Pulsars, transients and relativistic astrophysics

Abstract:

ABSTRACT Swift J1727.8–1613 is a newly discovered transient low-mass X-ray binary harbouring a stellar-mass ($\sim 10\,\mathrm{ M}_\odot$) black hole. We present state-resolved VLT/X-Shooter optical spectroscopy of its 2023 outburst, sampling the luminous hard-to-soft and late soft-to-hard transitions. During the onset of the brightest radio flare, He ii flux rises relative to adjacent epochs, with reduced peak-to-peak separation and full-width-half-maximum, consistent with enhanced irradiation shifting line emissivity to larger radii. We detect no contemporaneous change in the line base tracing the inner disc. The most dramatic change occurs at the onset of the dim-hard state, when strong, broad (higher-order) Balmer lines appear in absorption, and He ii remains in emission, but becomes highly asymmetric. While the hardening of the X-ray spectrum likely promotes the reappearance of an underlying disc photosphere, the kinematic alignment between the Balmer absorption ($v_w\sim -750\, \mathrm{km\, s^{-1}}$) and the suppressed blue peak of He ii suggests a unified origin in a massive, cool ($T\lesssim 10^{4}\, \mathrm{K}$) accretion disc wind. Radiative transfer simulations demonstrate that such asymmetric He ii profiles are naturally produced in a rotating and accelerating outflow. Using the Sobolev approximation, we estimate the wind mass-loss rate to be $\dot{M}_w\gtrsim 10^{-9}\, \mathrm{ M}_\odot \, \mathrm{yr^{-1}}$, comparable to the instantaneous accretion rate and a significant fraction of the secular mass-transfer rate from the donor. If persistent at quiescent-level X-ray luminosities, this outflow could strongly impact the system’s secular evolution.

Abstract:

Abstract Fast X-ray transients (FXTs) are brief, luminous bursts of soft X-ray emission whose physical origins remain uncertain. The Einstein Probe (EP) mission has recently enabled prompt discovery of these events, providing opportunities for rapid multi-wavelength follow-up. We present a coordinated radio observing campaign targeting 20 FXTs detected by the EP in 2024. The core consists of 59 epochs with the Allen Telescope Array (ATA), sampling post-burst timescales from ~1 to ~65 days and reaching typical 3σ sensitivities of 0.6–1.5 mJy across 1–8 GHz. We additionally incorporate higher-sensitivity observations from MeerKAT, the VLA, ATCA, eMERLIN, and AMI-LA. Two FXTs—EP240315a and EP241021a—have radio counterparts. For EP241021a, AMI-LA monitoring at 15.5 GHz reveals a light curve peaking at ~30 days with Fν ≈ 1.0 mJy; equipartition analysis implies a Newtonian equipartition energy of ~3 × 1049 erg and a mildly relativistic on-axis solution with Γon ≈ 1.3. Multi-frequency modelling of EP240315a, combining new VLA measurements with published MeerKAT, ATCA, and eMERLIN data, indicates highly relativistic early emission (Γ ≳ 3) that decelerates with time, consistent with jetted outflow. The detections and ATA non-detections span luminosities overlapping the brightest GRB and relativistic tidal disruption event (TDE) afterglows, suggesting FXTs comprise a heterogeneous population including both relativistic and non-relativistic explosions. Under a GRB-like redshift prior, our 3σ ATA limits correspond to observer-frame specific-luminosity thresholds of ~1032–1033 erg s−1 Hz−1; for nearby events (z ≲ 0.2), Lν ≲ 1030–31 erg s−1 Hz−1. Planned ATA upgrades and next-generation arrays (SKA, DSA, ngVLA) will enable sensitive, population-level radio studies of the FXT radio sky.

Abstract:

Abstract We study the influence of fuzzy-dark-matter cores on the orbits of stars at the Galactic centre. This dark matter candidate condenses into dense, solitonic cores, and, if a super-massive black hole is present at the centre of such a core, its central part forms a ‘gravitational atom’. Here, we calculate the atom’s contribution to the gravitational potential felt by a Galactic-centre star, for a general state of the atom. We study the angular-momentum dynamics this potential induces, and show that it is similar to vector resonant relaxation. Its influence is found to be potentially sufficiently strong that such a dynamical component should be accounted for in Galactic-centre modelling. For the Milky Way, the atom is expected to have some spherical asymmetry, and we use this to derive a stability condition for the disc of young, massive stars at the Galactic centre—if the atom’s mass is too large, then the disc would be destroyed. Thus, the existence of this disc constrains the mass of the particles comprising the solitonic core. We study an example model of the core, where all of the rotation of the core’s inner region is assumed to come from an l = 1 state, and its amplitude is determined by the halo’s spin parameter; such a core is found to be in tension with the stability of the clockwise stellar disc for 4.2 × 10−20 eV ≤ ma ≤ 5.4 × 10−20 eV at 2σ. Other core models could vary the constrained values of ma. These constraints will tighten significantly with future, improved data.