Narrow Results

Some of the most interesting types of astrophysical objects that have been intensively studied in the recent years are the Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs) seen usually as neutron stars pulsars with super strong magnetic fields. However, in the last two years two SGRs with low magnetic fields have been detected. Moreover, fast and very magnetic white dwarf pulsars have also been observed in the last years. Based on these new pulsar discoveries, white dwarf pulsars have been proposed as an alternative explanation to the observational features of SGRs and AXPs. Here we present several properties of these SGRs/AXPs as WD pulsar, in particular the surface magnetic field and the magnetic dipole momentum.

White dwarf stars are the final stage of most stars, born single or in multiple systems. We discuss the identification, magnetic fields, and mass distribution for white dwarfs detected from spectra obtained by the Sloan Digital Sky Survey up to Data Release 13 in 2016, which lead to the increase in the number of spectroscopically identified white dwarf stars from 5000 to 39000. This number includes only white dwarf stars with $logg\ge 6.5$, i.e., excluding the Extremely Low Mass white dwarfs, which are necessarily the byproduct of stellar interaction.

Electron captures by atomic nuclei in dense matter are among the most important processes governing the late evolution of stars, limiting in particular the stability of white dwarfs. Despite considerable progress in the determination of the equation of state of dense Coulomb plasmas, the threshold electron Fermi energies are still generally estimated from the corresponding Q values in vacuum. Moreover, most studies have focused on nonmagnetized matter. However, some white dwarfs are endowed with magnetic fields reaching 10⁹ G. Even more extreme magnetic fields might exist in super Chandrasekhar white dwarfs, the progenitors of overluminous type Ia supernovae like SN 2006gz and SN 2009dc. The roles of the dense stellar medium and magnetic fields on the onset of electron captures and on the structure of white dwarfs are briefly reviewed. New analytical formulas are derived to evaluate the threshold density for the onset of electron captures for arbitrary magnetic fields. Their influence on the structure of white dwarfs is illustrated by simple analytical formulas and numerical calculations.

Here we present results from an in-depth search for pulsed emission from both close binary systems AE Aquarii (AE Aqr) and AR Scorpii (AR Sco) in radio and gamma-ray energies. Both systems were observed recently with the MeerKAT telescope, and combined with this, we utilized the combined 10 year Pass 8 Fermi-LAT dataset to search for pulsed gamma-ray emission from both white dwarfs in these systems. Pulsed emission was detected in MeerKAT data from both these close binary systems at a period that is at, or close to, the spin period of the white dwarf. The search for pulsed gamma-ray emission revealed pulsed emission at the spin period of the white dwarf of AE Aqr after selecting data sets with duration of 2 weeks that show excess emission above the 2 σ significance level. Braking these two-week sets up in 10 minute intervals and stacking the power spectra revealed pulsed emission at both the spin (P * = 33.08 s) and its associated first harmonic (P₁ = 16.54 s). A full 10 year analysis of the AR Sco data revealed pulsed emission at the spin period/beat period of the white dwarf, albeit at a lower significance level. Several control analyses were performed to verify the authenticity of the emission in both radio and gamma-rays, which will be discussed in the main text. The results of this study definitely reveal that both white dwarfs in these systems contain a particle accelerator that accelerates charged particles to high energies resulting in associated non-thermal radio and gamma-ray emission.

Spectral modeling of the intermediate polars (IPs) and its application are introduced. The thermal X-ray spectral model is constructed with integrating the single temperature plasma emission along the post-shock accretion column (PSAC). The physical quantities distributions in the PSAC are calculated with hydrodynamics. The latest thermal model includes various physical effects and especially takes into account the difference of the specific accretion rate and the dipolar geometry of the PSAC. The reflection, another main component in the IPs spectrum, is modeled with Monte-Carlo simulation. In the simulation, the PSAC irradiates a cool and spherical WD with the various spectra determined by its position calculated in the thermal spectral modeling. Coherent and incoherent scatterings, photoelectric absorption and K_α and K_β reemissions of Fe and Ni are assumed to occur at the WD surface. The constructed spectral model were applied to EX Hya and V1223 Sgr observed by Suzaku and their WD masses are estimated at ${0.65}_{-0.12}^{+0.11}$M_⊙ and ${0.91}_{-0.03}^{+0.08}$M_⊙, respectively.