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Gamma-ray astronomy at the highest energies
Nuclear Instruments and Methods in Physics Research A280 (1989) 349-357 North-Holland, Amsterdam
349
G A M M A - R A Y A S T R O N O M Y AT T H E H I G H E S T E N E R G I E S Trevor C. W E E K E S Whspple Observatory, Harvard-Smithsonian Center for Astrophysics, Box 97, Amado, AZ 85645-0097, USA
One of the major astronomical discoveries of the past decade has been the detection of gamma rays of energy in excess of 0.1 TeV from a variety of galactic sources. These detections were made with ground-based telescopes using the earth's atmosphere as the detecting medium. The sources detected include the Crab Nebula (a 940 year old supernova remnant), some millisecond radio pulsars (rotating neutron stars), and binary X-ray sources (close-binaries systems containing accreting neutron stars).
1. Introduction As this is the only talk on the program that is concerned with any form of astronomy, it may be appropriate to preface it by a few words on high-energy astrophysics (HEA), that subdiscipline that is concerned with the most energetic, the most violent and, therefore, the most exciting areas of astrophysics. A source falls into the HEA regime if its luminosity far exceeds the norm for that class of object; thus a supernova is in, but a Main Sequence star which is producing energy by normal thermonuclear burning is out. A phenomenon is included if it involves the production of high-energy quanta even if the total luminosity is small. In general, thermal processes are excluded whereas nonthermal processes are the heart of the discipline. Also the astronomy of high-energy quanta, e.g. cosmic-ray particles or gamma rays, is by definition HEA. Astronomical research differs from the other branches of physical science in that it is exclusively an observational, rather than an experimental, science. The remoteness and scale of the source means that the astronomer must always play a passive role and must rely on observations (which often cannot be repeated) to give some insight into the processes under investigation. It is essential that the observations be carried out with the maximum sensitivity at every possible wavelength; not all of the electromagnetic spectrum has been available to the astronomer. Fifty years ago the available band was restricted to about 3500-7000 ,~. Today almost the entire electromagnetic spectrum from the longest radio wavelengths to the shortest gamma-ray wavelengths can be exploited. Here we will be concerned with the highest-energy quanta that have ever been observed - energies that exceed those that can be generated with our most efficient man-made particle accelerators. It goes without saying that the sky seen at different wavelengths is very different. Not only are the types of 0168-9002/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
objects that dominate the sky quite distinct but the time scales of the phenomenon observed change dramatically. As might be expected, as energies increase the time scales become shorter, a measure of the instabilities associated with the production of high-energy particles. In the energy region of interest to the HEA astronomer one would expect to see a sky with few sources, but these will all be powerful and many of them will be variable on short time scales. The astronomer is not allowed to program the phenomenon under investigation. This is not a serious limitation for many of the branches of classical astronomy where the time scales are longer than the epoch of modern astronomy and often millions to billions of years. In HEA the situation is quite different; without warning a previously quiescent X-ray binary source may give an outburst of radiation that makes it the brightest source in the sky in some wavelength band for a period of hours to days. Only if telescopes are trained in that direction at that time will the phenomenon be recorded and, if the source is poorly observed, e.g. with marginal statistics, then there is no second chance as it may never undergo another outburst or, if it does, it may have quite different characteristics. The consequence of this transient behaviour is that the cardinal rule of scientific research, that the results should be verifiable, may not always apply. The statistical significance of unexpected transient emissions thus becomes a controversial issue. The fundamental motivation to do gamma-ray astronomy at the highest energies is fairly obvious. Not only is it important to exploit every possible decade of the electromagnetic spectrum but the decades at the extreme end of the spectrum offer unique insights into high-energy phenomena.
2. Gamma-ray production and absorption It is almost inevitable that, if an object is the source of high-energy particles (or is a reservoir of high-energy III. FUNDAMENTAL RESEARCH
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particles produced elsewhere), it will also be a source of high-energy g a m m a rays. Whether the particles be electrons or hadrons, there will be conditions under which as much as 10% of the total energy will emerge in the form of high-energy photons. Like neutrons and neutrinos these quanta will travel in straight lines from the source to the detector and thus make high-energy astronomy possible. However, they are easy to detect (unlike neutrinos) and stable (unlike neutrons); also, as we shall see below, they are not easily absorbed and are hence very suitable probes of the sources of high-energy cosmic radiation. Relativistic electrons are relatively short-lived in most astrophysical situations, losing their energy by synchrotron radiation in weak magnetic fields, by curvature radiation or by pair production in strong fields, by bremsstrahlung in the presence of nuclei or by inverse C o m p t o n scattering on low-energy photons. All of these processes can give rise to secondary gamma rays, in some cases with energies only slightly less than the primary electrons. Relativistic hadrons, e.g. cosmic-ray protons, are longer-lived but in the presence of other hadronic matter, e.g. interstellar hydrogen, will interact to produce mesons which will eventually produce muons, electrons, neutrinos and g a m m a rays. The detailed study of the energy spectrum, spatial distribution and time variability of gamma-ray sources can thus elucidate the nature of the progenitor particles, the photon and particle densities in the source regions, and the magnetic fields. There is also the possibility that at these very high energies some new processes may come into play and thus these cosmic accelerators may proffer an insight into some new physics. Although high-energy gamma-ray photons may interact and thus be absorbed within the confines of a source, the conditions in interstellar and intergalactic space are such that the emitted beam of g a m m a rays is little attenuated. There is one process that is important for extragalactic sources and unique to this environment. A high-energy photon, E a, can pair-produce on a low-energy photon, E 2, if the condition E I E 2 > (mec2) 2 is satisfied. For photons of energy 0.1-1.0 PeV this condition is satisfied when they interact with the relatively dense (10 3 p h o t o n s / c m 2) 2.7 o black-body radiation field, the relic of the Big Bang. Even for a relatively close extragalactic source, e.g. the closest radiogalaxy, this is a large effect (fig. 1). This process is also serious for optically bright galactic sources where the absorption occurs between TeV photons and optical photons close to the source. This absorption process has not yet been observed in the laboratory or from a cosmic source. As several authors have pointed out, the observation of this absorption feature in a cosmic source of gamma rays would verify the physical process as well as demonstrate
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the physical extent of the microwave black-body field. It would also give an indication of the intergalactic magnetic field and could, in principle, provide an independent method of measuring distances to nearby extragalactic sources.
