GRB beaming, classifying stars

Andy Fruchter (STScI) gave the astrophysics seminar, on gamma-ray bursts and their host galaxies. He showed Modjaz's (and others) results on the metallicities of "broad-line type IIc" supernovae, which show that the ones associated with gamma-ray bursts are in much lower-metallicity environments than those not associated. I always react to this result by pointing out that this ought to put a very strong constraint on GRB beaming, because (if there is beaming) there ought to be "off-axis" bursts that we don't see as GRBs, but that we do see as a BLIIc. Both Fruchter and Modjaz claimed that the numbers make the constraint uninteresting, but I am surprised: The result is incredibly strong.

In group meeting, Fadely showed evidence that he can make a generative model of the colors and morphologies (think: angular sizes, or compactnesses) of faint, compact sources in the SDSS imaging data. That is, he can build a flexible model (using the "extreme deconvolution" method) that permits him to predict the compactness of a source given a noisy measurement of its five-band spectral energy distribution. This shows great promise to evolve into a non-parametric, model-free (that is: free of stellar or galaxy models) method for separating stars from galaxies in multi-band imaging. The cool thing is he might be able to create a data-driven star–galaxy classification system without training on any actual star or galaxy labels.

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