Pablo Rodriguez-Gil

Quiescent black hole low-mass X-ray binaries provide a key setting for probing accretion physics at low luminosities. A0620–00, the archetypal system in this class, has remained in X-ray quiescence for decades and exhibits complex optical variability, yet the long-term behaviour of its accretion flow remains poorly understood. Here, we report an analysis of long-term optical monitoring of A0620–00 from ZTF, LCO, and ATLAS. The full dataset spans nearly two decades, with the ZTF light curve providing the primary ∼2760-day baseline for the period analysis. We identify a superorbital cycle with a period of P = 261.9 ± 9.4 d and a peak-to-peak amplitude of ∼0.2 mag. The signal is recovered independently across all three surveys, and red-noise simulations indicate that it is unlikely to arise from stochastic variability alone. Furthermore, the relative occurrence of the ‘passive’ and ‘active’ quiescent states displayed by the system seems to depend on the superorbital phase, with passive states concentrated near the cycle minimum and active states more common near maximum. We find that, among the possible interpretations, retrograde nodal precession of a hot inner accretion flow might be able to explain the observed long-term modulation. In this interpretation, the periodic signal may arise from cyclic reorientation of the inner flow, which modulates the photometric contribution from the innermost regions. The inferred modulation period would correspond to a characteristic dynamical radius of ∼0.13 a (∼10 ⁴ R _g ), where a is the binary semi-major axis, broadly consistent with the expected transition between the outer thin disc and the inner hot accretion flow.

Mass transfer in stellar binaries has been well studied in most stellar mass ranges, with the notable exception of ultracool stars and substellar brown dwarfs. We report the discovery of ZTF J1239+8347 with the Zwicky Transient Facility (ZTF), a brown dwarf binary currently undergoing stable mass transfer with an orbital period of 57.41 minutes. Optical time-series photometry reveals an extremely high amplitude (>2 magnitude peak-to-trough) variability at short wavelengths, indicative of an orbiting hot spot slightly buried inside the atmosphere of the accretor. We use parallax measurements from Gaia along with optical and near-infrared spectra to infer an accretion temperature of T _eff = 8904 ± 54 K, an atmospheric temperature of the accretor of T _atmo ≈ 1500 K, and a slightly inflated accretor radius of R acc = 1.2 0 − 0.11 + 0.15 R Jup . ZTF J1239+8347 is a direct impact accretor, typically only seen in double degenerate white dwarf binaries, which are approximately a million times denser than the components in ZTF J1239+8347. The existence of an accreting brown dwarf binary suggests that angular momentum loss can be strong enough to make ultracool binaries interact in a Hubble time. The observed faintness (∼20 mag) and relative proximity (≈300 pc) of ZTF J1239+8347 suggest that many similar systems are likely to be found by the upcoming Rubin Observatory Legacy Survey of Space and Time.

Context. The Necklace nebula is a bipolar, post-common-envelope planetary nebula, the central star of which has been shown to have a dwarf carbon star companion. Aims. We aim to understand the origins of the Necklace and its dwarf carbon central star. Methods. We study the carbon abundance of the nebula through far-ultraviolet spectroscopy obtained with the Hubble Space Telescope. Furthermore, through simultaneous modelling of multi-band light and velocity curves, we attempt to constrain the parameters of the central star system. Results. Puzzlingly, we find that the region of the inner nebula observed with the Hubble Space Telescope is seemingly not carbon-rich, at odds with the dwarf carbon star nature of the companion of the central star. The initial mass of the nebular progenitor was likely very close to the limit to become carbon-rich, perhaps experiencing a very late thermal pulse. The dwarf carbon star companion is found to be significantly inflated with respect to that expected for an isolated main-sequence star of the same mass. Conclusions. The properties of the central binary are consistent with the progenitor having become carbon-rich and its companion having accreted a significant amount of that carbon-enriched material. However, it is unclear how this evolutionary hypothesis can be reconciled with the inner nebula potentially being carbon-poor.

