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Pursuant to recommendations of the United Nations Conference on the Exploration and Peaceful Uses of Outer Space (UNISPACE III) and deliberations of the United Nations Committee on the Peaceful Uses of Outer Space (UNCOPUOS), annual UN/European Space Agency workshops on basic space science have been held around the world since 1991. These workshops contributed to the development of astrophysics and space science, particularly in developing nations. Following a process of prioritization, the workshops identified the following elements as particularly important for international cooperation in the field: (i) operation of astronomical telescope facilities implementing TRIPOD, (ii) virtual observatories, (iii) astrophysical data systems, (iv) concurrent design capabilities for the development of international space missions, and (v) theoretical astrophysics such as applications of nonextensive statistical mechanics. Beginning in 2005, the workshops focus on preparations for the International Heliophysical Year 2007 (IHY2007). The workshops continue to facilitate the establishment of astronomical telescope facilities as pursued by Japan and the development of low-cost, ground-based, world-wide instrument arrays as lead by the IHY secretariat., Comment: PDF, 9 pages

In this paper we consider the problem of the limits concerning the physical information that can be extracted from the analysis of one or more time series (light curves) typical of astrophysical objects. On the basis of theoretical considerations and numerical simulations, we show that with no a priori physical model there are not so many possibilities to obtain interpretable results. For this reason, the practice to develop more and more sophisticated statistical methods of time series analysis is not very productive. Only techniques of data analysis developed in a specific physical context can be expected to provide useful results. The field of {\it stochastic dynamics} appears to be an useful framework for such an approach. In particular, it is shown that modelling the experimental time series by means of the stochastic differential equations (SDE) represents a valuable tool of analysis. For example, the use of SDE permits to make the analysis of a continuous signal independent from the frequency sampling with which the experimental time series have been obtained. In this respect, an efficient approach based on the extended Kalman-filter technique is presented. Freely downloadable software is made available., Comment: 6 Figures. submitted to A&A

It is well-known that the noise associated with the collection of an astronomical image by a CCD camera is, in large part, Poissonian. One would expect, therefore, that computational approaches that incorporate this a priori information will be more effective than those that do not. The Richardson-Lucy (RL) algorithm, for example, can be viewed as a maximum-likelihood (ML) method for image deblurring when the data noise is assumed to be Poissonian. Least-squares (LS) approaches, on the other hand, arises from the assumption that the noise is Gaussian with fixed variance across pixels, which is rarely accurate. Given this, it is surprising that in many cases results obtained using LS techniques are relatively insensitive to whether the noise is Poissonian or Gaussian. Furthermore, in the presence of Poisson noise, results obtained using LS techniques are often comparable with those obtained by the RL algorithm. We seek an explanation of these phenomena via an examination of the regularization properties of particular LS algorithms. In addition, a careful analysis of the RL algorithm yields an explanation as to why it is more effective than LS approaches for star-like objects, and why it provides similar reconstructions for extended objects. We finish with a convergence analysis of the RL algorithm. Numerical results are presented throughout the paper. It is important to stress that the subject treated in this paper is not academic. In fact, in comparison with many ML algorithms, the LS algorithms are much easier to use and to implement, often provide faster convergence rates, and are much more flexible regarding the incorporation of constraints on the solution. Consequently, if little to no improvement is gained in the use of an ML approach over an LS algorithm, the latter will often be the preferred approach., Comment: High resolution images are available upon request. submitted to A&A

The UN/ESA Workshops on Basic Space Science is a long-term effort for the development of astronomy and regional and international co-operation in this field on a world wide basis, particularly in developing nations. The first four workshops in this series (India 1991, Costa Rica and Colombia 1992, Nigeria 1993, and Egypt 1994) addressed the status of astronomy in Asia and the Pacific, Latin America and the Caribbean, Africa, and Western Asia, respectively. One major recommendation that emanated from the first four workshops was that small astronomical facilities should be established in developing nations for research and education programmes at the university level and that such facilities should be networked. Subsequently, material for teaching and observing programmes for small optical telescopes were developed or recommended and astronomical telescope facilities have been inaugurated at UN/ESA Workshops on Basic Space Science in Sri Lanka (1995), Honduras (1997), and Jordan (1999). UN/ESA Workshops on Basic Space Science in Germany (1996), France (2000), Mauritius (2001), Argentina (2002), and P.R. China (2004) emphasised the particular importance of astrophysical data systems and the virtual observatory concept for the development of astronomy on a world wide basis. Since 1996, the workshops are contributing to the development of the World Space Observatory concept. Achievements of the series of workshops are briefly summarised in this report., Comment: 3 pages

