Publications
A rotating white dwarf with two faces
Under review at Nature.
White dwarfs, the extremely dense remnants left behind by most stars after their death, are characterised by a mass comparable to that of the Sun compressed into the size of an Earth-like planet. In the resulting strong gravity, heavy elements sink toward the centre, and the upper layer of the atmosphere contains only the lightest element present, usually hydrogen, or, if the hydrogen content is low, helium. Helium-atmosphere white dwarfs account for about 20% of all white dwarfs; however, several mechanisms can compete with gravitational settling to change their surface composition as they cool, and the fraction of white dwarfs with helium atmospheres is not constant at all temperatures. This fraction is known to increase by a factor ~2.5 below a temperature of about 30,000 K; therefore, some white dwarfs that appear to have a hydrogen-dominated atmosphere above that temperature are bound to transition to a helium-dominated atmosphere as they cool below it. Here we report observations of ZTF J203349.8+322901.1, or "Janus", a white dwarf with two faces: one side of its atmosphere is dominated by hydrogen and the other one by helium. The peculiar double-faced nature of Janus is most likely caused by a small magnetic field, which creates an inhomogeneity in temperature, pressure, or mixing strength over the surface. We appear to have caught a white dwarf as it is undergoing the transition from a hydrogen-dominated to a helium-dominated atmosphere. If this is the case, Janus might be the most extreme member of a class of magnetic transitioning white dwarfs and could help shed light on the physical mechanisms behind white dwarf spectral evolution.
Polarized x-rays from a magnetar
Science, 3 Nov 2022, Vol 378, Issue 6620, pp. 646-650.
Magnetars are neutron stars with ultrastrong magnetic fields, which can be observed in x-rays. Polarization measurements could provide information on their magnetic fields and surface properties. We observed polarized x-rays from the magnetar 4U 0142+61 using the Imaging X-ray Polarimetry Explorer and found a linear polarization degree of 13.5 ± 0.8% averaged over the 2– to 8–kilo–electron volt band. The polarization changes with energy: The degree is 15.0 ± 1.0% at 2 to 4 kilo–electron volts, drops below the instrumental sensitivity ~4 to 5 kilo–electron volts, and rises to 35.2 ± 7.1% at 5.5 to 8 kilo–electron volts. The polarization angle also changes by 90° at ~4 to 5 kilo–electron volts. These results are consistent with a model in which thermal radiation from the magnetar surface is reprocessed by scattering off charged particles in the magnetosphere.
A highly magnetised and rapidly rotating white dwarf as small as the Moon
Nature, volume 595, pages39–42 (2021).
White dwarfs represent the last stage of evolution of stars with mass less than about eight times that of the Sun and, like other stars, are often found in binaries. If the orbital period of the binary is short enough, energy losses from gravitational-wave radiation can shrink the orbit until the two white dwarfs come into contact and merge. Depending on the component masses, the merger can lead to a supernova of type Ia or result in a massive white dwarf. In the latter case, the white dwarf remnant is expected to be highly magnetized because of the strong magnetic dynamo that should arise during the merger, and be rapidly spinning from the conservation of the orbital angular momentum. Here we report observations of a white dwarf, ZTF J190132.9+145808.7, that exhibits these properties, but to an extreme: a rotation period of 6.94 minutes, a magnetic field ranging between 600 megagauss and 900 megagauss over its surface, and a stellar radius of about 2100 kilometres, only slightly larger than the radius of the Moon. Such a small radius implies that the star’s mass is close to the maximum white dwarf mass, or Chandrasekhar mass. ZTF J190132.9+145808.7 is likely to be cooling through the Urca processes (neutrino emission from electron capture on sodium) because of the high densities reached in its core
Intermediate-mass Stars Become Magnetic White Dwarfs
The Astrophysical Journal Letters, Volume 901, Issue 1, id.L14, 9 pp.1
When a star exhausts its nuclear fuel, it either explodes as a supernova or more quiescently becomes a white dwarf, an object about half the mass of our Sun with a radius of about that of the Earth. About one-fifth of white dwarfs exhibit the presence of magnetic fields, whose origin has long been debated as either the product of previous stages of evolution or of binary interactions. We here report the discovery of two massive and magnetic white-dwarf members of young star clusters in the Gaia second data release (DR2) database, while a third massive and magnetic cluster white dwarf was already reported in a previous paper. These stars are most likely the product of single-star evolution and therefore challenge the merger scenario as the only way to produce magnetic white dwarfs. The progenitor masses of these stars are all above 5 solar masses, and there are only two other cluster white dwarfs whose distances have been unambiguously measured with Gaia and whose progenitors' masses fall in this range. This high incidence of magnetic white dwarfs indicates that intermediate-mass progenitors are more likely to produce magnetic remnants and that a fraction of magnetic white dwarfs forms from intermediate-mass stars.