3. Detection technique Although for all effective purposes the earth's atmosphere has a shielding power equivalent to a 1 m thickness of lead for all radiation in the X- and gammaray regimes, it is still possible to do gamma-ray astronomy at energies above 0.1 TeV from ground level. This is fortunate because even the large E G R E T detector on the G a m m a Ray Observatory (scheduled for launch in 1990) has an effective collection area of only 1000 cm 2. Although this is a factor of 20 greater than the collection area of any gamma-ray detector flown to date, it is still so small that at energies above 1 TeV even the strongest source known would give less than one observable photon per year. Ground-based detection is made possible by the generation of an air shower in the atmosphere whose secondary components survive at least until mountain altitude. At energies in excess of 0.1 PeV these can be detected with simple arrays of particle (electron) detectors. Arrival directions can be established by fast dif-
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The general characteristics of a very-high-energy (VHE) gamma-ray source are easy to define. There must be both a source of high-energy particles whose energy exceeds that of the gamma rays under study and there must be a target (beam dump) where the particles can interact to produce gamma rays. The particles can be electrons or hadrons and the target material can be gas (for bremsstrahlung or pion production), photons (for inverse Compton scattering) or a magnetic field (for synchrotron, curvature radiation or pair production). The actual mechanism for particle acceleration can be shock acceleration, direct acceleration across a large potential drop, or some other mechanism that has yet to be discovered. For practical purposes the accelerator can be considered a black box, although a prime motivation for the study is to understand the accelerating mechanism. The target must be thick enough to provide effective conversion efficiency but sufficiently thin to permit the gamma rays to escape. For instance, the most efficient thickness for gamma-ray production via production in the collision of relativistic protons with hydrogen gas is 50 g / c m 2. In this case up to 10% of the total energy in relativistic protons can emerge as VHE gamma rays with the average photon energy about 0.1 that of the incident proton. In some cases the acceleration and gamma-ray production can occur in the same region. Potential VHE gamma-ray sources can be chosen from lists of objects in which great amounts of energy are being released relative to the total rest mass or where observations at other wavelengths point to the presence of particles radiating by nonthermal processes. Supernova explosions (both at the time of the explosion and in the years following) are obvious candidates as are objects with apparent jets of relativistic particles (e.g. the galactic binary sources SS433 and Cygnus X-3, the radio-galaxy M87 and the quasar 3C273). In a supernova remnant the obvious source of highenergy particles is the rapidly spinning pulsar at its center (although there is no standard model for pulsar particle acceleration). The target material can be either the photons in the nebula or the gaseous shell thrown off by the explosion. In binary X-ray sources there is no obvious way in which the compact object can accelerate particles since the rotation rate is rather small; hence the source of energy is most likely accretion rather than rotation. The target material can be either the outer limb of the companion star, the edge of the accretion disk or the accretion wake.
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= 100 TeV-PeV) ranges. ferential timing between the spaced detectors. A simple array can have a collection area of 103 m2 and an angular resolution of 1°. If the air shower array indudes some muon detectors, then gamma-ray primaries can be identified by the low muon-to-electron ratios at detector level. The secondary particles in the air shower travel at velocities in excess of the speed of light in the atmosphere and hence cause the emission of Cherenkov radiation. This is beamed in the direction of the particle (the Cherenkov angle at sea level is - 1 . 3 ° ) , is concentrated in the blue and has a yield of about 8 200 photons per radiation length. At sea level the Cherenkov light component arrives as a pancake of photons with a diameter of 200 m and 1 m thickness (fig. 2). For a 1 TeV gamma ray the photon density is about 100 p h o t o n s / m 2. The air shower can be easily detected with a simple light receiver consisting of a mirror, a phototube and fast pulse-counting electronics. In the first instance the air shower resulting from a hadronic primary is similar and gamma-ray sources can only be identified as angular anisotropies. Recently techniques have been developed that identify the gamma-ray primary on the basis of the smaller angular size of the Cherenkov light image. In the past five years there has been a dramatic increase in the number of active ground-based gammaray observatories; the improvement in flux sensitivity
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T.C. Weekes / Gamma-rayastronomy at the highest energies
No attempt will be made to give a comprehensive description of the gamma-ray sky at very high energies. A catalog of sources and a complete description can be found in a recent review [1] where an extensive bibliography of the subject can be found. Galactic sources fall loosely under the headings: supernova (and supernova remnants) and close-binary X-ray sources. Two objects from each of these classifications will be discussed: it is no coincidence that three of these are the best established sources and the fourth is strongly predicted to become a source. 5. Supernova and supernova remnants
5.1. Supernova 1987a in the Large Magellanic Cloud If theoretical astrophysists were asked to make a short list of the objects most likely to be detected at energies greater than 100 GeV, it is likely that SN1987a would be near the top of their list. Although just outside the confines of our galaxy, it is still the closest supernova to be seen since the invention of the telescope, 300 years ago. Early models had suggested that the shock wave might give rise to a short burst of gamma-ray emission during the actual explosion. This would be of very short duration. If the supernova is to form a neutron star which becomes a regular fast-spinning radio pulsar, then it is very likely that the remnant will be a source and reservoir of relativistic particles. Initially the high-energy particle beam will be obscured by the thick expanding shell thrown off by the outburst; as the target thins it will gradually become an ideal gamma-ray target so that the presence of the particles (and the pulsar) will become evident by the production of a detectable beam of gamma rays at energies of 1 TeV and greater. As the pulsar slows down the strength of this beam may decline; it is predicted to have its maximum intensity 1-10 years after the outburst [2,3]. Given the small number of VHE observatories in the Southern Hemisphere it is unlikely that a telescope would be pointing at the supernova at the time of its outburst. The University of Durham group was operating a telescope in Narrabri, Australia the night of the outburst and by coincidence happened to be observing another source in the Large Magellanic Cloud so that the supernova was on the edge of their field of view. No detectable signal was recorded [4]. The same negative result has been reported from several observatories in the first year after the outburst [4]. The implication is that a pulsar has not formed or that the shell has not yet thinned sufficiently to permit the gamma rays to escape.
