Science of STACEE

The science of high energy gamma ray astronomy is compelling. It integrates many features of modern physics and touches on issues in particle physics, cosmology and astrophysics. The sources, primarily active galactic nuclei (AGNs) and pulsar-driven supernova remnants, are exotic objects which are only partially understood. The field is in need of more data to progress and experiments like STACEE can make decisive contributions. We review here some of the specific scientific motivations behind our project.

The major attraction of solar power plants for gamma-ray astronomy is their large numbers of heliostats. These mirrors can be used to track a candidate source across the sky and collect the Cherenkov light generated by the millions of electrons in the showers caused by the impact of high energy gamma-rays on the upper atmosphere. The light is reflected by the heliostats onto a secondary optics system atop the central tower where it is focussed onto a matrix of phototubes (PMTs) in a camera box, such that each phototube sees a separate heliostat. This granularity is required in order to form an image of the shower to aid in rejection of backgrounds due to charged cosmic rays (mostly protons).

This `air Cherenkov technique' (ACT) allows one to observe high energy gamma-rays from astrophysical sources using ground based instruments.

Interest in gamma-ray astronomy has intensified during the 1990's due to two key developments. The first was the breakthough made by the Whipple collaboration which established the existence of TeV gamma-ray emission from the Crab Nebula, a supernova remnant in the Milky Way galaxy. This was an ACT measurement and the key to its success was the use of an imaging detector. The second major development was the deployment, in 1991, of the EGRET telescope on board the Compton Gamma Ray Observatory (CGRO). This orbiting detector greatly expanded the catalog of astrophysical gamma-ray sources, both galactic and extra-galactic. Perhaps its most important contribution was the discovery that AGNs are bright sources of gamma-rays at GeV energies.

There are now several ACT detectors operating in the world. The Whipple telescope in Arizona remains the leader but there are others making contributions to the field; HEGRA in the Canary Islands and CANGAROO in Australia are two examples. Recently a new detector, CAT, was commissioned in the Pyrenees. All of these devices consist of a large, steerable mirror with a cluster of more than 100 PMTs forming a pixelated camera at the focus. The engineering problems associated with supporting and moving a large mirror have limited the mirror sizes to less than 100 square meter.

These ACT groups have firmly established the field of ground based gamma-ray astronomy. The Crab nebula has been seen repeatedly by all detectors which have the necessary sensitivity and has emerged as the standard candle of the field. Another galactic supernova remnant, PSR 1706-44, visible only from the southern hemisphere, has been seen by CANGAROO. This group has also detected TeV gamma-rays from the vicinity of the Vela pulsar.

Two extra-galactic sources, Markarian 421 (Mrk421) and Markarian 501 (Mrk501) have also been detected. These latter sources are BL Lacertae objects which belong to the blazar class of AGNs. (Another such object, 1ES 2344+514, has been seen by the Whipple collaboration, apparently during a flaring episode, but has not been confirmed by other detectors which have since come on line).

It is conjectured that the high energy gamma-rays are beamed towards the earth along the axes of relativistic jets emerging from the core of these sources. The core is believed to be the site of a super-massive (10^9 solar mass) black hole which powers the AGN by converting the gravitational energy from an accretion disk into collimated jets. The black hole hypothesis is supported by the rapid variability of the AGN radiation which is seen at all wavelengths including the TeV gamma radiation.

The present situation in gamma-ray astronomy can be summarized by the two plots in figure 1 and figure 2. In figure 1 we have the second EGRET catalog of sources seen at energies between 100 MeV and 10 GeV. The sources are plotted in galactic coordinates; pulsars cluster along the plane of the Milky Way and AGNs are seen at high galactic latitude. Unidentified sources (those for which, because of EGRET's large error box, do not have an unambiguous radio or optical counterpart) are also plotted. Clearly, there are many sources at GeV energies. In figure 2 we have the results of the ACT observations at energies from 300 GeV to 2 TeV. There are only the 5 sources discussed above.

