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1. Polarized radiation transfer in neutron star surface layers

   https://doi.org/10.1093/mnras/staa3541  

   Barchas, Joseph A ; Hu, Kun ; Baring, Matthew G ( December 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The study of polarized radiation transfer in the highly magnetized surface locales of neutron stars is of great interest to the understanding of accreting X-ray pulsars, rotation-powered pulsars, and magnetars. This paper explores scattering transport in the classical magnetic Thomson domain that is of broad applicability to these neutron star classes. The development of a Monte Carlo simulation for the polarized radiative transfer is detailed: it employs an electric field vector formalism to enable a breadth of utility in relating linear, circular, and elliptical polarizations. The simulation can be applied to any neutron star surface locale, and is adaptable to accretion column and magnetospheric problems. Validation of the code for both intensity and Stokes parameter determination is illustrated in a variety of ways. Representative results for emergent polarization signals from surface layers are presented for both polar and equatorial magnetic locales, exhibiting contrasting signatures between the two regions. There is also a strong dependence of these characteristics on the ratio of the frequency $\, \omega \,$ of a photon to the cyclotron frequency $\, \omega _{\mathrm{B}}=eB/mc\,$. Polarization signatures for high-opacity domains are presented, highlighting compact analytical approximations for the Stokes parameters and anisotropy relative to the local field directionmore »for an extended range of frequencies. These are very useful in defining injection conditions deep in the simulation slab geometries, expediting the generation of emission signals from highly opaque stellar atmospheres. The results are interpreted throughout using the polarization characteristics of the magnetic Thomson differential cross-section.« less
2. CLEAR: High-ionization [Ne v] λ3426 Emission-line Galaxies at 1.4 < z < 2.3

   https://doi.org/10.3847/1538-4357/acc1e6  

   Cleri, Nikko J. ; Yang, Guang ; Papovich, Casey ; Trump, Jonathan R. ; Backhaus, Bren E. ; Estrada-Carpenter, Vicente ; Finkelstein, Steven L. ; Giavalisco, Mauro ; Hutchison, Taylor A. ; Ji, Zhiyuan ; et al ( May 2023 , The Astrophysical Journal)

   We analyze a sample of 25 [Nev] (λ3426) emission-line galaxies at 1.4 <z< 2.3 using Hubble Space Telescope/Wide Field Camera 3 G102 and G141 grism observations from the CANDELS LyαEmission at Reionization (CLEAR) survey. [Nev] emission probes extremely energetic photoionization (creation potential of 97.11 eV) and is often attributed to energetic radiation from active galactic nuclei (AGNs), shocks from supernovae, or an otherwise very hard ionizing spectrum from the stellar continuum. In this work, we use [Nev] in conjunction with other rest-frame UV/optical emission lines ([Oii]λλ3726, 3729, [Neiii]λ3869, Hβ, [Oiii]λλ4959, 5007, Hα+[Nii]λλ6548, 6583, [Sii]λλ6716, 6731), deep (2–7 Ms) X-ray observations (from Chandra), and mid-infrared imaging (from Spitzer) to study the origin of this emission and to place constraints on the nature of the ionizing engine. The majority of the [Nev]-detected galaxies have properties consistent with ionization from AGNs. However, for our [Nev]-selected sample, the X-ray luminosities are consistent with local (z≲ 0.1) X-ray-selected Seyferts, but the [Nev] luminosities are more consistent with those fromz∼ 1 X-ray-selected QSOs. The excess [Nev] emission requires either reduced hard X-rays or a ∼0.1 keV excess. We discuss possible origins of the apparent [Nev] excess, which could be related to the “soft (X-ray) excess”more »observed in some QSOs and Seyferts and/or be a consequence of a complex/anisotropic geometry for the narrow-line region, combined with absorption from a warm, relativistic wind ejected from the accretion disk. We also consider implications for future studies of extreme high-ionization systems in the epoch of reionization (z≳ 6) with the James Webb Space Telescope.

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3. Three-dimensional Dynamics of Strongly Twisted Magnetar Magnetospheres: Kinking Flux Tubes and Global Eruptions

   https://doi.org/10.3847/2041-8213/accada  

   Mahlmann, J. F. ; Philippov, A. A. ; Mewes, V. ; Ripperda, B. ; Most, E. R. ; Sironi, L. ( April 2023 , The Astrophysical Journal Letters)

