X-ray emission from classical and recurrent-novae observed with ROSAT

We have analysed 350 pointed and serendipitous observations of 108 different classical and recurrent novae in outburst and in quiescence, contained in the ROSAT archive. One aim was to search for super-soft X-ray sources and we found only 3 of them among post-novae. Thus, the super-soft X-ray phase of novae is relatively short lived (up to 10 years) and is observed only for up to 20% of novae. Most classical and recurrent novae instead emit hard X-rays (in the ROSAT band) in the first months after the outburst, with peak X-ray luminosity of a few times 10(33) erg/s. The emission, which we attribute to shocks in the nova ejecta, lasts at least 2 years and even much longer under special circumstances (like preexisting circumstellar material, or a prolonged wind phase). We also investigate X-ray emission due to accretion in quiescent novae. Only 11 out of 81 Galactic classical and recurrent novae were detected. The average X-ray uminosity is not higher than for dwarf novae, and some novae are variable in X-rays on time scales of years.Comment: tex file of the text and 8 figure

Post-imperialism, postcolonialism and beyond: towards a periodisation of cultural discourse about colonial legacies

Taking German history and culture as a starting point, this essay suggests a historical approach to reconceptualising different forms of literary engagement with colonial discourse, colonial legacies and (post-) colonial memory in the context of Comparative Postcolonial Studies. The deliberate blending of a historical, a conceptual and a political understanding of the ‘postcolonial’ in postcolonial scholarship raises problems of periodisation and historical terminology when, for example, anti-colonial discourse from the colonial period or colonialist discourse in Weimar Germany are labelled ‘postcolonial’. The colonial revisionism of Germany’s interwar period is more usefully classed as post-imperial, as are particular strands of retrospective engagement with colonial history and legacy in British, French and other European literatures and cultures after 1945. At the same time, some recent developments in Francophone, Anglophone and German literature, e.g. Afropolitan writing, move beyond defining features of postcolonial discourse and raise the question of the post-postcolonial

Zusammenhänge zwischen der Konstitution von Karbonsäuren und der Kinetik ihrer Dissoziation und Rekombination

Protolytische Reaktionen spielen eine wichtige Rolle als Teilschritte zahlreicher Reaktionsmechanismen in vielen Bereichen der Chemie und Biochemie. Es erschien daher interessant, die Geschwindigkeitskonstanten der Dissoziation und Rekombination einer Reihe konstitutionell verschiedener Carbonsäuren in wäßriger Lösung zu bestimmen, um über eine hinreichende Anzahl kinetischer Daten zu verfügen, die eine sinnvolle Korrelation mit den Struktureigenschaften dieser Säuren und dieFormulierung einiger allgemeiner Regeln auf kinetischer Grundlage erlauben. Die Dissoziation und Rekombination von Carbonsäuren verläuft im allgemeinen in wäßriger Lösung sehr schnell. Beispielsweise liegen die Halbwertszeiten des Dissoziationsschrittes vielfach zwischen etwa 50 und nur 1/100 $\mu sec$ und sind in manchen Fällen noch kürzer. Derartig schnelle Vorgänge lassen sich wegen des unüberwindbaren Problems einer hinreichenden Mischzeitverkürzung nicht mit den Verfahren der klassischen Reaktionskinetik studieren, sondern erforderten die Entwicklung prinzipiell neuer methodischer Wege