Polarisation of Accreting X-ray Pulsars. I. A New Model
Polarisation of Accreting X-ray Pulsars. II. Hercules X-1
Monthly Notices of the Royal Astronomical Society, Volume 501, Issue 1, pp.109-128
Monthly Notices of the Royal Astronomical Society, Volume 501, Issue 1, pp.129-136
A new window is opening in high-energy astronomy: X-ray polarimetry. With many missions currently under development and scheduled to launch as early as 2021, observations of the X-ray polarisation of accreting X-ray pulsars will soon be available. As polarisation is particularly sensitive to the geometry of the emission region, the upcoming polarimeters will shed new light on the emission mechanism of these objects, provided that we have sound theoretical models that agree with current spectroscopic and timing observation and that can make predictions of the polarisation parameters of the emission. We here present a new model for the polarised emission of accreting X-ray pulsars in the accretion column scenario that for the first time takes into account the macroscopic structure and dynamics of the accretion region and the propagation of the radiation toward the observer, including relativistic beaming, gravitational lensing and quantum electrodynamics. In Paper I, we present all the details of the model, while in Paper II, we apply our model to predict the polarisation parameters of the bright X-ray pulsar Hercules X-1.
Hunting for ancient brown dwarfs: the developing field of brown dwarfs in globular clusters
Astro2020: Decadal Survey on Astronomy and Astrophysics, science white papers, no. 521; Bulletin of the American Astronomical Society, Vol. 51, Issue 3, id. 521 (2019)
The detection of brown dwarfs in globular star clusters will allow us to break the degeneracies in age, mass and composition that affect our current models, and therefore to constrain the physics of their atmospheres and interiors. Furthermore, detecting brown dwarfs will help us constraining the properties of the clusters themselves, as they carry information about the clusters’ age and dynamics. Great advancements in this field are to be expected in the next ten years, thanks to the extraordinary sensitivity in the infrared of upcoming telescopes like JWST and the ELTs.
Testing general relativity with accretion onto compact objects
Astro2020: Decadal Survey on Astronomy and Astrophysics, science white papers, no. 516; Bulletin of the American Astronomical Society, Vol. 51, Issue 3, id. 516 (2019)
The X-ray emission of neutron stars and black holes presents a rich phenomenology that can lead us to a better understanding of their nature and to address more general physics questions: Does general relativity (GR) apply in the strong gravity regime? Is spacetime around black holes described by the Kerr metric? This white paper considers how we can investigate these questions by studying reverberation mapping and quasi-periodic oscillations in accreting systems with a combination of high-spectral and high-timing resolution. In the near future, we will be able to study compact objects in the X-rays in a new way: advancements in transition-edge sensors (TES) technology will allow for electron-volt-resolution spectroscopy combined with nanoseconds-precision timing.
Polarimetry of Magnetars and Isolated Neutron Stars
Book Chapter. Astronomical Polarisation from the Infrared to Gamma Rays, Astrophysics and Space Science Library, Volume 460. ISBN 978-3-030-19714-8. Springer Nature Switzerland AG, 2019, p. 301.
Polarised radiation from isolated neutron stars provides key diagnostics on the structure of the neutron-star magnetosphere and the properties of its atmosphere. Furthermore, the detection of a large degree of polarisation is strong evidence for the presence of vacuum birefringence, which implies that photons of different polarisations travel at different speeds through the magnetised vacuum. We outline how polarisation is generated in neutron-star atmospheres and magnetospheres, how the polarisation is preserved as the radiation travels to us, what the current observations tell us and what are the prospects for future observations.
Vacuum birefringence and the x-ray polarization from black-hole accretion disks
Physical Review D, Volume 97, Issue 8, id.083001
In the next decade, x-ray polarimetry will open a new window on the high-energy Universe, as several missions that include an x-ray polarimeter are currently under development. Observations of the polarization of x rays coming from the accretion disks of stellar-mass and supermassive black holes are among the new polarimeters’ major objectives. In this paper, we show that these observations can be affected by the quantum electrodynamic (QED) effect of vacuum birefringence: after an x-ray photon is emitted from the accretion disk, its polarization changes as the photon travels through the accretion disk’s magnetosphere, as a result of the vacuum becoming birefringent in the presence of a magnetic field. We show that this effect can be important for black holes in the energy band of the upcoming polarimeters and has to be taken into account in a complete model of the x-ray polarization that we expect to detect from black-hole accretion disks, both for stellar mass and for supermassive black holes. We find that, for a chaotic magnetic field in the disk, QED can significantly decrease the linear polarization fraction of edge-on photons, depending on the spin of the hole and on the strength of the magnetic field. This effect can provide, for the first time, a direct way to probe the magnetic field strength close to the innermost stable orbit of black-hole accretion disks and to study the role of magnetic fields in astrophysical accretion in general.
Polluting white dwarfs with perturbed exo-comets
Monthly Notices of the Royal Astronomical Society, Volume 469, Issue 3, August 2017, Pages 2750–2759
We present a model to account for the observed debris discs around young white dwarfs and the presence of metal lines in their spectra. Stellar evolution models predict that the mass-loss on the AGB will be pulsed; furthermore, observations indicate that the bulk of the mass-loss occurs on the AGB. In this case, if the progenitors of the white dwarfs had remnants of planetary formation like the Sun’s Oort cloud or the Kuiper Belt and a planet lying within that cloud or nearby, we find that up to 2 per cent of the planetesimals will fall either into planet-crossing orbits or into chaotic regions after the mass-loss, depending on the location and mass of the planet (from Mars to Neptune). This yields a sufficient mass of comets that can be scattered towards the star, form a debris disc and pollute the atmosphere.