period = 33 ms) pulsar and to be a reservoir of electrons with energies up to 10 TeV. It was thus one of the earliest candidates for gamma-ray emission [5]. Satellite observations established that both the pulsar and the nebula were sources of 0.1 GeV gamma rays. There were early reports that this was also true at 0.1-1.0 TeV energies. It has proved difficult to detect a consistent signal at these energies from the pulsar; the conclusion therefore is that the pulsed flux that exhibits some variability at lower energies is much more variable at TeV energies. The same degree of variability is not expected from the nebula. The detection of TeV gamma rays from the nebula was first reported in 1972 [6]. This was confirmed by the Whipple Observatory Collaboration using a 10 m optical reflector as a large low-resolution fast camera. With 37 elements (each with a pixel scale of 0.5 o ), the group used the newly developed atmospheric Cherenkov imaging technique to select gamma-ray images based on their predicted properties [7]. More than 98% of the cosmic-ray background was rejected by this discrimination so that a signal that was only 0.2% of the total background was detected at the 9o level of significance. This observation is based on 100 hours of observation taken in 1986-88 and it confirms the flux levels determined in earlier observations indicating that this is a steady source; although weak, it is suitable for use as standard candle for comparing and improving the sensitivity of the atmospheric Cherenkov telescopes. The observed flux (fig. 3) can be explained in terms of the Compton-synchrotron model of the nebula [5] or by some sort of steady emission from the pulsar [8]. This detection is unusual in this field in that it combines good statistics (9a), good physics (the detected primaries are, by definition, gamma rays) and
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One of the major achievements of X-ray astronomy was the detection of a class of objects that have come to be known as close binaries. They consist of a compact object (neutron star or black hole) in close proximity to a normal star from which the compact object draws matter by accretion; as the matter falls into the compact object it forms a very hot accretion disk which is the source of the X-rays. Many of the neutron stars have detectable rotation periods, but these are less than those found in normal isolated radio pulsars. Because accretion gives off energy primarily by thermal radiation and because the rotation rates are low, it was not expected that there would be any high-energy particle acceleration in these sources despite the high X-ray luminosity. Therefore they did not figure in lists of potential sources in the early seventies and it was only with the discovery of emission from Cygnus X-3 (whose binary nature is in doubt) that they were studied. More than 20 of these galactic sources have well-established orbital and spin parameters; some of these are shown to scale in fig. 4 where the sources that have been observed to emit TeV gamma rays are so marked. Here we will only discuss two of the best-established sources, one of which may not belong to this class of objects at all.
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electrons) and because the one experiment that measured the muon-to-electron ratio indicated that the putative gamma-ray signal had a ratio more in accord with hadron primaries than gamma rays. This was followed by several underground experiments which appeared to detect the source in muons only; these results, which have been disputed, await explanation and confirmation. Recently the Fly's Eye group (who operate a large optical detector in Utah which detects very large air showers by atmospheric fluorescence) have reported [10] the detection of a flux of 0.5 EeV particles from the Cygnus X-3 direction (fig. 7). At these high energies the primaries could be neutrons but it seems most likely that they have the same nature as the primaries observed at lower energies. The most interesting consequence of these Cygnus X-3 observations may be the consideration of the total power that must be going into the high-energy particles (presumably hadrons) if we assume the observed primaries are gamma rays. If we consider just the particles at PeV energies we can calculate the total luminosity in PeV ganuna rays assuming a distance to the source of at least 10 kpc. Gamma-ray pair production in interstellar space introduces a factor of 3 absorption which must be corrected for; also the duty cycle is only 10% and if it assumed that the modulation is caused by the presence of the target, not an intrinsic beaming in the particle production, the observed luminosity must be of the order of 4 × 10 37 erg/s. The production efficiency, if pion production in proton-proton collisions is involved, must be less than 10% and the average energy of the progenitor particles must be a factor of 10 greater than the energy of the observed gamma rays.
T.C. Weekes / Gamma-ray astronomy at the highest energies
The inferred production of cosmic rays in the energy band from 10 to 100 PeV is then 4 × 103s erg/s. This is far greater than the X-ray luminosity and, more importantly, about equal to the cosmic-ray production rate necessary to replenish the cosmic-ray reservoir in the Galaxy (making reasonable assumptions about volume and lifetime [9]). Thus, if Cygnus X-3 were to produce gamma rays at the rate observed in the early eighties, it would appear to be sufficient to keep the Galaxy filled with cosmic rays, at least at these high energies. In practice, of course, because of the distance it is unlikely that the cosmic rays presently observed at earth could have come from Cygnus X-3 which is probably passing through a short-lived high-luminosity phase. Models for particle production in Cygnus X-3 are highly speculative and await a clearer understanding of the composition of the object as a whole. Much depends on whether the compact object, which should be present if the system is a close binary, is a fast pulsar; claims for the detection of periodicity in TeV gamma rays at 9.22 and 12.59 ms [11,12] have not been confirmed.
