5 The mid-IR spectrum of Mrk 279

Figure 3 shows the weighted mean 2.5-11.7$\mu$m spectrum of Mrk 279 obtained by averaging all 16 PHT-S spectra.

The spectrum of Mrk 279 is quite similar to the mean Seyfert 1 spectrum obtained by Clavel et al. (2000) from their sample of 28 type 1 AGNs. It shows a strong continuum, with a flux density per frequency unit that drops sharply with increasing frequency and only weak broad emission features. The continuum is well approximated by a power law ( $F_\nu \propto \nu^{\alpha}$) of spectral index $\alpha = -0.80 \pm 0.05$ (Fig. 3), close to the average Seyfert 1 mid-IR index $\alpha = -0.84 \pm 0.24$ (Clavel et al. 2000).

Figure 3: The weighted averaged mid-IR spectrum of Mrk 279 (dark line), the rms deviations about the mean (light line) and the error (dotted line). The best-fit power law with index $\alpha =-0.8$ is shown as a dashed line. The expected positions of PAH and Br$\alpha$ features are indicated. The x-axis shows the rest frequency at the bottom and the rest wavelength, in microns, at the top, both represented on a logarithmic scale

Figure 4: The spectral energy distribution (SED) of Mrk 279, from the far-IR to the X-rays. The IRAS data are represented as open squares whereas near-IR ground-based data are shown as stars. The PHT-S spectrum is shown as a dotted line, while ISOCAM fluxes are plotted as crosses where the horizontal bars indicate the filter range. The two open circles in the optical are the nuclear R- and B- band fluxes. The filled circle is the mean 5100Å flux. Optical and near IR data have been corrected for stellar light while UV and optical data have been corrected for galactic reddening. The best fit power laws mid-IR and UV ( $\alpha = -0.7$) continua are also displayed. The error bar on the UV flux represents the rms fluctuation about the mean 1500 Å due to variability. X-rays represent fits to EXOSAT and ASCA data. This SED was assembled from data gathered over a time span of $\sim$19 years

Its flux at a fiducial wavelength of 7$\mu$ is 103mJy. While the broad emission features of Polycyclic Aromatic Hydrocarbon (PAH) bands (Puget et al. 1985) at 3.3$\mu$m, 6.2$\mu$m, 7.7$\mu$m, and 8.6$\mu$m are ubiquitous in many different galactic and extragalactic line of sight, only the strongest band at 7.7$\mu$m is unambiguously detected in Mrk 279, with an intensity of $0.76\pm0.12$ mJy. Clavel et al. (2000) showed that PAH emission in AGNs originates in the interstellar medium (ISM) of the galaxy, whereas the mid-IR power-law continuum arises from near nuclear dust emission in the torus. Because Mrk 279 is a luminous AGN, almost a quasar, the apparent weakness of its PAH emission can be understood as a contrast effect whereby a faint ISM is observed against a bright nucleus. The 9.7$\mu$m silicate absorption feature, conspicuous in the mid-IR spectra of starburst galaxies (Moorwood et al. 1996; Rigopoulou et al. 1996; Acosta-Pulido et al. 1996), is absent from the Mrk 279 spectrum.

   

Table 3: Cross-correlation results

	This Work (H$\beta$)	Wise Obs. (H$\beta$)	Wise Obs. (H$\alpha$)	LAG (H$\alpha$)
Parameter	(1996)	(1988)	(1988)	(1990)
(1)	(2)	(3)	(4)	(5)
Cross-correlation centroid $\tau_{\rm cent}$ (days)	16.7+5.3-5.6	2.5+25.5-5.4	11.6+8.5-11.7	6.8+19.8-6.9
Cross-correlation peak $\tau_{\rm peak}$ (days)	21+2-9	3+28-5	11+11-11	2+29-3
Peak correlation coefficient $r_{\rm max}$	0.769	0.799	0.793	0.795
Continuum rms fractional variability $F_{\rm var}$	0.093	0.071	0.071	0.126
Mean time between observations (days)	2.3	4.1	4.1	6.0
Duration of campaign (days)	195	156	156	152