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Astron. Astrophys. 337, 25-30 (1998)

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3. Data sample

When selecting AGN as observation targets for the 3.8 m telescope we have used a number of criteria including the proximity of the source and measurements from X-ray and gamma-ray satellite experiments. As mentioned earlier, Stecker et al. 1996 have suggested that nearby XBLs are the most promising sources of detectable TeV gamma-ray emission. Multi-frequency studies of blazar emission (Sambruna et al. 1996) suggest that XBLs may be more compact and have higher electron energies and stronger magnetic fields than their radio selected counterparts. The current status of ground based TeV observations adds support to this belief. We present here the results of observations on two XBLs and two radio selected BL Lacs (RBLs). While RBLs are perhaps less promising as TeV sources, well placed upper limits in the TeV range could help to confirm fundamental differences between XBLs and RBLs. The four sources reported in this paper are described in the following sub-sections.

3.1. PKS0521-365

PKS0521-365 ([FORMULA] = [FORMULA], [FORMULA] =[FORMULA]23'2", z = 0.055) is a radio selected BL-Lac. It was first detected as a strong radio source more than 30 years ago (Bolton et al. 1964). The spectral energy distribution of PKS0521-365 shows a double peaked structure that is typical of blazars (Pian et al. 1996). The first peak in the energy distribution occurs in the far IR ([FORMULA]Hz) and is assumed to be from synchrotron emission from the electrons in the jet. The second component, possibly from IC emission from the same electron population, peaks at around 100MeV.

PKS0521-365 was viewed by the EGRET experiment on the CGRO during 1992 from May 14 to June 4 (Lin et al. 1995). A point source, consistent with the position of PKS0521-365 was detected at a statistical significance of 4.3 [FORMULA], with an integral source flux above 100MeV of [FORMULA] photons [FORMULA]. The photon spectrum from the EGRET observation can be fitted with a single power law


The hardness of the EGRET photon spectrum and the proximity of the source make PKS 0521-365 a candidate source for detectable levels of TeV gamma-ray emission.

The CANGAROO data set for this object consists of 52 on/off pairs of observations with a total of 89 hours of on-source and 84 hours of off-source data.

3.2. EXO 0423.4-0840

EXO 0423.4-0840 ([FORMULA] = [FORMULA], [FORMULA] =[FORMULA]33'43", z = 0.039) was reported as a serendipitous discovery as part of the high galactic latitude survey in the 0.05-2.0 keV range by EXOSAT (Giommi et al. 1991). Associating the source with the galaxy, MCG -01-12-005 (incorrectly given in Giommi et al. 1991 as MCG +01-12-005) and noting the high X-ray luminosity ([FORMULA] ergs [FORMULA]) of the source, Giommi et al. (1991) proposed the source as a candidate BL Lac object. This would make the source the third closest such object known (after Mkn 421 and Mkn 501), and the closest in the southern hemisphere. We note however that Kirhakos & Steiner 1990 determine a redshift of 0.0392 for the source they designate IRAS 04235-0840B and identify with the HEAO source 1H 0422-086, and which they classify as a type 2 Seyfert. Clearly higher resolution X-ray studies are required to firmly establish the identity of this source.

We have observed EXO 0423.4-0840 during October 1996 obtaining a total raw data set comprising 20 hours of on-source and 17 hours of off-source data.

3.3. PKS2005-489

PKS2005-489 ([FORMULA] = [FORMULA], [FORMULA] = [FORMULA]49'54", z=0.071) is an XBL. X-ray measurements of PKS2005-489 by EXOSAT show extremely large flux variations on timescales of hours (Giommi et al. 1990). Initially reported as a marginal EGRET detection (von Montigny et al. 1995), a more accurate background estimation decreased the significance below the level required for inclusion in the Second EGRET Catalog (Thompson et al. 1995). However the fluxes above 1 GeV are more significant than those above 100 MeV suggesting that the source, in a part of the sky that has received relatively poor EGRET exposure, may indeed be detectable at higher energies (Lin et al. 1997).

We have observed PKS2005-489 during August of 1993 and during August/September 1994 obtaining 41 hours of on-source and 38 hours of off-source data.

3.4. PKS2316-423

PKS2316-423 ([FORMULA] = [FORMULA], [FORMULA] = [FORMULA]06'48", z= 0.055) is a radio selected BL Lac object. Assuming a magnetic field strength of [FORMULA]G, the emission from radio through to X-ray wavelengths is consistent with synchrotron radiation from electrons with [FORMULA]eV (Crawford & Fabian 1994). PKS2316-423 is not detected by the EGRET telescope on the CGRO.

The CANGAROO 3.8 m telescope observed PKS2316-423 during July 1996 for a total of 26 hours of on-source data and 25 hours of off-source data.

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998