First Results from a Search for TeV Emission from BL Lacs Out to Z = 0.2.

Two active galactic nuclei have been detected at TeV energies using the atmospheric Cerenkov imaging technique. The Whipple Observatory γ-ray telescope has been used to observe all the BL Lacertae objects in the northern hemisphere out to a redshift of 0.1. We report the tentative detection of VHE emission from a third BL Lac object, 1ES 2344+514. Progress in extending this survey out to z=0.2 will also be reported. INTRODUCTION With the detection of very high energy (VHE, E > 250 GeV) emission from the two BL Lacertae objects (BL Lacs) , Markarian 421 (Mrk 421) (Punch et al. 1992) and Mrk 501 (Quinn et al. 1996), we began a survey of nearby BL Lacs to search for VHE emission. A collection of such sources could lead to constraintes on γ-ray emission models through investigation into the properties which are important for VHE emission and also an estimate of the density of extragalactic background IR light through its effect on the VHE γ-ray spectra (Gould and Schreder 1967; Stecker, de Jager, & Salamon 1993). BL Lacs are blazars, the only type of active galactic nucleus (AGN) detected above 100 MeV. The electromagnetic spectrum of blazars consists of synchrotron emission, which spans radio to UV or X-ray wavelengths, produced by electrons moving within jets oriented at small angles to our line of sight (Blandford & Konigl 1981), and a high energy part which can extend to γ-ray energies. Models of the high energy emission fall into three main categories: synchrotron self-Compton emission (e.g., Konigl 1981), inverse-Compton scattering of low energy photons arising outside the jet (e.g., Sikora, Begelman, and Rees 1994), and pair cascades initiated by protons (Mannheim 1993) or electrons. If protons produce the high energy emission, AGN could contribute significantly to the highest energy cosmic-ray flux (E > 1018eV) (Rachen, Stanev, and Biermann 1993). BL Lacs are particularly promising candidates for VHE emission because of two aspects of their low energy emission. First, BL Lacs may have less γ-ray absorbing material near the source because they have weak or no emission lines in their optical spectra (Dermer and Schlickeiser 1994). Second, in inverse Compton (IC) models of the high energy emission, the extension of the synchrotron emission of X-ray selected BL Lacs (e.g., Mrk 421 and Mrk 501) into the X-ray waveband implies a higher maximum γ-ray energy than for radio-loud BL Lacs (e.g., W Comae) and flat spectrum radio quasars (e.g., 3C 279) where the synchrotron emission ends in the optical to UV range. Table 1 lists the objects observed in our BL Lac survey so far. We have limited our search to BL Lacs with z 350 GeV) = 8.7× 10−11 photons cm−2 s−1 (Hillas et al. 1997). This procedure assumes that the Crab Nebula VHE γ-ray flux is constant, as 7 years of Whipple Observatory data indicate (Hillas et al. 1997), and that the object’s photon spectrum is identical to that of the Crab Nebula between 0.3 and 10 TeV, dN/dE ∝ E−2.4 (Hillas et al. 1997), which may not be the case. If no significant emission is seen from a candidate source, a 99.9% confidence upper limit is calculated using the method of Helene (1983). RESULTS In Table 1 we present the results of observations for which the analysis has been completed. With the exception of 1ES 2344+514, there is no evidence of emission from any of the objects in this survey. In particular, the EGRET sources W Comae (von Montigny et al. 1995) and BL Lacertae (Catanese et al. 1997a) are not detected despite long exposures. Also, only 1ES 2344+514 shows evidence of short term activity. Most of the excess from 1ES 2344+514 during 1995 comes from an apparent flare on 1995 December 20 (see Figure 1). We find a non-statistically significant excess from this object in 1996 which could simply mean that the average flux level dropped below the telescope sensitivity limit, as occasionally happens with Mrk 421 (Buckley et al. 1996). We currently consider the detection tentative because we see no evidence for a consistent signal nor is there independent confirmation of a high state for this object (e.g., from X-ray observations) during this period. Max. Observ. Exp. Daily Flux Object z Typea Epoch (hrs) Excess Exc. (Crab)b Fluxc 1ES 2344+514 0.044 X 1995/96 20.5 5.3σ 6.0σ 0.16±0.03 1.4±0.3 20-12-95 1.8 6.0σ 0.63±0.11 5.5±1.