   The origins of the various outbursts of hard X-rays from magnetars (highly magnetized neutron stars) are still unknown. We identify instabilities in relativistic magnetospheres that can explain a range of X-ray flare luminosities. Crustal surface motions can twist the magnetar magnetosphere by shifting the frozen-in footpoints of magnetic field lines in current-carrying flux bundles. Axisymmetric (2D) magnetospheres exhibit strong eruptive dynamics, i.e., catastrophic lateral instabilities triggered by a critical footpoint displacement ofψ_crit≳π. In contrast, our new three-dimensional (3D) twist models with finite surface extension capture important non-axisymmetric dynamics of twisted force-free flux bundles in dipolar magnetospheres. Besides the well-established global eruption resulting (as in 2D) from lateral instabilities, such 3D structures can develop helical, kink-like dynamics, and dissipate energy locally (confined eruptions). Up to 25% of the induced twist energy is dissipated and available to power X-ray flares in powerful global eruptions, with most of our models showing an energy release in the range of the most common X-ray outbursts, ≲10⁴³erg. Such events occur when significant energy builds up while deeply buried in the dipole magnetosphere. Less energetic outbursts likely precede powerful flares, due to intermittent instabilities and confined eruptions of a continuously twisting flux tube. Upon reaching amore »critical state, global eruptions produce the necessary Poynting-flux-dominated outflows required by models prescribing the fast radio burst production in the magnetar wind—for example, via relativistic magnetic reconnection or shocks.

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4. X-ray spectroscopy of the γ-ray brightest nova V906 Car (ASASSN-18fv)

   https://doi.org/10.1093/mnras/staa2104  

   Sokolovsky, Kirill V ; Mukai, Koji ; Chomiuk, Laura ; Lopes de Oliveira, Raimundo ; Aydi, Elias ; Li, Kwan-Lok ; Steinberg, Elad ; Vurm, Indrek ; Metzger, Brian D ; Kawash, Adam ; et al ( September 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Shocks in γ-ray emitting classical novae are expected to produce bright thermal and non-thermal X-rays. We test this prediction with simultaneous NuSTAR and Fermi/LAT observations of nova V906 Car, which exhibited the brightest GeV γ-ray emission to date. The nova is detected in hard X-rays while it is still γ-ray bright, but contrary to simple theoretical expectations, the detected 3.5–78 keV emission of V906 Car is much weaker than the simultaneously observed >100 MeV emission. No non-thermal X-ray emission is detected, and our deep limits imply that the γ-rays are likely hadronic. After correcting for substantial absorption (NH ≈ 2 × 1023 cm−2), the thermal X-ray luminosity (from a 9 keV optically thin plasma) is just ∼2 per cent of the γ-ray luminosity. We consider possible explanations for the low thermal X-ray luminosity, including the X-rays being suppressed by corrugated, radiative shock fronts or the X-rays from the γ-ray producing shock are hidden behind an even larger absorbing column (NH > 1025 cm−2). Adding XMM–Newton and Swift/XRT observations to our analysis, we find that the evolution of the intrinsic X-ray absorption requires the nova shell to be expelled 24 d after the outburst onset. The X-ray spectra show that the ejecta are enhanced in nitrogen and oxygen, and the nova occurredmore »on the surface of a CO-type white dwarf. We see no indication of a distinct supersoft phase in the X-ray light curve, which, after considering the absorption effects, may point to a low mass of the white dwarf hosting the nova.« less
5. Discovery and origin of the radio emission from the multiple stellar system KQ Vel

   https://doi.org/10.1093/mnras/stac2163  

   Leto, P. ; Oskinova, L. M. ; Buemi, C. S. ; Shultz, M. E. ; Cavallaro, F. ; Trigilio, C. ; Umana, G. ; Fossati, L. ; Pillitteri, I. ; Krtička, J. ; et al ( August 2022 , Monthly Notices of the Royal Astronomical Society)

   KQ Vel is a binary system composed of a slowly rotating magnetic Ap star with a companion of unknown nature. In this paper, we report the detection of its radio emission. We conducted a multifrequency radio campaign using the ATCA interferometer (band-names: 16 cm, 4 cm, and 15 mm). The target was detected in all bands. The most obvious explanation for the radio emission is that it originates in the magnetosphere of the Ap star, but this is shown unfeasible. The known stellar parameters of the Ap star enable us to exploit the scaling relationship for non-thermal gyro-synchrotron emission from early-type magnetic stars. This is a general relation demonstrating how radio emission from stars with centrifugal magnetospheres is supported by rotation. Using KQ Vel’s parameters the predicted radio luminosity is more than five orders of magnitudes lower than the measured one. The extremely long rotation period rules out the Ap star as the source of the observed radio emission. Other possible explanations for the radio emission from KQ Vel, involving its unknown companion, have been explored. A scenario that matches the observed features (i.e. radio luminosity and spectrum, correlation to X-rays) is a hierarchical stellar system, where the possible companion of the magnetic star ismore »a close binary (possibly of RS CVn type) with at least one magnetically active late-type star. To be compatible with the total mass of the system, the last scenario places strong constraints on the orbital inclination of the KQ Vel stellar system.

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