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three well-defined periodicities: a 1.24 s period (detected in X-rays and in visible light) that is associated with the neutron star rotation, a 1.7 day modulation (seen in X-rays and visible light) that is clearly the orbital motion of the star and a 35 day variation (seen only in X-rays) characterized by two active phases of emission which may be associated with precession of the accretion disk or the neutron star. In addition the system exhibits extended periods of low X-ray luminosity. The detection by the Durham group in 1983 [13] of a short (3 min) burst of pulsed radiation from the system was a surprise; it has since been seen as a short-lived source of 0.6-100 TeV emission on 20 occasions by five different groups. In each case the emission has been pulsed at a period close to the neutron star rotation period. In general the methodology used in studying this source has been first to detect a general increase in counting rate from the direction of the source; this is usually not statistically significant in itself but defines the duration of the putative outburst. This data is then subjected to a periodicity analysis using either epochfolding or the Rayleigh test (a form of Fourier analysis) at periods close to that of the Hercules X-1 pulsar period. The detection of a signal then depends on finding a power in this region that is statistically significant. An example of a typical detection (by the Whipple Observatory group) is shown in fig. 8. These detections are generally not very strong statistically; it is reassuring to find that on the one occasion that two TeV telescopes were trained on the source during an active phase both saw the same pulsed signal [9]. An attempt to correlate the active episodes of emission with phase on the 1.7 day and 35 day periods is shown in fig. 9. There is no clear correlation with orbital phase but it does appear that the TeV emission comes primarily during the "ON" periods of the 35 day cycle. There is as yet no completely satisfactory explanation of the origin of these episodes of emission nor is there any agreement on which is the mechanism by which particles are accelerated in an accreting system. One feature of the observations that any model must explain is that the frequency of the TeV gamma-ray emission is usually slightly to the blue of the X-ray emission, which is presumably the rotation frequency of the neutron star. This was particularly pronounced in a TeV observation on June, 1986 which was reported by the Whipple group in 1987 [14,15]. An episode lasting 25 minutes was found to have a strong periodic component at a frequency 0.16% off that of the pulsar. Although this was the strongest of the eight detections reported by this group, the frequency offset reduced the statistical significance to 0.9%. The same frequency was reported by the Haleakala group in an observation on May 13, 1986 [16]. The energy threshold claimed was
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Fig. 10. Power as a function of period for three sets of gamma-ray observations: (a) Haleakala Observatory, 13 May 1986, E = 0.2 TeV, P = 0.7% [16]; (b) Whipple Observatory, 11 June 1986, T = 0.6 TeV, P = 0.9% [15]; (c) Los Alamos A.S.A., 23 July 1986, E = 100 TeV, P = 0.002% [17]. The X-ray period is shown by the dashed line (from ref. [18]).
0.2 TeV and the chance probability was estimated at 0.7% (see fig. 10). The Los Alamos Air Shower Array was operating at this time also and they reported the detection of two 30 min bursts pulsed at this same offset frequency on July 23, 1988 [17]. Their energy threshold was 100 TeV and the statistical significance of the detection (independent of the earlier observations which might have been taken to define the expected period) was 2 × 10 -5. These three sets of observations taken over a period of three months constitute a strong case for the detection of 0.5-100 TeV emission from Hercules X-1. However, the Los Alamos detector has a large muon detector and when the response of this detector is used to determine the muon-to-electron ratio of the 14 showers that constitute the pulsed signal, it is found that the ratios are, if anything, greater than the average response of the array to hadronic showers. Similarly the Whipple Observatory group do not find the same photonic character in the Hercules X-1 signal images as they do in their signal from the Crab nebula; however, in this case the statistical significance is not as great as that of the Los Alamos
T.C. Weekes / Gamma-ray astronomy at the highest energies result. The Haleakala experiment does not discriminate between photons and hadron primaries. The implication is that the measured signal comes not from g a m m a rays, but from some other neutral component of the cosmic radiation. The problem is that there is no known component that could give this signal. Neutrons would have decayed en route and neutrinos would not have a high enough cross section to give sufficient showers in the atmosphere. An alternative explanation might be that there are some processes by which muons can be produced in air-shower interactions at energies above 1 TeV. Either explanation implies some new physics - and the offset frequency at which the pulsations are seen implies new astrophysics as well! This has increased the interest in further observations of this fascinating system with detectors with greater sensitivity.
7. Outlook The number of ground-based gamma-ray telescopes in both hemispheres has increased dramatically in the last few years so that one can expect a major leap forward in our understanding of the very-high-energy universe in the next decade. Outstanding questions such as the nature of the primaries at all energies should be resolved and the catalog of well established sources should grow. With the launch of satellites, G A M M A - 1 and G R O , the next decade may well be the decade of gamma-ray astronomy.
357
References [1] T.C. Weekes, Phys. Pep. 160 (1988) 1. [2] T. Gaisser, A. Harding and T. Stanev, Nature 329 (1987) 314. [3] V.S. Berezinsky and V.L. Ginzburg, Nature 329 (1987) 807. [4] P.M. Chadwick et al., Astrophys. J. Lett. 333 (1988) L19. [5] R.J. Gould, Phys. Rev. Lett. 15 (1965) 511. [6] G.G. Fazio, H.F. Helmken, E. O'Mongain, G.H. Rieke and T.C. Weekes, Astrophys. J. Lett. 175 (1972) Ll17. [7] P. Kwok et al., in: Proc. NATO Advanced Study Institute on Cosmic Gamma Rays and Cosmic Neutrinos, Erice, Italy, April 20-30, 1988 (Reidel, Dordrecht, 1989) in press. [8] K.S. Cheng, private communication, 1988. [9] R.C. Lamb and T.C. Weekes, Science 238 (1987) 1528. [10] G.L. Cassiday et al., Phys. Rev. Lett. 62 (1988) 383. [11] P.M. Chadwick et al., Nature 318 (1985) 642. [12] A.A. Stepanian, private communication, 1988. [13] J.C. Dowthwaite, A.B. Harrison, I.W. Kirkman, H.J. MacRae, K.J. Orford, K.E. Turver and M. Walmsley, Nature 309 (1984) 691. [14] R.C. Lamb et al., Bull. Am. Astron. Soc. 19 (1987) 1. [15] R.C. Lamb et al., Astrophys. J. Lett. 328 (1988) L13. [16] L.K. Resvanis et al., Astrophys. J. Lett. 328 (1988) L9. [17] B. Dingus et al., Phys. Rev. Lett. 61 (1988) 1906. [18] R.J. Protheroe, Proc. 20th Cosmic Ray Conf., Moscow, August 2-15, 1987, 8 (1987) 21.
Acknowledgements This research in V H E gamma-ray astronomy is supported by the U S Department of Energy and the Smithsonian Scholarly Studies Fund.
