeV RADIATION FROM THE CRAB NEBULA AND OTHER MATI'ERS
R. C. LAMB, Iowa State University, Ames, Ia 50011
ABSTRACT
The detection of the Crab Nebula via the Cherenkov imaging technique places TeV
astronomy on a secure observational footing. This paper presents the motivation for
TeV observations, a discussion of the atmospheric Cherenkov technique, the
experimental details of the Crab Nebula detection, and its scientific implications. The
present dilemma of VHE/UHE astronomy is that the Crab appears to be the only
source whose showers match theoretical expectations. The situation will be clarified as
improved ground-based detectors come on-line with sensitivities matching those of the
GRO instruments.
I. SCIENTIFIC MOTIVATION
Gamma rays provide direct information about the highest energy processes which
occur in nature. The various bands of the gamma ray portion of the electromagnetic
spectrum are shown in figure 1. The instruments of the Gamma Ray Observatory will
cover the five decades from 0.1 MeV to 10 GeV with a factor of 10 to 20 improvement
over previous satellite instruments; however, the 2000 cm 2 detection area of the highest
energy instrument, EGRET, means that only a few photons greater than 10 GeV will be
detected. Thus gamma rays of higher energy must be detected via shower techniques,
either by means of the Cherenkov light from the cascade particles or by means of the
particles themselves. In figure 1 I have indicated the approximate range of the
Cherenkov technique, 0.1 to 10 TeV, and the band covered by extensive air shower
arrays. Gaps exist between each of these bands. Developing instruments which fill these
gaps is a major challenge for gamma ray astronomy above 10 GeV.
I I I I I I I i
10 6 10 7 lO s 10 9 10 t° 10 tt 10 tz lO ts lO t" 10 t_ IO ta 10 t7 lO ts 10 t_
Photon Energy (eV)
Figure i. The gamma-ray portion of the electromagnetic spectrum.
The cross-hatched portions indicate bands which are covered by
present instruments. Gaps exist for -- 10 to _ 100 GeV and for
5 to _ 50 TeV.
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Two major motivations for ground-based gamma ray astronomy are the
identification of sources of VHE and UHE cosmic rays and understanding relativistic
objects.
For energies up to a few times 1013 eV, acceleration of protons in supernova shock
waves is a favored explanation for the origin of cosmic rays, however this mechanism
fails for higher energies (Hillas 1984). An accurate enumeration of the total power
output by the TeV and PeV sources will be a step toward direct verification of the
shock wave hypothesis and may lead to identification of other sources of VHEAJHE
cosmic rays.
It is now clear that some of the VHEAJHE sources contain rapidly spinning, highly
magnetized neutron stars, in which a substantial fraction of the energy output is in the
form of relativistic beams. A well developed TeV astronomy should help determine the
particle identify of the beams (electronic or hadronic) and illuminate the nature of the
various acceleration mechanisms which produce the beams.
Progress in understanding relativistic objects will be maximized through a multi-
wavelength approach in which observations at satellite energies are complemented by
coordinated ground-based observations.
II. THE ATMOSPHERIC CHERENKOV TECHNIQUE
A photon with an energy above a few tens of GeV may be detected on the
ground by means of the air shower Cherenkov light. This light is emitted by those
charged particles in the shower which have velocities, v/c, greater than 1/n where n is
the index of refraction of the atmosphere. For a 10 I2 eV shower approximately 106
Cherenkov photons are emitted spread over a region on the ground of a few x 108 cm 2.
The information content of the Cherenkov photons is high quality both from the
point-of-view of the Cherenkov photon statistics and because these photons are
dominantly from the region of the shower's maximum development and therefore less
subject to shower-to-shower fluctuations.
This information in principle allows air showers from gamma-rays and cosmic
rays to be distinguished from one another, as illustrated by the Whipple Observatory
Collaboration's detection of the Crab Nebula discussed below. Because of this feature
the technique has the ability to prove that the shower primary is in fact a gamma-ray,
and we can look forward to future improvements in the Cherenkov technique is which
the unwanted cosmic ray background is reduced substantially beyond what has already
been achieved.
Because of the large collection area per independent Cherenkov collection
mirror, TeV detectors are intrinsically high-rate devices, with detection rates for the
brightest sources of a few photons/minute and burst rates even higher. This feature can
be used to search for pulsars that would be hidden at x-ray and radio-wave energies.
The mean free path of gamma-rays is 5 x 1025 H-atoms/cm 2 vs < 1022 for x-rays.
