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Register a new userMagnetic Interaction in Ultra-compact Binary Systems
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Authors
Kinwah Wu
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This article reviews the current works on ultra-compact double-degenerate binaries in the presence of magnetic interaction, in particular, unipolar induction. The orbital dynamics and evolution of compact white-dwarf pairs are discussed in detail. Models and predictions of electron cyclotron masers from unipolar-inductor compact binaries and unipolar-inductor white-dwarf planetary systems are presented. Einstein-Laub effects in compact binaries are briefly discussed.
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Using neural networks, we integrate the ability to account for Doppler smearing due to a pulsars orbital motion with the pulsar population synthesis package psrpoppy to develop accurate modeling of the observed binary pulsar population. As a first application, we show that binary neutron star systems where the two components have highly unequal mass are, on average, easier to detect than systems which are symmetric in mass. We then investigate the population of ultra-compact ($1.5 , {rm min} leq P_{rm b} leq 15,rm min$) neutron star--white dwarf (NS--WD) and double neutron star (DNS) systems which are promising sources for the Laser Interferometer Space Antenna gravitational-wave detector. Given the non-detection of these systems in radio surveys thus far, we estimate a 95% confidence upper limit of $sim$1450 and $sim$1100 ultra-compact NS--WD and DNS systems in the Milky Way that are beaming towards the Earth respectively. We also show that using survey integration times in the range 20~s to 200~s with time-domain resampling will maximize the signal-to-noise ratio as well as the probability of detection of these ultra-compact binary systems. Among all the large scale radio pulsar surveys, those that are currently being carried out at the Arecibo radio telescope have $sim$50--80% chance of detecting at least one of these systems using current integration integration times and $sim$80--95% using optimal integration times in the next several years.
X-ray binary systems are very popular objects for astrophysical investigations. Compact objects in these systems are neutron stars, white dwarfs and black holes. Neutron stars and white dwarfs can have intrinsic magnetic fields. There is well known, famous theorem about absence of intrinsic magnetic fields of black holes. But magnetic field can exist in the accretion disk around a black hole. We present here the real estimates of the magnetic field strength at the radius of innermost stable orbit in an accretion disk of stellar mass black holes.
To confirm the nature of the donor star in the ultra-compact X-ray binary candidate 47 Tuc X9, we obtained optical spectra (3,000$-$10,000 {AA}) with the Hubble Space Telescope / Space Telescope Imaging Spectrograph. We find no strong emission or absorption features in the spectrum of X9. In particular, we place $3sigma$ upper limits on the H$alpha$ and HeII $lambda 4686$ emission line equivalent widths $-$EW$_{mathrm{Halpha}} lesssim 14$ {AA} and $-$EW$_{mathrm{HeII}} lesssim 9$ {AA}, respectively. This is much lower than seen for typical X-ray binaries at a similar X-ray luminosity (which, for $L_{mathrm{2-10 keV}} approx 10^{33}-10^{34}$ erg s$^{-1}$ is typically $-$EW$_{mathrm{Halpha}} sim 50$ {AA}). This supports our previous suggestion (by Bahramian et al.) of an H-poor donor in X9. We perform timing analysis on archival far-ultraviolet, $V$ and $I$-band data to search for periodicities. In the optical bands we recover the seven-day superorbital period initially discovered in X-rays, but we do not recover the orbital period. In the far-ultraviolet we find evidence for a 27.2 min period (shorter than the 28.2 min period seen in X-rays). We find that either a neutron star or black hole could explain the observed properties of X9. We also perform binary evolution calculations, showing that the formation of an initial black hole / He-star binary early in the life of a globular cluster could evolve into a present-day system such as X9 (should the compact object in this system indeed be a black hole) via mass-transfer driven by gravitational wave radiation.
Compact neutron star binary systems are produced from binary massive stars through stellar evolution involving up to two supernova explosions. The final stages in the formation of these systems have not been directly observed. We report the discovery of iPTF 14gqr (SN 2014ft), a Type Ic supernova with a fast evolving light curve indicating an extremely low ejecta mass ($approx 0.2$ solar masses) and low kinetic energy ($approx 2 times 10^{50}$ ergs). Early photometry and spectroscopy reveal evidence of shock cooling of an extended He-rich envelope, likely ejected in an intense pre-explosion mass loss episode of the progenitor. Taken together, we interpret iPTF 14gqr as evidence for ultra-stripped supernovae that form neutron stars in compact binary systems.
Variation of mass supply rate from the companion can be smeared out by viscous processes inside an accretion disk. By the time the flow reaches the inner edge, the variation in X-rays needs not reflect the true variation of the rate at the outer edge. However, if the viscosity fluctuates around a mean value, one would expect the viscous time scale also to spread around a mean value. In HMXBs, the size of the viscous Keplerian disk is smaller & thus such a spread could be lower as compared to the LMXBs. If there is an increasing or decreasing trend in viscosity, the interval between enhanced emission would be modified systematically. In the absence of a full knowledge about the variation of mass supply rates at the outer edge, we study ideal circumstances where modulation must take place exactly in orbital time scales when there is an ellipticity in the orbit. We study a few compact binaries using long term RXTE/ASM(1.5-12 keV) & Swift/BAT(15-50keV) data to look for such effects & to infer what these results can tell us about the viscous processes inside the respective disks. We employ three different methods to seek imprints of periodicity on the X-ray variation & found that in all the cases, the location of the peak in the power density spectra is consistent with the orbital frequencies. Interestingly, in HMXBs the peaks are sharp with high rms values, consistent with a small Keplerian disk in a wind fed system. However, in LMXBs with larger Keplerian disk, the peaks are spread out with much lower rms values. X-ray reflections, or superhump phenomena which may also cause such X-ray modulations would not be affected by the size of the Keplerian disk. Our result confirms different sizes of Keplerian disks in 2 classes of binaries. If the orbital period of a binary system is not known, it may be obtained with reasonable accuracy for HMXBs & with lesser accuracy for LMXBs by our method.
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