J.-E. Arlot¹ - W. Thuillot¹ - C. Ruatti¹ - H. Akasawa² - S. Baroni¹² - W. Beisker¹³ - J. Berthier¹ - C. Blanco³ - J. Boonstra¹⁶ - J. Bourgeois¹⁷ - H. Bulder¹⁸ - R. Casas¹⁹ - J. G. Castano³⁵ - F. Colas¹ - D. Collins²⁰ - J. Cuypers²¹ - W. Czech¹⁵ - V. D'Ambrosio³ - H. Denzau²² - P. Descamps¹ - A. Dimitrescu⁸ - N. Dinakarian³⁴ - G. Dourneau⁵ - N. Emelyanov¹¹ - J. M. Enriquez²³ - J. M. Fernandez²⁴ - D. Fernandez-Barba¹ - T. Flatres⁵ - S. Foglia¹⁴ - M. Goncalves²⁸ - K. Guhl²⁶ - G. Helmer⁶ - T. Hirose⁴⁴ - T. R. Irsmambetova⁹ - B. A. Krobusek³⁹ - J. Lecacheux⁷ - J.-F. Le Campion⁵ - M. Lou³⁰ - A. Mallama⁴² - F. Marchis⁴ - M. A. S. Navarro³⁰ - P. Nelson³⁸ - N. Okura³¹ - J. Park³⁸ - T. Pauwels²¹ - S. Pluchino³⁶ - V. Priban²⁹ - M. Rapaport⁵ - J.-J. Sacré²⁵ - F. Salvaggio⁴⁰ - M. A. Sanchez³⁰ - F. Sanchez-Bajo²⁷ - G. Stefanescu⁸ - P. Tanga³³ - V. G. Tejfel¹⁰ - J. L. Trisan³⁰ - E. M. Trunkovsky¹¹ - J. Van Gestel⁴³ - G. Vandenbulcke³² - R. Vasundhara³⁴ - G. Vass⁸ - P. Vingerhoets²¹ - D. T. Vu¹ - R. T. Wilds³⁷

1 - Institut de mécanique céleste et de calcul des éphémérides, Observatoire de Paris, UMR 8028 CNRS, 77 avenue Denfert-Rochereau, 75014 Paris, France
2 - Funaho, Japan
3 - Dipartimento di Fisica e Astronomia dell'Universita di Catania, Italy
4 - European Southern Observatory, La Silla, Chile
5 - Observatoire de Bordeaux, France
6 - Observatoire de la Côte d'Azur, France
7 - Observatoire de Paris, France
8 - Institutul Astronomic, Bucuresti, Romania
9 - Krimean Laboratory of the Sternberg Astronomical Institute, Moscow, Russia
10 - Fesenkov Astrophysical Institute, Almaty, Kazakhstan
11 - Sternberg Astronomical Institute, Moscow, Russia
12 - Milano, Italy
13 - GSF, Munchen, Germany
14 - Observatorio S. Zani, Lumezzane, Italy
15 - Stuttgart, Germany
16 - Buinen, Netherlands
17 - Reux, Belgium
18 - Boskoop, The Netherlands
19 - IAC, Tenerife, Spain
20 - WWC, Asheville, USA
21 - ORB, Bruxelles, Belgium
22 - Essen, Germany
23 - San Fernando, Spain
24 - Sevilla, Spain
25 - Chateaugiron, France
26 - Marwitz, Germany
27 - Universidade de Extramadura, Badajoz, Spain
28 - Lisboa, Portugal
29 - Planetarium Praha, Czech Rep.
30 - GAS, Zaragossa, Spain
31 - Okayama, Japan
32 - Oosterduinkerke, Belgium
33 - OCA, Nice, France
34 - IIA, Bangalore, India
35 - Madrid, Spain
36 - OAIN, Scicli, Ragusa, Italy
37 - Topeka, Kansas, USA
38 - Ellinbank, Australia
39 - Farmington, New-York, USA
40 - GAC, Catania, Italy
41 - Barcelona, Spain
42 - Bowie, Maryland, USA
43 - Belgium
44 - Tokyo, Japan