The World Space Observatory/Ultraviolet (WSO/UV) represents a new mission implementation model for large space missions for astrophysics. The process has been brought up to enable, fully scientific needs driven, a logic to be applied to the demands for large collection powers required to undertake space missions which are complementary to the continuously increasing sensitivity of ground-based telescopes. One of the assumptions associated with the idea of a WSO is to avoid the excessive complexity required for multipurpose missions. Although there may exist purely technological or programmatic policy issues, which would suggest such more complex missions to be more attractive, many other aspects, which do not need to be explored in this report, may argue against such a mission model. Following this precept and other reasons, the first implementation model for a WSO has been done for the ultraviolet domain WSO/UV. WSO/UV is a follow-up project of the UN/ESA Workshops on Basic Space Science, organised annually since 1991., Comment: 4 pages

We consider the estimation of the regularization parameter for the simultaneous deblurring of multiple noisy images via Tikhonov regularization. We approach the problem in three ways. We first reduce the problem to a single-image deblurring for which the regularization parameter can be estimated through a classic generalized cross-validation (GCV) method. A modification of this function is used for correcting the undersmoothing typical of the original technique. With a second method, we minimize an average least-squares fit to the images and define a new GCV function. In the last approach, we use the classical $GCV$ on a single higher-dimensional image obtained by concatanating all the images into a single vector. With a reliable estimator of the regularization parameter, one can fully exploit the excellent computational characteristics typical of direct deblurring methods, which, especially for large images, makes them competitive with the more flexible but much slower iterative algorithms. The performance of the techniques is analyzed through numerical experiments. We find that under the independent homoscedastic and Gaussian assumptions made on the noise, the three approaches provide almost identical results with the first single image providing the practical advantage that no new software is required and the same image can be used with other deblurring algorithms., Comment: To appear in Astronomy & Astrophysics

We consider the simultaneous deblurring of a set of noisy images whose point spread functions are different but known and spatially invariant, and the noise is Gaussian. Currently available iterative algorithms that are typically used for this type of problem are computationally expensive, which makes their application for very large images impractical. We present a simple extension of a classical least-squares (LS) method where the multi-image deblurring is efficiently reduced to a computationally efficient single-image deblurring. In particular, we show that it is possible to remarkably improve the ill-conditioning of the LS problem by means of stable operations on the corresponding normal equations, which in turn speed up the convergence rate of the iterative algorithms. The performance and limitations of the method are analyzed through numerical simulations. Its connection with a column weighted least-squares approach is also considered in an appendix., Comment: 9 pages, 16 figures. High resolution figures available upon demand. To appear in A&A

We present the intrinsic spectral energy distribution (SED) of the NLS1 Arakelian 564, constructed with contemporaneous data obtained during a multi-wavelength, multi-satellite observing campaign in 2000 and 2001. We compare it with that of the NLS1 Ton S180 and with those obtained for BLS1s to infer how the relative accretion rates vary among the Sy1 population. Although the peak of the SED is not well constrained, most of the energy is emitted in the 10-100 eV regime, constituting roughly half of the emitted energy in the optical/X-ray ranges. This is consistent with a primary spectral component peaking in the extreme UV/soft X-ray band, and disk-corona models, hence high accretion rates. Indeed, we estimate that \dot{m}~1. We examine the emission lines in its spectrum, and we constrain the physical properties of the line-emitting gas through photoionization modeling. The line-emitting gas is characterized by log n~11 and log U~0, and is stratified around log U~0. Our estimate of the radius of the H\beta-emitting region ~10 \pm 2 lt-days is consistent with the radius-luminosity relationships found for Sy1 galaxies. We also find evidence for super-solar metallicity in this NLS1. We show that the emission lines are not good diagnostics for the underlying SEDs and that the absorption line studies offer a far more powerful tool to determine the ionizing continuum of AGNs, especially if comparing the lower- and higher-ionization lines., Comment: 15 pages, 10 figures; accepted for publication in The Astrophysical Journal, LaTeX emulateapj.sty