Context: The Necklace nebula is a bipolar, post-common-envelope planetary nebula, the central star of which has been shown to have a dwarf carbon star companion. Aims: We aim to understand the origins of the Necklace and its dwarf carbon central star. Methods: We study the carbon abundance of the nebula through far ultraviolet spectroscopy obtained with the Hubble Space Telescope. Furthermore, through simultaneous modelling of multiband light and velocity curves, we attempt to constrain the parameters of the central star system. Results: Puzzlingly, we find that the region of the inner nebula observed with the Hubble Space Telescope is seemingly not carbon-rich, at odds with the dwarf carbon star nature of the companion of the central star. The initial mass of the nebular progenitor was likely very close to the limit to become carbon-rich, perhaps experiencing a very late thermal pulse. The dwarf carbon star companion is found to be significantly inflated with respect to that expected for an isolated main sequence star of the same mass. Conclusions: The properties of the central binary are consistent with the progenitor having become carbon-rich and its companion having accreted a significant amount of that carbon-enriched material. However, it is unclear how this evolutionary hypothesis can be reconciled with the inner nebula potentially being carbon poor.

Context. AM Canum Venaticorum (AM CVn) stars are ultra-compact binary systems composed of a white dwarf primary accreting from a hydrogen-deficient donor. They play a crucial role in astrophysics as potential progenitors of Type Ia supernovae and as laboratories for gravitational wave studies. However, their formation and evolutionary history remain incomplete. Three formation channels have been discussed in the literature: the white dwarf, He-star, and cataclysmic variable channels. Aims. The chemical composition of the accretor atmosphere reflects the material transferred from the donor. In this work we took the first accurate measurements of the fundamental parameters of the accreting white dwarf in ZTF J225237.05−051917.4, including the abundances of key elements such as carbon, nitrogen, and silicon, by analysing ultraviolet spectra obtained with the Hubble Space Telescope (HST). These measurements provide new insight into the evolutionary history of the system and, together with existing optical observations, establish it as a benchmark to develop our pipeline, paving the way for its application to a larger sample of AM CVn systems. Methods. We determined the binary parameters through photometric analysis and constrained the atmospheric parameters of the white dwarf accretor, including its effective temperature, surface gravity, and chemical abundances, by fitting the HST ultraviolet spectrum with synthetic spectral models. We then inferred the system’s formation channel by comparing the results with theoretical evolutionary models. Results. According to our measurements, the accretor’s effective temperature ( T _eff ) is 23 300 ± 600 K and the surface gravity (log g ) is 8.4 ± 0.3, which imply an accretor mass ( M _WD ) of 0.86 ± 0.16 M _⊙ . We find a high nitrogen-to-carbon abundance ratio by mass of > 153. Conclusions. The accretor is significantly hotter than previous estimates based on simplified blackbody fits to the spectral energy distribution, underscoring the importance of detailed spectral modelling for accurately determining system parameters. Our results show that ultraviolet spectroscopy is well suited to constraining the formation channels of AM CVn systems. Of the three proposed formation channels, the He-star channel can be excluded given the high nitrogen-to-carbon ratio. Our results are consistent with both the white dwarf and cataclysmic variable channels.

