4 Infrared observations  

The database contains infrared (IR) observations made through the eight filters J (1.25$\mu$m), H (1.65$\mu$m), K (2.2$\mu$m), L (3.45$\mu$m), $L^{\prime}$ (3.8$\mu$m), M (4.8$\mu$m), N (10$\mu$m) and Q (20$\mu$m), as well as very few far IR observations grouped together into the 60$\mu$m, the 100$\mu$m and the 240$\mu$m light curves (see Table 2).

The two main sources of observations in the IR are the 3.8m UKIRT on Mauna Kea, Hawaii and different ESO telescopes (1m, 2.2m and 3.6m) at La Silla, Chile. Since the flare of 1983, 3C 273 was intensively observed both from Mauna Kea and La Silla. The UKIRT observations of 3C 273 until 1993 and the ESO observations until 1990 were published by Litchfield et al. (1994). The same data are presented here, but with the recent unpublished ESO observations from 1990 to 1993 and some earlier measurements from the literature. Following Litchfield et al. (1994), all IR magnitudes were converted to fluxes, using the standard NASA Infrared Telescope Facility (IRTF) zero-magnitude fluxes, which are in Jy (10^-23ergcm^-2s^-1Hz^-1): 1600 (J), 1020 (H), 657 (K), 290 (L), 252 ($L^{\prime}$), 163 (M), 39.8 (N) and 10.4 (Q). The IRTF conversion has the advantage of being intermediate between the UKIRT and the ESO conversions (cf. Courvoisier et al. 1990). The use of a same conversion for all measurements enables the user to easily reconvert the fluxes into magnitudes if needed.

We included in the database the IR magnitude measurements from the literature reported by Allen (1976), Cutri et al. (1985), Elvis et al. (1994), Glass (1979), Hyland & Allen (1982), Kotilainen et al. (1992) (12$\hbox{$^{\prime\prime}$}$aperture), McLeod & Rieke (1994), O'Dell et al. (1978), Smith et al. (1987) and Takalo et al. (1992). All these magnitudes were converted into fluxes using the IRTF zero-magnitude fluxes given above. We also added the early 10 and 21$\mu$m flux measurements published by Rieke & Low (1972), as well as other flux densities given by Courvoisier et al. (1987), Robson et al. (1983, 1986), Roellig et al. (1986) and von Montigny et al. (1997). A light curve of early IR observations of 3C 273 from 1967 to 1978 is shown by Neugebauer et al. (1979), but unfortunately these data could not be obtained yet.

Above 20$\mu$m the only IR detections of 3C 273 are those performed by the Kuiper Airborne Observatory (KAO) at 107 and 240$\mu$m (Clegg et al. 1983), and by the Infrared Astronomical Satellite (IRAS) at 12, 25, 60 and 100$\mu$m (Neugebauer et al. 1986; Impey & Neugebauer 1988; Elvis et al. 1994).

The obtained H band light curve is compared to higher frequency light curves in Fig. 4. The contribution from the host galaxy in the H band is 6.4mJy. This value corresponds to $13-14\%$ of the mean H band flux and was obtained from the host galaxy's H magnitude of 13.0 (McLeod & Rieke 1994) by using the H band IRTF zero-magnitude flux (1020Jy). According to Fig. 1 of McLeod & Rieke (1995), the stellar contribution has been very roughly estimated to peak at $\lambda_{\mathrm{\,rest}}$$\sim$1$\mu$m with a value of $\sim$1.410^-11ergcm^-2s^-1 for 3C 273 (see Fig. 6).

  

Table 2: The infrared, optical and ultraviolet light curves of 3C 273 in the database. The parameters are as in Table 1 except that $\overline{F_{\nu}}$ and $\sigma_{\nu}$ are expressed in mJy

  

Figure 3: Passband functions of the seven filters in the Geneva photometric system. The observed position of the main emission-lines in 3C 273 is shown