We report on an observational campaign on the bright black hole X-ray binarySwift J1727.8$-$1613 centered around five observations by the Imaging X-rayPolarimetry Explorer (IXPE). These observations track for the first time theevolution of the X-ray polarization of a black hole X-ray binary across a hardto soft state transition. The 2--8 keV polarization degree decreased from$\sim$4\% to $\sim$3\% across the five observations, but the polarization angleremained oriented in the North-South direction throughout. Based onobservations with the Australia Telescope Compact Array (ATCA), we find thatthe intrinsic 7.25 GHz radio polarization aligns with the X-ray polarization.Assuming the radio polarization aligns with the jet direction (which can betested in the future with higher spatial resolution images of the jet), ourresults imply that the X-ray corona is extended in the disk plane, rather thanalong the jet axis, for the entire hard intermediate state. This in turnimplies that the long ($\gtrsim$10 ms) soft lags that we measure with theNeutron star Interior Composition ExploreR (NICER) are dominated by processesother than pure light-crossing delays. Moreover, we find that the evolution ofthe soft lag amplitude with spectral state does not follow the trend seen forother sources, implying that Swift J1727.8$-$1613 is a member of a hithertounder-sampled sub-population.
We critically discuss the results reported in arXiv:2311.17994v1 by L.Oppenheim, M. Koch-Janusz, S. Gazit, and Z. Ringel, on the multicriticalbehavior of the three-dimensional Ising-Gauge model at the multicritical point.We argue that their results do not contradict the theoretical scenario putforward in ``Multicritical point of the three-dimensional ${\mathbb Z}_2$ gaugeHiggs model'', Phys. Rev. B 105, 165138 (2022), arXiv:2112.01824, thatpredicted a multicritical behavior controlled by the stable $XY$ fixed point ofan effective three-dimensional ${\mathbb Z}_2\oplus {\mathbb Z}_2$Landau-Ginzburg-Wilson $\Phi^4$ field theory. Actually, their results, as wellas all numerical results reported so far in the literature, are consistent witha multicritical $XY$ scenario.
Weakly magnetized neutron stars in X-ray binaries show complex phenomenologywith several spectral components that can be associated with the accretiondisk, boundary and/or spreading layer, a corona, and a wind. Spectroscopicinformation alone is, however, not enough to disentangle these components.Additional information about the nature of the spectral components and inparticular the geometry of the emission region can be provided by X-raypolarimetry. One of the objects of the class, a bright, persistent, and ratherpeculiar galactic Type I X-ray burster was observed with the Imaging X-rayPolarimetry Explorer (IXPE) and the X-ray Multi-Mirror Mission Newton(XMM-Newton). Using the XMM-Newton data we estimated the current state of thesource as well as detected strong absorption lines associated with theaccretion disk wind. IXPE data showed the source to be significantly polarizedin the 2-8 keV energy band with the overall polarization degree (PD) of 1.4% ata polarization angle (PA) of -2 degrees (errors at 68% confidence level).During the two-day long observation, we detected rotation of the PA by about 70degrees with the corresponding changes in the PD from 2% to non-detectable andthen up to 5%. These variations in polarization properties are not accompaniedby visible changes in spectroscopic characteristics. The energy-resolvedpolarimetric analysis showed a significant change in polarization, from beingstrongly dependent on energy at the beginning of the observation to beingalmost constant with energy in the later parts of the observation. As apossible interpretation, we suggest the presence of a constant component ofpolarization, strong wind scattering, or different polarization of the two mainspectral components with individually peculiar behavior. The rotation of the PAsuggests a 30-degree misalignment of the neutron star spin from the orbitalaxis.
The present work describes the development of heterogeneous GPGPU implicitCFD coupled solvers, encompassing both density- and pressure- based approaches.In this setup, the assembled linear matrix is offloaded onto multiple GPUsusing specialized external libraries to solve the linear problem efficiently.These coupled solvers are applied to two industrial test cases representingcommon scenarios: the NASA CRM in a transonic regime and the externalaerodynamics study of the DriveAER car. Significant performance enhancementsare evident when compared to their CPU counterparts. Specifically, the NASA CRMcase achieves an overall speedup of more than 4x, while the DriveAER test casedemonstrates improved stability and reduced computational time compared tosegregated solvers. All calculations were carried out utilizing the GPU-basedpartition of the davinci-1 supercomputer at the Leonardo Labs, featuring 82GPU-accelerated nodes.
