NGC 253: This is the first starburst galaxy detected and also the closest (2.5 Mpc) source of extragalactic gamma rays. Starburst galaxies are the site of extraordinary supernovae activity and were postulated to be sources of VHE cosmic rays and gamma rays (34). The detection by CANGAROO-II in 2002 was at the 11 level (15). It was observed to have a very steep spectral index (-3.75) which implies that most of the signal is close to the telescope threshold. The source was extended with the same elongation as the optical source. A model of the source has been proposed (35). There are many other nearby starburst galaxies (e.g. M82, M81, IC342) so this detection opens up the possibility that there will be many more starburst detections even with the present generation of telescopes.
3C66A: This is, perhaps, the least certain of the TeV detections; it was reported by the Crimean Astrophysical Observatory at the 5.1 level of significance (16). Although a well-studied AGN and detected by EGRET, 3C66A seems an unlikely candidate for TeV emission because of its large redshift (z = 0.444) and its classification as an LBL. Upper limits have come from other observations (12); (36) but these were at other epochs and the source could be time variable on long time-scales. It was bright at longer wavelengths during the observations by the Crimean group.
Mrk421: This was the first AGN detected at TeV energies (13) and it remains the prototype of TeV AGN because its signal strength is, on average, 30% of the Crab (and often much stronger). It is the weakest blazar detected by EGRET in the 3rd Catalog (7) and also the closest.
Whipple observations of Mrk421 during 1994 revealed the first clear detection of flaring activity in the VHE emission from an AGN. A 10-fold increase in the flux, from an average level that year of approximately 15% of the Crab flux to approximately 150% of the Crab flux was observed. The observations of Mrk421 in 1995 (37) revealed several distinct episodes of flaring activity as in previous observations; perhaps more importantly though, they indicated that the VHE emission from Mrk421 was best characterized by a succession of day-scale or shorter flares with a baseline emission level below the sensitivity limit of the Whipple detector.
The VHE emission from Mrk421 was seen to flare on sub-day time-scales in 1996, with the observations of two short flares (38). In the first flare the flux increased monotonically during the course of ~ 2 hours of observations. This flux is the highest observed from any VHE source to date. The second flare, observed a week later, although weaker, was remarkable for its very short duration: The entire flare lasted approximately 30 minutes with a doubling and decay time of less than 15 minutes. These two flares exhibited the fastest time-scale variability, by far, seen from any blazar at any gamma-ray energy.
During 2001, Mrk421 exhibited exceptionally strong and long-lasting flaring activity (39). It was observed extensively with the Whipple telescope during this time and a large database of over 23,000 gamma-ray photons was collected allowing very accurate spectral information to be derived (29). The data are best described by a power law with an exponential cutoff:
|where E is in units of TeV.|
The data were binned according to the flux level that Mrk421 was in when they were recorded and spectra were derived for each flux level. A clear correlation was found to exist between the spectral index and the photon flux and, the spectra were all found to have exponential cutoffs consistent with the average value of 4.3 TeV (31). Spectral measurements of Mrk421 from previous observing seasons by the Whipple collaboration were also found to be consistent with this flux-spectral index correlation, suggesting this to be a long-term property of the source. The spectral index was found to vary between 1.89 ± 0.04stat ± 0.05syst in a high flux state and 2.72 ± 0.11stat ± 0.05syst in a low state.
During its dramatic outburst in 2001, Mrk421 was also detected by the STACEE detector at energies above 140 GeV (40) and by the CANGAROO-II telescope at energies above 10 TeV (41).
M87: This is one of the brightest nearby radio galaxies and is an obvious potential source of high energy radiation since the jet displays evidence for synchrotron radiation and time variability. The angle of the jet is about 30° (42) which means that it is unlikely to have the same observational gamma-ray properties as the blazars. In fact it was not detected by EGRET and the positive observation by the HEGRA group (17) was a surprise. Although the detection was only at the 4 level of significance (weaker than any of the other sources in the TeV catalog) it is a potentially exciting result as it opens up the possibility that many AGN may be observable whose axes are not pointing directly towards us. It is a weak source and its detection required 83 hours of observation. It was not seen in observations at lower energies (43); (44) but the exposures, and hence the flux sensitivities, were limited. The detection of M87 revives interest in the reported detection of Centaurus A in 1975 (45) which, although not confirmed in later, more sensitive, observations, was at a time when the source had an abnormally high microwave flux.