Context.Swift J1910.2–0546 (=MAXI J1910−057) is a Galactic X-ray transient discovered during a bright outburst in 2012. Its X-ray spectral and timing properties point to a black-hole accretor, yet the orbital period remains uncertain, and no reliable dynamical constraints on the binary parameters are available. The 2012 event, extensively monitored at X-ray and optical wavelengths, offers a rare opportunity to investigate the structure and dynamics of the system and to constrain its fundamental properties. Aims. We use time-series optical photometry and spectroscopy, obtained during outburst and quiescence, to estimate the orbital period, characterise the donor star, determine the interstellar extinction, distance, and system geometry, and constrain the component masses. Methods. Multi-site r -band and clear-filter light curves and WHT/ACAM spectra from the 2012 outburst were combined with time-series spectroscopy from GTC/OSIRIS and VLT/FORS2 in quiescence. Period searches were conducted using generalised Lomb–Scargle, phase-dispersion minimisation, and analysis-of-variance algorithms. We used diffuse interstellar bands to constrain E ( B − V ), while empirical correlations involving H α yielded estimates of K ₂ , q , and i . Results. We detected a coherent, double-humped modulation with a period of 0.0941 ± 0.0007 d (2.26 ± 0.02 h) during the outburst. Its morphology is consistent with an early superhump, suggesting that the true orbital period may be slightly shorter than 4.52 h. The H α radial velocity curves do not yield a definitive orbital period. In quiescence, TiO bands indicate an M3−M3.5 donor contributing ≃70% of the red continuum. Diffuse interstellar bands give E ( B − V ) = 0.60 ± 0.05 and N _H = (3.9 ± 1.3)×10 ²¹ cm ⁻² , placing the system at a distance of 2.8−4.0 kpc. The H α line width in quiescence (FWHM ₀ = 990 ± 45 km s ⁻¹ ), via a FWHM– K ₂ calibration, provides an estimate of K ₂ , while its double-peaked profile gives q and the orbital inclination. The latter appears much higher than estimates from X-ray studies. Adopting the resulting K ₂ = 230 ± 17 km s ⁻¹ and q = 0.032 ± 0.010, along with two orbital period scenarios (2.25 and 4.50 h), Monte Carlo sampling returns a compact object mass of M ₁ = 8 − 11 M _⊙ and an inclination of i = 13° −18° for plausible donor masses ( M ₂ = 0.25 − 0.35 M _⊙ ). Overall, we favour an orbital period of 4.5 h. Conclusions.Swift J1910.2–0546 may be a short-period, low-inclination black hole X-ray transient, although the possibility of it being a neutron star accretor cannot be completely ruled out. Subsequent phase-resolved spectroscopy and photometry during quiescence are needed to better determine its fundamental parameters.

White dwarfs (WDs) are the most abundant compact objects, and recent surveys have suggested that over a third of WDs in accreting binaries host a strong (B ≳ 1 MG) magnetic field. However, the origin and evolution of WD magnetism remain under debate. Two WD pulsars, AR Sco and J191213.72–441045.1 (J1912), have been found, which are non-accreting binaries hosting rapidly spinning (1.97 minutes and 5.30 minutes, respectively) magnetic WDs. The WD in AR Sco is slowing down on a P / P ̇ ≈ 5.6 × 1 0 6 yr timescale. It is believed they will eventually become polars, accreting systems in which a magnetic WD (B ≈ 10−240 MG) accretes from a Roche lobe-filling donor spinning in sync with the orbit (≳78 minutes). Here, we present multiwavelength data and analysis of Gaia22ayj, which outbursted in 2022 March. We find that Gaia22ayj is a magnetic accreting WD that is rapidly spinning down ( P / P ̇ = 6 . 1 − 0.2 + 0.3 × 1 0 6 yr) like WD pulsars, but shows clear evidence of accretion, like polars. Strong linear polarization (40%) is detected in Gaia22ayj; such high levels have only been seen in the WD pulsar AR Sco and demonstrate the WD is magnetic. High speed photometry reveals a 9.36 minutes period accompanying a high amplitude (∼2 mag) modulation. We associate this with a WD spin or spin–orbit beat period, not an orbital period as was previously suggested. Fast (60 s) optical spectroscopy reveals a broad “hump,” reminiscent of cyclotron emission in polars, between 4000 and 8000 Å. We find an X-ray luminosity of L X = 2 . 7 − 0.8 + 6.2 × 1 0 32 ergs − 1 in the 0.3–8 keV energy range, while two very large array radio campaigns resulted in a non-detection with a F _r < 15.8 μJy 3σ upper limit. The shared properties of both WD pulsars and polars suggest that Gaia22ayj is a missing link between the two classes of magnetic WD binaries.