The swan song: the disappearance of the nucleus of NGC 4051 and the echo of its past glory

BeppoSAX observed the low-luminous Seyfert 1 Galaxy NGC4051 in a ultra-dim X-ray state. The 2-10 keV flux (1.26 x 10^{-12} erg/cm^2/s) was about 20 times fainter than its historical average value, and remained steady along the whole observation (~2.3 days). The observed flat spectrum (\Gamma ~ 0.8) and intense iron line (EW ~600 eV) are best explained assuming that the active nucleus has switched off, leaving only a residual reflection component visible.Comment: 5 pages, Latex, 3 Postscript figures, accepted for publication in MNRA

Linear stability, transient energy growth and the role of viscosity stratification in compressible plane Couette flow

Linear stability and the non-modal transient energy growth in compressible plane Couette flow are investigated for two prototype mean flows: (a) the {\it uniform shear} flow with constant viscosity, and (b) the {\it non-uniform shear} flow with {\it stratified} viscosity. Both mean flows are linearly unstable for a range of supersonic Mach numbers ($M$). For a given $M$, the critical Reynolds number ($Re$) is significantly smaller for the uniform shear flow than its non-uniform shear counterpart. An analysis of perturbation energy reveals that the instability is primarily caused by an excess transfer of energy from mean-flow to perturbations. It is shown that the energy-transfer from mean-flow occurs close to the moving top-wall for ``mode I'' instability, whereas it occurs in the bulk of the flow domain for ``mode II''. For the non-modal analysis, it is shown that the maximum amplification of perturbation energy, $G_{\max}$, is significantly larger for the uniform shear case compared to its non-uniform counterpart. For $\alpha=0$, the linear stability operator can be partitioned into ${\cal L}\sim \bar{\cal L} + Re^2{\cal L}_p$, and the $Re$-dependent operator ${\cal L}_p$ is shown to have a negligibly small contribution to perturbation energy which is responsible for the validity of the well-known quadratic-scaling law in uniform shear flow: $G(t/{\it Re}) \sim {\it Re}^2$. A reduced inviscid model has been shown to capture all salient features of transient energy growth of full viscous problem. For both modal and non-modal instability, it is shown that the {\it viscosity-stratification} of the underlying mean flow would lead to a delayed transition in compressible Couette flow

Extended sudden approximation model for high-energy nucleon removal reactions

A model based on the sudden approximation has been developed to describe high energy single nucleon removal reactions. Within this approach, which takes as its starting point the formalism of Hansen \cite{Anne2}, the nucleon-removal cross section and the full 3-dimensional momentum distributions of the core fragments including absorption, diffraction, Coulomb and nuclear-Coulomb interference amplitudes, have been calculated. The Coulomb breakup has been treated to all orders for the dipole interaction. The model has been compared to experimental data for a range of light, neutron-rich psd-shell nuclei. Good agreement was found for both the inclusive cross sections and momentum distributions. In the case of $^{17}$C, comparison is also made with the results of calculations using the transfer-to-the-continuum model. The calculated 3-dimensional momentum distributions exhibit longitudinal and transverse momentum components that are strongly coupled by the reaction for s-wave states, whilst no such effect is apparent for d-waves. Incomplete detection of transverse momenta arising fromlimited experimental acceptances thus leads to a narrowing of the longitudinal distributions for nuclei with significant s-wave valence neutron configurations, as confirmed by the data. Asymmetries in the longitudinal momentum distributions attributed to diffractive dissociation are also explored.Comment: 16 figures, submitted to Phys. Rev.

Periodic magnetorotational dynamo action as a prototype of nonlinear magnetic field generation in shear flows

The nature of dynamo action in shear flows prone to magnetohydrodynamic instabilities is investigated using the magnetorotational dynamo in Keplerian shear flow as a prototype problem. Using direct numerical simulations and Newton's method, we compute an exact time-periodic magnetorotational dynamo solution to the three-dimensional dissipative incompressible magnetohydrodynamic equations with rotation and shear. We discuss the physical mechanism behind the cycle and show that it results from a combination of linear and nonlinear interactions between a large-scale axisymmetric toroidal magnetic field and non-axisymmetric perturbations amplified by the magnetorotational instability. We demonstrate that this large scale dynamo mechanism is overall intrinsically nonlinear and not reducible to the standard mean-field dynamo formalism. Our results therefore provide clear evidence for a generic nonlinear generation mechanism of time-dependent coherent large-scale magnetic fields in shear flows and call for new theoretical dynamo models. These findings may offer important clues to understand the transitional and statistical properties of subcritical magnetorotational turbulence.Comment: 10 pages, 6 figures, accepted for publication in Physical Review