We study the out-of-equilibrium behavior of statistical systems alongcritical relaxational flows arising from instantaneous quenches of thetemperature $T$ to the critical point $T_c$, starting from equilibriumconditions at time $t=0$. In the case of soft quenches, i.e. when the initialtemperature $T$ is assumed sufficiently close to $T_c$ (to keep the systemwithin the critical regime), the critical modes develop an out-of-equilibriumfinite-size scaling (FSS) behavior in terms of the rescaled time variable$\Theta=t/L^z$, where $t$ is the time interval after quenching, $L$ is the sizeof the system, and $z$ is the dynamic exponent associated with the dynamics.However, the realization of this picture is less clear when considering theenergy density, whose equilibrium scaling behavior (corresponding to thestarting point of the relaxational flow) is generally dominated by atemperature-dependent regular background term or mixing with the identityoperator. These issues are investigated by numerical analyses within thethree-dimensional lattice $N$-vector models, for $N=3$ and $N=4$, which provideexamples of critical behaviors with negative values of the specific-heatcritical exponent $\alpha$, implying that also the critical behavior of thespecific heat gets hidden by the background term. The results show that, aftersubtraction of its asymptotic critical value at $T_c$, the energy densitydevelops an asymptotic out-of-equilibrium FSS in terms of $\Theta$ as well,whose scaling function appears singular in the small-$\Theta$ limit.
We study the phase diagram and critical behaviors of three-dimensionallattice ${\mathbb Z}_2$-gauge $N$-vector models, in which an $N$-component realfield is minimally coupled with a ${\mathbb Z}_2$-gauge link variables. These models are invariant under global O($N$)and local ${\mathbb Z}_2$ transformations. They present three phasescharacterized by the spontaneous breaking of the global O($N$) symmetry and bythe different topological properties of the ${\mathbb Z}_2$-gauge correlations. We address the nature of the three transition linesseparating the three phases. The theoretical predictions are supported bynumerical finite-size scaling analyses of Monte Carlo data for the $N=2$ model.In this case, continuous transitions can be observed along both transitionlines where the spins order, in the regime of small and large inverse gaugecoupling $K$. Even though these continuous transitions belong to the same $XY$universality class, their critical modes turn out to be different. When thegauge variables are disordered (small $K$), the relevant order-parameter fieldis a gauge-invariant bilinear combination of the vector field. On the otherhand, when the gauge variables are ordered (large $K$), the order-parameterfield is the gauge-dependent $N$-vector field, whose critical behavior can onlybe probed by using a stochastic gauge fixing that reduces the gauge freedom.
We present the discovery of large radio shells around a massive pair ofinteracting galaxies and extended diffuse X-ray emission within the shells. Theradio data were obtained with the Australian Square Kilometer Array Pathfinder(ASKAP) in two frequency bands centred at 944 MHz and 1.4 GHz, respectively,while the X-ray data are from the XMM-Newton observatory. The host galaxy pair,which consists of the early-type galaxies ESO 184-G042 and LEDA 418116, is partof a loose group at a distance of only 75 Mpc (redshift z = 0.017). Theobserved outer radio shells (diameter ~ 145 kpc) and ridge-like centralemission of the system, ASKAP J1914-5433 (Physalis), are likely associated withmerger shocks during the formation of the central galaxy (ESO 184-G042) andresemble the new class of odd radio circles (ORCs). This is supported by thebrightest X-ray emission found offset from the centre of the Physalis system,instead centered at the less massive galaxy, LEDA 418116. The host galaxy pairis embedded in an irregular envelope of diffuse light, highlighting on-goinginteractions. We complement our combined radio and X-ray study withhigh-resolution simulations of the circumgalactic medium (CGM) around galaxymergers from the Magneticum project to analyse the evolutionary state of thePhysalis system. We argue that ORCs / radio shells could be produced by acombination of energy release from the central AGN and subsequent lightening upin radio emission by merger shocks traveling through the CGM of these systems.
Principal Component Analysis and biplots are so well-established and readilyimplemented that it is just too tempting to give for granted their internalworkings. In this note I get back to basics in comparing how PCA and biplotsare implemented in base-R and contributed R packages, leveraging animplementation-agnostic understanding of the computational structure of eachtechnique. I do so with a view to illustrating discrepancies that users mightfind elusive, as these arise from seemingly innocuous computational choicesmade under the hood. The proposed evaluation grid elevates aspects that areusually disregarded, including relationships that should hold if thecomputational rationale underpinning each technique is followed correctly.Strikingly, what is expected from these equivalences rarely follows withoutcaveats from the output of specific implementations alone.
Liquid argon is an excellent medium for detecting particles, given its yieldsand transport properties of light and charge. The technology of liquid argontime projection chambers has reached its full maturity after four decades ofcontinuous developments and is, or will be, used in world class experiments forneutrino and dark matter searches. The collection of ionization charge in thesedetectors allows to perform a complete tridimensional reconstruction of thetracks of charged particles, calorimetric measurements, particleidentification. This work proposes a novel approach to the problem of chargerecombination in liquid argon which moves from a microscopic model and isapplied to the cases of low energy electrons, alpha particles and nuclearrecoils. The model is able to describe precisely several sets of experimentaldata available in the literature, over wide ranges of electric field strengthsand kinetic energies and can be easily extended to other particles.