803 research outputs found
Transient jet formation and state transitions from large-scale magnetic reconnection in black hole accretion discs
Magnetically arrested accretion discs (MADs), where the magnetic pressure in
the inner disc is dynamically important, provide an alternative mechanism for
regulating accretion to what is commonly assumed in black hole systems. We show
that a global magnetic field inversion in the MAD state can destroy the jet,
significantly increase the accretion rate, and move the effective inner disc
edge in to the marginally stable orbit. Reconnection of the MAD field in the
inner radii launches a new type of transient outflow containing hot plasma
generated by magnetic dissipation. This transient outflow can be as powerful as
the steady magnetically-dominated Blandford-Znajek jet in the MAD state. The
field inversion qualitatively describes many of the observational features
associated with the high luminosity hard to soft state transition in black hole
X-ray binaries: the jet line, the transient ballistic jet, and the drop in rms
variability. These results demonstrate that the magnetic field configuration
can influence the accretion state directly, and hence the magnetic field
structure is an important second parameter in explaining observations of
accreting black holes across the mass and luminosity scales.Comment: 5 pages, 3 figures, submitted to MNRAS Letter
Magnetohydrodynamic Simulations of Gamma-Ray Burst Jets: Beyond the Progenitor Star
Achromatic breaks in afterglow light curves of gamma-ray bursts (GRBs) arise
naturally if the product of the jet's Lorentz factor \gamma and opening angle
\Theta_j satisfies (\gamma \Theta_j) >> 1 at the onset of the afterglow phase,
i.e., soon after the conclusion of the prompt emission. Magnetohydrodynamic
(MHD) simulations of collimated GRB jets generally give (\gamma \Theta_j) <~ 1,
suggesting that MHD models may be inconsistent with jet breaks. We work within
the collapsar paradigm and use axisymmetric relativistic MHD simulations to
explore the effect of a finite stellar envelope on the structure of the jet.
Our idealized models treat the jet-envelope interface as a collimating rigid
wall, which opens up outside the star to mimic loss of collimation. We find
that the onset of deconfinement causes a burst of acceleration accompanied by a
slight increase in the opening angle. In our fiducial model with a stellar
radius equal to 10^4.5 times that of the central compact object, the jet
achieves an asymptotic Lorentz factor \gamma ~ 500 far outside the star and an
asymptotic opening angle \Theta_j ~ 0.04 rad ~ 2 deg, giving (\gamma \Theta_j)
~ 20. These values are consistent with observations of typical long-duration
GRBs, and explain the occurrence of jet breaks. We provide approximate analytic
solutions that describe the numerical results well.Comment: Accepted to New Astronomy. 7 pages, 3 figures
Stem cells and fluid flow drive cyst formation in an invertebrate excretory organ.
Cystic kidney diseases (CKDs) affect millions of people worldwide. The defining pathological features are fluid-filled cysts developing from nephric tubules due to defective flow sensing, cell proliferation and differentiation. The underlying molecular mechanisms, however, remain poorly understood, and the derived excretory systems of established invertebrate models (Caenorhabditis elegans and Drosophila melanogaster) are unsuitable to model CKDs. Systematic structure/function comparisons revealed that the combination of ultrafiltration and flow-associated filtrate modification that is central to CKD etiology is remarkably conserved between the planarian excretory system and the vertebrate nephron. Consistently, both RNA-mediated genetic interference (RNAi) of planarian orthologues of human CKD genes and inhibition of tubule flow led to tubular cystogenesis that share many features with vertebrate CKDs, suggesting deep mechanistic conservation. Our results demonstrate a common evolutionary origin of animal excretory systems and establish planarians as a novel and experimentally accessible invertebrate model for the study of human kidney pathologies
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