9 Discussion and concluding remarks

The mid-IR spectrum of Mrk 279 shows a strong power-law continuum of spectral index $-0.80\pm0.05$, with weak PAH emission bands and no detectable silicate $\sim$9.7 $\mu$m feature. The mid-IR bump of Mrk 279 extends from roughly 1.25$\mu$m to 15-20$\mu$m and is wider than a single blackbody. It peaks near $\approx$3$\mu$m. In Fairall 9, the mid-IR bump most likely originates from the re-processing of UV and optical photons by nuclear dust (Sect. 1). We can estimate the distance to the central source of the innermost and hottest dust grains in Mrk 279, $r_{\rm in}$, by scaling directly from Fairall 9 (Clavel et al. 1989). Mrk 279 is approximately eight times less luminous than Fairall 9. Since the inner radius of the dust distribution is presumably controlled by sublimation, $r_{\rm in}$ should scale approximately as L^1/2, so $r_{\rm in}$ should be a factor of $\simeq2.8$ smaller in Mrk 279 than in Fairall 9, i.e. $r_{\rm in} \approx 140\pm36$ light-days.

During the ISO campaign, the mid-IR flux did not experience variations of amplitude larger than 10%, the detection limit of the PHT-S instrument. Optical data contemporaneous to the IR observations revealed significant fluctuations of the 5100Å flux with a relative rms amplitude of 9% and a ratio of the maximum to the minimum fluxes, $R_{\max}\,\sim\,1.64\pm0.11$. Any upper limit to the mid-IR variability in Mrk 279 has to be examined in the light of the UV and optical continuum variations over the same period of time. As noted earlier, in the dust reprocessing scenario the amplitude of the MIR flux variations will be reduced compared to that of the primary UV-optical source because of the finite propagation time of the photons. Imagine a short (duration $\leq$1 day) pulse of the UV-optical source illuminating a thin dust annulus, inclined by $i = 10^{\circ}$ with respect to the line of sight. The annulus IR response will be delayed by $\delta t\,=\,(1-\sin{i}) r_{\rm in}/c$and will last for $2(\sin{i}) r_{\rm in}/c$. For numerical values appropriate to Mrk 279, the duration of the IR reverberated pulse will be 38 days and its peak amplitude thereby reduced by a factor of order 38. Given the relatively low amplitude of the optical flux variations (Fig. 2), the absence of measurable variations of the MIR flux is consistent with the above scenario.

Though a detailed quantitative fit with a particular model is beyond the scope of this paper, it is nevertheless illustrative to perform a qualitative comparison of our data with the theoretical predictions from the torus model by Pier & Krolik (1992). This model predicts a mid-IR ``bump'' that is approximately 0.7 to 1 decade wide in wavelengths, in agreement with the Mrk 279 observations. In the Pier & Krolik (1992) model, the torus emission is expected to peak at a wavelength $\lambda_{\rm peak}$that depends primarily on the flux illuminating the torus inner surface and its inclination angle i with respect to the line of sight. The relatively high color temperature implied by $\lambda_{\rm peak} \approx3\,\mbox{$\mu$ m}$ constrains the inclination to be small ( $\cos{i} \geq 0.75$). The absence of silicate absorption also rules out very optically thick models and constrains the vertical column density at the torus inner edge, ${N_{\rm H}\,\leq\,10^{24}\,{\rm cm}^{-2}}$. Comparison of Fig. 4 with Fig. 5 of Pier & Krolik (1992) also suggests a moderately thick torus, with $r_{\rm in}/h = 0.3$.

The delay $\Delta T$ of H$\beta$ w.r.t. the optical continuum was 16.7^+5.3_-5.6 days during this campaign. Comparison with the results from previous monitoring campaigns (see Table 3) does not reveal any significant change of $\Delta T$ over a time span of $\sim$8 years. In other words, we find no evidence for a secular change in the structure of the BLR in Mrk 279. Equating ${c\,\times\,\Delta T}$ with the emissivity weighted radius ${R_{\rm BLR}}$ of the H$\beta$ emitting region, one sees that $r_{\rm in}$ is about 8 times larger than ${R_{\rm BLR}}$. In other words, the BLR lies well within the dust evaporation radius.

Acknowledgements

The authors are grateful to all the observatories involved for the generous allocation of observing time and José Acosta-Pulido for helpful discussions on the PHT-S instrument calibration. MS acknowledges partial support by Spanish CICYT grant PB-ESP95-0389-C02-02 and all the staff at the Laboratorio de Astrofísica Espacial y Física Fundamental, Spain where most of this work was done. Support for the ground-based observations was provided by the National Science Foundation through grant AST-9420080 to Ohio State University. Observations at the Wise Observatory are supported by grants from the Israel Science Foundation. This research has made use of the TARTARUS database, which is supported by Jane Turner and Kirpal Nandra under NASA grants NAG5-7385 and NAG5-7067.