| 1. |
- Edelson, R. A., et al.
(författare)
-
Multiwavelength observations of short-timescale variability in NGC 4151. IV. Analysis of multiwavelength continuum variability
- 1996
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 470:1, s. 364-377377
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Tidskriftsartikel (refereegranskat)abstract
- For pt.III see ibid., vol.470, no.1, p.349-63 (1996). Combines data from the three preceding papers in order to analyze the multi wave-band variability and spectral energy distribution of the Seyfert 1 galaxy NGC 4151 during the 1993 December monitoring campaign. The source, which was near its peak historical brightness, showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths. The strongest variations were seen in medium-energy (~1.5 keV) X-rays, with a normalized variability amplitude (NVA) of 24%. Weaker (NVA=6%) variations (uncorrelated with those at lower energies) were seen at soft gamma-ray energies of ~100 keV. No significant variability was seen in softer (0.1-1 keV) X-ray bands. In the ultraviolet/optical regime, the NVA decreased from 9% to 1% as the wavelength increased from 1275 to 6900 Aring. These data do not probe extreme ultraviolet (1200 Aring to 0.1 keV) or hard X-ray (250 keV) variability. The phase differences between variations in different bands were consistent with zero lag, with upper limits of lsim0.15 day between 1275 Aring and the other ultraviolet bands, lsim0.3 day between 1275 Aring and 1.5 keV, and lsim1 day between 1275 and 5125 Aring. These tight limits represent more than an order of magnitude improvement over those determined in previous multi-wave-band AGN monitoring campaigns. The ultraviolet fluctuation power spectra showed no evidence for periodicity, but were instead well fitted with a very steep, red power law (ales-2.5)
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| 2. |
- Sun, J. G., et al.
(författare)
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Optimal backward perturbation bounds for underdetermined systems
- 1997
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Ingår i: Siam Journal on Matrix Analysis and Applications. ; 18:2, s. 393-402
-
Tidskriftsartikel (refereegranskat)abstract
- Let A be an n x m (m < n) matrix, b be an m-dimensional vector, and y be an a-dimensional nonzero vector. In this paper are consider the following open problem: find an explicit expression of the optimal backward perturbation bound eta(theta) (y) defined by eta(theta)(y) = inf{(F-T,theta(g))(F):y is the minimum a-norm solution to (A + F)(T)x = b + g}, where theta is a positive number. This problem is solved.
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| 3. |
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| 4. |
- Crenshaw, D. M., et al.
(författare)
-
Multiwavelength observations of short-timescale variability in NGC 4151. I. Ultraviolet observations
- 1996
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 470:1, s. 322-335335
-
Tidskriftsartikel (refereegranskat)abstract
- Presents the results of an intensive ultraviolet monitoring campaign on the Seyfert 1 galaxy NGC 4151, as part of an effort to study its short-timescale variability over a broad range in wavelength. The nucleus of NGC 4151 was observed continuously with the International Ultraviolet Explorer for 9.3 days, yielding a pair of LWP and SWP spectra every ~70 minutes, and during 4 hr periods for 4 days prior to and 5 days after the continuous-monitoring period. The sampling frequency of the observations is an order of magnitude higher than that of any previous UV monitoring campaign on a Seyfert galaxy. The continuum fluxes in bands from 1275 to 2688 Aring went through four significant and well-defined ldquoeventsrdquo of duration 2-3 days during the continuous-monitoring period. The authors find that the amplitudes of the continuum variations decrease with increasing wavelength, which extends a general trend for this and other Seyfert galaxies to smaller timescales (i.e., a few days). The continuum variations in all the UV bands are simultaneous to within an accuracy of ~0.15 days, providing a strict constraint on continuum models. The emission-line light curves show only one major event during the continuous monitoring (a slow rise followed by a shallow dip) and do not correlate well with continuum light curves over the short duration of the campaign, because the timescale for continuum variations is apparently smaller than the response times of the emission lines
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| 5. |
- Ion, John, et al.
