Feeding the spinning top

Spin evolution of accretion-powered pulsars
in high-mass X-ray binaries

A collaboration to improve our understanding of the accretion-induced torques onto neutron stars orbiting in the wind of a high-mass stellar companion.


Thanks to a composite team including computational astrophysicists, observers and modelers, we aim at tackling a long-standing problem in High-Mass X-ray Binaries (HMXBs): how are the young neutron stars (NSs) they host spun up and down due to the interaction between the intense magnetic field and the inflowing plasma provided by the stellar companion? Through these workshops, we will build up on past modelling efforts and brand new observations in order to develop a unified view on the accretion of plasma onto highly magnetized NSs.


HMXBs represent an advanced stage in the tumultuous history of massive binaries. The multiple detections of mergers between compact objects since 2015, including between two neutron stars in 2017, have revived the interest for this field since HMXBs are thought to be a formation channel leading to these mergers.

In HMXBs, a compact stellar remnant, generally a young and highly magnetized NS, orbits a massive blue supergiant and accretes a fraction of its radiatively-driven stellar wind. As it accretes material, the neutron star spins up or down, depending on the properties of the accreted plasma when it couples to the extended NS magnetosphere, a few hundreds Schwarzschild radii away from the NS surface. Once the accreted material enters the NS magnetosphere, it is funneled towards the magnetic poles where it produces X-ray bright spots precessing around the NS spin axis, which manifest as periodic X-ray pulses.

These accretion-powered pulsars have proven to be more challenging than their lower magnetic field siblings. For decades, the spin periods of NSs in many HMXBs have been monitored. They were found to be slow rotators and to show episodes of steady spinning up/down, separated by sudden torque reversals whose origin remains unknown. Contrary to predictions, the evolution of the NS spin and of the X-ray luminosity are not correlated. It highlights that we still lack a unified model to understand how the wind couples to the magnetic field, penetrates the magnetosphere and spins up or down the NS.

Science Goals

A brief summary of the goals of our project

The motivation behind these workshops is twofold. It is based on new insights brought both by cutting-edge observations of pulsars in HMXBs and by state-of-the-art numerical simulations of mass transfer via wind accretion. At the bottom of the latter lies the wind launching mechanism of hot stars which involves the resonant line-absorption of UV photons by thousands of transition between excitation levels of metal elements (Sander et al. 2017). The derived line-acceleration profiles have been used in simulations of HMXBs where, in spite of the absence of stellar Roche lobe overflow, the captured material was found to circularize into a disk-like structure before reaching the extended magnetosphere of the accreting NS (El Mellah et al. 2019). This encouraging result was later reinforced by subsequent observations which identified hints in favor of a transient wind-captured disk in the archetypal HMXB Vela X-1 (Liao et al. 2020).

On the other hand, recent X-ray timing analysis focused on the evolution of the NS spin over much shorter lapses of time and found important variations over a few hours which had been overlooked until now (Malacaria et al. 2020, Martin-Carrillo, priv. comm.). The behavior of the spin of accretion-powered pulsars is dictated by the torques applied at the outer edge of the magnetosphere by the ionized material supplied by the stellar wind but in spite of the multiple models designed over the past decades in order to describe this coupling, observations are still inconsistent with available interpretations. In order to connect these observations to the properties of the flow upstream, we need to determine how the accreted fraction of the wind couples to the magnetosphere and how angular momentum is transferred.

With this ISSI team, we managed to bring together experts of accreting pulsars, studied either from an observational or a theoretical point of view. We gathered people knowledgeable with pulsar timing techniques and time-resolved X-ray spectroscopy, along with people familiar with magnetic reconnection and magneto-centrifugal processes. We believe these domains of expertise will help us to develop a more comprehensive view on the accretion-induced torques measured in NS-hosting HMXBs. In particular, we intend to address the following goals:


International experts in modeling, numerical simulations, and X-ray observations

Christian Malacaria Christian Malacaria X-ray observations, spectral & timing analysis, compact objects, accretion
Enrico Bozzo Enrico Bozzo Theoretical models of accretion onto neutron stars, stellar winds, and X-ray observations
Miki Miljenko Čemeljič Magneto-hydrodynamics simulations & modeling of accretion disks
Pushpita Pushpita Das Relativistic magneto-hydrodynamic simulations and accretion onto compact objects
Ileyk El Mellah Ileyk El Mellah (PI) Wind accretion, compact objects, numerical simulations, clumpy winds of massive stars
Jonathan Jonathan Ferreira Theory of disk accretion onto NSs, magnetically-driven outflows
Felix Fuerst Felix Fürst X-ray and γ-ray observations, orbital and long-term variability
Vici Victoria Grinberg Stellar winds, time-resolved X-ray spectroscopy
Peter Kretschmar Peter Kretschmar X-ray and γ-ray observations, accretion mechanisms, variability
Antonis Manousakis Antonios Manousakis Hydrodynamic simulations, stellar winds, accretion, X-ray observations
Silvia Martinez Silvia Martínez-Núñez Near-IR, X-ray and γ-ray observations
Antonio Antonio Martin-Carrillo Data analysis, pulsar timing variability, X-ray binaries
Kyle Kyle Parfrey Relativistic plasma astrophysics and accretion onto compact objects
Konstantin Postnov Konstantin Postnov Relativistic astrophysics and stellar hydrodynamics
Bart Bart Ripperda Simulations of magnetic reconnection around accreting compact objects
Abigail Abigail Stevens X-ray spectral-timing, accretion onto compact objects in X-ray binaries
Sasha Alexander Tchekhovskoy Theory and simulations of disk accretion onto compact objects


List of publications made possible through our meetings at ISSI.


The proposal containing the main information on our science goals and team members can be found here
Our first meeting will take place at the ISSI in Bern during September 13-17 2021.
A preliminary agenda will be announced soon.
The second meeting is expected to take place late 2022 - early 2023.