14,813 research outputs found
Bridging the gap between stellar-mass black holes and ultraluminous X-ray sources
The X-ray spectral and timing properties of ultraluminous X-ray sources
(ULXs) have many similarities with the very high state of stellar-mass black
holes (power-law dominated, at accretion rates greater than the Eddington
rate). On the other hand, their cool disk components, large characteristic
inner-disk radii and low characteristic timescales have been interpreted as
evidence of black hole masses ~ 1000 Msun (intermediate-mass black holes). Here
we re-examine the physical interpretation of the cool disk model, in the
context of accretion states of stellar-mass black holes. In particular, XTE
J1550-564 can be considered the missing link between ULXs and stellar-mass
black holes, because it exhibits a high-accretion-rate, low-disk-temperature
state (ultraluminous branch). On the ultraluminous branch, the accretion rate
is positively correlated with the disk truncation radius and the bolometric
disk luminosity, while it is anti-correlated with the peak temperature and the
frequency of quasi-periodic-oscillations. Two prototypical ULXs (NGC1313 X-1
and X-2) also seem to move along that branch. We use a phenomenological model
to show how the different range of spectral and timing parameters found in the
two classes of accreting black holes depends on both their masses and accretion
rates. We suggest that ULXs are consistent with black hole masses ~ 50-100
Msun, moderately inefficiently accreting at ~20 times Eddington.Comment: 11 pages, accepted for publication in Astrophysics and Space Science.
Based on work presented at the Fifth Stromlo Symposium, Australian National
University, Dec 200
RF amplification property of the MgO-based magnetic tunnel junction using field-induced ferromagnetic resonance
The radio-frequency (RF) voltage amplification property of a tunnel
magnetoresistance device driven by an RF external-magnetic-field-induced
ferromagnetic resonance was studied. The proposed device consists of a magnetic
tunnel junction (MTJ) and an electrically isolated coplanar waveguide. The
input RF voltage applied to the waveguide can excite the resonant dynamics in
the free layer magnetization, leading to the generation of an output RF voltage
under a DC bias current. The dependences of the RF voltage gain on the static
external magnetic field strength and angle were systematically investigated.
The design principles for the enhancement of the gain factor are also
discussed.Comment: 12 pages, 3 figure
Relativistic Iron Line Emission from the Neutron Star Low-mass X-ray Binary 4U 1636-536
We present an analysis of XMM-Newton and RXTE data from three observations of
the neutron star LMXB 4U 1636-536. The X-ray spectra show clear evidence of a
broad, asymmetric iron emission line extending over the energy range 4-9 keV.
The line profile is consistent with relativistically broadened Fe K-alpha
emission from the inner accretion disk. The Fe K-alpha line in 4U 1636-536 is
considerably broader than the asymmetric iron lines recently found in other
neutron star LMXBs, which indicates a high disk inclination. We find evidence
that the broad iron line feature is a combination of several K-alpha lines from
iron in different ionization states.Comment: 7 pages, 2 figures, Published in the Astrophysical Journa
Charge order and phase segregation in overdoped bilayer manganites
There have been recent reports of charge ordering around in the
bilayer manganites. At , there appears to be a coexistence region of
layered A-type antiferromagnetc and charge order. There are also reports of
orbital order in this region without any Jahn-Teller effect. Based on physical
grounds, this region is investigated from a model that incorporates the two
orbitals at each Mn site and a near-neighbour Coulomb repulsion. It is
shown that there indeed is both charge and orbital order close to the
half-doped region coincident with a layered magnetic structure. Although the
orbital order is known to drive the magnetic order, the layered magnetic
structure is also favoured in this system by the lack of coherent transport
across the planes and the reduced dimensionality of the lattice. The
anisotropic hopping across the orbitals and the underlying layered
structure largely determine the orbital arrangements in this region, while the
charge order is primarily due to the long range interactions.Comment: 6 pages, 6 figure
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