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G A M M A - R A Y A S T R O N O M Y AT T H E H I G H E S T E N E R G I E S Trevor C. W E E K E S Whspple Observatory, Harvard-Smithsonian Center for Astrophysics, Box 97, Amado, AZ 85645-0097, USA
One of the major astronomical discoveries of the past decade has been the detection of gamma rays of energy in excess of 0.1 TeV from a variety of galactic sources. These detections were made with ground-based telescopes using the earth's atmosphere as the detecting medium. The sources detected include the Crab Nebula (a 940 year old supernova remnant), some millisecond radio pulsars (rotating neutron stars), and binary X-ray sources (close-binaries systems containing accreting neutron stars).
1. Introduction As this is the only talk on the program that is concerned with any form of astronomy, it may be appropriate to preface it by a few words on high-energy astrophysics (HEA), that subdiscipline that is concerned with the most energetic, the most violent and, therefore, the most exciting areas of astrophysics. A source falls into the HEA regime if its luminosity far exceeds the norm for that class of object; thus a supernova is in, but a Main Sequence star which is producing energy by normal thermonuclear burning is out. A phenomenon is included if it involves the production of high-energy quanta even if the total luminosity is small. In general, thermal processes are excluded whereas nonthermal processes are the heart of the discipline. Also the astronomy of high-energy quanta, e.g. cosmic-ray particles or gamma rays, is by definition HEA. Astronomical research differs from the other branches of physical science in that it is exclusively an observational, rather than an experimental, science. The remoteness and scale of the source means that the astronomer must always play a passive role and must rely on observations (which often cannot be repeated) to give some insight into the processes under investigation. It is essential that the observations be carried out with the maximum sensitivity at every possible wavelength; not all of the electromagnetic spectrum has been available to the astronomer. Fifty years ago the available band was restricted to about 3500-7000 ,~. Today almost the entire electromagnetic spectrum from the longest radio wavelengths to the shortest gamma-ray wavelengths can be exploited. Here we will be concerned with the highest-energy quanta that have ever been observed - energies that exceed those that can be generated with our most efficient man-made particle accelerators. It goes without saying that the sky seen at different wavelengths is very different. Not only are the types of 0168-9002/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
objects that dominate the sky quite distinct but the time scales of the phenomenon observed change dramatically. As might be expected, as energies increase the time scales become shorter, a measure of the instabilities associated with the production of high-energy particles. In the energy region of interest to the HEA astronomer one would expect to see a sky with few sources, but these will all be powerful and many of them will be variable on short time scales. The astronomer is not allowed to program the phenomenon under investigation. This is not a serious limitation for many of the branches of classical astronomy where the time scales are longer than the epoch of modern astronomy and often millions to billions of years. In HEA the situation is quite different; without warning a previously quiescent X-ray binary source may give an outburst of radiation that makes it the brightest source in the sky in some wavelength band for a period of hours to days. Only if telescopes are trained in that direction at that time will the phenomenon be recorded and, if the source is poorly observed, e.g. with marginal statistics, then there is no second chance as it may never undergo another outburst or, if it does, it may have quite different characteristics. The consequence of this transient behaviour is that the cardinal rule of scientific research, that the results should be verifiable, may not always apply. The statistical significance of unexpected transient emissions thus becomes a controversial issue. The fundamental motivation to do gamma-ray astronomy at the highest energies is fairly obvious. Not only is it important to exploit every possible decade of the electromagnetic spectrum but the decades at the extreme end of the spectrum offer unique insights into high-energy phenomena.
2. Gamma-ray production and absorption It is almost inevitable that, if an object is the source of high-energy particles (or is a reservoir of high-energy III. FUNDAMENTAL RESEARCH
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particles produced elsewhere), it will also be a source of high-energy g a m m a rays. Whether the particles be electrons or hadrons, there will be conditions under which as much as 10% of the total energy will emerge in the form of high-energy photons. Like neutrons and neutrinos these quanta will travel in straight lines from the source to the detector and thus make high-energy astronomy possible. However, they are easy to detect (unlike neutrinos) and stable (unlike neutrons); also, as we shall see below, they are not easily absorbed and are hence very suitable probes of the sources of high-energy cosmic radiation. Relativistic electrons are relatively short-lived in most astrophysical situations, losing their energy by synchrotron radiation in weak magnetic fields, by curvature radiation or by pair production in strong fields, by bremsstrahlung in the presence of nuclei or by inverse C o m p t o n scattering on low-energy photons. All of these processes can give rise to secondary gamma rays, in some cases with energies only slightly less than the primary electrons. Relativistic hadrons, e.g. cosmic-ray protons, are longer-lived but in the presence of other hadronic matter, e.g. interstellar hydrogen, will interact to produce mesons which will eventually produce muons, electrons, neutrinos and g a m m a rays. The detailed study of the energy spectrum, spatial distribution and time variability of gamma-ray sources can thus elucidate the nature of the progenitor particles, the photon and particle densities in the source regions, and the magnetic fields. There is also the possibility that at these very high energies some new processes may come into play and thus these cosmic accelerators may proffer an insight into some new physics. Although high-energy gamma-ray photons may interact and thus be absorbed within the confines of a source, the conditions in interstellar and intergalactic space are such that the emitted beam of g a m m a rays is little attenuated. There is one process that is important for extragalactic sources and unique to this environment. A high-energy photon, E a, can pair-produce on a low-energy photon, E 2, if the condition E I E 2 > (mec2) 2 is satisfied. For photons of energy 0.1-1.0 PeV this condition is satisfied when they interact with the relatively dense (10 3 p h o t o n s / c m 2) 2.7 o black-body radiation field, the relic of the Big Bang. Even for a relatively close extragalactic source, e.g. the closest radiogalaxy, this is a large effect (fig. 1). This process is also serious for optically bright galactic sources where the absorption occurs between TeV photons and optical photons close to the source. This absorption process has not yet been observed in the laboratory or from a cosmic source. As several authors have pointed out, the observation of this absorption feature in a cosmic source of gamma rays would verify the physical process as well as demonstrate
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the physical extent of the microwave black-body field. It would also give an indication of the intergalactic magnetic field and could, in principle, provide an independent method of measuring distances to nearby extragalactic sources.
