34,156 research outputs found
Historic Light Curve and Long-term Optical Variation of BL Lacertae 2200+420
In this paper, historical optical(UBVRI) data and newly observed data from
the Yunnan Observatory of China(about100 years) are presented for BL Lacertae.
Maximum variations in UBVRI: 5.12, 5.31, 4.73, 2.59, and 2.54 and color indices
of U-B = -0.11 +/- 0.20, B-V= 1.0 +/- 0.11, V-R= 0.73 +/- 0.19, V-I= 1.42 +/-
0.25, R-I= 0.82 +/- 0.11, and B-I= 2.44 +/- 0.29 have been obtained from the
literature; The Jurkevich method is used to investigate the existence of
periods in the B band light curve, and a long-term period of 14 years is found.
The 0.6 and 0.88 year periods reported by Webb et al.(1988) are confirmed. In
addition, a close relation between B-I and B is found, suggesting that the
spectra flattens when the source brightens.Comment: 21 pages, 6 figures, 2 table, aasms4.sty, to be published in ApJ,
Vol. 507, 199
Anomalous behavior of trapping on a fractal scale-free network
It is known that the heterogeneity of scale-free networks helps enhancing the
efficiency of trapping processes performed on them. In this paper, we show that
transport efficiency is much lower in a fractal scale-free network than in
non-fractal networks. To this end, we examine a simple random walk with a fixed
trap at a given position on a fractal scale-free network. We calculate
analytically the mean first-passage time (MFPT) as a measure of the efficiency
for the trapping process, and obtain a closed-form expression for MFPT, which
agrees with direct numerical calculations. We find that, in the limit of a
large network order , the MFPT behaves superlinearly as with an exponent 3/2 much larger than 1, which is in sharp contrast
to the scaling with , previously obtained
for non-fractal scale-free networks. Our results indicate that the degree
distribution of scale-free networks is not sufficient to characterize trapping
processes taking place on them. Since various real-world networks are
simultaneously scale-free and fractal, our results may shed light on the
understanding of trapping processes running on real-life systems.Comment: 6 pages, 5 figures; Definitive version accepted for publication in
EPL (Europhysics Letters
The central black hole masses and Doppler factors of the -ray loud blazars
In this paper, The central black hole masses and the Doppler factors are
derived for PKS 0528+134, PKS 0537-441, 3C279,
PKS 1406-074, PKS 1622-297, Q1633+382, Mkn 501, and BL Lacertae.
The masses obtained are in the range of (1 -7) and
compared with that obtained with the Klein-Nishina cross section considered
(Dermer & Gehrels 1995). If we considered only the Thomson cross section, the
masses are in the range of 2.6 - 2.
The masses obtained from our method are less sensitive to the flux than those
obtained from Dermer & Gehrels (1995) method. The masses obtained from two
flares (1991 and 1996 flares) of 3C279 are almost the same. For 3C279 and BL
Lacertae, viewing angle, , and Lorentz factor, , are estimated
from the derived Doppler factor and the measured superluminal velocity.
For 3C279,
,
= 2.4-14.4 for = 3.37;
, = 2.95-11.20 for =
4.89;
For BL Lacertae, ,
= 2.0-4.0.Comment: 5 pages, A&AS, 136, 13-18 (1999
Strain relaxation in InGaN/GaN micro-pillars evidenced by high resolution cathodoluminescence hyperspectral imaging
A size-dependent strain relaxation and its effects on the optical properties of InGaN/GaN multiple quantum wells (QWs) in micro-pillars have been investigated through a combination of high spatial resolution cathodoluminescence (CL) hyperspectral imaging and numerical modeling. The pillars have diameters (d) ranging from 2 to 150 μm and were fabricated from a III-nitride light-emitting diode (LED) structure optimized for yellow-green emission at ∼560 nm. The CL mapping enables us to investigate strain relaxation in these pillars on a sub-micron scale and to confirm for the first time that a narrow (≤2 μm) edge blue-shift occurs even for the large InGaN/GaN pillars (d > 10 μm). The observed maximum blue-shift at the pillar edge exceeds 7 nm with respect to the pillar centre for the pillars with diameters in the 2–16 μm range. For the smallest pillar (d = 2 μm), the total blue-shift at the edge is 17.5 nm including an 8.2 nm “global” blue-shift at the pillar centre in comparison with the unetched wafer. By using a finite element method with a boundary condition taking account of a strained GaN buffer layer which was neglected in previous simulation works, the strain distribution in the QWs of these pillars was simulated as a function of pillar diameter. The blue-shift in the QWs emission wavelength was then calculated from the strain-dependent changes in piezoelectric field, and the consequent modification of transition energy in the QWs. The simulation and experimental results agree well, confirming the necessity for considering the strained buffer layer in the strain simulation. These results provide not only significant insights into the mechanism of strain relaxation in these micro-pillars but also practical guidance for design of micro/nano LEDs
A Force-Balanced Control Volume Finite Element Method for Multi-Phase Porous Media Flow Modelling
Dr D. Pavlidis would like to acknowledge the support from the following research grants: Innovate UK ‘Octopus’, EPSRC ‘Reactor Core-Structure Re-location Modelling for Severe Nuclear Accidents’) and Horizon 2020 ‘In-Vessel Melt Retention’. Funding for Dr P. Salinas from ExxonMobil is gratefully acknowledged. Dr Z. Xie is supported by EPSRC ‘Multi-Scale Exploration of Multi-phase Physics in Flows’. Part funding for Prof Jackson under the TOTAL Chairs programme at Imperial College is also acknowledged. The authors would also like to acknowledge Mr Y. Debbabi for supplying analytic solutions.Peer reviewedPublisher PD
Observation of coherent Josephson response in the non-linear ab-plane microwave impedance of single crystals
We report novel non-linear phenomena in the -plane microwave impedance of
single crystals. The vs. data are well
described by the non-linear RSJ model : . The entire crystal behaves like a single Josephson junction. The
extraordinary coherence of the data suggests an intrinsic mechanism.Comment: 2 pages,1 figure, Submitted to Proc. of M^2SHTSC-V (Beijing), also
available at http://sagar.physics.neu.edu/preprint
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