2 Observations and data reduction

2.1 ISAAC/VLT observations

Near-infrared photometry and spectroscopy have been performed at the first 8-m Unit (Antu) of the Very Large Telescope (VLT, ESO, Chile), with the ISAAC IR spectro-imager in its imaging mode (Morwood et al. 1999; Cuby et al. 2000). In its SW mode (1-2.5 $\mu$m spectral range), ISAAC is equipped with a $1024 \times 1024$Hg:Cd:Te array, and has a field of view of $2.5\hbox{$^\prime$ }\times 2.5\hbox{$^\prime$ }$.

   
2.1.1 Near-infrared photometry

J, H, and $K{\rm s}$ photometry has been performed on 24th May 1999. We used the J, H, and $K{\rm s}$ filters, respectively centered at 1.25, 1.65, and 2.16 $\mu$m. Exposure times were 5 min in J filter, and 6 min in H and $K{\rm s}$ filters.

Observations were carried out using the jitter imaging technique, as usual in the infrared, that is, applying telescope offsets between frames (dither pattern). Data reduction was carried out with both the ESO-MIDAS and eclipse (Devillard 1997) data reduction packages. A combined image was generated using the jitter routine from the eclipse package. The main steps of this procedure are: i) flatfielding, using twilight flats, ii) estimate of a sky frame by median filtering of the individual frames in the stack, after scaling with the median of the number of counts in each image to account for sky intensity variations, iii) subtraction of the sky frame from all the individual frames, after proper scaling, iv) determination of the offsets between frames, based on approximate values present in the FITS header and using an auto-correlation technique, v) final image registration. More details about the routine can be found at http://www.eso.org/eclipse/.

The magnitude of Sycorax was then measured using classical aperture photometry for each filter. The results are presented in Table 2.

 

 

Table 2: Magnitudes of Sycorax in visible and near-infrared filters.

Date - Time (UT)	Instrument	Filter	Magnitude
24th May 1999 05:17	ISAAC	J	$19.53 \pm 0.10$
22th Aug. 2000 01:19	ISAAC	Js	$19.72 \pm 0.10$
9th Aug. 2000 23:58	ISAAC	Js	$19.74 \pm 0.15$
24th May 1999 08:35	ISAAC	H	$19.28 \pm 0.05$
24th May 1999 08:43	ISAAC	Ks	$19.07 \pm 0.05$
8th Aug. 2000 01:45	OIG	B	$21.48 \pm 0.07$
8th Aug. 2000 01:36	OIG	V	$20.74 \pm 0.06$
8th Aug. 2000 01:40	OIG	V	$20.76 \pm 0.07$
8th Aug. 2000 01:49	OIG	V	$20.77 \pm 0.07$
8th Aug. 2000 01:54	OIG	V	$20.74 \pm 0.06$
8th Aug. 2000 02:01	OIG	V	$20.72 \pm 0.06$
8th Aug. 2000 01:52	OIG	R	$20.24 \pm 0.03$
8th Aug. 2000 01:57	OIG	I	$19.68 \pm 0.02$
8th Aug. 2000 02:33	ARNICA	J	$19.63 \pm 0.15$

The zero points of the photometric calibration were measured from observations of S889-E, from the HST/Nicmos list of faint IR standard stars.

Further photometry was achieved in the $J{\rm s}$ filter by using spectroscopic acquisition images. The characteristics of the ISAAC J and $J{\rm s}$ and ARNICA filters are given in Table 3.

 

 

Table 3: Characteristics of the J filters used (for comparison of the different measurements of the J magnitude).

Instrument	Filter	Central wav. ($\mu$m)	Bandwidth ($\mu$m)
ISAAC	J	1.25	0.29
ISAAC	Js	1.24	0.16
ARNICA	J	1.251	0.27

2.1.2 Near-infrared spectroscopy

The spectroscopic observations took place at the VLT-Antu, in May 1999 and August-September 2000. The slit width was 1 arcsec, providing a spectral resolution R of $\sim$500. Three spectral domains corresponding to the J, H, and K bands were covered separately (see Table 4).

 

 

Table 4: Description of the spectroscopic observations.

Date (UT)	Spectral	Exp. Time	Airmass	Calib.
	range
May 24 1999	1.1-1.4	$1 \times 48$ min	1.01-1.04	128
08:56:21	$\mu$m			Nemesis
Aug. 22 2000	1.45-1.8	$4 \times 48$ min	1.01-1.17	HD
01:51:09	$\mu$m			1835
Sep. 07 2000	1.9-2.45	$6 \times 48$ min	1.01-1.53	HD
00:28:00	$\mu$m			144585

The object was nodded along the slit, between 2 positions A and B. In addition, some jittering was added around each of the A and B positions (except in May 1999), to allow a better removal of bad pixels. Between 4 and 6 images were taken at each A or B position (one cycle), and between 1 and 6 AB or BA cycles were executed sequentially, depending on the spectral range and integration time (see Table 4).