Radio pulses from x-ray binaries may not escape due to plasma absorption. Thus there
may be pulsars that are only detectable in gamma-rays. Recognition of this fact has
motivated several groups to search for a possible pulsar associated with Cygnus X-3
(Chadwick et al. 1985 and Zyskin et al. 1987)
136
The high-rate capability can also be used to explore short time scale phenomena
indicative of time varying acceleration and/or changes in beam-target geometry.
The ultimate capabilities of the atmospheric Cherenkov technique have not been
fully explored. Most recently Hillas (1989) has shown that angular resolutions of 2 - 3
arc minutes are possible with the technique with an ideal imaging detector.
III. DETECTION OF THE CRAB NEBULA
The Whipple Observatory's gamma ray telescope consists of a 10 meter reflector
in which an array of fast photomultipliers, located in the focal plane, is used to image
individual air showers. Showers are detected at a rate of approximately 3 Hz, more
than 99% of which are due to cosmic rays. The shower images are recorded and
gamma ray candidates are selected in off-line analysis, using a procedure d.etermined by
Hillas (1985) from Monte Carlo simulations tailored to the Whipple instrument.
A simple moment analysis of the pattern of light is used in which second
moments of the light around its centroid are used to derive size and orientation
parameters for each shower image. Simulations and observations agree that in the
original 37 element (0.5 ° pixel spacing) camera cosmic-ray background showers have a
mean size of 1.0 ° (full-width half height) whereas gamma-ray showers have a mean size
of 0.5 ° . Furthermore, gamma-ray showers are more elongated (except for showers
whose axis falls within 50 meters of the reflector). The elongation is such than the
major axis of the light is oriented preferentially radially from the center of the field-of-
view. These differences can be summarized by saying that the Cherenkov light images
from a gamma-ray source are more compact and point to the source.
Although six parameters were originally introduced by Hillas, one parameter, the
"azwidth", appears to be more effective than any other single parameter and as effective
as any combination of parameters. Azwidth is defined to be the rms width of the
shower images in the azimuthal direction of the image plane.
With this parameter and with the cut values of azwidth as set in advance by the
Hillas' simulations the Whipple group has published a 9 cr detection of the Crab Nebula
(Weekes et al. 1989) using a relatively coarse resolution camera (0.5 ° pixel separation)
and, more recently, an independent 15 cr detection (Lang et al. 1990) with a higher
resolution camera (0.25 ° pixel).
The distribution of azwidth values for the 9 tr detection (Weekes et al. 1989) is
shown in figure 2b, with a comparison of the expected behavior of azwidth values for
simulated gamma-ray and proton-initiated showers in figure 2a. The peak of the
observed azwidth distribution, near 0.4 °, is in agreement with the simulations for proton
showers. The only significant difference in the on-source and off-source distributions is
in the region of small azwidth as expected if the on-source region is a source of TeV
gamma rays. The difference of the two distributions in this region is shown in the inset
to figure 2b.
137
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Figure 2. (a) Distribution of AZWIDTH for simulated shower events
(Hillas 1985}: gamma-ray initiated showers, solid line (-);
proton-initiated showers, dashed line (--); (b) Distribution of
AZWIDTH for observed showers (Weekes et al. 1989). On source,
solid line (-); off source, dotted (...). The difference in the
two distributions is shown in the inset.
The Whipple's observations of the Crab Nebula have been subjected to a
number of tests for possible systematic errors. For example, the signal does not depend
on the order of the comparison between on-source and off-source regions. If the signal
were noise related then it should be most apparent for showers with total light near
threshold; it is not. There is a third magnitude star, Zeta Taurus, in the field of view of
138
the camera. In order to test for the effect of such a star on the Cherenkov images, a
control region of the sky was chosen in which a third magnitude star was present as a
false "on-Crab" region whereas no such star was present in the false "off-Crab" region.
Comparison of the two false regions showed no excess either before or after the
azwidth selection was made. Many other tests were made, as described in Weekes et
al. (1989), with similar results. The Whipple's detection of the Crab Nebula appears to
be a solid result of TeV astronomy.
Independent support for this result comes from the University of Michigan's
observations. With twin 11 meter diameter mirrors, each with 7 element cameras,
Akerlof et al. (1989) have a 5.8 or detection of the Crab Nebula produced by the
application of imaging type cuts on a raw 2.3 or excess.
