Pulsars, transients and relativistic astrophysics

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A multi-wavelength view of the outflowing short-period X-ray binary UW CrB

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) (2025) staf1790

Authors:

S Fijma, N Degenaar, N Castro Segura, TJ Maccarone, C Knigge, M Armas Padilla, D Mata Sánchez, T Muñoz-Darias, JV Hernández Santisteban, L Rhodes, J Bright, J van den Eijnden, DA Green

Abstract:

Abstract Previous work detected transient ultraviolet outflow features for the short-period (Porb ≈ 111 min), low-mass X-ray binary (LMXB) UW CrB, suggesting the presence of a disc wind in the system. However, because of the transient nature of the outflow features, and the limited amount of data available, the features were challenging to interpret. To follow up on this work, we present a comprehensive multi-wavelength campaign on UW CrB. We observe complex phenomenology and find several features that could be naturally interpreted as being associated with a persistent disc wind. Moreover, we identify a blue-shifted absorption in the Hβ line during one of the epochs, which might be the signature of such an outflow. We present an X-ray to radio campaign of the source, discuss our results in the context of accretion disc wind outflows, present a ‘toy model’ interpretation of the outflow scattering the X-ray emission into our line of sight, and explore the implications for binary evolution models. If correct, our preferred scenario of a persistent disc wind suggests that mass transfer for LMXBs can be non-conservative down to short orbital periods, and thereby opens an important parameter space for angular momentum loss in compact binaries.
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Gamma-ray lines, electron–positron annihilation, and possible radio emission in X-ray pulsars

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) 543:4 (2025) 3993-4002

Authors:

Alexander A Mushtukov, Emir Tataroglu, Alex J Cooper, Sergey S Tsygankov

Abstract:

ABSTRACT Accretion on to neutron stars (NSs) in X-ray pulsars (XRPs) results in intense X-ray emission, and under specific conditions, high-energy nuclear interactions that produce gamma-ray photons at discrete energies. These interactions are enabled by the high free-fall velocities of accreting nuclei near the NS surface and give rise to characteristic gamma-ray lines, notably at 2.2, 5.5, and 67.5 MeV. We investigate the production mechanisms of these lines and estimate the resulting gamma-ray luminosities, accounting for the suppression effects of radiative deceleration in bright XRPs and the creation of electron–positron pairs in strong magnetic fields. The resulting annihilation of these pairs leads to a secondary emission line at ${\sim} 511$ keV. We also discuss the possibility that non-stationary pair creation in the polar cap region could drive coherent radio emission, though its detectability in accreting systems remains uncertain. Using a numerical framework incorporating general relativistic light bending and magnetic absorption, we compute the escape fraction of photons and distinguish between actual and apparent gamma-ray luminosities. Our results identify the parameter space – defined by magnetic field strength, accretion luminosity, and NS compactness – where these gamma-ray signatures may be observable by upcoming MeV gamma-ray missions. In particular, we highlight the diagnostic potential of detecting gravitationally redshifted gamma-ray lines and annihilation features for probing the mass–radius relation and magnetospheric structure of NSs.
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Hydrodynamic simulations of black hole evolution in AGN discs II: inclination damping for partially embedded satellites

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) 543:4 (2025) 3768-3782

Authors:

Henry Whitehead, Connar Rowan, Bence Kocsis

Abstract:

