66,456 research outputs found
The unique rapid variabilities of the iron K line profiles in NGC 4151
We present a detailed analysis of the iron K line variabilities in
NGC 4151 by using long ASCA observation data obtained in May 1995. Despite the
relatively small amplitude variations in the continuum flux, the iron K
line flux and profile show dramatic variations. Particularly, the line profile
changes from single peak to seeming double peaks and back in time scales of a
few 10 sec. The seemingly double-peaked profiles can be well interpreted as
line emission from a Keplerian ring around a massive black hole. An absorption
line at around 5.9 keV is also marginnaly detected. We discussed current Fe K
line models, but none of them can well explain the observed line and continuum
variations.Comment: 18 pages, latex, 3 figures, ApJ accepte
Entanglement reciprocation using three level atoms in a lambda configuration
We propose a scheme in which entanglement can be transferred from atoms
(discrete variables) to entangled states of cavity fields (continuous
variables). The cavities play the role of a kind of quantum memory for
entanglement, in such a way that it is possible to retrieve it back to the
atoms. In our method, two three level atoms in a lambda configuration,
previously entangled, are set to interact with single mode cavity fields
prepared in coherent states. During the process, one e-bit of entanglement may
be deposited in the cavities in an efficient way. We also show that the stored
entanglement may be transferred back to flying atoms
The broad Fe K line profile in NGC 4151
We present an analysis of the Fe K line profile of NGC 4151 by using long
ASCA observation data obtained in May 1995. The unprecedented good data
quality, which is much better in the energy band around 6.4 keV than that of
the famous 4.2-day ASCA observation of MCG -6-30-15 in July 1994, offers a
unique opportunity to study the details of Fe K line profile. Apart from those
characteristics already noticed in earlier ASCA observations on this object
(Yaqoob et al. 1995): a broad and skewed profile, with a strong peak at about
6.4 keV and a large red wing extending to 4 - 5 keV, which is remarkably
similar to that of MCG -6-30-15, we also find a weak blue wing extending to
about 8 keV, thanks to the good quality of the data. When fitted by a
relativistic accretion disk line plus a narrow core at 6.4 keV, the data
constrain the accretion disk to be nearly face-on, contrary to the edge-on
geometry inferred from optical and UV observations. However, the extended blue
wing can not be well fitted even after we include corresponding Fe K
components. Ni K line emission by an amount of 12% of Fe K is
statistically required. An alternative explanation is a model consisting of a
narrow core and two disk lines with inclinations of 58 and 0, respectively. We suppose that the component with inclination of 58 was observed directly, consistent with its edge-on geometry, and the
component with inclination of 0 was scattered into our line of sight
by a Compton mirror, which might be the cool accretion disk corona proposed by
Poutanen et al. (1996).Comment: 15 pages, including 2 figures, aasms4.sty. To appear in ApJL 52
Entanglement in spin-one Heisenberg chains
By using the concept of negativity, we study entanglement in spin-one
Heisenberg chains. Both the bilinear chain and the bilinear-biquadratic chain
are considered. Due to the SU(2) symmetry, the negativity can be determined by
two correlators, which greatly facilitate the study of entanglement properties.
