3FGL DEMOGRAPHICS OUTSIDE THE GALACTIC PLANE USING SUPERVISED MACHINE LEARNING: PULSAR AND DARK MATTER SUBHALO INTERPRETATIONS
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Type of Work8 pages
Citation of Original PublicationN. Mirabal et al., 3FGL DEMOGRAPHICS OUTSIDE THE GALACTIC PLANE USING SUPERVISED MACHINE LEARNING: PULSAR AND DARK MATTER SUBHALO INTERPRETATIONS,The Astrophysical Journal, Volume 825, Number 1 (2016), doi: 10.3847/0004-637x/825/1/69
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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law
Nearly one-third of the sources listed in the Third Fermi Large Area Telescope (LAT) catalog (3FGL) remain unassociated. It is possible that predicted and even unanticipated gamma-ray source classes are present in these data waiting to be discovered. Taking advantage of the excellent spectral capabilities achieved by the Fermi LAT, we use machine-learning classifiers (Random Forest and XGBoost) to pinpoint potentially novel source classes in the unassociated 3FGL sample outside the Galactic plane. Here we report a total of 34 high-confidence Galactic candidates at | b|>= 5°. The currently favored standard astrophysical interpretations for these objects are pulsars or low-luminosity globular clusters hosting millisecond pulsars (MSPs). Yet these objects could also be interpreted as dark matter annihilation taking place in ultra-faint dwarf galaxies or dark matter subhalos. Unfortunately, Fermi LAT spectra are not sufficient to break degeneracies between the different scenarios. Careful visual inspection of archival optical images reveals no obvious evidence for low-luminosity globular clusters or ultra-faint dwarf galaxies inside the 95% error ellipses. If these are pulsars, this would bring the total number of MSPs at | b| >= 5° to 106, down to an energy flux ≈4.0 × 10⁻¹² erg cm⁻² s⁻¹ between 100 MeV and 100 GeV. We find this number to be in excellent agreement with predictions from a new population synthesis of MSPs that predicts 100–126 high-latitude 3FGL MSPs depending on the choice of high-energy emission model. If, however, these are dark matter substructures, we can place upper limits on the number of Galactic subhalos surviving today and on dark matter annihilation cross sections. These limits are beginning to approach the canonical thermal relic cross section for dark matter particle masses below ~100 GeV in the bottom quark (bb) annihilation channel.
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