Constraining the GRB Collimation with a Survey for Orphan Afterglows

Gamma-ray bursts are believed to be produced in highly-relativistic collimated outflows. Support for this comes among others from the association of the times of detected breaks in the decay of afterglow light curves with the collimation angle of the jets. An alternative approach to estimate a limit on the collimation angle uses GRB afterglows without detected prompt-emission counterparts. Here we report on the analysis of a dedicated survey for the search of these orphan afterglows using the Wide Field Imager at the 2.2m MPI/ESO telescope at La Silla, Chile. We monitored ~12 square degrees. in up to 25 nights typically spaced by one to two nights with a limiting magnitude of R=23. Four previously unknown optical transients were discovered and three of these associated with a flare star, a cataclysmic variable and a dwarf nova. The fourth source shows indications for an extragalactic origin but the sparse sampling of the light curve prevents a reliable classification. We discuss the results in the context of the collimation of GRBs.Comment: 11 pages, A&A 449, 79-8

Nazism in Austria

Nazism emerged and thrived in Austria before World War II. A philosopher and writer on central European history examines some of the reasons why

Origin of the `He/N' and `Fe II' Spectral Classes of Novae

The spectra of postoutburst novae display either He+N or Fe II lines as the most prominent non-Balmer lines at maximum light. Spectral diagnostics indicate physical conditions for 'He/N' spectra that are consistent with their origin in the white dwarf (WD) ejecta, whereas 'Fe II' spectra point to their formation in a large circumbinary envelope of gas whose origin is the secondary star. A determining parameter for which of the two types of spectra predominates may be the binary mass ratio Msec/MWD. The increasing fraction of novae that are observed to be 'hybrid' objects, where both classes of spectra appear sequentially, is explained by changing parameters in the two emitting regions during the postoutburst decline. We argue that most novae may be hybrids that show both types of spectra during decline. The emission line intensity ratio O I {\lambda}8446/{\lambda}7773 is suggested as a good density diagnostic for the ejecta, and a finding list of emission lines identified in recent spectroscopic surveys of novae is presented as an aid to future line identification work.Comment: 14 pages, 2 table