arXiv subjects

Anke van Dyk

Publications and source records attributed to Anke van Dyk.

4 recordsLinked to original sources

arXiv2026

Chromospherically Active Stars in the MeerKAT Galactic Plane Survey

Stellar magnetic activity has long been established to occur in subsets of late-type main-sequence stars and manifests itself in the form of chromospheric and coronal activity. This activity is driven by magnetic fields generated by dynamo mechanisms associated with stellar rotation and convection, which channel and dissipate magnetic energy to the outer atmospheric layers. Optical spectroscopic follow-up was carried out for 99 radio-X-ray coronally active stars of G-, K-, and M-spectral types in the SARAO MeerKAT Galactic plane survey point sources. There are 46 stars that exhibited chromospheric activity. Specifically, 39 stars show Ca II H&K emission lines, whereas 18 show Ca II IRT, 11 show both H&K and IRT lines. Of the 39 stars displaying H&K emission, 37 show activity levels greater than the Solar minimum activity. Their X-ray to bolometric luminosity ratios are within the coronal thresholds of > -4.5. In addition, we explored optical photometry to investigate the rotation information and found that these stars show short rotation periods; in comparison with activity levels, the activity levels dampen as the rotation periods increase. We also analysed optical light curves for a subset of stars, and they show flaring behaviour at the same epoch as enhanced chromospheric activity, indicating the flares and chromospheric emission are manifestations of the same underlying magnetic activity process.

astro-ph.SR↗

arXiv2026

A probable inside-out dwarf nova outburst from the period bouncer candidate ASASSN-25dc

We report optical time-resolved photometric observations of a newly-discovered outbursting system, ASASSN-25dc. Its 8-mag amplitude, 40-day duration, 1-mag dip in the outburst plateau, and positive superhumps are characteristic of a dwarf nova superoutburst in a non-magnetic cataclysmic variable. We establish its stage-A and stage-B superhump periods as 0.059387(5) d and 0.058864(3) d, respectively. The negative superhump period derivative ($-$1.4(2)$\times10^{-5}$ cycle$^{-1}$) during the stage-B superhumps and the empirical relation indicate the mass ratio is 0.054(7), below the period bounce range. The long outburst decline timescale (35.2(1) d mag$^{-1}$) and small superhump amplitude ($\simeq$0.08 mag) observed in ASASSN-25dc are also seen in some period bouncer systems, but not seen in systems well before the period bounce. Despite its short superhump period and indicated small mass ratio, we find no evidence of the excitement of the 2:1 tidal resonance. Moreover, its outburst rise timescale (1.62(9) d mag$^{-1}$) is significantly longer than those measured at less than 0.4 d mag$^{-1}$ in other dwarf nova outbursts around the period minimum. Overall, an inside-out dwarf nova outburst from a massive disc in a system with a mass ratio around or even below the period minimum, but lacking the 2:1 tidal resonance, may explain all these observations. However, this challenges the existing models of dwarf nova superoutbursts, which do not predict these outburst properties in low-mass-ratio systems.

astro-ph.SR↗

arXiv2024

A Broadband X-ray Investigation of Fast-Spinning Intermediate Polar CTCV J2056-3014

We report on XMM-Newton, NuSTAR, and NICER X-ray observations of CTCV J2056-3014, a cataclysmic variable (CV) with one of the fastest-spinning white dwarfs (WDs) at P = 29.6 s. While previously classified as an intermediate polar (IP), CJ2056 also exhibits the properties of WZ-Sge-type CVs, such as dwarf novae and superoutbursts. With XMM-Newton and NICER, we detected the spin period up to approximately 2 keV with 7-$σ$ significance. We constrained its derivative to |$\dot{P}$| < 1.8e-12 s/s after correcting for binary orbital motion. The pulsed profile is characterized by a single broad peak with approximately 25% modulation. NuSTAR detected a four-fold increase in unabsorbed X-ray flux coincident with an optical flare in November 2022. The XMM-Newton and NICER X-ray spectra in 0.3-10 keV are best characterized by an absorbed optically-thin three-temperature thermal plasma model (kT = 0.3, 1.0, and 4.9 keV), while the NuSTAR spectra in 3-30 keV are best fit by a single-temperature thermal plasma model (kT = 8.4 keV), both with Fe abundance $Z_{Fe}/Z_\odot$ = 0.3. CJ2056 exhibits similarities to other fast-spinning CVs, such as low plasma temperatures, and no significant X-ray absorption at low energies. As the WD's magnetic field strength is unknown, we applied both non-magnetic and magnetic CV spectral models (MKCFLOW and MCVSPEC) to determine the WD mass. The derived WD mass range (M = 0.7-1.0 $M_\odot$) is above the centrifugal break-up mass limit of 0.56 $M_\odot$ and consistent with the mean WD mass of local CVs (M $\approx$ 0.8-0.9 $M_\odot$).

astro-ph.HE↗

arXiv2023

Rapid Evolution of the White Dwarf Pulsar AR Scorpii

Analysis of AR Sco optical light curves spanning nine years show a secular change in the relative amplitudes of the beat pulse pairs generated by the two magnetic poles of its rotating white dwarf. Recent photometry now shows that the primary and secondary beat pulses have similar amplitudes, while in 2015 the primary pulse was approximately twice that of the secondary peak. The equalization in the beat pulse amplitudes is also seen in the linearly polarized flux. This rapid evolution is consistent with precession of the white dwarf spin axis. The observations imply that the pulse amplitudes cycle over a period of $\gtrsim 40$ yrs, but that the upper limit is currently poorly constrained. If precession is the mechanism driving the evolution, then over the next 10 years the ratio of the beat pulse amplitudes will reach a maximum followed by a return to asymmetric beat pulses.

astro-ph.SR↗