In this second paper in a series dedicated to developing efficient numerical techniques for the deblurring Cosmic Microwave Background (CMB) maps, we consider the case of asymmetric point spread functions (PSF). Although conceptually this problem is not different from the symmetric case, there are important differences from the computational point of view because it is no longer possible to use some of the efficient numerical techniques that work with symmetric PSFs. We present procedures that permit the use of efficient techniques even when this condition is not met. In particular, two methods are considered: a procedure based on a Kronecker approximation technique that can be implemented with the numerical methods used with symmetric PSFs but that has the limitation of requiring only mildly asymmetric PSFs. The second is a variant of the classic Tikhonov technique that works even with very asymmetric PSFs but that requires discarding the edges of the maps. We provide details for efficient implementations of the algorithms. Their performance is tested on simulated CMB maps., Comment: 9 pages, 13 Figures

Digital deblurring of images is an important problem that arises in multifrequency observations of the Cosmic Microwave Background (CMB) where, because of the width of the point spread functions (PSF), maps at different frequencies suffer a different loss of spatial resolution. Deblurring is useful for various reasons: first, it helps to restore high frequency components lost through the smoothing effect of the instrument's PSF; second, emissions at various frequencies observed with different resolutions can be better studied on a comparable resolution; third, some map-based component separation algorithms require maps with similar level of degradation. Because of computational efficiency, deblurring is usually done in the frequency domain. But this approach has some limitations as it requires spatial invariance of the PSF, stationarity of the noise, and is not flexible in the selection of more appropriate boundary conditions. Deblurring in real space is more flexible but usually not used because of its high computational cost. In this paper (the first in a series on the subject) we present new algorithms that allow the use of real space deblurring techniques even for very large images. In particular, we consider the use of Tikhonov deblurring of noisy maps with applications to {\it PLANCK}. We provide details for efficient implementations of the algorithms. Their performance is tested on Gaussian and non-Gaussian simulated CMB maps, and PSFs with both circular and elliptical symmetry. Matlab code is made available., Comment: 14 pages, 16, figures, A&A in press; high quality figures available upon request to the authors

The World Space Observatory is an unconventional space project proceeding via distributed studies. The present design, verified for feasibilty, consists of a 1.7-meter telescope operating at the second Largangian point of the Earth-Sun system. The focal plane instruments consist of three UV spectrometers covering the spectral band from Lyman alpha to the atmospheric cutoff with R~55,000 and offering long-slit capability over the same band with R~1,000. In addition, a number of UV and optical imagers view adjacent fields to that sampled by the spectrometers. Their performance compares well with that of HST/ACS and the spectral capabilities of WSO rival those of HST/COS., Comment: 11 pages, including 7 figures and 5 tables To be published in Proc SPIE, vol 4854

We summarize the capabilities of the World Space Observatory (UV) Project (WSO/UV). An example of the importance of this project (with a planned launch date of 2007/8) for the study of Classical Novae is given., Comment: 4 pages, To appear in the proceeedings of the "Classical Nova Explosions" conference, eds. M. Hernanz and J. Jose, AIP

We provide theoretical procedures and practical recipes to simulate non-Gaussian correlated, homogeneous random fields with prescribed marginal distributions and cross-correlation structure, either in a N-dimensional Cartesian space or on the celestial sphere. We illustrate our methods using far-infrared maps obtained with the Infrared Space Observatory. However, the methodology presented here can be used in other astrophysical applications that require modeling correlated features in sky maps, for example, the simulation of multifrequency sky maps where backgrounds, sources and noise are correlated and can be modeled by random fields., Comment: 24 pages, 4 figures. To appear in PASP

It is the purpose of the paper to describe the virtues of time-frequency methods for signal processing applications, having astronomical time series in mind. Different methods are considered and their potential usefulness respectively drawbacks are discussed and illustrated by examples. As areas where one can hope for a successful application of joint time-frequency analysis (JTFA), we describe specifically the problem of signal denoising as well as the question of signal separation which allows to separate signals (possibly overlapping in time or frequency, but) which are living on disjoint parts of the time-frequency plane. Some recipes for practical use of the algorithms are also provided., Comment: 16 pages 19 figures. A&A in press

Point-source contamination in high-precision Cosmic Microwave Background (CMB) maps severely affects the precision of cosmological parameter estimates. Among the methods that have been proposed for source detection, wavelet techniques based on ``optimal'' filters have been proposed.In this paper we show that these filters are in fact only restrictive cases of a more general class of matched filters that optimize signal-to-noise ratio and that have, in general, better source detection capabilities, especially for lower amplitude sources. These conclusions are confirmed by some numerical experiments. \keywords{Methods: data analysis -- Methods: statistical, Comment: 6 pages, 3 figures

