arXiv subjects

I. D. Sharp

Publications and source records attributed to I. D. Sharp.

7 recordsLinked to original sources

arXiv2025

Eclipse timing variations in 7 post-common envelope binaries: an update on recent circumbinary models

Eclipsing, short-period post-common envelope binaries (PCEBs) have been studied for several decades by eclipse timing variations (ETVs) which have been interpreted as being caused by circumbinary bodies. In this paper we report 355 new observations of 7 PECBs (HS0705+6700, NN Ser, NSVS 07826147, NSVS 14256825, NY Vir, QS Vir and RR Cae) and examine how the recent proposed models of these systems compare with our new observations. We find that none of the 18 recent models fit accurately with our data. We review alternative mechanisms of the ETVs, including magnetic effects, but conclude that they do not predict our results. Although we cannot exclude the presence of circumbinary bodies a combination of several mechanisms may be required to explain the observed ETVs.

astro-ph.SR↗

arXiv2025

GOTO065054+593624: a 8.5 mag amplitude dwarf nova identified in real time via Kilonova Seekers

Dwarf novae are astrophysical laboratories for probing the nature of accretion, binary mass transfer, and binary evolution -- yet their diverse observational characteristics continue to challenge our theoretical understanding. We here present the discovery of, and subsequent observing campaign on GOTO065054+593624 (hereafter GOTO0650), a dwarf nova of the WZ Sge type, discovered in real-time by citizen scientists via the Kilonova Seekers citizen science project, which has an outburst amplitude of 8.5 mag. An extensive dataset charts the photometric and spectroscopic evolution of this object, covering the 2024 superoutburst. GOTO0650 shows an absence of visible emission lines during the high state, strong H and barely-detected HeII emission, and high-amplitude echo outbursts with a rapidly decreasing timescale. The comprehensive dataset presented here marks GOTO0650 as a candidate period bouncer, and highlights the important contribution that citizen scientists can make to the study of Galactic transients.

astro-ph.SR↗

arXiv2022

Eclipse timing variations in post-common envelope binaries: Are they a reliable indicator of circumbinary companions?

Post-common envelope binary systems evolve when matter is transferred from the primary star at a rate that cannot be accommodated by its secondary companion. A common envelope forms which is subsequently ejected resulting in a system with a binary period frequently between 2 and 3 hours. Where circumbinary companions are predicted, it remains unclear whether they form before or after the common envelope ejection. From observations of eclipse time variations (ETVs), exoplanet databases e.g. NASA Exoplanet Archive, list typically a dozen systems with confirmed circumbinary planets. Here we examine seven of these systems, discuss other possible causes and consider whether, for these dynamic systems, the ETV methodology is a reliable indicator of planetary companions. The systems selected were those where we could determine precise eclipse timings, free from significant extraneous effects such as pulsations, and present 163 new times of minima permitting us to test existing models. Over thirty circumbinary models have been proposed for these seven systems and note all, other than the latest model for NY Vir which remains to be fully tested, fail within a year to accurately predict eclipse times. In examining alternative mechanisms we find that magnetic effects could contribute significantly in two of the seven systems studied. We conclude that the structure of these dynamic systems, with the extreme temperature differences and small binary separations, are not fully understood and that many factors may contribute to the observed ETVs.

astro-ph.SR↗

arXiv2020

Can surface-transfer doping and UV irradiation during annealing improve shallow implanted Nitrogen-Vacancy centers in diamond?

It has been reported that conversion yield and coherence time of ion-implanted NV centers improve if the Fermi level is raised or lowered during the annealing step following implantation. Here we investigate whether surface transfer doping and surface charging, by UV light, can be harnessed to induce this effect. We analyze the coherence times and the yield of NV centers created by ion implantation and annealing, applying various conditions during annealing. Specifically, we study coating the diamond with nickel, palladium or aluminum oxide, to induce positive surface transfer doping, as well as annealing under UV illumination to trigger vacancy charging. The metal coated diamonds display a two times higher formation yield than the other samples. The coherence time $T_2$ varies by less than a factor of two between the investigated samples. Both effects are weaker than previous reports, suggesting that stronger modifications of the band structure are necessary to find a pronounced effect. UV irradiation has no effect on yield and $T_2$ times.

cond-mat.mtrl-sci↗

arXiv2006

Compositional tuning of ferromagnetism in Ga1-xMnxP

We report the magnetic and transport properties of Ga1-xMnxP synthesized via ion implantation followed by pulsed laser melting over a range of x, namely 0.018 to 0.042. Like Ga1-xMnxAs, Ga1-xMnxP displays a monotonic increase of the ferromagnetic Curie temperature with x associated with the hole-mediated ferromagnetic phase while thermal annealing above 300 C leads to a quenching of ferromagnetism that is accompanied by a reduction of the substitutional fraction of Mn. However, contrary to observations in Ga1-xMnxAs, Ga1-xMnxP is non-metallic over the entire composition range. At the lower temperatures over which the films are ferromagnetic, hole transport occurs via hopping conduction in a Mn-derived band; at higher temperatures it arises from holes in the valence band which are thermally excited across an energy gap that shrinks with x.

cond-mat.mtrl-sci↗

arXiv2006

Mn L3,2 X-ray Absorption Spectroscopy And Magnetic Circular Dichroism In Ferromagnetic (Ga,Mn)P

We have measured the X-ray absorption (XAS) and X-ray magnetic circular dichroism (XMCD) at the Mn L3,2 edges in ferromagnetic Ga1-xMnxP films for 0.018<x<0.042. Large XMCD asymmetries at the L3 edge indicate significant spin-polarization of the density of states at the Fermi energy. The spectral shapes of the XAS and XMCD are nearly identical with those for Ga1-xMnxAs indicating that the hybridization of Mn d states and anion p states is similar in the two materials. Finally, compensation with sulfur donors not only lowers the ferromagnetic Curie temperature but also reduces the spin polarization of the hole states.

cond-mat.mtrl-sci↗

arXiv2006

Mn L3,2 X-ray absorption and magnetic circular dichroism in ferromagnetic Ga1-xMnxP

We have measured the X-ray absorption (XAS) and X-ray magnetic circular dichroism (XMCD)at the Mn L3,2 edges in ferromagnetic Ga1-xMnxP for 0.018<x<0.042. Large XMCD asymmetries at the L3 edge indicate significant spin-polarization of the density of states at the Fermi energy. The temperature dependence of the XMCD and moment per Mn of 2.67 Bohr magnetons calculated using sum rules are consistent with magnetometry values. The spectral shapes of the XAS and XMCD are nearly identical with those for Ga1-xMnxAs indicating that the hybridization of Mn d and anion p states is similar in the two materials.

cond-mat.mtrl-sci↗