A: The Payload for Ultrahigh Energy Observations (PUEO) long-duration balloon experiment is designed to have world-leading sensitivity to ultrahigh-energy neutrinos at energies above 1 EeV. Probing this energy region is essential for understanding the extreme-energy universe at all distance scales. PUEO leverages experience from and supersedes the successful Antarctic Impulsive Transient Antenna (ANITA) program, with an improved design that drastically improves sensitivity by more than an order of magnitude at energies below 30 EeV. PUEO will either make the first significant detection of or set the best limits on ultrahigh-energy neutrino fluxes. K: Balloon instrumentation; Large detector systems for particle and astroparticle physics; Neutrino detectors A X P : 2010.02892

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The accretion-powered X-ray pulsar GX 301−2 was observed with the balloon-borne X-Calibur hard X-ray polarimeter during late December 2018, with contiguous observations by the NICER X-ray telescope, the Swift X-ray Telescope and Burst Alert Telescope, and the Fermi Gamma-ray Burst Monitor spanning several months. The observations detected the pulsar in a rare apastron flaring state coinciding with a significant spin-up of the pulsar discovered with the Fermi GBM. The X-Calibur, NICER, and Swift observations reveal a pulse profile strongly dominated by one main peak, and the NICER and Swift data show strong variation of the profile from pulse to pulse. The X-Calibur observations constrain for the first time the linear polarization of the 15-35 keV emission from a highly magnetized accreting neutron star, indicating a polarization degree of (27 +38 −27 )% (90% confidence limit) averaged over all pulse phases. We discuss the spin-up and the X-ray spectral and polarimetric results in the context of theoretical predictions. We conclude with a discussion of the scientific potential of future observations of highly magnetized neutron stars with the more sensitive follow-up mission XL-Calibur.

It is commonly assumed that in black hole accretion disks the angular momenta of the disk and the black hole are aligned. However, for a significant fraction of stellar mass black holes and supermassive black holes, the momenta may not be aligned. In such systems, the interplay of disk viscosity and general relativistic frame dragging can cause the disk to warp or break into two (or more) distinct planes; this is called the Bardeen-Petterson effect. We have developed a general relativistic ray-tracing code to find the energy spectra and polarization of warped accretion disks, accounting for the emission from the disk and for photons reflecting one or multiple times off the warped accretion disk segments. We find that polarization angle can be used to give a lower limit on the misalignment angle when a previous measurement of the jet, which is thought be aligned with the black hole angular momentum, can be spatially resolved.