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We introduce Semstem, a new method for the reconstruction of so called stemmatic trees, i.e., trees encoding the copying relationships among a set of textual variants. Our method is based on a structural expectation-maximization (structural EM) algorithm. It is the first computer-based method able to estimate general latent tree structures, unlike earlier methods that are usually restricted to bifurcating trees where all the extant texts are placed in the leaf nodes. We present experiments on two well known benchmark data sets, showing that the new method outperforms current state- of-the-art both in terms of a numerical score as well as interpretability.
We study BIC-like model selection criteria. In particular, we approximate the lower-order terms, which typically include the constant $\log \int \sqrt \det I(\theta) d\theta$, where $I(\theta)$ is the Fisher information at parameter value $\theta$. We observe that the constant can sometimes be a huge negative number that dominates the other terms in the criterion for moderate sample sizes. At least in the case of Markov sources, including the lower-order terms in the criteria dramatically degrades model selection accuracy. A take-home lesson is to keep it simple.
Comment: 15 pages, 7 figures, MNRAS accepted
Inverse Compton (IC) scattering is one of two viable mechanisms that can produce prompt non-thermal soft gamma-ray emission in gamma-ray bursts. IC requires low-energy seed photons and a population of relativistic electrons that upscatter them. The same electrons will upscatter the gamma-ray photons to even higher energies in the TeV range. Using the current upper limits on the prompt optical emission, we show that under general conservative assumption the IC mechanism suffers from an 'energy crisis'. Namely, IC will overproduce a very high energy component that would carry much more energy than the observed prompt gamma-rays, or alternatively it will require a low-energy seed that is more energetic than the prompt gamma-rays. Our analysis is general, and it makes no assumptions on the specific mechanism that produces the relativistic ...
The extremely bright optical flash that accompanied GRB 080319B suggested, at first glance, that the prompt γ-rays in this burst were produced by synchrotron self-Compton (SSC). We analyze here the observed optical and γ spectra. We find that the very strong optical emission imposes, due to self-absorption, very strong constraints on the emission processes and puts the origin of the optical emission at a very large radius, almost inconsistent with internal shock. Alternatively, it requires a very large random Lorentz factor for the electrons. We find that SSC could not have produced the prompt γ-rays. We also show that the optical emission and the γ-rays could not have been produced by synchrotron emission from two populations of electrons within the same emitting region. Thus, we must conclude that the optical emission and the γ-rays ...
Comment: Accepted for publication in MNRAS, the interpretation of GRB 080916C has been extended, main conclusions are unchanged
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