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

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

Fluidodinamica bifase in condotti elicoidali per applicazioni nucleari

The widespread use of the helicoidal pipes in heat exchangers for air conditioning and refrigeration systems, chemical reactors, and nuclear power plants is due to their heat transfer high efficiency and their compactness. Depending on specific applications of these components, for adequate design purposes, it is important to have a full knowledge of fluid dynamics behavior, both in single and in two phase flow conditions. In fact, as it is well known, the secondary flow due to the centrifugal force in the cross section of the helicoidal pipes is a significant factor affecting the thermal-fluid dynamic

Fluidodinamica bifase in condotti elicoidali per applicazioni nucleari

The widespread use of the helicoidal pipes in heat exchangers for air conditioning and refrigeration systems, chemical reactors, and nuclear power plants is due to their heat transfer high efficiency and their compactness. Depending on specific applications of these components, for adequate design purposes, it is important to have a full knowledge of fluid dynamics behavior, both in single and in two phase flow conditions. In fact, as it is well known, the secondary flow due to the centrifugal force in the cross section of the helicoidal pipes is a significant factor affecting the thermal-fluid dynamics

Analysis of a XMM-Newton Spectrum of the Extremely Hot White Dwarf in Nova V4743 Sgr

Classical novae occur in close binary systems (main-sequence star + white dwarf). V4743 Sgr is a very fast nova exhibiting a X-ray spectrum which is consistent with a view on the optically thick atmosphere of an extremely hot white dwarf (WD). It exhibits strong absorption features of the C V, C VI, N VI, N VII, and O VII resonance lines indicating an effective temperature of about 610 kK. We present preliminary results of a spectral analysis by means of line-blanketed non-LTE atmosphere models of a XMM-Newton RGS-1 spectrum of V4743 Sgr, taken about half a year after its outburst.Comment: 3 pages, 1 figure, proceedings ITAMP Workshop: X-ray Diagnostics for Astrophysical Plasmas: Theory, Experiment, and Observation. 2004 November 15-17 Cambridge, US