ABSTRACT We investigate the evolution of black holes on orbits with small inclinations ($i < 2^\circ$) to the gaseous discs of active galactic nuclei (AGNs). We perform 3D adiabatic hydrodynamic simulations within a shearing frame, studying the damping of inclination by black hole-gas gravitation. We find that for objects with $i< 3H_0R_0^{-1}$, where $H_0R_0^{-1}$ is the disc aspect ratio, the inclination lost per mid-plane crossing is proportional to the inclination preceding the crossing, resulting in a net exponential decay in inclination. For objects with $i>3H_0R_0^{-1}$, damping efficiency decreases for higher inclinations. We consider a variety of different AGN environments, finding that damping is stronger for systems with a higher ambient Hill mass: the initial gas mass within the BH sphere of influence. We provide a fitting formula for the inclination changes as a function of Hill mass. We find reasonable agreement between the damping driven by gas gravity in the simulations and the damping driven by accretion under a Hill-limited Bondi–Hoyle–Lyttleton prescription. We find that gas dynamical friction consistently overestimates the strength of damping, especially for lower inclination systems, by at least an order of magnitude. For regions in the AGN disc where coplanar binary black hole formation by gas dissipation is efficient, we find that the simulated damping time-scales are especially short with $\tau _d < 10P_\mathrm{SMBH}$. We conclude that as the time-scales for inclination damping are shorter than the expected interaction time between isolated black holes, the vast majority of binaries formed from gas capture should form from components with negligible inclination to the AGN disc.
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Thermal Electrons in the Radio Afterglow of Relativistic Tidal Disruption Event ZTF22aaajecp/AT 2022cmc

The Astrophysical Journal American Astronomical Society 992:1 (2025) 146-146

Authors:

Lauren Rhodes, Ben Margalit, Joe S Bright, Hannah Dykaar, Rob Fender, David A Green, Daryl Haggard, Assaf Horesh, Alexander J van der Horst, Andrew K Hughes, Kunal Mooley, Itai Sfaradi, David Titterington, David Williams-Baldwin

Abstract:

Abstract A tidal disruption event (TDE) occurs when a star travels too close to a supermassive black hole. In some cases, accretion of the disrupted material onto the black hole launches a relativistic jet. In this paper, we present a long-term observing campaign to study the radio and submillimeter emission associated with the fifth jetted/relativistic TDE: AT 2022cmc. Our campaign reveals a long-lived counterpart. We fit three different models to our data: a nonthermal jet, a spherical outflow consisting of both thermal and nonthermal electrons, and a jet with thermal and nonthermal electrons. We find that the data are best described by a relativistic spherical outflow propagating into an environment with a density profile following R −1.8. Comparison of AT 2022cmc to other TDEs finds agreement in the density profile of the environment but also that AT 2022cmc is twice as energetic as the other well-studied relativistic TDE, Swift J1644. Our observations of AT 2022cmc allow a thermal electron population to be inferred for the first time in a jetted transient, providing new insights into the microphysics of relativistic transients jets.
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TiDES: The 4MOST Time Domain Extragalactic Survey

The Astrophysical Journal American Astronomical Society 992:1 (2025) 158-158

Authors:

C Frohmaier, M Vincenzi, M Sullivan, SF Hönig, M Smith, H Addison, T Collett, G Dimitriadis, RS Ellis, P Gandhi, O Graur, I Hook, L Kelsey, Y-L Kim, C Lidman, K Maguire, L Makrygianni, B Martin, A Möller, RC Nichol, M Nicholl, P Schady, BD Simmons, SJ Smartt, E Tempel, P Wiseman

Abstract:

Abstract The Time Domain Extragalactic Survey (TiDES) conducted on the 4 m Multi-Object Spectroscopic Telescope will perform spectroscopic follow-up of extragalactic transients discovered in the era of the NSF-DOE Vera C. Rubin Observatory. TiDES will conduct a 5 yr survey, covering > 14 , 000 square degrees , and use around 250,000 fibre hours to address three main science goals: (i) spectroscopic observations of >30,000 live transients, (ii) comprehensive follow-up of >200,000 host galaxies to obtain redshift measurements, and (iii) repeat spectroscopic observations of active galactic nuclei to enable reverberation mapping studies. The live spectra from TiDES will be used to reveal the diversity and astrophysics of both normal and exotic supernovae across the luminosity-timescale plane. The extensive host-galaxy redshift campaign will allow exploitation of the larger sample of supernovae and improve photometric classification, providing the largest-ever sample of SNe Ia, capable of a sub-2% measurement of the equation-of-state of dark energy. Finally, the TiDES reverberation mapping experiment of 700–1000 AGN will complement the SN Ia sample and extend the Hubble diagram to z ∼ 2.5.
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