Pulsars, transients and relativistic astrophysics

Abstract:

We report results from a multi-mission observational campaign of the transient X-ray pulsar 2S 1417−624 during its 2025 outburst, using data from NICER, IXPE, and NuSTAR. Phase-averaged and phase-resolved spectroscopy with NICER and NuSTAR reveal that a typical broken power-law model with a high-energy cutoff describes the broadband spectra well. Several spectral parameters, however, show clear and systematic modulations with pulse phase, indicating variations in the physical conditions of the emitting plasma over the neutron star’s rotation. IXPE provides the first polarimetric measurements of this source, yielding a phase-averaged polarization degree (PD) of 4.8 ± 1.2% and a polarization angle (PA) of 17° ±7°, both quoted at the 1 σ confidence level. Fitting the phase-resolved PA with the rotating vector model (RVM) gives a magnetic obliquity of θ = 69 −29 +13 deg, indicating a significantly inclined magnetic geometry that may approach a quasi-orthogonal configuration. In addition, using the unbinned photon-by-photon method, we obtain a PD of 5.9 ± 1.2% across the pulse phase, together with a pulsar geometry consistent with that inferred from the binned analysis, assuming the variable PA predicted by the RVM. A simultaneous RVM fit across the three energy bands, 2–5 keV, 5–6 keV, and 6–8 keV, provides the strongest constraints on the geometrical parameters, yielding θ = 84 −6 +4 deg. Together, these findings demonstrate pronounced phase-dependent spectral and polarization variability, offering valuable constraints on the geometry and emission processes within the accretion region of this transient X-ray pulsar.

Abstract:

Abstract The origin of the complex orbital structure of young massive stars at the Galactic centre remains an open question. If these stars formed in a single episode from a gaseous accretion disc, they may initially have constituted a single, coherently rotating stellar disc. We investigate whether perturbations from an unseen intermediate-mass black hole (IMBH) could fragment and/or disrupt such a disc into the multiple orbital components observed today. First, we derive a theoretical criterion for when and where the IMBH’s torque overcomes the disc’s self-torque and tears it apart. We then test this picture with direct N-body simulations of a stellar disc interacting with an inclined IMBH around a central supermassive black hole. We find that the outcome depends strongly on the IMBH’s orbit and mass. A prograde IMBH rapidly aligns with the stellar disc, while a massive retrograde IMBH (m• ≃ 0.67 Md) anti-aligns relative to the radially overlapping stars and efficiently fragments the original disc into three components in angular-momentum space: an inner disc, a misaligned overlapping region, and an unperturbed outer disc. The IMBH also excites eccentricities in the overlapping region, driving stars away from initially circular orbits. These features emerge for an IMBH mass of 2000 ⊙ and a disc mass of 3000 ⊙ within 10–20 Myr, a timescale comparable to the age of the young Galactic centre stellar population, and provide a plausible explanation for the observed multiple orbital planes, warped geometry, and broad eccentricity distribution.

Abstract:

Abstract We investigate magnetospheric accretion in disc-fed X-ray pulsars assuming that the neutron star spin axis is not perpendicular to the disc plane. We focus on X-ray pulsars, where a geometrically thin disc is truncated far from the stellar surface the channelled part of the accretion flow is expected to be guided by the large-scale dipolar magnetic field after coupling to it near the disc–magnetosphere boundary. Using numerical simulations of plasma motion from the inner disc edge to the neutron star surface, we show that a finite inclination between the disc normal and the stellar spin axis leads to periodic modulation of the mass accretion rate onto the magnetic poles even for a steady mass supply through the disc. This purely geometrical effect arises because stellar rotation changes the orientation of the magnetosphere relative to the disc, producing phase-dependent mass loading of magnetic field lines. The amplitude and shape of the modulation are determined by the system geometry and by the ratio of the stellar spin period to the flow time through the magnetosphere. The resulting variability affects the structure and luminosity of emitting regions near the neutron star surface and leads to asymmetric X-ray pulse profiles. Even without intrinsic asymmetries of the emission regions, this mechanism breaks the time-reversal symmetry expected for stationary accretion and naturally contributes to the observed asymmetry of pulse profiles and phase-resolved spectral features. The effect may also be relevant for ULX pulsars, where intrinsic accretion rate modulation can help preserve strong pulsations in the presence of geometric beaming.