Pulsars, transients and relativistic astrophysics

Abstract:

Accretion onto strongly magnetized neutron stars in X-ray pulsars (XRPs) produces intense X-ray emission and gamma-ray photons, the latter arising from nuclear reactions and high-energy particle collisions in the stellar atmosphere. These gamma-rays interact with the magnetic field via one- and two-photon pair creation processes, generating electron-positron pairs. We investigate one-photon pair production in sub-critical XRPs, with a focus on how surface magnetic field strength affects gamma-ray absorption in the magnetosphere. Using general relativistic photon trajectory simulations, we map the spatial distribution of pair creation sites and quantify absorption efficiencies. We find that XRPs with surface fields B ≲ 10 12 G are largely transparent to 2.2MeV gamma-rays, while fields B ≳ 3 × 10 12 G lead to efficient absorption within a few tens of centimeters from the surface. For lower field strengths, absorption can occur at larger distances and outside the accretion column, offering a potential channel for radio emission. Our results provide new insight into the interplay between nuclear processes, magnetospheric structure, and multiwavelength radiation in XRPs.

Abstract:

We present observations of the accretion-powered X-ray pulsar 4U 1907+09 conducted with the Imaging X-ray Polarimetry Explorer, which has delivered the first high-quality polarization measurements of this source. 4U 1907+09 was observed twice during its brightest periods, close to the periastron. We observe a stronger polarization in the first observation, with a phase-averaged polarization degree (PD) of 6.0 ± 1.6% and a polarization angle (PA) of 69° ±8°. The second observation provides weaker constraints on the polarimetric properties, PD = 2.2 ± 1.6% and PA = 46° ±23°, as determined from the spectro-polarimetric analysis. Combining the data from the two observations results in PD = 3.7 ± 1.1% and PA = 63° ±9°. We detect an energy-dependent PA in the phase-averaged analyses with a significance of 1.7 σ . In the phase-resolved analyses, we observe a potential PA rotation of approximately 90° between adjacent energy bands (4–5 and 5–6 keV) within the single phase bin of 0.25–0.375. We also investigate the influence of short flares on the polarization properties of this source. The results suggest that flares do not significantly affect the energy-phase-dependent PA, implying that the pulsar’s geometry remains stable during flare events.

Abstract:

A set of analytic solutions for the plunging region thermodynamics has been developed recently under the assumption that the fluid undergoes a gravity-dominated geodesic plunge into the black hole. We test this model against a dedicated 3D global general relativistic magnetohydrodynamics simulation of a thin accretion disc around a Schwarzschild black hole using the code athenak . Provided that we include the effects of non-adiabatic heating (plausibly from grid-scale magnetic dissipation), we find excellent agreement between the analytic model and the simulated quantities. These results are particularly important for existing and future electromagnetic black hole spin measurements, many of which do not include the plunging fluid in their emission modelling. This exclusion typically stems from the assumption of a zero-stress boundary condition at the innermost stable circular orbit (ISCO), forcing all thermodynamic quantities to vanish. Instead, we find a non-zero drop in the angular momentum over the plunging region, which is consistent with both prior simulations and observations. We demonstrate that this stress is small enough for the dynamics of the fluid in the plunging region to be well-described by geodesic trajectories, yet large enough to cause measurable dissipation near to the ISCO – keeping thermodynamic quantities from vanishing. In the plunging region, constant -disc models are a physically inappropriate framework.

Abstract:

The sensitivity and field of view of the MeerKAT radio telescope provide excellent opportunities for commensal transient searches. We carry out a commensal transient search in supernova and short gamma-ray burst fields using methodologies established by S. I. Chastain et al. We search for transients in MeerKAT L-band images with integration times of 30 minutes, finding 13 variable sources. We compare these sources to the VLASS and RACS survey data, and examine possible explanations for the variability. Additionally, for one of these sources we examine archival Chandra ACIS data. We find that 12 of these sources are consistent with variability due to interstellar scintillation. The remaining source could possibly have some intrinsic variability. We also split the MeerKAT L band into upper and lower halves, and search for transients in images with an integration time of 8 s. We find a source with a duration of 8–16 s that is highly polarized at the lowest frequencies. This source is spatially coincident with a star detected by the Transiting Exoplanet Survey Satellite. We conclude that this source may be consistent with a stellar flare. Finally, we calculate accurate upper and lower limits on the transient rate using transient simulations.