Pulsars, transients and relativistic astrophysics

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.

Abstract:

Recently, a class of long-period radio transients (LPTs) has been discovered, exhibiting emission thousands of times longer than radio pulsars. These findings, enabled by advances in wide-field radio surveys, challenge existing models of rotationally powered pulsars. Proposed models include highly magnetized neutron stars, white-dwarf pulsars and white-dwarf binary systems with low-mass companions. Although some models predict X-ray emission, no LPTs have been detected in X-rays despite extensive searches Here we report the discovery of an extremely bright LPT (10–20 Jy in radio), ASKAP J1832−0911, which has coincident radio and X-ray emission, both with a 44.2-minute period. Its correlated and highly variable X-ray and radio luminosities, combined with other observational properties, are unlike any known Galactic object. The source could be an old magnetar or an ultra-magnetized white dwarf; however, both interpretations present theoretical challenges. This X-ray detection from an LPT reveals that these objects are more energetic than previously thought and establishes a class of hour-scale periodic X-ray transients with a luminosity of about 1033 erg s−1 linked to exceptionally bright coherent radio emission.