We use Hubble Space Telescope UV and optical spectra of the narrow-line Seyfert 1 (NLS1) galaxy Akn 564 to investigate its internal reddening and properties of its emission-line and intrinsic UV absorption gas. We find that the extinction curve of Akn 564, derived from a comparison of its UV/optical continuum to that of an unreddened NLS1, lacks a 2200 A bump and turns up towards the UV at a longer wavelength (4000 A) than the standard Galactic, LMC, and SMC curves. However, it does not show the extremely steep rise to 1200 A that characterizes the extinction curve of the Seyfert 1 galaxy NGC 3227. The emission-lines and continuum experience the same amount of reddening, indicating the presence of a dust screen that is external to the narrow-line region (NLR). Echelle spectra from the Space Telescope Imaging Spectrograph show intrinsic UV absorption lines due to Ly-alpha, N V, C IV, Si IV, and Si III, centered at a radial velocity of -190 km/s (relative to the host galaxy). Photoionization models of the UV absorber indicate that it has a sufficient columnand is at a sufficient distance from the nucleus (D > 95 pc) to be the source of the dust screen. Thus, Akn 564 contains a dusty ``lukewarm absorber'' similar to that seen in NGC 3227., Comment: 25 pages, 4 figures. LaTeX with encapsulated postscript figures. Accepted for publication in the Astrophysical Journal

We present results of an intensive two-month campaign of approximately daily spectrophotometric monitoring of the narrow-line Seyfert 1 galaxy Akn 564 with HST. The fractional variability amplitude of the continuum variations between 1365-3000 A is ~6%, about a factor 3 less than that found in typical Seyfert 1 galaxies over a similar period of time. However, large amplitude, short time-scale flaring behavior is evident, with trough-to-peak flux changes of about 18% in approximately 3 days. We present evidence for wavelength-dependent continuum time delays, with the variations at 3000 A lagging behind those at 1365 A by about 1 day. These delays may be interpreted as evidence for a stratified continuum reprocessing region, possibly an accretion-disk structure. The Lyman-alpha 1216 emission-line exhibits flux variations of about 1% amplitude., Comment: 27 pages, 14 figures. Accepted by Astrophysical Journal

This paper deals with the definition of the input fluxes used for the calibration of the IUE Final Archive. The method adopted consists on the determination of the shape of the detector's sensitivity curves using IUE low resolution observations with model fluxes of the DA white dwarf G191-B2B. A scale factor was then determined so that the IUE observations of some bright OAO-2 standards match the original measurements from Meade (1978) in the spectral region 2100-2300 A. The ultraviolet fluxes of six standard stars used as input for the Final Archive photometric calibration together with the model fluxes of G191-B2B normalized to the OAO-2 scale are given. A comparison with the independent FOS calibration shows that the IUE flux scale for the Ultraviolet is 7.2% lower. We consider this mainly to be caused by the different normalization procedures. It is shown that the present flux calibration applies to spectra processed with the INES low resolution extraction software., Comment: 17 pages, 8 figures. To appear in Astronomy and Astrophysics

This paper discusses the overall quality of IUE high resolution data processed with the NEWSIPS software in terms of flux and wavelength accuracy. It also describes the processing of NEWSIPS high resolution spectra within the framework of the ESA ``IUE Newly Extracted Spectra'' (INES) System. This system provides the IUE high resolution data in two formats: the high resolution ``concatenated'' spectra, in which the spectral orders are connected, eliminating the overlap regions through a procedure designed to optimize the signal-to-noise ratio at the edges of the orders, and the ``rebinned'' spectra, i.e. the high resolution concatenated spectra resampled into the low resolution wavelength domain. Our study reveals the existence of a significant discrepancy in the wavelength scales of short and long wavelength NEWSIPS high resolution spectra. The INES processing applies a correction of +17.7 km/s to the wavelength scale of the high resolution SWP spectra in order to provide an internally consistent velocity scale. Similarly, suitable corrections have been applied to long wavelength small aperture spectra. The new wavelength scale is, within the errors, in good agreement with the optical velocity scale., Comment: 12 pages, 8 figures, accepted for publication in A&A Supplement Series