3. Detection technique Although for all effective purposes the earth's atmosphere has a shielding power equivalent to a 1 m thickness of lead for all radiation in the X- and gammaray regimes, it is still possible to do gamma-ray astronomy at energies above 0.1 TeV from ground level. This is fortunate because even the large E G R E T detector on the G a m m a Ray Observatory (scheduled for launch in 1990) has an effective collection area of only 1000 cm 2. Although this is a factor of 20 greater than the collection area of any gamma-ray detector flown to date, it is still so small that at energies above 1 TeV even the strongest source known would give less than one observable photon per year. Ground-based detection is made possible by the generation of an air shower in the atmosphere whose secondary components survive at least until mountain altitude. At energies in excess of 0.1 PeV these can be detected with simple arrays of particle (electron) detectors. Arrival directions can be established by fast dif-
T.C. Weekes / Gamma-ray astronomy at the highest energies
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The general characteristics of a very-high-energy (VHE) gamma-ray source are easy to define. There must be both a source of high-energy particles whose energy exceeds that of the gamma rays under study and there must be a target (beam dump) where the particles can interact to produce gamma rays. The particles can be electrons or hadrons and the target material can be gas (for bremsstrahlung or pion production), photons (for inverse Compton scattering) or a magnetic field (for synchrotron, curvature radiation or pair production). The actual mechanism for particle acceleration can be shock acceleration, direct acceleration across a large potential drop, or some other mechanism that has yet to be discovered. For practical purposes the accelerator can be considered a black box, although a prime motivation for the study is to understand the accelerating mechanism. The target must be thick enough to provide effective conversion efficiency but sufficiently thin to permit the gamma rays to escape. For instance, the most efficient thickness for gamma-ray production via production in the collision of relativistic protons with hydrogen gas is 50 g / c m 2. In this case up to 10% of the total energy in relativistic protons can emerge as VHE gamma rays with the average photon energy about 0.1 that of the incident proton. In some cases the acceleration and gamma-ray production can occur in the same region. Potential VHE gamma-ray sources can be chosen from lists of objects in which great amounts of energy are being released relative to the total rest mass or where observations at other wavelengths point to the presence of particles radiating by nonthermal processes. Supernova explosions (both at the time of the explosion and in the years following) are obvious candidates as are objects with apparent jets of relativistic particles (e.g. the galactic binary sources SS433 and Cygnus X-3, the radio-galaxy M87 and the quasar 3C273). In a supernova remnant the obvious source of highenergy particles is the rapidly spinning pulsar at its center (although there is no standard model for pulsar particle acceleration). The target material can be either the photons in the nebula or the gaseous shell thrown off by the explosion. In binary X-ray sources there is no obvious way in which the compact object can accelerate particles since the rotation rate is rather small; hence the source of energy is most likely accretion rather than rotation. The target material can be either the outer limb of the companion star, the edge of the accretion disk or the accretion wake.
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= 100 TeV-PeV) ranges. ferential timing between the spaced detectors. A simple array can have a collection area of 103 m2 and an angular resolution of 1°. If the air shower array indudes some muon detectors, then gamma-ray primaries can be identified by the low muon-to-electron ratios at detector level. The secondary particles in the air shower travel at velocities in excess of the speed of light in the atmosphere and hence cause the emission of Cherenkov radiation. This is beamed in the direction of the particle (the Cherenkov angle at sea level is - 1 . 3 ° ) , is concentrated in the blue and has a yield of about 8 200 photons per radiation length. At sea level the Cherenkov light component arrives as a pancake of photons with a diameter of 200 m and 1 m thickness (fig. 2). For a 1 TeV gamma ray the photon density is about 100 p h o t o n s / m 2. The air shower can be easily detected with a simple light receiver consisting of a mirror, a phototube and fast pulse-counting electronics. In the first instance the air shower resulting from a hadronic primary is similar and gamma-ray sources can only be identified as angular anisotropies. Recently techniques have been developed that identify the gamma-ray primary on the basis of the smaller angular size of the Cherenkov light image. In the past five years there has been a dramatic increase in the number of active ground-based gammaray observatories; the improvement in flux sensitivity
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No attempt will be made to give a comprehensive description of the gamma-ray sky at very high energies. A catalog of sources and a complete description can be found in a recent review [1] where an extensive bibliography of the subject can be found. Galactic sources fall loosely under the headings: supernova (and supernova remnants) and close-binary X-ray sources. Two objects from each of these classifications will be discussed: it is no coincidence that three of these are the best established sources and the fourth is strongly predicted to become a source. 5. Supernova and supernova remnants
5.1. Supernova 1987a in the Large Magellanic Cloud If theoretical astrophysists were asked to make a short list of the objects most likely to be detected at energies greater than 100 GeV, it is likely that SN1987a would be near the top of their list. Although just outside the confines of our galaxy, it is still the closest supernova to be seen since the invention of the telescope, 300 years ago. Early models had suggested that the shock wave might give rise to a short burst of gamma-ray emission during the actual explosion. This would be of very short duration. If the supernova is to form a neutron star which becomes a regular fast-spinning radio pulsar, then it is very likely that the remnant will be a source and reservoir of relativistic particles. Initially the high-energy particle beam will be obscured by the thick expanding shell thrown off by the outburst; as the target thins it will gradually become an ideal gamma-ray target so that the presence of the particles (and the pulsar) will become evident by the production of a detectable beam of gamma rays at energies of 1 TeV and greater. As the pulsar slows down the strength of this beam may decline; it is predicted to have its maximum intensity 1-10 years after the outburst [2,3]. Given the small number of VHE observatories in the Southern Hemisphere it is unlikely that a telescope would be pointing at the supernova at the time of its outburst. The University of Durham group was operating a telescope in Narrabri, Australia the night of the outburst and by coincidence happened to be observing another source in the Large Magellanic Cloud so that the supernova was on the edge of their field of view. No detectable signal was recorded [4]. The same negative result has been reported from several observatories in the first year after the outburst [4]. The implication is that a pulsar has not formed or that the shell has not yet thinned sufficiently to permit the gamma rays to escape.
