When two black holes merge, a tremendous amount of energy is released in the form of gravitational radiation in a short span of time, making such events among the most luminous phenomenon in the Universe. Models that predict the peak luminosity of black hole mergers are of interest to the gravitational wave community, with potential applications in tests of general relativity. We present a surrogate model for the peak luminosity that is directly trained on numerical relativity simulations of precessing binary black holes. Using Gaussian process regression, we interpolate the peak luminosity in the seven-dimensional parameter space of precessing binaries with mass ratios q ≤ 4 and spin magnitudes χ 1 , χ 2 ≤ 0.8. We demonstrate that our errors in estimating the peak luminosity are lower than those of existing fitting formulas by about an order of magnitude. In addition, we construct a model for the peak luminosity of aligned-spin binaries with mass ratios q ≤ 8 and spin magnitudes jχ 1z j; jχ 2z j ≤ 0.8. We apply our precessing model to infer the peak luminosity of the GW event GW190521 and find the results to be consistent with previous predictions.
Located ∼100 pc from the dynamic center of the Milky Way, the molecular cloud Sagittarius B2 (Sgr B2) is the most massive such object in the Galactic Center region. In X-rays, Sgr B2 shows a prominent neutral Fe K line at 6.4 keV and continuum emission beyond 10 keV, indicating high-energy, non-thermal processes in the cloud. The Sgr B2 complex is an X-ray reflection nebula whose total emissions have decreased since the year 2001 as it reprocesses what are likely one or more past energetic outbursts from the supermassive black hole Sagittarius A*. The X-ray reflection model explains the observed time-variability of the Fe K and hard X-ray emissions, and it provides a window into the luminous history of our nearest supermassive black hole. In light of evidence of elevated cosmic particle populations in the Galactic Center, recent interest has also focused on X-rays from Sgr B2 as a probe of low-energy (sub-GeV) cosmic particles. In contrast to X-ray reflection, in this case we can assume that the X-ray flux contribution from ionization by low-energy cosmic particles is constant in time, such that upper limits on low-energy cosmic particle populations may be obtained using the lowest flux levels observed from the cloud. Here, we present the most recent and correspondingly dimmest NuSTAR and XMM-Newton observations of Sgr B2, from 2018. These reveal small-scale variations within lower density portions of the Sgr B complex, including brightening features, and enable the best upper limits on ionization of molecular clouds by low-energy cosmic particles in the inner ∼ 100 pc of the Galaxy.
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