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1. On the Interpretation of d o ( x )do(x)

   https://doi.org/10.1515/jci-2019-2002  

   Pearl, Judea (April 2019, Journal of Causal Inference)

   Abstract This paper provides empirical interpretation of the do(x) operator when applied to non-manipulable variables such as race, obesity, or cholesterol level. We view do(x) as an ideal intervention that provides valuable information on the effects of manipulable variables and is thus empirically testable. We draw parallels between this interpretation and ways of enabling machines to learn effects of untried actions from those tried. We end with the conclusion that researchers need not distinguish manipulable from non-manipulable variables; both types are equally eligible to receive the do(x) operator and to produce useful information for decision makers. 

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2. The X-Ray Polarimetry View of the Accreting Pulsar Cen X-3

   https://doi.org/10.3847/2041-8213/aca486  

   Tsygankov, Sergey S.; Doroshenko, Victor; Poutanen, Juri; Heyl, Jeremy; Mushtukov, Alexander A.; Caiazzo, Ilaria; Di Marco, Alessandro; Forsblom, Sofia V.; González-Caniulef, Denis; Klawin, Moritz; et al (December 2022, The Astrophysical Journal Letters)

   Abstract The first X-ray pulsar, Cen X-3, was discovered 50 yr ago. Radiation from such objects is expected to be highly polarized due to birefringence of plasma and vacuum associated with propagation of photons in the presence of the strong magnetic field. Here we present results of the observations of Cen X-3 performed with the Imaging X-ray Polarimetry Explorer. The source exhibited significant flux variability and was observed in two states different by a factor of ∼20 in flux. In the low-luminosity state, no significant polarization was found in either pulse phase-averaged (with a 3 σ upper limit of 12%) or phase-resolved (the 3 σ upper limits are 20%–30%) data. In the bright state, the polarization degree of 5.8% ± 0.3% and polarization angle of 49.°6 ± 1.°5 with a significance of about 20 σ were measured from the spectropolarimetric analysis of the phase-averaged data. The phase-resolved analysis showed a significant anticorrelation between the flux and the polarization degree, as well as strong variations of the polarization angle. The fit with the rotating vector model indicates a position angle of the pulsar spin axis of about 49° and a magnetic obliquity of 17°. The detected relatively low polarization can be explained if the upper layers of the neutron star surface are overheated by the accreted matter and the conversion of the polarization modes occurs within the transition region between the upper hot layer and a cooler underlying atmosphere. A fraction of polarization signal can also be produced by reflection of radiation from the neutron star surface and the accretion curtain. 

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3. Polarized x-rays constrain the disk-jet geometry in the black hole x-ray binary Cygnus X-1

   https://doi.org/10.1126/science.add5399  

   Krawczynski, Henric; Muleri, Fabio; Dovčiak, Michal; Veledina, Alexandra; Rodriguez Cavero, Nicole; Svoboda, Jiri; Ingram, Adam; Matt, Giorgio; Garcia, Javier A.; Loktev, Vladislav; et al (November 2022, Science)

   x-ray polarization measurements determine the geometric arrangement of hot material accreting onto a black hole. 

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4. The role of zero-mode contributions in the matching for the twist-3 PDFs e(x) and hL(x)

   S. Bhattacharya, K. Cichy (January 2020, Physical review)

   null (Ed.)
5. Localising pulsations in the hard X-ray and microwave emission of an X-class flare

   https://doi.org/10.1051/0004-6361/202348652  

   Collier, Hannah; Hayes, Laura A.; Yu, Sijie; Battaglia, Andrea F.; Ashfield, William; Polito, Vanessa; Harra, Louise K.; Krucker, Säm (April 2024, Astronomy & Astrophysics)

   Aims.The aim of this work is to identify the mechanism driving pulsations in hard X-ray (HXR) and microwave emission during solar flares. Using combined HXR and microwave observations from Solar Orbiter/STIX and EOVSA, we investigate an X1.3 GOES class flare, 2022-03-30T17:21:00, which displays pulsations on timescales evolving from ∼7 s in the impulsive phase to ∼35 s later in the flare. Methods.We analysed the temporal, spatial, and spectral evolution of the HXR and microwave pulsations during the impulsive phase of the flare. We reconstructed images for individual peaks in the impulsive phase and performed spectral fitting at high cadence throughout the first phase of pulsations. Results.Our imaging analysis demonstrates that the HXR and microwave emission originates from multiple sites along the flare ribbons. The brightest sources and the location of the emission change in time. Through HXR spectral analysis, the electron spectral index is found to be anti-correlated with the HXR flux, showing a “soft-hard-soft” spectral index evolution for each pulsation. The timing of the associated filament eruption coincides with the early impulsive phase. Conclusions.Our results indicate that periodic acceleration and/or injection of electrons from multiple sites along the flare arcade is responsible for the pulsations observed in HXR and microwave emission. The evolution of pulsation timescales is likely a result of changes in the 3D magnetic field configuration over time related to the associated filament eruption. 

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