H1426+428: This source is of interest primarily because, at a redshift of 0.129, it is the most distant confirmed source of TeV gamma rays; three different groups have reported significant detections (46); (47); (48). It is weak source (typically 6% of the Crab) and, having its synchrotron peak located at higher frequencies than any of the other TeV blazars, is classified as an "extreme" HBL (49). It was not seen by EGRET. The initial detection by the VERITAS group was at the 5.8 level of significance and was based on 44.4 hours of observation. The source is also significant in that it was predicted to be a detectable TeV emitter based on its hard X-ray spectrum (50). The source is definitely variable on time-scales of a year and maybe on times as short as a day. The energy spectrum, which has been derived by three groups, is found to be quite steep. It is well described by a power law with a spectral index between 250 GeV and 1 TeV of 3.50 ± 0.15 derived by the VERITAS Collaboration 51) and of 3.66 ± 0.41 by CAT (48). The HEGRA group derived a spectral index of 2.6 ± 0.6 between 700 GeV 1.4 TeV; above this energy, consistent with the expected signature of absorption of the TeV gamma rays by the extragalactic infra-red photons, evidence for a break in the spectrum was found (33).
Mrk501: Historically Mrk501 is important because, when it was detected in 1995 (19), it was the first TeV source to be detected that had not previously been detected by EGRET; hence it established TeV extragalactic astronomy as a discipline in its own right. The properties of Mrk501 are very similar to those of Mrk421 although in general the characteristic time-scales seem longer with the flux levels varying less rapidly. In 1997, the VHE emission from Mrk501 increased dramatically. Fortunately this was a time when several new telescopes were coming on-line so that it was well-observed (52); (53). After being the weakest known source in the VHE sky, in 1995-96 it became the brightest, with an average flux greater than that of the Crab Nebula (whereas previous observations had never revealed a flux > 50% of the Crab flux). The amount of day-scale flaring increased and, for the first time, significant hour-scale variations were seen. It was also detected by EGRET for the first time. The Mrk501 spectrum is similar to that of Mrk421 and can be represented by:
(28) where E is in units of TeV.
1ES1959+650: This was first reported in conference proceedings by the Telescope Array group operating in Dugway, Utah (20) in 2000. Upper limits were reported by other groups (54). In 2002, the HEGRA Collaboration reported their detection of 1ES1959+650 (55). The detection was dramatically confirmed later in 2002 when an outburst was seen by several groups (24); (56); (57). Although the quiescent level was only about 5% of the Crab, when it flared its flux was 5 times that of the Crab. Correlations were reported with optical and X-ray observations (58). 1ES1959+650 was not seen by EGRET. The differential energy spectrum has been derived by Aharonian et al.(56) and is well described by a power law with an exponential cut-off during flaring states:
|where E is in units of TeV.|
The low state spectrum is best represented by a pure power law (56) with a spectral index of 3.18 ± 0.17stat ± 0.08syst.
Extensive multiwavelength observations on 1ES1959+650 were carried out during May-July 2002 (59). During this time, a TeV gamma-ray flare was observed which had no counterpart at X-ray energies. This "orphan flare" is difficult to explain in terms of one-zone synchrotron self-Compton (SSC) models. Several possibilities are explored in (60) including multiple-component SSC models, external Compton models and proton models. The latter seems the least likely explanation since the X-ray and gamma-ray flux from 1ES1959+650 were found to be correlated during the rest of the observing campaign.
PKS2155-304: This BL Lac is very bright in the ultraviolet and is classified as a HBL. It is highly variable in X-rays. It is strongly detected by EGRET with a hard spectrum (index -1.71 ± 0.24). It was first reported by the University of Durham group working in Narrabri, Australia (21) who saw a signal at the 6.8 level of significance. Upper limits were also reported (61); (62). It was dramatically confirmed by the report at this symposium by the HESS group who saw it at the 10.1 level in just 2.2 hours of observation. This is the first blazar detected in TeV gamma rays in the southern hemisphere.
This is the object after which this class of AGN is named. It is now classified as a LBL like many of the EGRET-detected AGN. The paper (63) that reported the detection of BL Lacertae by EGRET also reported an upper limit at TeV energies from the Whipple group. Subsequently the Crimean group reported the detection of this source at the 7.2 level of significance (22). It was optically quite bright at this epoch (July-September, 1998). BL Lac and 3C66A are the only LBLs that have been reported at TeV energies; it is important that these detections at TeV energies be confirmed as they place severe constraints on source models.
1ES2344+514: This AGN was reported by the Whipple group in 1998 (23) as a TeV source based on observations made in 1995. Most of the reported signal came in one night so, if real, the source is highly variable. Indeed, BeppoSAX observations of 1ES2344+514 have revealed it to be highly variable at hard X-ray energies (64) with the overall X-ray spectral shape varying with intensity. Its synchrotron peak frequency was seen to shift by a factor of 30 between observations taken in 1996 and in 1998. This behaviour is typical of HBLs and has, for example, been also observed in Mrk501 (65). A confirmation of 1ES2344+514 has been reported by the HEGRA group (66) and by the Whipple group (67). No spectrum has been reported. At the peak of its flaring activity it was 60% of the strength of the Crab.