Lei Huang

Black hole shadow image and visibility analysis of Sagittarius A

The compact dark objects with very large masses residing at the centres of galaxies are believed to be black holes. Due to the gravitational lensing effect, they would cast a shadow larger than their horizon size over the background; the shape and size of this shadow can be calculated. For the supermassive black hole candidate Sgr A*, this shadow spans an angular size of about 50 μas, which is under the resolution attainable with the current astronomical instruments. Such a shadow image of Sgr A* will be observable at about 1 mm wavelength, considering the scatter broadening by the interstellar medium. By simulating the black hole shadow image of Sgr A* with the radiatively inefficient accretion flow model, we demonstrate that analysing the properties of the visibility function can help us determine some parameters of the black hole configuration, which is instructive for the submillimetre Very Long Baseline Interferometry (VLBI) observations of Sgr A* to be made in the near future.

Images of the radiatively inefficient accretion flow surrounding a Kerr black hole: application in Sgr A*

In fully general relativity, we calculate the images of the radiatively inefficient accretion flow (RIAF) surrounding a Kerr black hole (BH) with arbitrary spins, inclination angles, and observational wavelengths. For the same initial conditions, such as the fixed accretion rate, it is found that the intrinsic size and radiation intensity of the images become larger, but the images become more compact in the inner region, while the size of the BH shadow decreases with the increase of the BH spin. With the increase of the inclination angles, the shapes of the BH shadows change and become smaller, even disappear completely due to the obscuration by the thick disks. For median inclination angles, the radial velocity observed at infinity is larger because of both the rotation and radial motion of the fluid in the disk, which results in the luminous part of the images being much brighter. For larger inclination angles, such as the disk is edge-on, the emission becomes dimmer at longer observational wavelengths (such as at 7.0 mm and 3.5 mm wavelengths), or brighter at shorter observational wavelengths (such as at 1.3 mm wavelength) than that of the face on case, except for the high-spin and high-inclination images. These complex behaviors are due to the combination of the Lorentz boosting effect and the radiative absorption in the disk. We hope our results are helpful to determine the spin parameter of the BH in low-luminosity sources, such as the Galactic center. A primary analysis by comparison with the observed sizes of Sgr A* at millimeter wavelengths strongly suggests that the disk around the central BH at Sgr A* is highly inclined or the central BH is rotating fast.

TESTING THE ACCRETION FLOW WITH PLASMA WAVE HEATING MECHANISM FOR SAGITTARIUS A* BY THE 1.3 mm VLBI MEASUREMENTS

The vicinity of the supermassive black hole associated with the compact radio source Sagittarius (Sgr) A* is believed to dominate the observed emission at wavelengths near and shorter than similar to 1 millimeter. We show that a general relativistic accretion flow, heated via the plasma wave heating mechanism, is consistent with the polarization and recent millimeter-VLBI observations of Sgr A* for an inclination angle of similar to 45 degrees, position angle of similar to 140 degrees, and spin less than or similar to 0.9. Structure in visibilities produced by the black hole shadow can potentially be observed by 1.3 mm-VLBI on the existing Hawaii-CARMA and Hawaii-SMT baselines. We also consider eight additional potential millimeter-VLBI stations, including sites in Chile and New Zealand, finding that with these the basic geometry of the emission region can be reliably estimated.

Testing the radiatively inefficient accretion flow model for sagittarius A* using the size measurements

Recent radio observations by the Very Long Baseline Array at 7 and 3.5 mm produced the high-resolution images of the compact radio source located at the center of our Galaxy ( Sgr A*) and detected its wavelength-dependent intrinsic sizes at the two wavelengths. This provides us with a good chance of testing previously proposed theoretical models for Sgr A*. In this Letter, we calculate the size based on the radiatively inefficient accretion flow ( RIAF) model proposed by Yuan, Quataert, & Narayan. We find that after taking into account the scattering of the interstellar electrons, the predicted sizes are consistent with the observations. We further predict an image of Sgr A* at 1.3 mm that can be tested by future observations.Recent radio observations by the VLBA at 7 and 3.5 mm produced the high-resolution images of the compact radio source located at the center of our Galaxy--Sgr A*, and detected its wavelength-dependent intrinsic sizes at the two wavelengths. This provides us with a good chance of testing previously-proposed theoretical models for Sgr A*. In this {\em Letter}, we calculate the size based on the radiatively inefficient accretion flow (RIAF) model proposed by Yuan, Quataert & Narayan (2003). We find that the predicted sizes after taking into account the scattering of the interstellar electrons are consistent with the observations. We further predict an image of Sgr A* at 1.3 mm which can be tested by future observations.

Faraday conversion and rotation in uniformly magnetized relativistic plasmas

We provide precise fitting formulae for Faraday conversion and rotation coefficients in uniformly magnetized relativistic plasma. The formulae are immediately applicable to Rotation Measure and Circular Polarization (CP) production in jets and hot accretion flows. We show the recipe and results for arbitrary isotropic particle distributions, in particular thermal and power-law. The exact Faraday conversion coefficient is found to approach zero with the increasing particle energy. The non-linear corrections of Faraday conversion and rotation coefficients are found essential for reliable CP inter- pretation of Sgr A*.

