In Search of X-rays from Uranus

<p>Within the solar system, X-ray emissions have been detected from every planet except the Ice Giants: Uranus and Neptune. Here, we present three Chandra X-ray Observations of Uranus (each 24-30 ks duration): an Advanced CCD Imaging Spectrometer (ACIS) observation during solar maximum on 7 August 2002 and two High Resolution Camera (HRC) observations during solar minimum on 11 and 12 November 2017. The ACIS observation from 2002 shows a low signal but statistically significant detection of X-rays from Uranus. The measured Uranus X-ray fluxes of 10<sup>-15</sup>-10<sup>-16 </sup>erg/cm<sup>2</sup>/s from this detection are consistent with upper limits and modelling predictions in previous work (Ness & Schmidt. 2000; Cravens et al. 2006).  The photon energy distribution from this observation is consistent with an X-ray emission from charge exchange or scattering of solar photons, as observed for Jupiter and Saturn. The two HRC observations from 2017 constitute non-detections. For 11 Nov 2017, the X-ray emission coincident with Uranus’ location is dimmer than 98% of the Field of View. 12 November 2017, was also a non-detection, but with tentative hints of planetary X-ray signatures: Uranus was 4 times brighter than the previous day, and brighter than 94% of the Field of View (1.6 standard deviations > Field of View mean). At this time, the Uranus coincident X-ray signature also exhibited timing variation distinct from the field of view. Further and longer observations will be required to better characterise the nature of the X-ray emissions from Uranus.</p>

Optimizing the role of impact ionization in conventional insulators

A mechanism for multiple carrier generation through impact ionization (IA) proposed earlier for bulk systems of strongly correlated insulators is generalized to the case of conventional insulators that contain localized bands a few eV above and below the highest occupied band. Specifically, we study the case of hybridization of localized orbitals with more dispersive bands near the Fermi level, where the generated multiple carriers, which ultimately decay to the edges of the dispersive bands by means of IA processes, acquire lighter mass and this could allow their more efficient separation before recombination. We argue that this may be applicable to the case of halide perovskites and it could be one of the reasons for their observed photovoltaic efficiency. We discuss the criteria one should use to uncover the appropriate material in order to harvest the optimum effect of IA for the spectrum of the solar photon energy distribution.

FCNC DECAYS OF B MESONS

The paper reviews recent results from the Belle experiment on the flavor changing neutral current processes b→sγ, b→dγ and b→sℓ+ℓ-. We report on a new measurement of the b→sγ inclusive rate and of the moments of the photon energy distribution. We also present the measurements of rate, isospin asymmetry and CP violation in the B→K*γ decays, as well as the CP asymmetry measurement in the inclusive B→Xsγ transitions. We report on the evidence for exclusive b→dγ transitions, and finally present the first observation of the B→K*ℓ+ℓ- decay and updated measurements of the B→Kℓ+ℓ- and B→Xsℓ+ℓ- branching fractions.

b→s+γ: A QCD-CONSISTENT ANALYSIS OF THE PHOTON ENERGY DISTRIBUTION

The photon energy distribution in the inclusive b→s+γ transitions is a combination of two components: the first component, soft physics, is determined by the so-called primordial distribution function, while the second component, perturbative physics, is governed by hard gluon emission. A simple ansatz is suggested for the primordial distribution function which obeys the QCD constraints known so far. We then discuss in detail how hard gluon emission affects the energy distribution. An extension of the Sudakov approximation is worked out incorporating the Brodsky-Lepage-Mackenzie prescription and its generalizations. We explicitly calculate the marriage of nonperturbative and perturbative effects in the way required by OPE, introducing separation scale µ. A few parameters, such as mb and [Formula: see text] affect the shape of the distribution and, thus, can be determined by matching to the experimental data. The data, still scarce, while not giving precise values for these parameters, yield consistency with theory: the current values of the above parameters lie within experimental uncertainty. On the theoretical side we outline a method allowing one to go beyond the practical version of OPE.