Narrow Results

The string inspired higher dimensional theories suggest modification of Newton's law at submillimeter length scales. Inter-particle distances in white dwarf and neutron stars are 10^-10 cms and 10^-13 cms respectively, and therefore, the effects of short distance corrections to gravity deserve investigation. We show, by carrying out explicit analytical many-body calculations that, in the presence of corrections, the normal state of these compact stars become metastable. The actual quantum mechanical ground state of the stars turns out to be unstable. However, the tunneling probability to the unstable ground state is so small that the stars may remain trapped in the metastable state practically forever.

In the framework of $f(R,T)=R+2\beta T$ gravity, wherein R denotes the Ricci scalar and T represents the trace of the energy–momentum tensor, the equilibrium structure of charged anisotropic white dwarfs is investigated. The stellar equations for the general scenario are derived, and numerical solutions are obtained for the Chandrasekhar equation of state alongside the charge density distribution proportional to the energy density, ${\rho }_{\mathrm{\text{ch}}}=\alpha \rho$. By scrutinizing the properties of the solutions under various parameter combinations, we conducted an exhaustive analysis of the influences of electric charge, modified gravity, and anisotropy on the structural characteristics of white dwarfs. Crucially, white dwarfs can transcend the trivial instances of General Relativity in various ways, thereby demonstrating super-Chandrasekhar phenomena. As an inquiry into the structure of white dwarfs, the findings of this research may contribute to elucidating astronomical observations such as superluminous type Ia supernovae.

We study the structure of pulsating white dwarf stars, which present multi-periodic light variations with periods around minutes, defining instability strips along their cooling sequence. We analyze the ultraviolet (HST) and optical (WET) light curves of the pulsating white dwarf G185-32. Among all pulsating white dwarfs, this star has the shortest periodicity so far observed. We detected 18 periodicities, including non-linear effects, which can be used to study the stellar structure, constraining the physics of the degenerate matter.

Stable end-point stars currently fall into two distinct classes — white dwarfs and neutron stars — differing enormously in central density and radial size. No stable cold dead stars are thought to span the intervening densities or have masses beyond ~2–3 solar masses. I show, however, that the general-relativistic condition of hydrostatic equilibrium augmented by the equation of state of a neutron condensate at 0 K generates stable sequences of cold stars that span the density gap and can have masses well beyond prevailing limits. The radial sizes and mass limit of each sequence are determined by the mass and scattering length of the composite bosons. Solutions for hypothetical bosons of ultrasmall mass and large scattering length yield huge self-gravitating systems of low density, resembling galactic dark matter halos.

In quantum gravity phenomenology, the effect of the generalized uncertainty principle (GUP) on white dwarf structure has been given much attention in recent literature. However, these studies assume a zero temperature equation of state (EoS), excluding young white dwarfs whose initial temperatures are substantially high. To that cause, this paper calculates the Chandrasekhar EoS and resulting mass-radius relations of finite temperature white dwarfs modified by the quadratic GUP, an approach that extends Heisenberg’s uncertainty principle by a quadratic term in momenta. The EoS was first approximated by treating the quadratic GUP parameter as perturbative, causing the EoS to exhibit expected thermal deviations at low pressures, and conflicting behaviors at high pressures, depending on the order of approximation. We then proceeded with a full numerical simulation of the modified EoS, and showed that in general, finite temperatures cause the EoS at low pressures to soften, while the quadratic GUP stiffens the EoS at high pressures. This modified EoS was then applied to the Tolman–Oppenheimer–Volkoff equations and its classical approximation to obtain the modified mass-radius relations for general relativistic and Newtonian white dwarfs. The relations for both cases were found to exhibit the expected thermal deviations at small masses, where low-mass white dwarfs are shifted to the high-mass regime at large radii, while high-mass white dwarfs acquire larger masses, beyond the Chandrasekhar limit. Additionally, we find that for sufficiently large values of the GUP parameter and temperature, we obtain mass-radius relations that are completely removed from the ideal case, as high-mass deviations due to GUP and low-mass deviations due to temperature are no longer mutually exclusive.

