1,126 research outputs found
State estimation: direct state measurement vs. tomography
We compare direct state measurement (DST or weak state tomography) to
conventional state reconstruction (tomography) through accurate Monte-Carlo
simulations. We show that DST is surprisingly robust to its inherent bias. We
propose a method to estimate such bias (which introduces an unavoidable error
in the reconstruction) from the experimental data. As expected we find that DST
is much less precise than tomography. We consider both finite and
infinite-dimensional states of the DST pointer, showing that they provide
comparable reconstructions.Comment: 4 pages, 4 figure
Testing the Kerr black hole hypothesis
It is thought that the final product of the gravitational collapse is a Kerr
black hole and astronomers have discovered several good astrophysical
candidates. While there is some indirect evidence suggesting that the latter
have an event horizon, and therefore that they are black holes, a proof that
the space-time around these objects is described by the Kerr geometry is still
lacking. Recently, there has been an increasing interest in the possibility of
testing the Kerr black hole hypothesis with present and future experiments. In
this paper, I briefly review the state of the art of the field, focussing on
some recent results and work in progress.Comment: Brief review; 15 pages, no figures. v3: references added, some typos
correcte
Linear Stability Analysis of a Levitated Nanomagnet in a Static Magnetic Field: Quantum Spin Stabilized Magnetic Levitation
We theoretically study the levitation of a single magnetic domain nanosphere
in an external static magnetic field. We show that apart from the stability
provided by the mechanical rotation of the nanomagnet (as in the classical
Levitron), the quantum spin origin of its magnetization provides two additional
mechanisms to stably levitate the system. Despite of the Earnshaw theorem, such
stable phases are present even in the absence of mechanical rotation. For large
magnetic fields, the Larmor precession of the quantum magnetic moment
stabilizes the system in full analogy with magnetic trapping of a neutral atom.
For low magnetic fields, the magnetic anisotropy stabilizes the system via the
Einstein-de Haas effect. These results are obtained with a linear stability
analysis of a single magnetic domain rigid nanosphere with uniaxial anisotropy
in a Ioffe-Pritchard magnetic field.Comment: Published version. 10 pages, 4 figures, 3 table
Gravitomagnetism in superconductors and compact stars
There are three experimentally observed effects in rotating superconductors
that are so far unexplained. Some authors have tried to interpret such a
phenomena as possible new gravitational properties of coherent quantum systems:
in particular, they suggest that the gravitomagnetic field of that kind of
matter may be many orders of magnitude stronger than the one expected in the
standard theory. Here I show that this interpretation would be in conflict with
the common belief that neutron stars have neutrons in superfluid state and
protons in superconductive one.Comment: 9 pages, no figur
Possible ~ 1 hour quasi-periodic oscillation in narrow-line Seyfert 1 galaxy MCG--06--30--15
We found a possible ~ 1 hour quasi-periodic oscillation (QPO) in a ~ 55 ks
X-ray observation of the narrow-line Seyfert 1 galaxy MCG--06--30--15 made with
the XMM-Newton EPIC/pn detector in the energy range 0.3 -- 10 keV. We identify
a total modulation of ~ 16% in the light curve and find a 3670~s
quasi-period using Lomb-Scargle periodogram (LSP) and weighted wavelet
Z-transform (WWZ) techniques. Our analyses of eight light curves of
MCG--06--30--15, indicated the possible presence of an oscillation during one
of them. The LSP indicates a statistically significant ( 3) QPO
detection. A WWZ analysis shows that the signal at this possible roughly 3670s
period is present, and rather persistent, throughout the observation; however,
a signal around 8735s is more persistent. We briefly discuss models that can
produce X-ray QPOs with such periods in narrow line Seyfert 1 galaxies, as both
other claimed QPO detections in this class of AGN had very similar periods.Comment: 5 pages, 4 figures, Published in A&A Letter
Quantum Spin Stabilized Magnetic Levitation
We theoretically show that, despite Earnshaw's theorem, a non-rotating single
magnetic domain nanoparticle can be stably levitated in an external static
magnetic field. The stabilization relies on the quantum spin origin of
magnetization, namely the gyromagnetic effect. We predict the existence of two
stable phases related to the Einstein--de Haas effect and the Larmor
precession. At a stable point, we derive a quadratic Hamiltonian that describes
the quantum fluctuations of the degrees of freedom of the system. We show that
in the absence of thermal fluctuations, the quantum state of the nanomagnet at
the equilibrium point contains entanglement and squeezing.Comment: Published version. 5 pages, 2 figure
3D simulations of the accretion process in Kerr space-time with arbitrary value of the spin parameter
We present the results of three-dimensional general relativistic hydrodynamic
simulations of adiabatic and spherically symmetric accretion in Kerr
space-time. We consider compact objects with spin parameter
(black holes) and with (super-spinars). Our full three-dimensional
simulations confirm the formation of equatorial outflows for high values of
, as found in our previous work in 2.5 dimensions. We show that the
critical value of determining the onset of powerful outflows depends
mainly on the radius of the compact object. The phenomenon of equatorial
outflows can hardly occur around a black hole and may thus be used to test the
bound for astrophysical black hole candidates.Comment: 13 pages, 9 figures. v2: refereed versio
Detection of a quasi-periodic oscillation in gamma-ray light curve of the high redshift blazar B2 1520+31
We detected a possible quasi-periodic oscillation (QPO) of ~ 71 days in the
0.1 -- 300 GeV gamma-ray Fermi-LAT light curve of the high redshift flat
spectrum radio quasar B2 1520+31. We identify and confirm that quasi-period by
Lomb Scargle periodogram (LSP), and weighted wavelet z-transform (WWZ)
analyses. Using this QPO period, and assuming it originates from accretion-disc
fluctuations at the innermost stable circular orbit, we estimate the central
supermassive black hole mass to range between ~ 5.4 * 10 M for
a non-rotating black hole and ~ 6.0 * 10 M for a maximally
rotating black hole. We briefly discuss other possible radio-loud active
galactic nuclei emission models capable of producing a gamma-ray QPO of such a
period in a blazar.Comment: 6 pages, 4 figures, Submitted to MNRA
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