period = 33 ms) pulsar and to be a reservoir of electrons with energies up to 10 TeV. It was thus one of the earliest candidates for gamma-ray emission [5]. Satellite observations established that both the pulsar and the nebula were sources of 0.1 GeV gamma rays. There were early reports that this was also true at 0.1-1.0 TeV energies. It has proved difficult to detect a consistent signal at these energies from the pulsar; the conclusion therefore is that the pulsed flux that exhibits some variability at lower energies is much more variable at TeV energies. The same degree of variability is not expected from the nebula. The detection of TeV gamma rays from the nebula was first reported in 1972 [6]. This was confirmed by the Whipple Observatory Collaboration using a 10 m optical reflector as a large low-resolution fast camera. With 37 elements (each with a pixel scale of 0.5 o ), the group used the newly developed atmospheric Cherenkov imaging technique to select gamma-ray images based on their predicted properties [7]. More than 98% of the cosmic-ray background was rejected by this discrimination so that a signal that was only 0.2% of the total background was detected at the 9o level of significance. This observation is based on 100 hours of observation taken in 1986-88 and it confirms the flux levels determined in earlier observations indicating that this is a steady source; although weak, it is suitable for use as standard candle for comparing and improving the sensitivity of the atmospheric Cherenkov telescopes. The observed flux (fig. 3) can be explained in terms of the Compton-synchrotron model of the nebula [5] or by some sort of steady emission from the pulsar [8]. This detection is unusual in this field in that it combines good statistics (9a), good physics (the detected primaries are, by definition, gamma rays) and
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One of the major achievements of X-ray astronomy was the detection of a class of objects that have come to be known as close binaries. They consist of a compact object (neutron star or black hole) in close proximity to a normal star from which the compact object draws matter by accretion; as the matter falls into the compact object it forms a very hot accretion disk which is the source of the X-rays. Many of the neutron stars have detectable rotation periods, but these are less than those found in normal isolated radio pulsars. Because accretion gives off energy primarily by thermal radiation and because the rotation rates are low, it was not expected that there would be any high-energy particle acceleration in these sources despite the high X-ray luminosity. Therefore they did not figure in lists of potential sources in the early seventies and it was only with the discovery of emission from Cygnus X-3 (whose binary nature is in doubt) that they were studied. More than 20 of these galactic sources have well-established orbital and spin parameters; some of these are shown to scale in fig. 4 where the sources that have been observed to emit TeV gamma rays are so marked. Here we will only discuss two of the best-established sources, one of which may not belong to this class of objects at all.
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electrons) and because the one experiment that measured the muon-to-electron ratio indicated that the putative gamma-ray signal had a ratio more in accord with hadron primaries than gamma rays. This was followed by several underground experiments which appeared to detect the source in muons only; these results, which have been disputed, await explanation and confirmation. Recently the Fly's Eye group (who operate a large optical detector in Utah which detects very large air showers by atmospheric fluorescence) have reported [10] the detection of a flux of 0.5 EeV particles from the Cygnus X-3 direction (fig. 7). At these high energies the primaries could be neutrons but it seems most likely that they have the same nature as the primaries observed at lower energies. The most interesting consequence of these Cygnus X-3 observations may be the consideration of the total power that must be going into the high-energy particles (presumably hadrons) if we assume the observed primaries are gamma rays. If we consider just the particles at PeV energies we can calculate the total luminosity in PeV ganuna rays assuming a distance to the source of at least 10 kpc. Gamma-ray pair production in interstellar space introduces a factor of 3 absorption which must be corrected for; also the duty cycle is only 10% and if it assumed that the modulation is caused by the presence of the target, not an intrinsic beaming in the particle production, the observed luminosity must be of the order of 4 × 10 37 erg/s. The production efficiency, if pion production in proton-proton collisions is involved, must be less than 10% and the average energy of the progenitor particles must be a factor of 10 greater than the energy of the observed gamma rays.
T.C. Weekes / Gamma-ray astronomy at the highest energies
The inferred production of cosmic rays in the energy band from 10 to 100 PeV is then 4 × 103s erg/s. This is far greater than the X-ray luminosity and, more importantly, about equal to the cosmic-ray production rate necessary to replenish the cosmic-ray reservoir in the Galaxy (making reasonable assumptions about volume and lifetime [9]). Thus, if Cygnus X-3 were to produce gamma rays at the rate observed in the early eighties, it would appear to be sufficient to keep the Galaxy filled with cosmic rays, at least at these high energies. In practice, of course, because of the distance it is unlikely that the cosmic rays presently observed at earth could have come from Cygnus X-3 which is probably passing through a short-lived high-luminosity phase. Models for particle production in Cygnus X-3 are highly speculative and await a clearer understanding of the composition of the object as a whole. Much depends on whether the compact object, which should be present if the system is a close binary, is a fast pulsar; claims for the detection of periodicity in TeV gamma rays at 9.22 and 12.59 ms [11,12] have not been confirmed.