MAGNETAR GIANT FLARES AND THEIR PRECURSORS—FLUX ROPE ERUPTIONS WITH CURRENT SHEETS

We propose a catastrophic magnetospheric model for magnetar precursors and their successive giant flares. Axisymmetric models of the magnetosphere, which contain both a helically twisted flux rope and a current sheet, are established based on force-free field configurations. In this model, the helically twisted flux rope would lose its equilibrium and erupt abruptly in response to the slow and quasi-static variations at the ultra-strongly magnetized neutron stars surface. In a previous model without current sheets, only one critical point exists in the flux rope equilibrium curve. New features show up in the equilibrium curves for the flux rope when current sheets appear in the magnetosphere. The causal connection between the precursor and the giant flare, as well as the temporary re-entry of the quiescent state between the precursor and the giant flare, can be naturally explained. Magnetic energy would be released during the catastrophic state transitions. The detailed energetics of the model are also discussed. The current sheet created by the catastrophic loss of equilibrium of the flux rope provides an ideal place for magnetic reconnection. We point out the importance of magnetic reconnection for further enhancement of the energy release during eruptions.

Magnetar giant flares in multipolar magnetic fields. II. Flux rope eruptions with current sheets

We propose a physical mechanism to explain giant flares and radio afterglows in terms of a magnetospheric model containing both a helically twisted flux rope and a current sheet (CS). With the appearance of a CS, we solve a mixed boundary value problem to get the magnetospheric field based on a domain decomposition method. We investigate properties of the equilibrium curve of the flux rope when the CS is present in background multipolar fields. In response to the variations at the magnetar surface, it quasi-statically evolves in stable equilibrium states. The loss of equilibrium occurs at a critical point and, beyond that point, it erupts catastrophically. New features show up when the CS is considered. In particular, we find two kinds of physical behaviors, i.e., catastrophic state transition and catastrophic escape. Magnetic energy would be released during state transitions. This released magnetic energy is sufficient to drive giant flares, and the flux rope would, therefore, go away from the magnetar quasi-statically, which is inconsistent with the radio afterglow. Fortunately, in the latter case, i.e., the catastrophic escape, the flux rope could escape the magnetar and go to infinity in a dynamical way. This is more consistent with radio afterglow observations of giant flares. We find that the minor radius of the flux rope has important implications for its eruption. Flux ropes with larger minor radii are more prone to erupt. We stress that the CS provides an ideal place for magnetic reconnection, which would further enhance the energy release during eruptions.

Linearly and Circularly Polarized Emission in Sagittarius A

We perform general relativistic ray-tracing calculations of the transfer of polarized synchrotron radiation through the relativistic accretion flow in Sagittarius (Sgr) A*. Based on a two-temperature magnetorotational instability (MRI) induced accretion mode, the birefringence effects are treated self-consistently. By fitting the spectrum and polarization of Sgr A* from millimeter to near-infrared bands, we are able to not only constrain the basic parameters related to the MRI and the electron heating rate, but also limit the orientation of the accretion torus. These constraints lead to unique polarimetric images, which may be compared with future millimeter and submillimeter VLBI observations. In combination with general relativistic MHD simulations, the model has the potential to test the MRI with observations of Sgr A*.

MAGNETAR GIANT FLARES IN MULTIPOLAR MAGNETIC FIELDS. I. FULLY AND PARTIALLY OPEN ERUPTIONS OF FLUX ROPES

We propose a catastrophic eruption model for the enormous energy release of magnetars during giant flares, in which a toroidal and helically twisted flux rope is embedded within a force-free magnetosphere. The flux rope stays in stable equilibrium states initially and evolves quasi-statically. Upon the loss of equilibrium, the flux rope cannot sustain the stable equilibrium states and erupts catastrophically. During the process, the magnetic energy stored in the magnetosphere is rapidly released as the result of destabilization of global magnetic topology. The magnetospheric energy that could be accumulated is of vital importance for the outbursts of magnetars. We carefully establish the fully open fields and partially open fields for various boundary conditions at the magnetar surface and study the relevant energy thresholds. By investigating the magnetic energy accumulated at the critical catastrophic point, we find that it is possible to drive fully open eruptions for dipole-dominated background fields. Nevertheless, it is hard to generate fully open magnetic eruptions for multipolar background fields. Given the observational importance of the multipolar magnetic fields in the vicinity of the magnetar surface, it would be worthwhile to explore the possibility of the alternative eruption approach in multipolar background fields. Fortunately, we find that flux ropes may give rise to partially open eruptions in the multipolar fields, which involve only partial opening of background fields. The energy release fractions are greater for cases with central-arcaded multipoles than those with central-caved multipoles that emerged in background fields. Eruptions would fail only when the centrally caved multipoles become extremely strong.

Short-term spectroscopic variability in the pre-main sequence Herbig AE star AB Aurigae during the MUSICOS 96 campaign

We present results of the spectroscopic monitoring of AB Aur obtained during the MUSICOS 96 campaign. The analysis is mainly focussed on the He I D3 line, on the H line, and on a set of photospheric lines. The star was monitored irregularly for more than 200 hours. We confirm the high level of variability of spectral lines in AB Aur. We find that the photospheric lines have a profile differing significantly from a classical rotational profile. The dominant features of this abnormal photospheric profile are a blue component, in absorption, whose velocity is modulated with a 34hr period, and a red component, stable in velocity but of variable intensity, with a possible periodicity near 43 hrs. The He I D3 line exhibits two well-defined components: a blue component, always in emission with a velocity modulated with a 45hr period, and a red component of variable intensity, alternatively in emission and in absorption, occurring at a fixed velocity, with a variable intensity possibly modulated with a 45 hr period.