There is no evidence for time variability on a month-a-month basis in the
Whipple's Crab signals. If we assume little or no variation over two decades it is
meaningful to compare the earliest Whipple Crab detection (Fazio et al. 1972) without
imaging with the more recent imaging results. Table I gives the comparison. As the
reader can see, an overall gain in sensitivity of a factor of 10 has been achieved, i.e. the
signal/background has improved by a factor of 100.
TABLE I
SENSITIVITIES OF DIFFERENT DETECTORS
FOR OBSERVATIONS OF CRAB NEBULA
IV. SIGNIFICANCE
The bulk of the radiation reported by both the Whipple group (Weekes et al.
1989) and by the University of Michigan group (Akerlof et al. 1989) is not pulsed at the
Crab pulsar frequency. The integral photon flux values from these measurements is
shown in figure 3, along with the extrapolations from lower energies of the COS-B
results (Clear, et al. 1987) for both the pulsed and unpulsed components. The TeV
points falls approximately midway between the extrapolations of the lower energy
observations.
The TeV observations and a synchrotron-self Compton model for TeV emission
(Rieke and Weekes 1969) constrain the magnetic field of the Nebula to be
approximately what is expected on the basis of energy equipartition arguments, namely
6x10 "4 G, Measurements of the differential energy spectrum of the source in this
spectral region should be forthcoming, and, in the context of the synchrotron-self
Compton model, determine the spectral behavior of the parent population of relativistic
electrons.
139
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Figure 3. The gamma-ray energy spectrum of the Crab nebula and
pulsar above 70 MeV.
V. OTHER SOURCES
More than a dozen other sources have been reported at both TeV and PeV
energies. See Weekes (1988) for a recent compilation. A potential major
embarrassment of the field of VHE/UHE gamma ray astronomy is that so far the Crab
Nebula is the only source for which the signals behave as expected of photons. The
optimist would say that this strong evidence that "new physics" is afoot. The
pessimist/realist would say that this is strong evidence that no source (other than the
Crab) has been seen. Bonnet-Bidaud and Chardin (1988) have expressed this latter
view in their review of Cygnus X-3. My own view is that the number of independent
sightings of such binary sources as Cygnus X-3, Hercules X-l, and Vela X-1 are too
140
numerous and too strong to ignore. The question of "new physics", however, is a much
bigger question and one that is a long way from being proved. As someone said
recently, "extraordinary claims require extraordinary evidence." The extraordinary
evidence is not at hand (yet).
The failure of other source signals to behave as expected of photons has been
emphasized by Hillas (1987) at the Moscow ICRC. At air shower energies two
traditional methods of reducing the background exist, one of which (muon content) is
on a secure calculational footing; however, the other method, which uses age cuts, is
not.
Calculations (Protheroe and Turver 1979; Stanev, Vankov, and Halzen 1985)
show that gamma-ray showers are relatively poor in muons (10% or less of the content
of proton showers of the same size); thus a cut on muon poor showers should improve
sensitivity. In fact the original Cygnus X-3 PeV detection (Samorski and Stamm 1983)
showed signals with a muon content roughly equal to that of background cosmic ray
showers.
The other selection technique used at air shower energies is an age parameter
cut. The use of this parameter has been called into question by simulation of PeV
gamma-ray and proton air showers by Hillas (1987). These simulations make two
points: 1) age is a relatively poor discriminant between gamma-ray and cosmic-ray
showers, and 2) what discrimination exists is such that gamma-ray showers are younger
on average rather than older than the cosmic-ray background showers of the same
energy. The selection of Samorski and Stamm picked showers with age values greater
than 1.3, relatively old and inconsistent with Monte Carlo expectations for gamma-rays.
The CYGNUS group, operating an air shower array at Los Alamos National
Laboratory, has reported evidence for burst signals from Hercules X-1 (Dingus et al.
1988) on a time scale of 30 minutes. The apparatus is sensitive to air showers of
energies greater than 50 TeV. Their signals are anomalous in that they are not poor in
muons.
Signals reported by the Whipple Collaboration at TeV energies from the
direction of Hercules X-1 (Reynolds et al 1990) are also anomalous in that they
disappear when the azwidth imaging cut is applied to them.
