Observations of Classical and Recurrent Novae with X-ray Gratings

X-ray grating spectra have opened a new window on the nova physics. High signal-to-noise spectra have been obtained for 12 novae after the outburst in the last 13 years with the Chandra and XMM-Newton gratings. They offer the only way to probe the temperature, effective gravity and chemical composition of the hydrogen burning white dwarf before it turns off. These spectra also allow an analysis of the ejecta, which can be photoionized by the hot white dwarf, but more often seem to undergo collisional ionization. The long observations required for the gratings have revealed semi-regular and irregular variability in X-ray flux and spectra. Large short term variability is especially evident in the first weeks after the ejecta have become transparent to the central supersoft X-ray source. Thanks to Chandra and XMM-Newton, we have discovered violent phenomena in the ejecta, discrete shell ejection, and clumpy emission regions. As expected, we have also unveiled the white dwarf characteristics. The peak white dwarf effective temperature in the targets of our samples varies between ~400,000 K and over a million K, with most cases closer to the upper end, although for two novae only upper limits around 200,000 K were obtained. A combination of results from different X-ray satellites and instruments, including Swift and ROSAT, shows that the shorter is the supersoft X-ray phase, the lower is the white dwarf peak effective temperature, consistently with theoretical predictions. The peak temperature is also inversely correlated with t(2) the time for a decay by 2 mag in optical. I strongly advocate the use of white dwarf atmospheric models to obtain a coherent physical picture of the hydrogen burning process and of the surrounding ejecta.Comment: Accepted in August of 2012 for the publication in the Bullettin of the Astronomical Society of India (BASI

Governance and The New Economic Order: At The Roots of Uncertainty. Managing Risks in The Service Economy

The Club of Rome has achieved worldwide renown, sometimes stimulated by heavy criticism, after the publication in 1972 of its report on «Limits to Growth». This was a very critical time since the high rate of growth of the economy in most of the industrialized countries after the World War II had been until then of around 6% per year. From 1973 until now, such rate of growth has declined to about 2 to 3 % per year in the average. The "scandal" of the Club of Rome consisted in the fact that doubts were expressed on the possibility of a continued, and as one would say today of a "sustainable", growth. This article summarizes another point of view: during those years, there has been a fundamental change in the way in which wealth is produced. The industrial revolution based essentially on investment in new machines, tools and products, had given way to the emergence of service functions- in all sectors of the economy - as the key factors of production. This issue is therefore essentially a view from the supply side of the economy. A series of reports were proposed through the Club of Rome to support this analysis based on over two decades of experience in the manufacturing as well as in the traditional service sector. The difficulty, which persists today, is that the classical and neo-economic analysis is still bound essentially to fundamentals linked to a reality in which the manufacturing system would be dominant. When services become determinant for the production of the wealth of nations, the very basic notion of economic value changes its connotations and the issue is at the end philosophical: value can not anymore be defined as the result of an equilibrium system where the disequilibria have to be considered as a matter of imperfect information. Such information is bound, in the service economy, to remain constantly imperfect because it involves the utilization of products and systems in time. A larger and larger part of costs in the performance of such systems in time is linked to future events where even that duration of utilization is uncertain. The value system is therefore basically dependent on the uncertainties of reality. The assumption is that the deterministic model, which is still dominant in the traditional macroeconomic analysis, has in fact given way to in deterministic systems. As a major consequence, the key economic issue today is that of understanding and managing, as fundamental problems, risks, uncertainty and vulnerability.

Suzaku observation of the classical nova V2491 Cyg in quiescence

We present Suzaku XIS observation of V2491 Cyg (Nova Cyg 2008 No. 2) obtained in quiescence, more than two years after the outburst. The nova was detected as a very luminous source in a wide spectral range from soft to hard X-rays. A very soft blackbody-like component peaking at 0.5 keV indicates that either we observe remaining, localized hydrogen burning on the surface of the white dwarf, or accretion onto a magnetized polar cap. In the second case, V2491 Cyg is a candidate "soft intermediate polar". We obtained the best fit for the X-ray spectra with several components: two of thermal plasma, a blackbody and a complex absorber. The later is typical of intermediate polars. The X-ray light-curve shows a modulation with a $\sim$38 min period. The amplitude of this modulation is strongly energy dependent and reaches maximum in the 0.8--2.0 keV range. We discuss the origin of the X-ray emission and pulsations, and the likelihood of the intermediate polar scenario.Comment: 8 pages, 7 figures, accepted to Astrophysical Journa

Variability and multi-periodic oscillations in the X-ray light curve of the classical nova V4743 Sgr

The classical nova V4743 Sgr was observed with XMM-Newton for about 10 hours on April 4 2003, 6.5 months after optical maximum. At this time, this nova had become the brightest supersoft X-ray source ever observed. We present the results of a time series analysis performed on the X-ray light curve obtained in this observation, and in a previous shorter observation done with Chandra 16 days earlier. Intense variability, with amplitude as large as 40% of the total flux, was observed both times. Similarities can be found between the two observations in the structure of the variations. Most of the variability is well represented as a combination of oscillations at a set of discrete frequencies lower than 1.7 mHz. At least five frequencies are constant over the 16 day time interval between the two observations. We suggest that a periods in the power spectrum of both light curves at the frequency of 0.75 mHz and its first harmonic are related to the spin period of the white dwarf in the system, and that other observed frequencies are signatures of nonradial white dwarf pulsations. A possible signal with a 24000 sec period is also found in the XMM-Newton light curve: a cycle and a half are clearly identified. This period is consistent with the 24278 s periodicity discovered in the optical light curve of the source and thought to be the orbital period of the nova binary system.Comment: In press in Monthly Notices of the Royal Astronomical Societ

Guided Tours Across a Collection of Historical Digital Images

We propose the use of short lectures, called narratives, embedded in a digital archive as a personalization method to support and guide users within a collection of historical material. The effectiveness of the approach has been evaluated with two groups of users. An analysis of the results has been conducted enabling the presentation of preliminary results

XMM-Newton observations of Nova Sgr 1998

We report on X-ray observations of Nova Sagittarius 1998 (V4633 Sgr), performed with XMM-Newton at three different epochs, 934, 1083 and 1265 days after discovery. The nova was detected with the EPIC cameras at all three epochs, with emission spanning the whole energy range from 0.2 to 10 keV. The X-ray spectra do not change significantly at the different epochs, and are well fitted for the first and third observations with a multi-temperature optically thin thermal plasma, while lower statistics in the second observations lead to a poorer fit. The thermal plasma emission is most probably originated in the shock heated ejecta, with chemical composition similar to that of a CO nova. However, we can not completely rule out reestablished accretion as the origin of the emission. We also obtain upper limits for the temperature and luminosity of a potential white dwarf atmospheric component, and conclude that hydrogen burning had already turned-off by the time of our observations.Comment: 18 pages, 3 figures. Accepted in Astrophysical Journa