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three well-defined periodicities: a 1.24 s period (detected in X-rays and in visible light) that is associated with the neutron star rotation, a 1.7 day modulation (seen in X-rays and visible light) that is clearly the orbital motion of the star and a 35 day variation (seen only in X-rays) characterized by two active phases of emission which may be associated with precession of the accretion disk or the neutron star. In addition the system exhibits extended periods of low X-ray luminosity. The detection by the Durham group in 1983 [13] of a short (3 min) burst of pulsed radiation from the system was a surprise; it has since been seen as a short-lived source of 0.6-100 TeV emission on 20 occasions by five different groups. In each case the emission has been pulsed at a period close to the neutron star rotation period. In general the methodology used in studying this source has been first to detect a general increase in counting rate from the direction of the source; this is usually not statistically significant in itself but defines the duration of the putative outburst. This data is then subjected to a periodicity analysis using either epochfolding or the Rayleigh test (a form of Fourier analysis) at periods close to that of the Hercules X-1 pulsar period. The detection of a signal then depends on finding a power in this region that is statistically significant. An example of a typical detection (by the Whipple Observatory group) is shown in fig. 8. These detections are generally not very strong statistically; it is reassuring to find that on the one occasion that two TeV telescopes were trained on the source during an active phase both saw the same pulsed signal [9]. An attempt to correlate the active episodes of emission with phase on the 1.7 day and 35 day periods is shown in fig. 9. There is no clear correlation with orbital phase but it does appear that the TeV emission comes primarily during the "ON" periods of the 35 day cycle. There is as yet no completely satisfactory explanation of the origin of these episodes of emission nor is there any agreement on which is the mechanism by which particles are accelerated in an accreting system. One feature of the observations that any model must explain is that the frequency of the TeV gamma-ray emission is usually slightly to the blue of the X-ray emission, which is presumably the rotation frequency of the neutron star. This was particularly pronounced in a TeV observation on June, 1986 which was reported by the Whipple group in 1987 [14,15]. An episode lasting 25 minutes was found to have a strong periodic component at a frequency 0.16% off that of the pulsar. Although this was the strongest of the eight detections reported by this group, the frequency offset reduced the statistical significance to 0.9%. The same frequency was reported by the Haleakala group in an observation on May 13, 1986 [16]. The energy threshold claimed was
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Fig. 10. Power as a function of period for three sets of gamma-ray observations: (a) Haleakala Observatory, 13 May 1986, E = 0.2 TeV, P = 0.7% [16]; (b) Whipple Observatory, 11 June 1986, T = 0.6 TeV, P = 0.9% [15]; (c) Los Alamos A.S.A., 23 July 1986, E = 100 TeV, P = 0.002% [17]. The X-ray period is shown by the dashed line (from ref. [18]).
0.2 TeV and the chance probability was estimated at 0.7% (see fig. 10). The Los Alamos Air Shower Array was operating at this time also and they reported the detection of two 30 min bursts pulsed at this same offset frequency on July 23, 1988 [17]. Their energy threshold was 100 TeV and the statistical significance of the detection (independent of the earlier observations which might have been taken to define the expected period) was 2 × 10 -5. These three sets of observations taken over a period of three months constitute a strong case for the detection of 0.5-100 TeV emission from Hercules X-1. However, the Los Alamos detector has a large muon detector and when the response of this detector is used to determine the muon-to-electron ratio of the 14 showers that constitute the pulsed signal, it is found that the ratios are, if anything, greater than the average response of the array to hadronic showers. Similarly the Whipple Observatory group do not find the same photonic character in the Hercules X-1 signal images as they do in their signal from the Crab nebula; however, in this case the statistical significance is not as great as that of the Los Alamos
T.C. Weekes / Gamma-ray astronomy at the highest energies result. The Haleakala experiment does not discriminate between photons and hadron primaries. The implication is that the measured signal comes not from g a m m a rays, but from some other neutral component of the cosmic radiation. The problem is that there is no known component that could give this signal. Neutrons would have decayed en route and neutrinos would not have a high enough cross section to give sufficient showers in the atmosphere. An alternative explanation might be that there are some processes by which muons can be produced in air-shower interactions at energies above 1 TeV. Either explanation implies some new physics - and the offset frequency at which the pulsations are seen implies new astrophysics as well! This has increased the interest in further observations of this fascinating system with detectors with greater sensitivity.
7. Outlook The number of ground-based gamma-ray telescopes in both hemispheres has increased dramatically in the last few years so that one can expect a major leap forward in our understanding of the very-high-energy universe in the next decade. Outstanding questions such as the nature of the primaries at all energies should be resolved and the catalog of well established sources should grow. With the launch of satellites, G A M M A - 1 and G R O , the next decade may well be the decade of gamma-ray astronomy.
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References [1] T.C. Weekes, Phys. Pep. 160 (1988) 1. [2] T. Gaisser, A. Harding and T. Stanev, Nature 329 (1987) 314. [3] V.S. Berezinsky and V.L. Ginzburg, Nature 329 (1987) 807. [4] P.M. Chadwick et al., Astrophys. J. Lett. 333 (1988) L19. [5] R.J. Gould, Phys. Rev. Lett. 15 (1965) 511. [6] G.G. Fazio, H.F. Helmken, E. O'Mongain, G.H. Rieke and T.C. Weekes, Astrophys. J. Lett. 175 (1972) Ll17. [7] P. Kwok et al., in: Proc. NATO Advanced Study Institute on Cosmic Gamma Rays and Cosmic Neutrinos, Erice, Italy, April 20-30, 1988 (Reidel, Dordrecht, 1989) in press. [8] K.S. Cheng, private communication, 1988. [9] R.C. Lamb and T.C. Weekes, Science 238 (1987) 1528. [10] G.L. Cassiday et al., Phys. Rev. Lett. 62 (1988) 383. [11] P.M. Chadwick et al., Nature 318 (1985) 642. [12] A.A. Stepanian, private communication, 1988. [13] J.C. Dowthwaite, A.B. Harrison, I.W. Kirkman, H.J. MacRae, K.J. Orford, K.E. Turver and M. Walmsley, Nature 309 (1984) 691. [14] R.C. Lamb et al., Bull. Am. Astron. Soc. 19 (1987) 1. [15] R.C. Lamb et al., Astrophys. J. Lett. 328 (1988) L13. [16] L.K. Resvanis et al., Astrophys. J. Lett. 328 (1988) L9. [17] B. Dingus et al., Phys. Rev. Lett. 61 (1988) 1906. [18] R.J. Protheroe, Proc. 20th Cosmic Ray Conf., Moscow, August 2-15, 1987, 8 (1987) 21.
Acknowledgements This research in V H E gamma-ray astronomy is supported by the U S Department of Energy and the Smithsonian Scholarly Studies Fund.
III. FUNDAMENTAL RESEARCH