VI. TeV ASTRONOMY IN THE GRO ERA
The sensitivities of detectors operating in the VI-IE/-t./HE energy range are
improving. In addition to the Whipple instrument Cherenkov imaging detectors exist at
the Crimean Astrophysical Observatory (operational 1989) and the Yerevan Physical
Institute (construction begun 1988), and plans for a high angular resolution instrument
have been proposed by the JANZOS collaboration to be located in Australia. At
higher energies the CYGNUS collaboration is extending their array to a ground
coverage area of 0.8 x 105 m 2 and the Chicago, Michigan, Utah installation will have a
coverage of 2.5 x 105 m 2 within two years.
The Whipple instrument will be upgraded with the addition of a second reflector
11 meters in diameter (GRANITE project). It should be complete and ready to take
data in early 1991. Figure 4 shows the sensitivities of the new ground-based
instruments in comparison with the sensitivities expected of the GRO instruments. Note
that the vertical scale of the differential sensitivity has been multiplied by the 2.5 power
of the gamma-ray energy. As one can see there is a good match by the ground-based
instruments to the sensitivities of GRO instruments.
141
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gamma-ray spectrum.
A vigorous program of coordinated observations between GRO and the ground-
based Cherenkov receivers is anticipated. If GRO is pointed at a target of mutual
interest during a dark moon period and the source transits near midnight, then the on-
time of Cherenkov receivers can approach 25% under the assumption of good weather.
Thus a good overlap in simultaneous observations can be anticipated. Of course, for a
reasonably steady source like the Crab Nebula, simultaneity is not necessary. For the
Crab, many interesting questions regarding spectral changes for the pulsed and unpulsed
components can be addressed by combining data from EGRET and the most sensitive
of the Cherenkov receivers.
Acknowledgements: I wish to acknowledge helpful conversations with D.A. Lewis, G.
Vacanti, T.C. Weekes, and G.B. Yodh. This work was supported in part by the U.S.
Department of Energy.
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Akerlof, C. et al. 1989, Proc. of the Gamma Ray Observatory Science Workshop,
NASA/Goddard Space Flight Center, 4-49.
Bonnet-Bidaud, J.M. and Chardin, G. 1988, Physics Reports, 170, 325.
Chadwick, P.M. et al. 1985, Nature, 318, 642.
Clear, J. et al. 1987, Astro. Ap., 174, 85.
Dingus, B.L. et al. 1988, Phys. Rev. Letters 61, 1906.
Fazio, G.G. et al. 1972, Ap.J. Letters, 175, Ll17
Hillas, A.M. 1984, Am. Rev. Astron. Ap., 22, 425.
Hillas, A.M. 1985, 19th Int. Cosmic Ray Conf., La Jolla, 3, 445.
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Hillas, A.M. 1987, 20th Int. Cosmic Ray Conf., Moscow, ._, 262.
Hillas, A.M. 1989, Proc. of the Crimean Workshop on VHE Gamma Ray Astronomy,
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Lang, M.J. et al. 1990, 21st Int. Cosmic Ray Conf., Adelaide, paper OG4.3-2.
Protheroe, R.J. and Turver, K.E. 1979, J. Phys. G., _ 1613.
Reynolds, P.T. et al. 1990, 21st Inst. Cosmic Ray Conf., Adelaide, paper OG4.2-2.
Rieke, G. H. and Weekes, T.C. 1969, Ap.J., 155, 429.
Samorski, M. and Stamm, W. 1983, Ap.J. Letters, 268, 47.
Stanev, T., Vankov, Ch.P. and Halzen, F. 1985, 19th Int. Cosmic Ray Conf., La Jolla,
7_, 219.
Weekes, T.C. 1988, Physics Report, 160, 1.
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Zyskin, Yu. L. et al. 1987, Astron. Circ. Bur. Astron. Info., USSR Acad. Sci. No. 1508.
DISCUSSION
Carl Fichtel."
Will any of the other Cerenkov observatories have either a fine grid of photomultipliers
comparable to the Whipple one or two telescope system?
Dick Lamb:
The Crimean Astrophysical Observatory has a Cherenkov imaging system with a 37-
element camera which is essentially ready to begin observations. An imaging system is
under construction at the Yereban Physical Institute, and the JANZOS collaboration plans
a high resolution camera (pixel size 0.15 °) for operation in Australia.
R. Buccheri:
Does Whipple Observatory confirm the 12.6 ms pulsar detected by the Durham group in
the Cyg X-3 source?
Dick Lamb:
The Whipple Observatory collaboration does not confirm either the 12.6 ms pulsation
reported by Durham nor a 9.22 ms pulsation reported by the Crimean Astrophysical
Observatory.
143



TeV radiation from the Crab nebula and other matters

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