next previous

5. Discussion

Comparison between the photometric lightcurves illustrated in Figs. 2 (click here) and 3 (click here) for the A, B, C and D lensed components of H1413+117 reveals a remarkably good match between the measurements derived using the two independent image analysis methods described in the previous section. Figure 4 (click here) compares the photometric measurements and their uncertainties derived using the two independent methods. Except for a few isolated data points, there is a very good overall agreement between these independent photometric determinations.

Figure 4: Relative magnitudes for the A, B, C and D lensed components of H1413+117 (all filters), with respect to the reference photometric star, derived using the automatic image decomposition technique (1) and the interactive CLEAN algorithm (2). The diagonal lines represent curves of perfect match between the two methods. Note that for the sake of clarity, the measurements pertaining to B, C, D and their associated diagonal lines have been translated by constant values with respect to the results for A

From our two observations in the B band (see Table 1), we note that the B component is weaker than the C component, while being almost equal in the V band (see Fig. 2 (click here) and Fig. 3 (click here)). Image B is also markedly stronger than image C in the I band than in the R band (see Table 1 for the independent photometric measurements). This is consistent with differential reddening caused by dust in the lensing galaxy along the different light paths of the lensed components. The prominence of absorption lines at redshifts z = 1.44, 1.66 in the spectrum of component B supports this interpretation (Magain et al. 1988) and
suggests searching for the lensing object(s) at those redshifts, in the vicinity of the B component.

The lightcurves for the A, B and C components clearly vary in a parallel fashion in all filters, starting at a
minimum luminosity in 1988, at the beginning of the ESO observation series, and peaking in 1991. The time delay for this system is expected to be shorter than one month (Kayser et al. 1990), so the quasi-simultaneous variations observed for these three components are consistent with intrinsic variations, as previously noted by Arnould et al. (1993) and Remy et al. (1996).

In order to avoid windowing effects on the time delay determinations (see Kayser 1993), the V and R lightcurves of component A, derived with method 1, have been smoothed using a simple low-pass spline with cut-off frequency of about (2 weeks)-1. The magnitude differences between the data and the smoothed curve fall approximately within the tex2html_wrap_inline1265 limit. Nevertheless, the tex2html_wrap_inline1101 for those magnitude differences, in a given filter, is found to be too high by a factor 4, at least. This could indicate the existence of photometric variations at higher frequencies but also of non-Gaussian errors or an oversmoothing of the data. In the next step of the analysis, the two smoothed lightcurves for component A (one for R and one for V) have been numerically translated in the epoch-magnitude diagram (tex2html_wrap_inline1275, tex2html_wrap_inline1277). For a given value of (tex2html_wrap_inline1275, tex2html_wrap_inline1277), the corresponding tex2html_wrap_inline1101 between the observations of a given component and the point of the translated smoothed lightcurve at the same epoch has been computed. As the high frequency variations disallow a direct statistical interpretation of the tex2html_wrap_inline1101 values, the latter ones have been normalized in two ways.

If we assume that the smoothed lightcurve of A should be compatible with the A data, the tex2html_wrap_inline1101 for each component have been normalized to the minimum tex2html_wrap_inline1101 of A (solid contours in Fig. 5 (click here)). Considering the A data points, the derived tex2html_wrap_inline1299 contour at the 99% confidence level (combined for the V and R filters in Fig. 5 (click here)), extends to about tex2html_wrap_inline1305 days. This sets a lower limit on the accuracy of the time delay determination based on this technique with the present data. For the B and C components, the time delays corresponding to a minimum tex2html_wrap_inline1101 computed independently for the two filters (not represented) are found to be non-consistent. This accounts for the relatively smaller size of the 99% compatibility domain of the combined tex2html_wrap_inline1101 for components B and C (see Fig. 5 (click here)). No acceptable values for the tex2html_wrap_inline1101 were found for the D component at the 99% confidence level. This is a definite indication for an abnormal behavior of the D lightcurve, incompatible with the simple interpretation in terms of time delay.

The tex2html_wrap_inline1101 for each component has also been normalized to the local minimum of that component (dashed contours in Fig. 5 (click here)). With this normalization, differences in the curves which would be incompatible with the hypothesis of the time delay phenomenon are hidden. An upper limit of about 150 days on the absolute value for the time delays remains. All the curves are thus also compatible with zero time delay.

Figure 5: 99% confidence level contours for the tex2html_wrap_inline1101 of the differences between a smoothed lightcurve of component A, translated by tex2html_wrap_inline1275 in epoch and tex2html_wrap_inline1277 in magnitude, and the measurements of the four components, as derived by method 1. The smoothed lightcurve has been defined by a spline interpolation of the A measurements; hence the differences in epoch and magnitudes are relative to A. The tex2html_wrap_inline1101 computed independently for the V and R data have been combined in the present diagram and normalized to the minimum tex2html_wrap_inline1101 for A (solid contours) or to the minimum tex2html_wrap_inline1101 for the given component (dashed contours). Crosses indicate the positions of the minima

We conclude that the present lightcurves of the Cloverleaf quasar are not sufficiently well sampled and/or accurate in order to estimate precise values for the time delays, only upper limits can be set. However, given the noticeable intrinsic photometric variability of H1413+117, we feel confident that a better sampling of the lightcurves will lead to an accurate determination of the time delays for this system.

Analysis of our observations does not show any clear sign of micro-lensing activity for the A, B and C components. The photometric variations for the D component, however, deviate somewhat from the parallel behavior seen for the other three lensed QSO images. Particularly during the 1992 season, there is a steep decrease in the flux of this component by approximately 0.1 magnitude during three months, while the other components remain relatively constant. The event is seen in both the I and R bands, whereas the V and B bands have insufficient observations to substantiate this. Variations during 1988 for the D component reported by Remy et al. (1996) in the V band were also interpreted as being possibly caused by micro-lensing. Angonin et al. concluded from spectra obtained on March 7, 1989 that the continuum of image D was slightly bluer than the other three images. From our observations in 1988 and 1994 we find no significant blue excess in the D component to within
0.03 mag. Unfortunately the observations from 1989 are not of sufficient quality to draw any definite conclusions.

In conclusion, pending a direct identification of the lensing galaxy, the multiply imaged QSO H1413+117

constitutes a very good candidate for a direct and independent determination of the Hubble parameter H0. A photometric monitoring of this optical source, with optimal time coverage, should be organized in the near future. International collaboration will be essential for its success.


RØ would like to thank the STScI, the Norwegian Research Council and the Stockholm Observatory for financial support during a two month stay at STScI. MR and JS would like to thank B. Revenaz for his help at STScI and the PRODEX grant "Gravitational Lensing with HST'' for financial support. AS thanks for financial support under grant no 781-73-058 from the Netherlands Foundation for Research in Astronomy (ASTRON) which receives its funds from the Netherlands Organisation for Scientific Research (NWO).

next previous

Copyright by the European Southern Observatory (ESO)