In this paper we document the results of the study which led to the ripple correction and absolute calibration algorithms applied to the high resolution spectra processed with the NEWSIPS software for the Final Archive of the IUE Project. In this analysis, based on a very large number of spectra, we find that both K and the alpha parameters (not only the former as previously believed) vary with order number. This fact, together with the finding that the central peaks of the blaze function vary also as a function of the THDA temperature (for the SWP camera) and of the date of observations (for the LWP and LWR cameras), makes the ripple correction algorithm more complex than previously considered but, at the same time, considerably more reliable. As for the high resolution absolute calibration, the method followed is similar to the one implemented in IUESIPS. The internal accuracy of the high resolution calibration is about 4%. We note that the ripple correction and absolute calibration algorithms here described apply also to IUE data processed and distributed with the INES system., Comment: 11 pages, 16 figures, accepted for publication in A&A Supplement Series

We present the extraction and processing of the IUE Low Dispersion spectra within the framework of the ESA ``IUE Newly Extracted Spectra'' (INES) System. Weak points of SWET, the optimal extraction implementation to produce the NEWSIPS output products (extracted spectra) are discussed, and the procedures implemented in INES to solve these problems are outlined. The more relevant modifications are: 1) the use of a new noise model, 2) a more accurate representation of the spatial profile of the spectrum and 3) a more reliable determination of the background. The INES extraction also includes a correction for the contamination by solar light in long wavelength spectra. Examples showing the improvements obtained in INES with respect to SWET are described. Finally, the linearity and repeatability characteristics of INES data are evaluated and the validity of the errors provided in the extraction is discussed., Comment: 15 pages, 18 figures, accepted for A&ASS

We present here the analysis of ultraviolet spectra from IUE and an X-ray spectrum from ROSAT PSPC observations of the X-ray weak, far-infrared loud AGN, PG 1126-041 (Mrk 1298). The first UV spectra taken in June 1992, simultaneously with ROSAT, show strong absorption lines of NV, CIV and SiIV, extending over a velocity range from -1000 to -5000 km/s with respect to the corresponding line centre. Our analysis shows that the Broad Emission Line Region (BELR) is, at least partially, covered by the material causing these absorption lines. In the IUE spectrum taken in Jan. 1995, the continuum was a factor of two brighter and the UV absorption lines are found to be considerably weaker than in 1992, but only little variation in the emission line fluxes is found. With UV spectral indices of A_{uv} \simeq 1.82 and 1.46 for the 1992 and 1995 data, the far UV spectrum is steep. Based on the emission line ratios and the broad band spectral energy distribution, we argue that the steepness of the UV spectrum is unlikely to be due to reddening. The soft X-ray emission in the ROSAT band is weak. A simple power-law model yields a very poor fit with a UV-to-X-ray spectral index A_{uvx}=2.32. Highly ionized (warm) absorption is suggested by the ROSAT data. After correcting for a warm absorber, the optical to X-ray spectral slope is close to the average of A{uvx}\simeq 1.67 for radio quiet quasars., Comment: 8 pages, 9 postscript figures. Mon. Not. Roy. Astr. Soc., accepted

We present a wide multi-wavelength database of most observations of the quasar 3C 273 obtained during the last 30 years. This database is the most complete set of observations available for an active galactic nucleus (AGN). It contains nearly 20'000 observations grouped together into 70 light curves covering 16 orders of magnitude in frequency from the radio to the gamma-ray domain. The database is constituted of many previously unpublished observations and of most publicly available data gathered in the literature and on the World Wide Web (WWW). It is complete to the best of our knowledge, except in the optical (UBV) domain where we chose not to add all observations from the literature. In addition to the photometric data, we present the spectra of 3C 273 obtained by the International Ultraviolet Explorer (IUE) satellite. In the X-ray domain, we used the spectral fit parameters from the literature to construct the light curves. Apart from describing the data, we show the most representative light curves and the average spectrum of 3C 273. The database is available on the WWW in a homogeneous and clear form and we wish to update it regularly by adding new observations., Comment: 12 pages, 6 figures, to be published in A&AS, data available at: http://obswww.unige.ch/3c273/