Abstract
In 1997 the Sun and the Earth passed through the equatorial plane of Jupiter and therefore through the orbital planes of its main satellites. During this period, mutual eclipses and occultations occurred and were observed. We investigate the precision of the catalogue to produce improved data for the development of dynamical models. Light curves of mutual eclipses and occultations were recorded by the observers of the international campaign PHEMU97 organized by the Institut de Mécanique Céleste, Paris, France. We made 275 observations of 148 mutual events from 42 sites. For each observation, information is given about the telescope, the receiver, the site and the observational conditions. This paper gathers together the data and gives a first estimate of the precision. The catalogue of these rare events represents a collection of improved accurate astrometric data useful for the development of dynamical models.

Key words: eclipses - occultations - astrometry - astronomical data bases: miscellaneous

1 Introduction

Observations of mutual events of natural satellites have been performed intensively since 1973 and they have proved to be a very accurate way to obtain astrometric measurements of natural satellites. Many such events involving the Galilean satellites of Jupiter have been observed. In 1994-1996, similar events occurred in the Saturnian system. In 1997, we organized and coordinated an international campaign to observe these rare events. This campaign, named PHEMU97, allowed us to collect 275 lightcurves of 148 mutual events by our international network of 42 sites.

We present our results here. Another paper (Vasundhara et al. 2003) provides the astrometric data extracted from the lightcurves using a sophisticated photometric model including the albedo map deduced from the space probe images. The aim of the present paper is to provide the photometric data and the observational parameters useful for future work on the improvement of dynamical models as well as of models of the surfaces of the satellites. These data will be available through the data base NSDC dedicated to the natural satellites (http://www.imcce.fr/nsdc).

2 The mutual events

The Earth and the Sun cross the equatorial plane of Jupiter every six years. The Jovian declinations of the Earth and the Sun become zero and since the orbital plane of the Galilean satellites is very close to the equatorial plane of Jupiter, the satellites occult and eclipse each other.

The 1997 period was favorable since it occurred during the opposition of Jupiter and the Sun.

Arlot (1996) made predictions of all the 1997 events using the G5 ephemerides based on Lieske's theory (Lieske 1977) of the motion of the Galilean satellites. 182 dates of mutual events were computed. Several campaigns of observations took place during the previous occurrences. Table 1 shows the results obtained during each campaign. Our goal was to observe as many events as possible. Two observations of each event are needed to eliminate observational errors.

Since there is no thick atmosphere around the Galilean satellites, the photometric observations of such phenomena are very accurate for astrometric purposes. The results previously obtained after similar observations of the Galilean satellites (Arlot et al. 1997) show that an accuracy better than 30 mas can be expected.

This allows us to provide data necessary for the improvement of theoretical models of orbital motion and the determination of tide effects in the dynamics of Galilean satellites.

3 The PHEMU97 campaign

The observation of these phenomena required a coordinated international campaign in order to obtain a significant amount of data. These events occur over a short span of time so numerous observers located at several sites are necessary in order to avoid meteorological problems and observe from different longitudes to record different events. Thus observers previously involved in the PHEMU campaigns of observations of the mutual events of the Galilean satellites were invited to join the new campaign.

3.1 Receivers

For the observations of the mutual events only relative photometry is generally possible. Since the elevation of Jupiter above the horizon may be very small, the air mass is often too large and absolute photometry is not possible. The telescopes were equipped with the receivers listed in Table 2. Three kinds of receivers were used: photoelectric photometric single channel receivers, video cameras and two-dimensional CCD receivers. Visual observations are reported for comparison.

3.2 Sites of observation

This campaign, coordinated by the Institut de Mécanique Céleste, involved the different locations given in Table 3. This table gives the names, longitudes and latitudes of these sites.

4 Lightcurve reduction procedure

Lightcurves were derived from photometric measurements either with relative photometry performed with photoelectric photometers or with CCD cameras.

For observations made with CCD cameras in video mode the digitized signal was computed with digitizing boards. The lightcurves were obtained by aperture photometry, as were the observations for which a value of the diaphragm is provided. For CCD observations made in France, images were measured with the Gaussian photometry package of the ASTROL software (Colas 1996). Two dimensional measurements allow us to calibrate the signal of the satellite involved to the signal of a nearby satellite and to use data obtained under very difficult conditions (see for example Arlot et al. 1997).

