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Register a new userMeasuring the Density Structure of an Accretion Hot Spot
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Added by
Catherine Espaillat
Publication date
2021
fields
Physics
and research's language is
English
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Magnetospheric accretion models predict that matter from protoplanetary disks accretes onto the star via funnel flows which follow the stellar field lines and shock on the stellar surface leaving a hot spot with a density gradient. Previous work has inferred different densities in the hot spot, but has not been sensitive to the radial density distribution. Attempts have been made to measure this with X-ray observations, but X-ray emission only traces a fraction of the hot spot and also coronal emission. Here we report periodic ultraviolet and optical light curves of the accreting star GM Aur that display a time lag of about 1 day between their peaks. The periodicity arises as the source of the ultraviolet and optical emission moves into and out of view as it rotates along with the star. The time-lag indicates a difference in the spatial distribution of ultraviolet and optical brightness over the stellar surface. Within the framework of the magnetospheric accretion model, this indicates a radial density gradient in a hot spot on the stellar surface since different density parts of the hot spot are expected to emit radiation at different wavelengths. These results are the first observational confirmation of the magnetospheric accretion models prediction of a density gradient in the hot spot and demonstrate the insights gained from focusing on the wavelengths where the bulk of the accretion energy can be observed.
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We present a coherent timing analysis of the 2003 outburst of the accreting millisecond pulsar XTE J1807-294. We find an upper limit for the spin frequency derivative of 5E-14 Hz/s. The sinusoidal fractional amplitudes of the pulsations are the highest observed among the accreting millisecond pulsars and can reach values of up to 27% (2.5-30 keV). The pulse arrival time residuals of the fundamental follow a linear anti-correlation with the fractional amplitudes that suggests hot spot motion over the surface of the neutron star both in longitude and latitude. An anti-correlation between residuals and X-ray flux suggests an influence of accretion rate on pulse phase, and casts doubts on the use of standard timing techniques to measure spin frequencies and torques on the neutron star.
We have carried out simulations of supersonic light jets in order to model the features observed in optical and radio images of the western hot-spot in the radio galaxy Pictor A. We have considered jets with density ratios eta=1e-2 - 1e-4, and Mach numbers ranging between 5 and 50. From each simulation, we have generated ray-traced maps of radio surface brightness at a variety of jet inclinations, in order to study the appearance of time-dependent luminous structures in the vicinity of the western hot-spot. We compare these rendered images with observed features of Pictor A. A remarkable feature of Pictor A observations is a bar-shaped filament inclined almost at right angles to the inferred jet direction and extending 24 (10.8 /h kpc) along its longest axis. The constraints of reproducing the appearance of this structure in simulations indicate that the jet of Pictor A lies nearly in the plane of the sky. The results of the simulation are also consistent with other features found in the radio image of Pictor A. This filament arises from the surging behaviour of the jet near the hot-spot; the surging is provoked by alternate compression and decompression of the jet by the turbulent backflow in the cocoon. We also examine the arguments for the jet in Pictor A being at a more acute angle to the line of sight and find that our preferred orientation is just consistent with the limits on the brightness ratio of the X-ray jet and counter-jet. We determine from our simulations, the structure function of hot-spot brightness and also the cumulative distribution of the ratio of intrinsic hot-spot brightnesses. The latter may be used to quantify the use of hot-spot ratios for the estimation of relativistic effects.
Here we report on recent near-infrared observations of the Sgr A* counterpart associated with the super-massive ~ 4x10^6 M_sun black hole at the Galactic Center. We find that the May 2007 flare shows the highest sub-flare contrast observed until now, as well as evidence for variations in the profile of consecutive sub-flares. We modeled the flare profile variations according to the elongation and change of the shape of a spot due to differential rotation within the accretion disk.
Helicity is a fundamental property of a magnetic field but to date it has only been possible to observe its evolution in one star - the Sun. In this paper we provide a simple technique for mapping the large-scale helicity density across the surface of any star using only observable quantities: the poloidal and toroidal magnetic field components (which can be determined from Zeeman-Doppler imaging) and the stellar radius. We use a sample of 51 stars across a mass range of 0.1-1.34 M$_odot$ to show how the helicity density relates to stellar mass, Rossby number, magnetic energy and age. We find that the large-scale helicity density increases with decreasing Rossby number $R_o$, peaking at $R_o simeq 0.1$, with a saturation or decrease below that. For both fully- and partially-convective stars we find that the mean absolute helicity density scales with the mean squared toroidal magnetic flux density according to the power law: $|langle{h,}rangle|$ $propto$ $langle{rm{B_{tor}}^2_{},rangle}^{0.86,pm,0.04}$. The scatter in this relation is consistent with the variation across a solar cycle, which we compute using simulations and observations across solar cycles 23 and 24 respectively. We find a significant decrease in helicity density with age.
We solved the set of two-dimensional magnetohydrodynamic (MHD) equations for optically thin black hole accretion flows incorporating toroidal component of magnetic field. Following global and local MHD simulations of black hole accretion disks, the magnetic field inside the disk is decomposed into a large scale field and a fluctuating field. The effects of the fluctuating magnetic field in transferring the angular momentum and dissipating the energy are described through the usual $ alpha $ description. We solved the MHD equations by assuming steady state and radially self-similar approximation in $ r-theta $ plane of spherical coordinate system. We found that as the amount of magnetic field at the equatorial plane increases, the heating by the viscosity decreases. In addition, the maximum amount of the heating by the viscous dissipation is produced at the mid-plane of the disk, while that of the heating by the magnetic field dissipation is produced at the surface of the disk. Our main conclusion is that in terms of the no-outflow solution, thermal equilibrium still exists for the strong magnetic filed at the equatorial plane of the disk.
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