Interpretation of Two Point Angular Data

Data such as those discussed in the previous section are studied to investigate the possibility of there being information in air shower directional data pertaining to possible cosmic ray source directions. Figure 1a shows the SUGAR directional distribution for the highest energy included in table 3. As we discussed above, based on the two point angular autocorrelation function, there may be underlying evidence for clustering on a scale of a few tens of degrees. In examining figure 1a, one can subjectively see the origin of this effect in an apparent non uniform distribution within the accessible range of the sky on scales of tens of degrees. Without our shuffling method or some equivalent, this is difficult to quantify. It appears that the data show some groupings within the overall non uniformity together with an admixture of non grouped directions. These groupings are arguably at galactic longitudes of about , near the galactic equator (RA , dec ) plus a region at about of galactic longitude and of galactic latitude (RA , dec ). Figure 1b shows the same data but with a rather higher energy threshold in which the apparent clustering is clearer.

Figure 1: Distribution in galactic coordinates of SUGAR events with energies (a) above

eV and (b)

eV.

We hope that by studying the arrival directions of the highest energy cosmic rays, we might derive directional information such as these which will lead us to identifying some sources. Such sources are believed to be extragalactic because, despite there being no significant clustering of arrival directions along the Milky Way, known galactic magnetic fields are believed to be incapable of randomising those directions from such nearby sources.

Our directional data will be limited by the angular uncertainty of the detection array and by any intergalactic magnetic deviations. Even with an angular uncertainty limited only by the array to the order of one degree, many astronomical objects will be within the 'beam'. It will be necessary to have a selected catalogue of potential extragalactic sources to limit such confusion.

In order to investigate this process, we have chosen some catalogue selection criteria and used those to select potential sources from a complete, but less selective, catalogue. We took the catalogue of strong radio sources produced by Robertson (1973). This is complete above 10 Jy. We are thus assuming that strong radio sources are potential sources of the highest energy cosmic rays. This is consistent with a number of proposals such as those discussed by Hillas (1984) as potential source regions. For instance, the catalogue includes FR II sources and clusters of galaxies. The selection criterion of strong emission should ensure that the catalogued sources have a combination of closeness and strong activity.

Of course, if the sources were to be active galactic nuclei which are variable, possibly short lived and are beamed (such that we only observe a subset of all possible sources), this process could reasonably be regarded as hopeless since magnetic deviations result in the cosmic rays lagging the presently observable light or radio waves by millions of years. Such sources are intrinsically variable but they are also likely to beamed and a natural result of long period precession of the central black hole would be to move the most intense part of the photon beam away from the observer for large periods of time (e.g. Thorne et al. 1986, Jackson 1999). It could be that the appropriate sources of the highest energy cosmic rays are not even visible as bright photon sources at the present time.

In developing our catalogue from that of Robertson, we assumed that any potential source must have a redshift below 0.07. This is a generous allowance for the distance limitation due to photopion energy losses. This reduces the number of catalogued objects from 160 to 42. Based on the NASA Extragalactic Database, there are two sources in the catalogue within of the centre of the first clustering noted above. These are IRAS 06343-2032 and Hydra A. There are also two sources for the second cluster centre but they are both within the same structure, the cluster of galaxies, A3667. The IRAS source is an FR II object and Hydra A is a cluster of galaxies with significant x-ray luminosity (Ikebe et al. 1997). At the centre of Hydra A is 3C218, a complex, strong, radio source with the highest Faraday rotation ever measured for a radio galaxy. A3667 is a source which has been mentioned as a likely type of source for the highest energy cosmic rays. It is a huge galactic cluster with strong x-ray emission and two very large shock structures. All of these sources have redshifts which are a little over 0.05. If one were to use the propagation model of Clay et al. (1998), a fit to the angular spreads would result from a characteristic intergalactic magnetic field strength of G together with a largest turbulence scale of 100kpc.

It is clear that the process which we have described, of defining a general potential source direction and then examining a purpose developed catalogue of potential sources, can select a managable number of plausible cosmic ray sources for further study. It is then possible for interesting further astrophysical information to become apparent.

© Copyright Astronomical Society of Australia 1997