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1. Cosmic ray feedback in the FIRE simulations: constraining cosmic ray propagation with GeV gamma ray emission

   https://doi.org/10.1093/mnras/stz1895  

   Chan, T K; Kereš, D; Hopkins, P F; Quataert, E; Su, K-Y; Hayward, C C; Faucher-Giguère, C-A (July 2019, Monthly Notices of the Royal Astronomical Society)

   Abstract We present the implementation and the first results of cosmic ray (CR) feedback in the Feedback In Realistic Environments (FIRE) simulations. We investigate CR feedback in non-cosmological simulations of dwarf, sub-L⋆ starburst, and L⋆ galaxies with different propagation models, including advection, isotropic and anisotropic diffusion, and streaming along field lines with different transport coefficients. We simulate CR diffusion and streaming simultaneously in galaxies with high resolution, using a two moment method. We forward-model and compare to observations of γ-ray emission from nearby and starburst galaxies. We reproduce the γ-ray observations of dwarf and L⋆ galaxies with constant isotropic diffusion coefficient κ ∼ 3 × 1029 cm2 s−1. Advection-only and streaming-only models produce order-of-magnitude too large γ-ray luminosities in dwarf and L⋆ galaxies. We show that in models that match the γ-ray observations, most CRs escape low-gas-density galaxies (e.g. dwarfs) before significant collisional losses, while starburst galaxies are CR proton calorimeters. While adiabatic losses can be significant, they occur only after CRs escape galaxies, so they are only of secondary importance for γ-ray emissivities. Models where CRs are “trapped” in the star-forming disk have lower star formation efficiency, but these models are ruled out by γ-ray observations. For models with constant κ that match the γ-ray observations, CRs form extended halos with scale heights of several kpc to several tens of kpc. 

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2. Gamma ray signals from cosmic ray scattering on axionlike particles

   https://doi.org/10.1103/PhysRevD.104.055044  

   Dent, James B.; Dutta, Bhaskar; Newstead, Jayden L.; Rodriguez, Alejandro; Shoemaker, Ian M.; Tabrizi, Zahra; Arellano, Natalia Tapia (September 2021, Physical Review D)
3. Exploring composition mixing in kilonova ejecta with ray-by-ray simulations

   https://doi.org/10.1103/phzv-ldhz  

   Zhai, Ruocheng; Radice, David; Magistrelli, Fabio; Bernuzzi, Sebastiano; Perego, Albino (April 2026, Physical Review D)

   Binary neutron star merger (BNSM) ejecta are considered a primary repository of r-process nucleosynthesis and a source of the observed heavy-element abundances. We implement composition mixing into ray-by-ray radiation-hydrodynamic simulations of BNSM ejecta, coupled with an online nuclear network (NN). We model mixing via a gradient-based mixing approximation that evolves simultaneously with the hydrodynamics. We find that mixing occurs in regions where the electron fraction changes rapidly. While mixing smooths composition gradients in transition regions, it has a negligible impact on the heavy-element yields. This is because the primary r-process site (the equatorial ejecta) is initially homogeneous in free neutrons, leaving no strong gradients for mixing to act upon. In each angular ray, the abundances of the most produced elements are robust under mixing, while the less abundant ones are more affected. The total global abundances change only slightly from mixing, since each angular ray contributes its most abundant elements. Furthermore, the predicted kilonova light curves show only minor reddening, with differences below the detectability of state-of-the-art telescopes. In general, we do not observe significant effects from mixing in the time span of the r-process. Consequently, mixing only leads to minor variations in abundances and light curves in ray-by-ray simulations. 

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4. X-ray polarimetry of X-ray pulsar X Persei: another orthogonal rotator?

   https://doi.org/10.1093/mnras/stad1961  

   Mushtukov, A A; Tsygankov, S S; Poutanen, J; Doroshenko, V; Salganik, A; Costa, E; Marco, A Di; Heyl, J; Monaca, F La; Lutovinov, A A; et al (July 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT X Persei is a persistent low-luminosity X-ray pulsar of period of ≈ 835 s in a Be binary system. The field strength at the neutron star surface is not known precisely, but indirect signs indicate a magnetic field above 1013 G, which makes the object one of the most magnetized known X-ray pulsars. Here we present the results of observations X Persei performed with the Imaging X-ray Polarimetry Explorer (IXPE). The X-ray polarization signal was found to be strongly dependent on the spin phase of the pulsar. The energy-averaged polarization degree in 3–8 keV band varied from several to ∼20 per cent over the pulse with a phase dependence resembling the pulse profile. The polarization angle shows significant variation and makes two complete revolutions during the pulse period, resulting in nearly nil pulse-phase averaged polarization. Applying the rotating vector model to the IXPE data we obtain the estimates for the rotation axis inclination and its position angle on the sky, as well as for the magnetic obliquity. The derived inclination is close to the orbital inclination, reported earlier for X Persei. The polarimetric data imply a large angle between the rotation and magnetic dipole axes, which is similar to the result reported recently for the X-ray pulsar GRO J1008−57. After eliminating the effect of polarization angle rotation over the pulsar phase using the best-fitting rotating vector model, the strong dependence of the polarization degree with energy was discovered, with its value increasing from 0 at ∼2 keV to 30per cent at 8 keV. 

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5. X-Ray-to-radio Offset Inference from Low-count X-Ray Jets

   https://doi.org/10.3847/1538-4365/abd8d7  

   Reddy, Karthik; Georganopoulos, Markos; Meyer, Eileen T. (March 2021, The Astrophysical Journal Supplement Series)

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