Pulsars, transients and relativistic astrophysics

Abstract:

The Imaging X-ray Polarimetry Explorer (IXPE) observations of accreting X-ray pulsars (XRPs) continue to provide novel insights into the physics and geometry of these sources. We present the first X-ray polarimetric study of the persistent wind-fed XRP 4U 1538−52, based on five IXPE observations totaling 360 ks, conducted in March and October 2024. We detect marginally significant polarization in the combined data set in the full 2–8 keV energy band, with a polarization degree (PD) of 3.0 ± 1.1% and polarization angle (PA) of −18° ±11°. The energy-resolved analysis shows a clear energy dependence of the polarization properties, with a remarkable ∼70° switch in PA between low and high energies. Similarly, the pulse phase-resolved spectro-polarimetric analysis reveals different signatures at low and high energies. At low energies (2–3 keV), the PD spans from ∼2% up to ∼18%, accompanied by large-amplitude swings in the PA. At higher energies (4–8 keV), the PD varies between ∼3% and ∼12%, and the PA not only is similarly highly variable but also exhibits a markedly different phase dependence. Fitting the rotating vector model to the pulse phase dependence of the PA at the lower energies, we constrain the geometric configuration of the pulsar. The analysis favors a high spin-axis inclination of > 50°, which agrees with both previous pulse-phase-dependent spectral fitting of the cyclotron line region and the known high orbital inclination of the binary system. The magnetic obliquity is estimated to be 30° and the spin position angle to be 19°. A sharp switch in PA around 3 keV presents a particular theoretical challenge, as it is not consistent with the right-angle switch that was only seen in one other pulsar, Vela X-1.

Abstract:

PSR J0901−4046, a likely radio-loud neutron star with a period of 75.88 s, challenges conventional models of neutron star radio emission. Here, we showcase results from 46 h of follow-up observations of PSR J0901−4046 using the MeerKAT, Murriyang, Giant Metrewave Radio Telescope, and Murchison Widefield Array radio telescopes. We demonstrate the intriguing stability of the source’s timing solution over more than 3 yr, leading to an RMS arrival-time uncertainty of just of the rotation period. Furthermore, non-detection below 500 MHz may indicate a low-frequency turnover in the source’s spectrum, while no secular decline in the flux density of the source over time, as was apparent from previous observations, has been observed. Using high time-resolution MeerKAT data, we demonstrate two distinct quasi-periodic oscillation modes present in single pulses, with characteristic time-scales of 73 and 21 ms. We also observe a statistically significant change in the relative prevalence of distinct pulse morphologies compared to previous observations, possibly indicating a shift in the magnetospheric composition over time. Finally, we show that the W pulse width is nearly constant from 544 to 4032 MHz, consistent with zero radius-to-frequency mapping. The very short duty cycle () is more similar to radio pulsars with periods >5 s than to radio-loud magnetars. This, along with the lack of magnetar-like outbursts or timing glitches, complicates the identification of the source with ultralong period magnetar models.

Abstract:

Tidal disruption events (TDEs) can generate non-spherical, relativistic, and optically thick outflows. Simulations show that the radiation we observe is reprocessed by these outflows. According to a unified model suggested by these simulations, the spectral energy distributions (SEDs) of TDEs depend strongly on viewing angle: low [high] optical-to-X-ray ratios (OXRs) correspond to face-on [edge-on] orientations. Post-processing with radiative transfer codes has simulated the emergent spectra but has so far been carried out only in a quasi-1D framework, with three atomic species (H, He, and O). Here, we present 2.5D Monte Carlo radiative transfer simulations which model the emission from a non-spherical outflow, including a more comprehensive set of cosmically abundant species. While the basic trend of OXR increasing with inclination is preserved, the inherently multi-D nature of photon transport through the non-spherical outflow significantly affects the emergent SEDs. Relaxing the quasi-1D approximation allows photons to preferentially escape in (polar) directions of lower optical depth, resulting in a greater variation of bolometric luminosity as a function of inclination. According to our simulations, inclination alone may not fully explain the large dynamic range of observed TDE OXRs. We also find that including metals, other than O, changes the emergent spectra significantly, resulting in stronger absorption and emission lines in the extreme ultraviolet, as well as a greater variation in the OXR as a function of inclination. Whilst our results support previously proposed unified models for TDEs, they also highlight the critical importance of multi-D ionization and radiative transfer.