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The evolution of accreting X-ray binary systems is closely coupled to the properties of their donor stars. Consequently, we can constrain the evolutionary track a system is by establishing the nature of its donor. Here, we present far-ultraviolet (far-UV) spectroscopy of the transient neutron-star low-mass X-ray binary J1858 in different accretion states (low-hard, high-hard, and soft). All of these spectra exhibit anomalous N v, C iv, Si iv, and He ii lines, suggesting that its donor star has undergone CNO processing. We also determine the donor’s effective temperature, Td ≃ 5700 K, and radius, Rd ≃ 1.7 R⊙, based on photometric observations obtained during quiescence. Lastly, we leverage the transient nature of the system to set an upper limit of $\dot{M}_{\rm acc} \lesssim 10^{-8.5}~{\rm M}_{\odot }~\mathrm{ yr}^{-1}$ on the present-day mass-transfer rate. Combining these with the orbital period of the system, Porb = 21.3 h, we search for viable evolution paths. The initial donor masses in the allowed solutions span the range 1 M⊙ ≲ Md,i ≲ 3.5 M⊙. All but the lowest masses in this range are consistent with the strong CNO-processing signature in the UV line ratios. The present-day donor mass in the permitted tracks are 0.5 M⊙ ≲ Md,obs ≲ 1.3 M⊙, higher than suggested by eclipse modelling. Since Porb is close to the so-called bifurcation period, both converging and diverging binary tracks are permitted. If the former is confirmed, J1858 will end its life as an ultracompact system with a substellar donor.

Extragalactic fast X-ray transients (FXTs) are a class of soft (0.3–10 keV) X-ray transients lasting a few hundred seconds to several hours. Several progenitor mechanisms have been suggested to produce FXTs, including supernova shock breakouts, binary neutron star mergers, or tidal disruptions involving an intermediate-mass black hole and a white dwarf. We present detailed host studies, including spectroscopic observations of the host galaxies of seven XMM-Newton-discovered FXTs. The candidate hosts lie at redshifts 0.0928 <z < 0.645 implying peak X-ray luminosities of 1043 erg s−1<LX < 1045 erg s−1 and physical offsets of 1 kpc < rproj < 22 kpc. These observations increase the number of FXTs with a spectroscopic redshift measurement by a factor of 2, although we note that one event is re-identified as a Galactic flare star. We infer host star formation rates and stellar masses by fitting the combined spectroscopic and archival photometric data. We also report on a contemporaneous optical counterpart search to the FXTs in Pan-STARRS and ATLAS by performing forced photometry at the position of the FXTs. We do not find any counterpart in our search. Given our constraints, including peak X-ray luminosities, optical limits, and host properties, we find that XRT 110 621 is consistent with an supernova shock breakout (SN SBO) event. Spectroscopic redshifts of likely host galaxies for four events imply peak X-ray luminosities that are too high to be consistent with SN SBOs, but we are unable to discard either the binary neutron star or white dwarf–intermediate-mass black hole tidal disruption event scenarios for these FXTs.

Over the past few years, ∼30 extragalactic fast X-ray transients (FXRTs) have been discovered, mainly in Chandra and XMM-Newton data. Their nature remains unclear, with proposed origins, including a double neutron star merger, a tidal disruption event involving an intermediate-mass black hole and a white dwarf, or a supernova shock breakout. A decisive differentiation between these three promising mechanisms for their origin requires an understanding of the FXRT energetics, environments, and/or host properties. We present optical observations obtained with the Very Large Telescope for the FXRTs XRT 000519 and XRT 110103 and Gran Telescopio Canarias observations for XRT 000519 designed to search for host galaxies of these FXRTs. In the gs, rs, and R-band images, we detect an extended source on the north-west side of the $\sim \, 1^{\prime \prime }$ (68 per cent confidence) error circle of the X-ray position of XRT 000519 with a Kron magnitude of gs = 26.29 ± 0.09 (AB magnitude). We discuss the XRT 000519 association with the probable host candidate for various possible distances, and we conclude that if XRT 000519 is associated with the host candidate a supernova shock breakout scenario is likely excluded. No host galaxy is found near XRT 110103 down to a limiting magnitude of R > 25.8.

We use very long baseline interferometry to measure the proper motions of three black hole X-ray binaries (BHXBs). Using these results together with data from the literature and Gaia DR2 to collate the best available constraints on proper motion, parallax, distance, and systemic radial velocity of 16 BHXBs, we determined their three-dimensional Galactocentric orbits. We extended this analysis to estimate the probability distribution for the potential kick velocity (PKV) a BHXB system could have received on formation. Constraining the kicks imparted to BHXBs provides insight into the birth mechanism of black holes (BHs). Kicks also have a significant effect on BH–BH merger rates, merger sites, and binary evolution, and can be responsible for spin–orbit misalignment in BH binary systems. 75 per cent of our systems have potential kicks $\gt 70\, \rm {km\,s^{-1}}$. This suggests that strong kicks and hence spin–orbit misalignment might be common among BHXBs, in agreement with the observed quasi-periodic X-ray variability in their power density spectra. We used a Bayesian hierarchical methodology to analyse the PKV distribution of the BHXB population, and suggest that a unimodal Gaussian model with a mean of 107 $\pm \,\,16\, \rm {km\,s^{-1}}$ is a statistically favourable fit. Such relatively high PKVs would also reduce the number of BHs likely to be retained in globular clusters. We found no significant correlation between the BH mass and PKV, suggesting a lack of correlation between BH mass and the BH birth mechanism. Our python code allows the estimation of the PKV for any system with sufficient observational constraints.