4,840 research outputs found
Magnetically Driven Accretion in the Kerr Metric III: Unbound Outflows
We have carried out fully relativistic numerical simulations of accretion
disks in the Kerr metric. In this paper we focus on the unbound outflows that
emerge self-consistently from the accretion flow. These outflows are found in
the axial funnel region and consist of two components: a hot, fast, tenuous
outflow in the axial funnel proper, and a colder, slower, denser jet along the
funnel wall. Although a rotating black hole is not required to produce these
unbound outflows, their strength is enhanced by black hole spin. The
funnel-wall jet is excluded from the axial funnel due to elevated angular
momentum, and is also pressure-confined by a magnetized corona. The tenuous
funnel outflow accounts for a significant fraction of the energy transported to
large distances in the higher-spin simulations. We compare the outflows
observed in our simulations with those seen in other simulations.Comment: 33 pages, 8 figures, ApJ submitte
Phosphorylation by Akt within the ST loop of AMPK-Ī±1 down-regulates its activation in tumour cells
The insulin/IGF-1 (insulin-like growth factor 1)-activated protein kinase Akt (also known as protein kinase B) phosphorylates Ser(487) in the āST loopā (serine/threonine-rich loop) within the C-terminal domain of AMPK-Ī±1 (AMP-activated protein kinase-Ī±1), leading to inhibition of phosphorylation by upstream kinases at the activating site, Thr(172). Surprisingly, the equivalent site on AMPK-Ī±2, Ser(491), is not an Akt target and is modified instead by autophosphorylation. Stimulation of HEK (human embryonic kidney)-293 cells with IGF-1 caused reduced subsequent Thr(172) phosphorylation and activation of AMPK-Ī±1Ā in response to the activator A769662 and the Ca(2+) ionophore A23187, effects we show to be dependent on Akt activation and Ser(487) phosphorylation. Consistent with this, in three PTEN (phosphatase and tensin homologue deleted on chromosome 10)-null tumour cell lines (in which the lipid phosphatase PTEN that normally restrains the Akt pathway is absent and Akt is thus hyperactivated), AMPK was resistant to activation by A769662. However, full AMPK activation could be restored by pharmacological inhibition of Akt, or by re-expression of active PTEN. We also show that inhibition of Thr(172) phosphorylation is due to interaction of the phosphorylated ST loop with basic side chains within the Ī±C-helix of the kinase domain. Our findings reveal that a previously unrecognized effect of hyperactivation of Akt in tumour cells is to restrain activation of the LKB1 (liver kinase B1)āAMPK pathway, which would otherwise inhibit cell growth and proliferation
Hot Settling Accretion Flow onto a Spinning Black Hole
We study the structure and properties of hot MHD accretion onto a Kerr black
hole. In such a system, the hole is magnetically coupled to the inflowing gas
and exerts a torque onto the accretion flow. A hot settling flow can form
around the hole and transport the angular momentum outward, to the outer edge
of the flow. Unlike other hot flows, such as advection- and
convection-dominated flows and inflow-outflow solutions (ADAFs, CDAFs, and
ADIOS), the properties of the hot settling flow are determined by the spin of
the central black hole, but are insensitive to the mass accretion rate.
Therefore, it may be possible to identify rapidly spinning BHs simply from
their broad-band spectra.
Observationally, the hot settling flow around a Kerr hole is somewhat similar
to other hot flows in that they all have hard, power-law spectra and relatively
low luminosities. Thus, most black hole candidates in the low/hard and,
perhaps, intermediate X-ray state may potentially accrete via the hot settling
flow. However, a settling flow will be somewhat more luminous than
ADAFs/CDAFs/ADIOS, will exhibit high variability in X-rays, and may have
relativistic jets. This suggests that galactic microquasars and active galactic
nuclei may be powered by hot settling flows. We identify several galactic X-ray
sources as the best candidates.Comment: 7 pages, 1 figure. Submitted to Ap
Synchrotron Radiation From Radiatively Inefficient Accretion Flow Simulations: Applications to Sgr A*
We calculate synchrotron radiation in three-dimensional pseudo-Newtonian
magnetohydrodynamic simulations of radiatively inefficient accretion flows. We
show that the emission is highly variable at optically thin frequencies, with
order of magnitude variability on time-scales as short as the orbital period
near the last stable orbit; this emission is linearly polarized at the 20-50 %
level due to the coherent toroidal magnetic field in the flow. At optically
thick frequencies, both the variability amplitude and polarization fraction
decrease significantly with decreasing photon frequency. We argue that these
results are broadly consistent with the observed properties of Sgr A* at the
Galactic Center, including the rapid infrared flaring.Comment: Accepted for publication in Ap
Seawater alkalinity determination by the pH method
The coefficient fH used in the seawater alkalinity method of Anderson and Robinson (1946), has been redetermined at 25Ā°C. We have found that fH = 0.741 Ā±0,005 for salinities between 30ā° and 41ā°ā¦
BOSS Ultracool Dwarfs I: Colors and Magnetic Activity of M and L dwarfs
We present the colors and activity of ultracool (M7-L8) dwarfs from the Tenth
Data Release of the Sloan Digital Sky Survey (SDSS). We combine previous
samples of SDSS M and L dwarfs with new data obtained from the Baryon
Oscillation Sky Survey (BOSS) to produce the BOSS Ultracool Dwarf (BUD) sample
of 11820 M7-L8 dwarfs. By combining SDSS data with photometry from the Two
Micron All Sky Survey and the Wide-Field Infrared Sky Explorer mission, we
present ultracool dwarf colors from to as a function of spectral
type, and extend the SDSS-2MASS-WISE color locus to include ultracool dwarfs.