As part of an extensive multi-wavelength monitoring campaign, the International Ultraviolet Explorer satellite was used to observe the broad-line radio galaxy 3C 390.3 during the period 1994 December 31 to 1996 March 5. Spectra were obtained every 6-10 days. The UV continuum varied by a factor of 7 through the campaign, while the broad emission-lines varied by factors of 2-5. Unlike previously monitored Seyfert 1 galaxies, in which the X-ray continuum generally varies with a larger amplitude than the UV, in 3C 390.3 the UV continuum light-curve is similar in both amplitude and shape to the X-ray light-curve observed by ROSAT. The UV broad emission-line variability lags that of the UV continuum by 35-70 days for Ly-alpha and CIV 1549; values larger than those found for Seyfert 1 galaxies of comparable UV luminosity. These lags are also larger than those found for the Balmer lines in 3C 390.3 over the same period. The red and blue wings of CIV and Ly-alpha vary in phase, suggesting that radial motion does not dominate the kinematics of the UV line-emitting gas. Comparison with archival data provides evidence for velocity-dependent changes in the Ly-alpha and CIV line profiles, indicating evolution in the detailed properties and/or distribution of the broad-line emitting gas., Comment: Accepted for publication in the Astrophysical Journal. Paper of 29 pages, plus 10 pages of tables and 7 figures

We report on observations of redshifted HeII 304 A absorption in the QSO HE 2347-4342 (z=2.885, V=16.1) with the GHRS on board HST (FWHM=0.7 A). At the expected position of HeII 304 A absorption, HE 2347-4342 is the most UV-bright high redshift QSO discovered so far. We show that the HeII opacity as a function of redshift is patchy showing spectral regions with low HeII opacity (voids) and regions with high HeII opacity (troughs) and no detectable flux. Combination with high-resolution optical spectra of the Ly_alpha forest using CASPEC at the ESO 3.6m telescope shows that the voids can be explained either exclusively by Ly_alpha forest cloud absorption with a moderate N(HeII)/N(HI) ratio of 100 and turbulent line broadening or by a combination of Ly_alpha forest with N(HeII)/N(HI)=45 and thermal broadening plus a diffuse medium with tau_GP_HeII=0.3. Thus a strict upper limit to a diffuse medium is Omega_diff<= 0.02 h_50^(-1.5) at z=2.8. In the troughs an additional continuous HeII 304 A opacity tau > 2.8 is required. In case of photoionization, the troughs would require a diffuse component with a density close to Omega = 0.059 h_50^(-1.5), i.e. all baryons in the universe, which is inconsistent with the observed absence of such a component in the voids. A tentative interpretation is that we observe the epoch of partial HeII reionization of the universe with patches not yet reionized. In that case a diffuse component with Omega_diff >= 1.3 10^(-4) h_50^(-1) would be sufficient to explain the ``trough'' opacity., Comment: 12 pages, 6 figures, uses A&A l-aa.sty. To appear in A&A

The presence of narrow high-temperature emission lines from nitrogen-rich gas close to SN 1987A has been the principal observational constraint on the evolu- tionary status of the supernova's progenitor. A new analysis of the complete five-year set of low and high resolution IUE ultraviolet spectra of SN 1987A (1987.2--1992.3) provide fluxes for the N V 1240, N IV] 1486, He II 1640, OIII] 1665, NIII] 1751, and CIII] 1908 lines with significantly reduced random and systematic errors and reveals significant short-term fluctuations in the light curves. The N V, N IV] and N III] lines turn on sequentially over 15 to 20 days and show a progression from high to low ionization potential, implying an ioni- zation gradient in the emitting region. The line emission turns on suddenly at 83+/-4 days after the explosion, as defined by N IV]. The N III] line reaches peak luminosity at 399+/-15 days. A ring radius of (6.24+/-0.20)E{17} cm and inclination of 41.0+/-3.9 is derived from these times, assuming a circular ring. The probable role of resonant scattering in the N V light curve introduces systematic errors that leads us to exclude this line from the timing analysis. A new nebular analysis yields improved CNO abundance ratios N/C=6.1+/-1.1 and N/O=1.7+/-0.5, confirming the nitrogen enrichment found in our previous paper. From the late-time behavior of the light curves we find that the emission origi- nates from progressively lower density gas. We estimate the emitting mass near maximum (roughly 400 days) to be roughly 4.7E{-2} solar masses, assuming a filling factor of unity and an electron density of 2.6E4 cm^{-3}. These results are discussed in the context of current models for the emission and hydrodynamics of the ring., Comment: 38 pages, AASTeX v.4.0, 13 Postscript figures; ApJ, in press