Analytical results of negativity are obtained in the bilinear model up to four
spins and the two-spin bilinear-biquadratic model, and numerical results of
negativity are presented. We determine the threshold temperature before which
the thermal state is doomed to be entangled.Comment: 7 pages and 4 figure
Rapidly variable Fe K line in NGC 4051
We present a detailed analysis on the variability of the Fe K emission line
in NGC 4051 using ASCA data. Through simple Gaussian line fits, we find not
only obvious Fe K line variability with no significant difference in the X-ray
continuum flux between two ASCA observations which were separated by 440
days, but also rapid variability of Fe K line on time scales s
within the second observation. During the second observation, the line is
strong (EW = 733 eV) and broad (
keV) when the source is brightest, and become weaker (EW = 165
eV) and narrower ( keV) whilst the source is weakest. The
equivalent width of Fe K line correlates positively with the continuum flux,
which shows an opposite trend with another Seyfert 1 galaxy MCG --6-30-15.Comment: 12 pages with 5 figures, to appear in ApJ Vol. 516, L6
Quasiparticle Scattering Interference in (K,Tl)FexSe2 Superconductors
We model the quasiparticle interference (QPI) pattern in the recently
discovered (K,Tl)Fe_xSe2 superconductors. We show in the superconducting state
that, due to the absence of hole pockets at the Brillouin zone center, the
quasiparticle scattering occurs around the momentum transfer q=(0,0) and (\pm
\pi, \pm \pi) between electron pockets located at the zone boundary. More
importantly, although both d_{x^2-y^2}-wave and s-wave pairing symmetry lead to
nodeless quasiparticle excitations, distinct QPI features are predicted between
both types of pairing symmetry. In the presence of a nonmagnetic impurity
scattering, the QPI exhibits strongest scattering with q=(\pm \pi, \pm \pi) for
the d_{x^2-y^2}-wave pairing symmetry; while the strongest scattering exhibits
a ring-like structure centered around both q=(0,0) and (\pm \pi, \pm \pi) for
the isotropic s-wave pairing symmetry. A unique QPI pattern has also been
predicted due to a local pair-potential-type impurity scattering. The
significant contrast in the QPI pattern between the d_{x^2-y^2}-wave and the
isotropic s-wave pairing symmetry can be used to probe the pairing symmetry
within the Fourier-transform STM technique.Comment: 4+ pages, 3 embedded eps figure
Multistage Random Growing Small-World Networks with Power-law degree Distribution
In this paper, a simply rule that generates scale-free networks with very
large clustering coefficient and very small average distance is presented.
These networks are called {\bf Multistage Random Growing Networks}(MRGN) as the
adding process of a new node to the network is composed of two stages. The
analytic results of power-law exponent and clustering coefficient
are obtained, which agree with the simulation results approximately.
In addition, the average distance of the networks increases logarithmical with
the number of the network vertices is proved analytically. Since many real-life
networks are both scale-free and small-world networks, MRGN may perform well in
mimicking reality.Comment: 3 figures, 4 page
Instability of two dimensional graphene: Breaking sp2 bonds with soft X-rays
We study the stability of various kinds of graphene samples under soft X-ray
irradiation. Our results show that in single layer exfoliated graphene (a
closer analogue to two dimensional material), the in-plane carbon-carbon bonds
are unstable under X-ray irradiation, resulting in nanocrystalline structures.
As the interaction along the third dimension increases by increasing the number
of graphene layers or through the interaction with the substrate (epitaxial
graphene), the effect of X-ray irradiation decreases and eventually becomes
negligible for graphite and epitaxial graphene. Our results demonstrate the
importance of the interaction along the third dimension in stabilizing the long
range in-plane carbon-carbon bonding, and suggest the possibility of using
X-ray to pattern graphene nanostructures in exfoliated graphene.Comment: 4 pages, 3 figures, Phys. Rev. B rapid communication, in pres
A Framework of Robust Transmission Design for IRS-Aided MISO Communications With Imperfect Cascaded Channels
Intelligent reflection surface (IRS) has recently been recognized as a
promising technique to enhance the performance of wireless systems due to its
ability of reconfiguring the signal propagation environment. However, the
perfect channel state information (CSI) is challenging to obtain at the base
station (BS) due to the lack of radio frequency (RF) chains at the IRS. Since
most of the existing channel estimation methods were developed to acquire the
cascaded BS-IRS-user channels, this paper is the first work to study the robust
beamforming based on the imperfect cascaded BS-IRS-user channels at the
transmitter (CBIUT). Specifically, the transmit power minimization problems are
formulated subject to the worst-case rate constraints under the bounded CSI
error model and the rate outage probability constraints under the statistical
CSI error model, respectively. After approximating the worst-case rate
constraints by using the S-procedure and the rate outage probability
constraints by using the Bernstein-type inequality, the reformulated problems
can be efficiently solved. Numerical results show that the negative impact of
the CBIUT error on the system performance is greater than that of the direct
CSI error.Comment: Accepted in IEEE Transactions on Signal Processing. Keywords:
Intelligent reflecting surface (IRS), reconfigurable intelligent surface
(RIS), robust transmission desig
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