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Astron. Astrophys. 353, 465-472 (2000)

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5. Fluxes, SED and variability

The spectral energy distribution (SED) of the central region of the AGN is an essential parameter in the modeling. To derive this quantity, spatial resolution is obviously needed to disentangle the different sources of emission - dust, stars, non-thermal source - and, in that respect, AO observations bring precious information.

Fluxes at 3.5 and 4.8µm have been measured through circular apertures centered on the near-infrared peak, with a radius varying from 0.3" to 1.5" (22 to 100 pc). They are depicted in Table 2. A weak PAH line emission at 3.3µm has been detected as well, although no flux calibration is available for this observation, unfortunately.


[TABLE]

Table 2. Photometric data for NGC 1068 . Observed fluxes are in Jy.


The aperture flux density as a function of radius, over the region 22 pc [FORMULA] 100 pc, can be fitted with a power law: we find [FORMULA] and [FORMULA] (where the flux unit is Jy/arcsec2).

From the set of AO images (this paper and Rouan et al. 1998), as well as high resolution images obtained at 10µm (Alloin et al. 1999), we have reconstructed the SED of the core emission through an 0.6" diameter diaphragm, as shown in Fig. 2. Therefore, this SED refers only to the central engine and its immediate environment (including the inner parts of the dusty/molecular torus as well as some contribution from the NS extended structure). In this plot, the contribution from the stellar component has been removed in the J and H bands, following the spatial profile analysis performed by Rouan et al. (1998), while this is not the case in the K band. The part of the near-infrared to mid-infrared emission which is arising from hot to warm dust grains can be represented by a series of grey-bodies of different temperatures and this summation is expected to result in a smooth distribution. Yet, an emission bump at 2.2µm is observed, which could be interpreted as the unremoved stellar contribution in the K band, within the 0.6" diameter diaphragm. Under this assumption, an upper limit of 75% for the stellar contribution to the flux at 2.2µm, can be derived. This upper limit remains far larger than the 6% effectively derived by Thatte et al. (1997) within a 1" diameter diaphragm, on the basis of the dilution of the equivalent width of a CO absorption feature arising from cold stars.

[FIGURE] Fig. 2. SED of the central 0.6" diameter core (the value for the N band is given for 0.8", hence the dashed line between the M and N bands). The dotted line represents an interpolation for establishing the lower-limit of the dust contribution in the K band.

In any case, a revision of the AGN modeling for NGC 1068 should incorporate the 1 to 10µm SED derived for the innermost region around the central engine and therefore less affected by dilution from other surrounding components (in particular cleaned from the stellar contribution).

It is interesting as well to compare these 1996/1997 measurements in the infrared to those performed by Rieke & Low as early as 1975. Therefore, we have derived, for their 3" diameter diaphragm, the 1996/1997 observed fluxes at 1.25, 1.65 (both uncorrected for the stellar contribution), 2.2, 3.4, 4.8 and 10µm, from Rouan et al. (1998), from the current data set and from Alloin et al. (1999).

Before examining the temporal behavior of the near-infrared emission in NGC 1068 , the 1997 flux measurements in the K band can be compared to previous determinations. We consider again a 3" diameter diaphragm and compare the 1997 AO measurement to classical photometric measurements. Generally those are performed through much larger diaphragms. However Penston et al. (1974) provide a full set of measurements through diaphragms ranging from 12" to 2" diameter. Leaving aside the measurements from this paper which have been flagged down for bad transparency or other flaws, we obtain the K magnitude offset when one moves from a 12" diaphragm to a 3" diaphragm, [FORMULA]K = 0.85. This magnitude offset is not expected to vary with time because it refers to the outer parts of the AGN ([FORMULA] 110 pc). Then, from the 12" diameter diaphragm measurements by Glass (1995), who analyzed the variability properties of NGC 1068 , we can infer/extrapolate the K magnitude at the date of the AO measurement by Rouan et al. (1998): K [FORMULA] 6.81. Applying the magnitude offset computed above between the 12" and 3" diameter diaphragms, we predict that the K magnitude should be of 7.66 at the date of the Rouan et al. (1998) observation, while the K magnitude measured is of 7.26. This agreement is quite satisfactory, given the rather large error-bars involved in the Penston et al. (1974) data set which was obtained more than 25 years ago.

A comparison of the fluxes within a 3" diameter diaphragm at both epochs, 1975 (Rieke & Low 1975) and 1997 (this paper) is depicted in Table 3. One notices immediately that a flux increase has occurred over this time interval: by a factor 2 between 2.2 to 4.8µm, and by a factor 1.2, at 10µm. According to the independent photometric monitoring by Glass (1997), the L band (3.5µm) flux has doubled in 18 years, from 1974 to 1992: our result is in very good agreement with his finding.


[TABLE]

Table 3. Comparison of the infrared fluxes (given in Jy) at two epochs in a 3" diameter diaphragm


Because of the possibly complex way through which the UV-optical photons illuminate and heat the dust grains (direct and/or indirect illumination via scattering on the mirror to the N of the central engine or on dusty regions further out, Miller et al. 1991), it is not possible to infer the size of the dust component from light-echo effects acting differentially in the near-infrared and mid-infrared bands.

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999
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