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Monte-Carlo radiation hydrodynamic simulations of line-driven disc winds: relaxing the isothermal approximation

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) (2025) staf1101

Authors:

Amin Mosallanezhad, Christian Knigge, Nicolas Scepi, James H Matthews, Knox S Long, Stuart A Sim, Austen Wallis

Abstract:

Abstract Disc winds play a crucial role in many accreting astrophysical systems across all scales. In accreting white dwarfs (AWDs) and active galactic nuclei (AGN), radiation pressure on spectral lines is a promising wind-driving mechanism. However, the efficiency of line driving is extremely sensitive to the ionization state of the flow, making it difficult to construct a reliable physical picture of these winds. Recently, we presented the first radiation-hydrodynamic (RHD) simulations for AWDs that incorporated detailed, multi-dimensional ionization calculations via fully frequency-dependent radiative transfer, using the Sirocco code coupled to Pluto. These simulations produced much weaker line-driven winds ($\dot{M}_{\rm wind}/\dot{M}_{\rm acc} < 10^{-5}$ for our adopted parameters) than earlier studies using more approximate treatments of ionization and radiative transfer (which yielded $\dot{M}_{\rm wind}/\dot{M}_{\rm acc} \simeq 10^{-4}$). One remaining limitation of our work was the assumption of an isothermal outflow. Here, we relax this by adopting an ideal gas equation of state and explicitly solving for the multi-dimensional temperature structure of the flow. In the AWD setting, accounting for the thermal state of the wind does not change the overall conclusions drawn from the isothermal approximation. Our new simulations confirm the line-driving efficiency problem: the predicted outflows are too highly ionized, meaning they neither create optimal driving conditions, nor reproduce the observed ultraviolet wind signatures. Possible solutions include wind clumping on sub-grid scales, a softer-than-expected spectral energy distribution or additional driving mechanisms. With the physics now built into our simulations, we are well-equipped to also explore line-driven disc winds in AGN.
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Joint Radiative and Kinematic Modelling of X-ray Binary Ejecta: Energy Estimate and Reverse Shock Detection

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) (2025)

Authors:

AJ Cooper, JH Matthews, F Carotenuto, R Fender, GP Lamb, TD Russell, N Sarin, K Savard, AA Zdziarski

Abstract:

Abstract Black hole X-ray binaries in outburst launch discrete, large-scale jet ejections which can propagate to parsec scales. The kinematics of these ejecta appear to be well described by relativistic blast wave models original devised for gamma-ray burst afterglows. In previous kinematic-only modelling, a crucial degeneracy prevented the initial ejecta energy and the interstellar medium density from being accurately determined. In this work, we present the first joint Bayesian modelling of the radiation and kinematics of a large-scale jet ejection from the X-ray binary MAXI J1535-571. We demonstrate that a reverse shock powers the bright, early ejecta emission. The joint model breaks the energetic degeneracy, and we find the ejecta has an initial energy of E0 ∼ 3 × 1043 erg, and propagates into a low density interstellar medium of nism ∼ 4 × 10−5 cm−3. The ejecta is consistent with being launched perpendicular to the disc and could be powered by an efficient conversion of available accretion power alone. This work lays the foundation for future parameter estimation studies using all available data of X-ray binary jet ejecta.
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Polarized Multiwavelength Emission from Pulsar Wind—Accretion Disk Interaction in a Transitional Millisecond Pulsar

The Astrophysical Journal Letters American Astronomical Society 987:1 (2025) L19-L19

Authors:

Maria Cristina Baglio, Francesco Coti Zelati, Alessandro Di Marco, Fabio La Monaca, Alessandro Papitto, Andrew K Hughes, Sergio Campana, David M Russell, Diego F Torres, Francesco Carotenuto, Stefano Covino, Domitilla de Martino, Stefano Giarratana, Sara E Motta, Kevin Alabarta, Paolo D’Avanzo, Giulia Illiano, Marco M Messa, Arianna Miraval Zanon, Nanda Rea

Abstract:

Abstract Transitional millisecond pulsars (tMSPs) bridge the evolutionary gap between accreting neutron stars in low-mass X-ray binaries and millisecond radio pulsars. These systems exhibit a unique subluminous X-ray state characterized by the presence of an accretion disk and rapid switches between high and low X-ray emission modes. The high mode features coherent millisecond pulsations spanning from the X-ray to the optical band. We present multiwavelength polarimetric observations of the tMSP PSR J1023+0038 aimed at conclusively identifying the physical mechanism powering its emission in the subluminous X-ray state. During the high mode, we report a probable detection of polarized emission in the 2–6 keV energy range, with a polarization degree of (12 ± 3)% and a polarization angle of −2∘ ± 9∘measured counterclockwise from the north celestial pole toward the east (99.7% confidence level, c.l.; uncertainties are quoted at 1σ). At optical wavelengths, we find a polarization degree of (1.41 ± 0.04)% and a polarization angle aligned with that in the X-rays, suggesting a common physical mechanism operating across these bands. Remarkably, the polarized flux spectrum matches the pulsed emission spectrum from optical to X-rays. The polarization properties differ markedly from those observed in other accreting neutron stars and isolated rotation-powered pulsars and are also inconsistent with an origin in a compact jet. Our results provide direct evidence that the polarized and pulsed emissions both originate from synchrotron radiation at the boundary region formed where the pulsar wind interacts with the inner regions of the accretion disk.
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Are FRBs emitted from rotating magnetospheres? Searching for periodicity in polarized bursts

Monthly Notices of the Royal Astronomical Society: Letters Oxford University Press (OUP) 542:1 (2025) L43-L47

Authors:

KM Rajwade, A Karastergiou

Abstract:

ABSTRACT One of the potential sources of repeating fast radio bursts (FRBs) is a rotating magnetosphere of a compact object, as suggested by the similarities in the polarization properties of FRBs and radio pulsars. Attempts to measure an underlying period in the times of arrival of repeating FRBs have nevertheless been unsuccessful. To explain this lack of observed periodicity, it is often suggested that the line of sight towards the source must be sampling active parts of the emitting magnetosphere throughout the rotation of the compact object, i.e. has a large duty cycle, as can be the case in a neutron star with near-aligned magnetic and rotation axes. This may lead to apparently aperiodic bursts; however, the polarization angle of the bursts should be tied to the rotational phase from which they occur. This is true for radio pulsars. We therefore propose a new test to identify a possible stable rotation period under the assumptions above, based on a periodogram of the measured polarization angle time series for repeating FRBs. We show that this test is highly sensitive when the duty cycle is large, where standard time-of-arrival periodicity searches fail. Therefore, we can directly test the hypothesis of repeating FRBs of magnetospheric origin with a stable rotation period. Both positive and negative results of the test applied to FRB data will provide important information.
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HI Intensity Mapping with the MIGHTEE Survey: First Results of the H i Power Spectrum

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) (2025) staf975

Authors:

Aishrila Mazumder, Laura Wolz, Zhaoting Chen, Sourabh Paul, Mario G Santos, Matt Jarvis, Junaid Townsend, Srikrishna Sekhar, Russ Taylor
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