The determination of the time of the minimum of light and of the value of the magnitude drop was based on a fit of the lightcurve with a sample polynomial. The errors in these determinations are also given. The error on the timing of the minimum is determined as follows: we calculate the noise in magnitude and transform it into a time error through the highest value of the decreasing speed in magnitude during the event. The largest errors occur for faint noisy events and the smallest for the fast ones. The errors are comparable only if the integration time is the same.

5 The catalogue

5.1 The data

These data and light-curves are available at the Natural Satellite Data Center (NSDC) server (http://www.imcce.fr/nsdc).

5.2 Discussion

This catalogue provides observational information and reduced data issued from the PHEMU97 campaign. Another paper (Vasundhara et al. 2003) provides the astrometric data extracted from the lightcurves.

The quality of each lightcurve may be judged either by the value of the errors on the determined parameters (time of the minimum of light and lightflux drop) or the appearance of the lightcurve itself.

As for the previous catalogues of such events, we computed the errors on the determined parameter as follows. The error on the lightflux drop is deduced from the standard deviation of the fit to the model light curve. The error on the date of the minimum is determined from the error on the magnitude drop combined with the speed of the decrease of the lightflux during the event. This explains that this error depends on the number of points, on the integrating time and on the depth of the light curve. Because of that, the error bars may be compared only between events recorded with the same time constants and, preferably, with the same equipment in order to obtain the observational error and a measurement of the quality of the observation.

6 Conclusion

We give in this paper the results of the PHEMU97 campaign. To record the maximum of events, it was necessary to organize an international campaign. The phenomena recorded occur every 6 years and they lead to very accurate astrometric measurements which are very difficult to obtain with ground-based techniques. Such data may allow us to determine surface parameters by comparison of lightcurves with synthetic models.

References

Online Material

$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich1.ps} \end{figure}$	Figure 1: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich2.ps} \end{figure}$	Figure 2: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich3.ps} \end{figure}$	Figure 3: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich4.ps} \end{figure}$	Figure 4: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich5.ps} \end{figure}$	Figure 5: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich6.ps} \end{figure}$	Figure 6: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich7.ps} \end{figure}$	Figure 7: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich8.ps} \end{figure}$	Figure 8: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich9.ps} \end{figure}$	Figure 9: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich10.ps} \end{figure}$	Figure 10: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich11.ps} \end{figure}$	Figure 11: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich12.ps} \end{figure}$	Figure 12: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich13.ps} \end{figure}$	Figure 13: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich14.ps} \end{figure}$	Figure 14: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich15.ps} \end{figure}$	Figure 15: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich16.ps} \end{figure}$	Figure 16: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich17.ps} \end{figure}$	Figure 17: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich18.ps} \end{figure}$	Figure 18: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich19.ps} \end{figure}$	Figure 19: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich20.ps} \end{figure}$	Figure 20: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich21.ps} \end{figure}$	Figure 21: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich22.ps} \end{figure}$	Figure 22: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich23.ps} \end{figure}$	Figure 23: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich24.ps} \end{figure}$	Figure 24: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich25.ps} \end{figure}$	Figure 25: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich26.ps} \end{figure}$	Figure 26: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich27.ps} \end{figure}$	Figure 27: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich28.ps} \end{figure}$	Figure 28: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich29.ps} \end{figure}$	Figure 29: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich30.ps} \end{figure}$	Figure 30: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich31.ps} \end{figure}$	Figure 31: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich32.ps} \end{figure}$	Figure 32: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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$\begin{figure} \par\includegraphics[width=10.3cm,clip]{zfich33.ps} \end{figure}$	Figure 33: Lightcurves from the observations of the mutual events of the Galilean satellites in 1996-1997.
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The PHEMU97 catalogue of observations of the mutual phenomena of the Galilean satellites of Jupiter

Online Material