The data reduction was carried out, as for the imaging, with the ESO-MIDAS and eclipse packages. The frames were first flatfielded and corrected for distortions (spatial and spectral). The frames in the J band obtained in May 1999 were further corrected for bad pixels. Then A and Bimages belonging to one AB or BA cycle are averaged and subtracted. All A-B images from one sequence are then registered taking into account the telescope offsets as recorded in the FITS headers, and combined. The resultant image has one positive and one negative spectrum of the object, and some sky residuals resulting from the sky variations. The A-B image is then combined, after adequate offsetting, with the B-A image: this superimposes all the useful signal corresponding to the A and B positions, while removing the sky residuals. Some offsets (in wavelength) were occasionally noted within a sequence of frames, which were corrected for. All final images corresponding to different sequences with the same filter were then combined together, from which the object spectrum was finally extracted and wavelength calibrated using Xe and Ar arc frames taken in the morning following the observations.

Further processing was done for the removal of the telluric and solar features, dividing the Sycorax spectra by spectra of solar analogs: HD 1835 (G3V) and HD 144585 (G5V) (Hardorp 1978). A C-type asteroid (128 Nemesis) was also observed and its spectrum was used for removal of the telluric and solar features. C-types asteroids are known to have featureless and generally flat spectra in the near-infrared range, so they can be considered as good solar analogs.

The resulting spectra were finally smoothed with a Gaussian filter using $\sigma = 30$ pixels, degrading the final spectral resolution to $\sim$50 (see Fig. 1). The final signal-to-noise ratio obtained is about 20 to 30, depending on wavelength.

Figure 1: Spectral reflectivity of Sycorax in the J, H, and K ranges. The spectra have been adjusted using the colors derived from the photometric measurements. The lower curves correspond to the full resolution spectra (R=500), whereas the upper curves show the same spectra at a spectral resolution of 50. The upper curves have been offset by 1.5.

2.2 TNG observations - BVRI and J photometry

BVRI and J photometry of Sycorax has been carried out on 8th August 2000 at the Observatorio del Roque de los Muchachos (La Palma, Canary Islands). Observations were performed using the Telescopio Nazionale Galileo (TNG). TNG is a 3.5-m alt-azimuth telescope, with two Nasmyth foci and active optics control.

For the visible photometry, we used the Optical Imager Galileo (OIG) CCD camera mounted on the Nasmyth A focus. For the infrared photometry, we used the ARNICA near-infrared camera mounted on the same focus. This configuration allowed us to switch from OIG to ARNICA in order to obtain quasi-simultaneous visible and near-infrared observations, and avoid rotational effects. Details about the observations are given in Table 2.

   
2.2.1 Visible photometry

The OIG CCD camera is equiped with two $2048 \times 4096$ pixels chips, and has a field of view of $4.9\hbox{$^\prime$ }\times 4.9\hbox{$^\prime$ }$(binning $2 \times 2$). We used Bessel B, V, Cousins R, and Moult I filters, centered respectively at 436 nm, 533 nm, 625 nm, and 825 nm. Exposure times were 60 s for V and R filters, and 120 s for B and I filters.

Data reduction was performed using MIDAS software. Frames were first corrected from bias and flat-field. Then, instrumental magnitudes were measured using specific data reduction techniques developed for observations of TNOs (see Barucci et al. 2000 for details about the data reduction techniques used). Photometric calibration was performed using the usual calibration technique: 7 standard stars were observed at different airmasses during the night; zero-point and extinction coefficients were computed using a least square method. The resulting magnitudes are reported in Table 2. We compared the measurements previously published with our data, which are more accurate. Gladman et al. (1998) obtained for Sycorax visible colors of $B-R = 1.6 \pm 0.2$ and $R-I = 0.6 \pm 0.2$. Our measurement of the R-I color index ( $R-I = 0.57 \pm 0.04$) is fully consistent with the previous one, whereas our B-R value is lower ( $B-R = 1.23 \pm 0.08$).

2.2.2 J photometry

The ARNICA near-infrared camera is a HgTeCd array detector ( $256 \times 256$, i.e. a field of view of $90'' \times 90''$). We used the J broadband filter, centred at 1.251 $\mu$m (see Table 3). Exposure time was 4 min (four frames of 1 min, each frame being the co-average of four 15 s-frames). Observations were carried out using the jitter imaging technique. Data reduction was performed using both IRAF and MIDAS. The steps of the data reduction process are roughly the same as those used for VLT near-infrared photometric observations (see 2.1.1). Instrumental magnitudes were then obtained from the resulting frames using the MIDAS procedures developped for the visible observations (see 2.2.1). Calibration of the instrumental response was obtained by monitoring standard star fields at different airmasses (Hunt et al. 1998).