(författare)
-
Process diagrams for laser beam welding of carbon manganese steels
- 1996
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Ingår i: Welding Journal. - 0043-2296. ; 75:7, s. 225-
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Tidskriftsartikel (refereegranskat)abstract
- Diagrams are presented which illustrate the ranges of process variables that can be used to produce acceptable laser welds in carbon manganese steels. The most practical type uses axes of beam power and welding speed. An empirical operating window is displayed, which describes the limits of full joint penetration welding for a particular steel and plate thickness. Theoretical contours of heat-affected zone (HAZ) hardness, established using analytical models, are also displayed on the diagram. Appropriate combinations of laser beam power and welding speed can thus be selected. A more comprehensive diagram uses axes of carbon equivalent and absorbed laser beam energy, on which theoretical HAZ hardness contours are displayed. The effects of changes in steel composition and welding variables on HAZ hardness can thus be assessed. Model results agree well with experimental data, as well as more sophisticated predictive methods. The diagrams provide guidance on the selection of steel composition and beam parameters during the initial stages in the development of procedures for laser welding of carbon manganese steels.
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| 6. |
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| 7. |
- Rao, K. Hanumantha, et al.
(författare)
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Size effect of the mode II fracture toughness of rock
- 1999
-
Ingår i: Rock mechanics for industry. - Rotterdam : Balkema Publishers, A.A. / Taylor & Francis The Netherlands. - 9058091007 ; , s. 1117-1123
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Konferensbidrag (refereegranskat)abstract
- A new method, compression-shear test, has been developed for determining the mode II fracture toughness K (sub IIC) of rock. The effect of the specimen thickness B, the dimensionless notch length a/W as well as the notch inclination angle alpha on the determination of K (sub IIC) has been experimentally studied. The results show that the apparent mode II fracture toughness K (sub c) decreases as B and a/W increase and becomes constant when B> or =70 mm (i.e. cubic specimen) and a/W>0.5. The K (sub c) is almost independent of alpha in the range of 65 degrees -75 degrees . The constant K (sub c) is about 2-3 times that of mode I fracture toughness K (sub IC) and can be considered as the true mode II fracture toughness K (sub IIC) of rock.
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| 8. |
- Rao, Q.H., et al.
(författare)
-
Mode II fracture toughness testing of rock
- 1999
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Ingår i: Comptes-rendus / 9 Congrès international de mécanique de roches, Paris, France 1999. - Rotterdam : Balkema Publishers, A.A. / Taylor & Francis The Netherlands. - 905809071X ; , s. 731-734
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Konferensbidrag (refereegranskat)
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| 9. |
- Sun, H., et al.
(författare)
-
New Band Structures and an Unpaired Crossing in 78Kr
- 1999
-
Ingår i: Physical Review C (Nuclear Physics). - 0556-2813. ; 59:2, s. 655-664
-
Tidskriftsartikel (refereegranskat)abstract
- High-spin states in Kr-78 were studied using the Ni-58(Na-23,3p) reaction at 70 MeV and the Ni-58(Si-23,alpha 4p) reaction at 130 MeV. Prompt gamma-gamma coincidences were measured using the Pitt-FSU detector array and the GAMMASPHERE-MICROBALL array. Results from these experiments have led to 26 new excitation levels, some of which have been grouped into 3 new bands. Spins were assigned based on directional correlations of oriented nuclei. Two of the new negative-parity bands appear to form a signature-partner pair based on a two-quasineutron structure, in contrast to the previously known two-quasiproton negative-parity bands. A forking has been observed at the 24(+) state in the yrast band, which calculations suggest may result from an unpaired crossing. The available evidence suggests oblate shapes in the yrast band coexist with prolate shapes in the negative-parity bands. [S0556-2813(99)04602-6].
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| 10. |
- Sun, Z., et al.
(författare)
-
Laser welding of dissimilar metal combinations
- 1995
-
Ingår i: Journal of Materials Science. - 0022-2461 .- 1573-4803. ; 30:17, s. 4205-4214
-
Tidskriftsartikel (refereegranskat)abstract
- The ability to manufacture a product using a number of different metals and alloys greatly increases flexibility in design and production. Properties such as heat, wear and corrosion resistance can be optimized, and benefits in terms of production economics are often gained. Joining of dissimilar metal combinations is, however, a challenging task owing to the large differences in physical and chemical properties which may be present. Laser welding, a high power density but low energy-input process, provides solutions to a number of problems commonly encountered with conventional joining techniques. Accurate positioning of the weld bead, rapid heating and cooling, low distortion, process flexibility, and opportunities for product redesign are its principal characteristics. The review describes the principles underlying laser welding of dissimilar metal combinations and highlights the above benefits in a number of practical applications. It is concluded that there is potential for its application in many industrial sectors.
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