Since their initial discovery, the origin of isolated white dwarfs (WDs) with magnetic fields in excess of ~1 MG has remained a mystery. Recently, the formation of these high-field magnetic WDs has been observationally linked to strong binary interactions incurred during post-main-sequence evolution. Planetary, brown dwarf or stellar companions located within a few AU of main-sequence stars may become engulfed during the primary's expansion off the main sequence. Sufficiently low-mass companions in-spiral inside a common envelope until they are tidally shredded near the natal white dwarf. Formation of an accretion disk from the disrupted companion provides a source of turbulence and shear which act to amplify magnetic fields and transport them to the WD surface. We show that these disk-generated fields explain the observed range of magnetic field strengths for isolated, high-field magnetic WDs. Additionally, we discuss a high-mass binary analogue which generates a strongly-magnetized WD core inside a pre-collapse, massive star. Subsequent core-collapse to a neutron star may produce a magnetar.

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.

AE Aquarii is a cataclysmic variable with the fastest known rotating magnetized white dwarf (P = 33.08 s). It was observed with the NuSTAR imaging hard X-ray observatory to search for possible thermal emission above 10 keV and to identify a reported narrow feature in its pulse profile. The 3–30 keV phase-averaged spectrum is found to be well-fit by either an optically-thin thermal plasma model with a temperature of 3.9 keV plus power-law component with photon index of 2.2, or an optically-thin thermal plasma model with two temperatures of 2.3 keV and 8.7 keV, with the latter model being slightly preferred statistically. In the 3–30 keV hard energy band, we confirm a sinusoidal pulse profile with a pulsed fraction of 16.4 ± 2.4%. However, we were not able to reproduce the sharp pulse feature in the light curve in this band reported to be seen in a previous observation.

In order to search for possible non-thermal emission like AE Aquarii, we picked up magnetized isolated white dwarfs (WDs) in addition to binaries including magnetized cataclysmic variables. Recently, many magnetized WDs are found by Sloan Digital Sky Survey project reaching 9000 objects including WDs showing a magnetic field strength of B ~ 10⁹ G. 82 objects were selected with known magnetic field strength and spin periods in terms of magnetic dipole momentum. The most promising object is EUVE J0317-855, which locates above death line of pulsars and has magnetic dipole radiation Erad ~ 1.1 × 10²⁹ erg/s. Then, we researched for non-thermal emission from isolated magnetized WDs with INTEGRAL, MAXI and Suzaku. As a result, no significant emissions were detected from isolated magnetized WDs with these instruments sensitivities.

Among various types of astrophysical particle acceleration sites, one important prototype is rotation-powered pulsars, namely highly magnetized (10¹² G) and rapidly rotating (1 sec) neutronstar. On the first successful detection of pulsating hard X-rays from AE Aqr, the possibility of particle acceleration in the magnetosphere was pointed out. We consider that such magnetized whited dwarfs have similar system as pulsars. The particles are accelerated by the electrostatic potential which is induced by a strong surface field but the acceleration mechanism is still controversal. Here, for simplicity, we demonstrated particle simulation for isolated Rotating White Dwarf magnetosphere. We considered the effect of higher order stellar magnetic field such as quadrupole magnetic field. Magnetic field is assumed to be dipole or quadrepole. The numerical code is originately developed for particle simulation for axissymetric pulsar magnetosphere. We have obtained steady solution with accelerating regions around the star. Because of superimposition of quadrepole field, the location of accelerating region differs around the equatorial plane in contrast to dipole magnetic field case.

Accretion-driven spin-up of a magnetized white dwarf in a close binary system is discussed. We address a situation in which the magnetic field of the white dwarf is screening during the accretion phase and re-generating due to the field diffusion through the accreted material after it. We find this scenario to be effective for a formation of massive pulsar-like white dwarfs.