1,139 research outputs found
Active shape correction of a thin glass/plastic X-ray mirror
Optics for future X-ray telescopes will be characterized by very large
aperture and focal length, and will be made of lightweight materials like glass
or plastic in order to keep the total mass within acceptable limits. Optics
based on thin slumped glass foils are currently in use in the NuSTAR telescope
and are being developed at various institutes like INAF/OAB, aiming at
improving the angular resolution to a few arcsec HEW. Another possibility would
be the use of thin plastic foils, being developed at SAO and the Palermo
University. Even if relevant progresses in the achieved angular resolution were
recently made, a viable possibility to further improve the mirror figure would
be the application of piezoelectric actuators onto the non-optical side of the
mirrors. In fact, thin mirrors are prone to deform, so they require a careful
integration to avoid deformations and even correct forming errors. This however
offers the possibility to actively correct the residual deformation. Even if
other groups are already at work on this idea, we are pursuing the concept of
active integration of thin glass or plastic foils with piezoelectric patches,
fed by voltages driven by the feedback provided by X-rays, in intra-focal setup
at the XACT facility at INAF/OAPA. In this work, we show the preliminary
simulations and the first steps taken in this project
Avoiding barren plateaus via transferability of smooth solutions in a Hamiltonian variational ansatz
A large ongoing research effort focuses on variational quantum algorithms (VQAs), representing leading candidates to achieve computational speed-ups on current quantum devices. The scalability of VQAs to a large number of qubits, beyond the simulation capabilities of classical computers, is still debated. Two major hurdles are the proliferation of low-quality variational local minima, and the exponential vanishing of gradients in the cost-function landscape, a phenomenon referred to as barren plateaus. In this work, we show that by employing iterative search schemes, one can effectively prepare the ground state of paradigmatic quantum many-body models, also circumventing the barren plateau phenomenon. This is accomplished by leveraging the transferability to larger system sizes of a class of iterative solutions, displaying an intrinsic smoothness of the variational parameters, a result that does not extend to other solutions found via random-start local optimization. Our scheme could be directly tested on near-term quantum devices, running a refinement optimization in a favorable local landscape with nonvanishing gradients
Search for Optical Pulsation in M82 X-2
We report on a search for optical pulsation from M82 X-2 over a range of periods. M82 X-2 is an X-ray pulsar with a 1.37s average spin period and a 2.5 day sinusoidal modulation. The observations were done with the ARray Camera for Optical to Near-IR Spectrophotometry at the 200 inch Hale telescope at the Palomar Observatory. We performed H test and χ^2 statistical analysis. No significant optical pulsations were found in the wavelength range of 3000–11000 Å with a pulsation period between 1.36262 and 1.37462 s. We found an upper limit on pulsed emission in the 4000–8000 Å wavelength range to be fainter than ~20.5 mag_(AB) , corresponding to ~23 μJy
Adiabatic dynamics of an inhomogeneous quantum phase transition: the case of z > 1 dynamical exponent
We consider an inhomogeneous quantum phase transition across a multicritical
point of the XY quantum spin chain. This is an example of a Lifshitz transition
with a dynamical exponent z = 2. Just like in the case z = 1 considered in New
J. Phys. 12, 055007 (2010) when a critical front propagates much faster than
the maximal group velocity of quasiparticles vq, then the transition is
effectively homogeneous: density of excitations obeys a generalized
Kibble-Zurek mechanism and scales with the sixth root of the transition rate.
However, unlike for z = 1, the inhomogeneous transition becomes adiabatic not
below vq but a lower threshold velocity v', proportional to inhomogeneity of
the transition, where the excitations are suppressed exponentially.
Interestingly, the adiabatic threshold v' is nonzero despite vanishing minimal
group velocity of low energy quasiparticles. In the adiabatic regime below v'
the inhomogeneous transition can be used for efficient adiabatic quantum state
preparation in a quantum simulator: the time required for the critical front to
sweep across a chain of N spins adiabatically is merely linear in N, while the
corresponding time for a homogeneous transition across the multicritical point
scales with the sixth power of N. What is more, excitations after the adiabatic
inhomogeneous transition, if any, are brushed away by the critical front to the
end of the spin chain.Comment: 10 pages, 6 figures, improved version accepted in NJ
An X-ray and Optical Investigation of the Starburst-driven Superwind in the Galaxy Merger Arp 299
We present a detailed investigation of the X-ray and optical properties of
the starburst-merger system Arp299 (NGC 3690, Mrk 171), with an emphasis on its
spectacular gaseous nebula. We analyse \rosat and \asca X-ray data and optical
spectra and narrow-band images. We suggest that the on-going galaxy collision
has tidally-redistributed the ISM of the merging galaxies. The optical
emission-line nebula results as this gas is photoionized by radiation that
escapes from the starburst, and is shock-heated, accelerated, and pressurized
by a `superwind' driven by the collective effect of the starburst supernovae
and stellar winds. The X-ray nebula in Arp 299 is is plausibly a mass-loaded
flow of adiabatically-cooling gas that carries out a substantial fraction of
the energy and metals injected by the starburst at close to the escape velocity
from Arp 299. The mass outflow rate likely exceeds the star-formation rate in
this system. We conclude that powerful starbursts are able to heat (and
possibly eject) a significant fraction of the ISM in merging galaxies.Comment: 54 pages, 17 postscript figures, AAS late
Analysis of response of PVA-GTA Fricke gel dosimeters through magnetic resonance imaging.
The experimental activities here reported are focused on the analysis of the clinical applications of Fricke gel dosimeters produced with a matrix of Polyvinyl alcohol (PVA) cross-linked by adding glutaraldehyde (GTA). The analyses were performed by means of a 1.5 T clinical magnetic resonance imaging (MRI) scanner. The sensitivity of these dosimetric gels to clinical photon beams as well as the signal stability were suitably investigated. Furthermore, the effect of the xylenol orange on the MRI signal was analyzed. The results obtained show that these PVA-GTA gel dosimeters show good dosimetric features for possible future use for 3D dose mapping in clinical applications
Gradient critical phenomena in the Ising quantum chain: surface behaviour
We consider the influence of a power-law deviation from the critical coupling
such that the system is critical at its surface. We develop a scaling theory
showing that such a perturbation introduces a new length scale which governs
the scaling behaviour of the density profiles as well as the finite-size
behaviour of the surface properties. Exact results are obtained for the Ising
quantum chain when the perturbation varies linearly whereas the quadratic
perturbation is mainly studied numerically. The scaling theory is well
confirmed in both cases
Transport in out-of-equilibrium XXZ chains: Nonballistic behavior and correlation functions
We consider the nonequilibrium protocol where two semi-infinite gapped XXZ chains, initially prepared in different equilibrium states, are suddenly joint together. At large times, a generalized hydrodynamic description applies, according to which the system can locally be represented by space- and time- dependent stationary states. The magnetization displays an unusual behavior: depending on the initial state, its profile may exhibit abrupt jumps that can not be predicted directly from the standard hydrodynamic equations and which signal non-ballistic spin transport. We ascribe this phenomenon to the structure of the local conservation laws and make a prediction for the exact location of the jumps. We find that the jumps propagate at the velocities of the heaviest quasiparticles. By means of tDMRG simulations we show that our theory yields a complete description of the long-time steady profiles of conserved charges, currents, and local correlations
- …