The obvious question is `what happens to the EGRET sources above 10 GeV?' Two possiblities are that the gamma-ray production mechanism, whatever it may be, is not always capable of producing TeV photons; ie. some sources may have an intrinsic spectral cut-off. Another possibility is that something between the source and the earth is systematically attenuating the flux of high energy photons. This latter scenario is motivated by the fact that at least some AGNs (the two Markarian galaxies observed so far) can produce TeV photons. The spectrum of Mrk 421, shown in figure 3, follows a simple power law from GeV to TeV energies. 3C279, another AGN observed by EGRET, has a similar power law at low energies, implying possibly similar physics at the source. This AGN is the brightest in the EGRET catalog and should outshine Mrk421 at TeV energies. However it has not been detected and the upper limits set by the experiments are well below a simple power law extrapolation. The attenuation effect may explain these data since 3C279 (at a redshift of z = 0.54) is much further away than Mrk 421 (z = 0.03).

What causes the attenuation? One mechanism is photon-photon collisions which lead to the production of electron positron pairs. At TeV energies the collisions are between the gamma-ray and infra red (IR) photons. The wavelengths of the photons involved are determined by the centre of mass energy required to produce an electron-positron pair; for lower energy gamma-rays to experience attenuation we need higher energy `target' photons. (At 1 TeV, for example the threshold wavelength is 5 microns.) The IR photons are believed to exist as relics of early galaxies. It is important to learn what we can about the IR photons since galaxy formation and in particular when it began, is an important feature of cosmology and depends on parameters such as the quantity and nature of dark matter and on the cosmological constant. In principle STACEE can probe extragalactic IR fields with a greater sensitivity than other, more direct, detectors, such as the DIRBE instrument aboard the COBE satellite, because it is insensitive to nearby IR sources.

Attenuation of photons from distant AGNs is not the only motivation for a study of their energy spectra at energies between 20 and 200 GeV. The current paradigm states that the production mechanism for gammas from these objects is synchrotron emission from accelerated electrons at low energies and inverse Compton scattering at higher energies. This model predicts two broad peaks in the energy spectrum separated by a valley. For some sources, the lower peak (synchrotron photons) occurs at X-ray energies and the upper peak (inverse Compton) at TeV energies although the entire structure can be moved up or down in energy depending on the details of the source. A continuous spectrum over many decades in energy is needed to properly test this. The upper peak is broad enough that with the current generation of ACT telescopes, it is not clear if one is observing the peak maximum or some part of its edge.

The question of time correlations between X-rays and gamma rays during a flare is especially interesting. If the electrons which produce the synchrotron emission are also those which upscatter photons to high energy (the inverse Compton effect) then there should be correlations and their relative timing can give information about source details. Early results, obtained from the 1997 flaring of Mrk 501, show such correlations. More data, at other energies, are in great demand.

EGRET runs out of statistics at about 10 GeV. This upper limit is a function of its small size (about 0.15 square meters) and finite observing time coupled to the steeply falling spectra of astrophysical sources. The current generation ACT telescopes are limited to observations above (about 250 GeV) because of the finite size of their collector mirrors; at lower energies the density of Cherenkov photons at ground level is too low for measurements to be made. Thus there is a gap in an important part of the spectrum. Most of the gamma-ray sources in the sky disappear in this gap and it is of vital interest to make measurements here; this is the goal of the STACEE project.

There are other reasons to fill in the gap. In our own galaxy, the Crab is the brightest source of gamma rays. It is seen by both EGRET and by the ACT experiments but only EGRET detects pulsed emission (presumably from the pulsar which drives the nebula). At energies of several hundred GeV the gammas are continuous and this implies a different production mechanism. Measuring the transition from pulsed to steady emission will provide important information to model builders.

The Crab Nebula is a supernova remnant (SNR) driven by a pulsar at its centre. It is believed that the shocks associated with expanding SNRs are where high energy cosmic rays originate; if this is true then SNRs should also produce high energy gamma rays (from neutral pion decay). Again, at EGRET energies, one sees gammas from SNRs but at ACT energies only two candidate SNRs have been detected. (The Crab Nebula and PSR1706-44 in the southern skies.) Independent of the gamma ray observations, the SNR/pulsar situation is not completely clear since not all pulsars are seen to have an associated SNR and vice versa. Thus a high sensitivity experiment with the timing and angular resolution capable of detecting pulsed emission and measuring the extent of the emission region in the energy range from 20 to 200 GeV is desirable.