The , , and colors provide the best indication of spectral type
for M7-L3 dwarfs. We also examine ultracool dwarf chromospheric activity
through the presence and strength of H emission. The fraction of active
dwarfs rises through the M spectral sequence until it reaches 90% at
spectral type L0. The fraction of active dwarfs then declines to 50% at
spectral type L5; no H emission is observed in the late-L dwarfs in the
BUD sample. The fraction of active L0-L5 dwarfs is much higher than previously
observed. The strength of activity declines with spectral type from M7 through
L3, after which the data do not show a clear trend. Using one-dimensional
chromosphere models, we explore the range of filling factors and chromospheric
temperature structures that are consistent with H observations of M0-L7
dwarfs. M dwarf chromospheres have a similar, smoothly varying range of
temperature and surface coverage while L dwarf chromospheres are cooler and
have smaller filling factors.Comment: 24 pages and 13 figures, submitted to AJ. A short video describing
these results can be found at https://www.youtube.com/watch?v=wwX5WkuJCU
Observation of Buried Phosphorus Dopants near Clean Si(100)-(2x1) with Scanning Tunneling Microscopy
We have used scanning tunneling microscopy to identify individual phosphorus
dopant atoms near the clean silicon (100)-(2x1) reconstructed surface. The
charge-induced band bending signature associated with the dopants shows up as
an enhancement in both filled and empty states and is consistent with the
appearance of n-type dopants on compound semiconductor surfaces and passivated
Si(100)-(2x1). We observe dopants at different depths and see a strong
dependence of the signature on the magnitude of the sample voltage. Our results
suggest that, on this clean surface, the antibonding surface state band acts as
an extension of the bulk conduction band into the gap. The positively charged
dimer vacancies that have been observed previously appear as depressions in the
filled states, as opposed to enhancements, because they disrupt these surface
bands.Comment: 4 pages, 3 figures. TeX for OSX from Wierde
First Detection of a Strong Magnetic Field on a Bursty Brown Dwarf: Puzzle Solved
We report the first direct detection of a strong, 5 kG magnetic field on the
surface of an active brown dwarf. LSR J1835+3259 is an M8.5 dwarf exhibiting
transient radio and optical emission bursts modulated by fast rotation. We have
detected the surface magnetic field as circularly polarized signatures in the
819 nm sodium lines when an active emission region faced the Earth. Modeling
Stokes profiles of these lines reveals the effective temperature of 2800 K and
log gravity acceleration of 4.5. These parameters place LSR J1835+3259 on
evolutionary tracks as a young brown dwarf with the mass of 554 M and age of 224 Myr. Its magnetic field is at least 5.1 kG and covers
at least 11% of the visible hemisphere. The active region topology recovered
using line profile inversions comprises hot plasma loops with a vertical
stratification of optical and radio emission sources. These loops rotate with
the dwarf in and out of view causing periodic emission bursts. The magnetic
field is detected at the base of the loops. This is the first time that we can
quantitatively associate brown dwarf non-thermal bursts with a strong, 5 kG
surface magnetic field and solve the puzzle of their driving mechanism. This is
also the coolest known dwarf with such a strong surface magnetic field. The
young age of LSR J1835+3259 implies that it may still maintain a disk, which
may facilitate bursts via magnetospheric accretion, like in higher-mass T
Tau-type stars. Our results pave a path toward magnetic studies of brown dwarfs
and hot Jupiters.Comment: ApJ, in pres
- ā¦