One-neutron removal reactions on neutron-rich psd-shell nuclei

A systematic study of high energy, one-neutron removal reactions on 23 neutron-rich, psd--shell nuclei (Z=5-9, A=12-25) has been carried out. The longitudinal momentum distributions of the core fragments and corresponding single-neutron removal cross sections are reported for reactions on a carbon target. Extended Glauber model calculations, weighted by the spectroscopic factors obtained from shell model calculations, are compared to the experimental results. Conclusions are drawn regarding the use of such reactions as a spectroscopic tool and spin-parity assignments are proposed for 15B, 17C, 19-21N, 21,23O, 23-25F. The nature of the weakly bound systems 14B and 15,17C is discussed.Comment: 11 pages + 2 figure

Assessment of smallholder seed groups performance and market linkages in Southern Malawi. Series Paper Number 12

ICRISAT in partnership with the Rural Livelihoods Support Project was implementing a two year project since 2009 to promote production of groundnuts and pigeon peas in Chiradzulu and Thyolo districts. The project was providing start-up seed of improved pigeon peas and groundnut varieties and technical advice. The variety received for pigeon peas was ICP40 while for groundnuts farmers were given Nsinjilo and CG7. The groups were multiplying the seed and sharing the seed on pass-on scheme. This study was thus conducted to assess the performance of the smallholder seed groups and explore opportunities for linking with the market for seed through the agro-dealer network and other initiatives as marketing outlets for seed. The study methodology involved interviews with selected agro-dealers operating in the project area and focus group discussions with selected farmer groups involved in seed multiplication. In total the study involved nine community seed groups, six in Chiradzulu district and three in Thyolo district and three agro-dealers (2 from Chiradzulu and one from Thyolo). The groups were clear of their objectives to multiply seed to increase access to seed within their communities and engage in small scale seed business. The outcome of these activities would be increased production of pigeon peas and groundnuts and improved livelihoods and incomes. The membership composition for the visited groups shows that most groups were involving more women than men. This means that women were being actively engaged in seed multiplication for the legumes as such they would be primary beneficiaries of the accrued benefits of the project support. Community members joined the groups on voluntary basis based on their willingness to participate in growing the crop. The project provided training to some of the groups in Chiradzulu and in Thyolo. But some groups did not yet receive training. For the groups that received training, the training mainly comprised of crop management aspects. For most groups, the seed support was received once when they started in 2009 and they have been able to multiply it and pass on to others and realized some excess seed for sale. Most of the groups had been able to realize some significant and increasing harvest in the two years so that other farmers have also benefited from the produced seed including sale of excess produce by some groups. For example, Nankuyu group (a group of 20) in Chiradzulu district produced 500kg of pigeon peas in 2009/10 season and increased to 1000kg in 2010/11 season. A number of other farmers also benefited from the produced seed as the groups have been able to share the seed with other farmers thereby expanding access to improve seed of both groundnuts and pigeon peas. This means that the objective of increasing access to seed to other needy farmers was being realized through these groups. The study has highlighted a number of challenges faced by the groups during the two years which affected their production levels. These included lack of training and extension services; late distribution of seed by the project; poor weather; occurrence of pests and diseases; and poor storage condition and storage pests for both pigeon peas and groundnuts. The groups also faced a number of marketing challenges which included: low production limiting the amount for sale; no linkage with reliable and competitive markets; lack of information on profitable market opportunities; high transport costs to markets in the city; and poor road network The seed groups have not yet had any interactions with agro dealers in the area or any other organized seed marketing organizations. However the interviewed agro-dealers are eager to link up with the seed groups to purchase from them the seed and sell in their input retail outlets as long as the seed is of good quality and prices are not exorbitant. The main constraints affecting the seed trading for the agro-dealers include: lack of adequate capital to buy seed stock and run their business; high transport costs; and competition with large seed companies. The groups indicated that the market available is mostly for grain and not seed as the local seed market is not developed and linkages are not yet developed. Apart from selling to vendors and local markets, some groups (e.g. Nankuyu group) have been able to explore and link up with larger buyers with support from ICRISAT. They have been able to sell their grain produce for pigeon peas collectively to Export Trading in Blantyre with assistance from the project and this has helped them to realize better returns. Overall the groups have not been very successful as smallholder seed producing groups. The ICRISAT support may have provided the start-up seed and increased access to good seed for the targeted communities but the groups were not systematically developed and supported to become sustainable seed producing groups. The main reasons include lack of or inadequate training on seed production and government standards and requirements for seed production. Almost all farmer groups interviewed from the two districts indicated that they are not aware of the requirements for seed production as prescribed by either government or seed companies. There is need to link the farmer groups in the project area to organizations which focus on producing and marketing improved seeds in tandem with requirements for seed production as prescribed by either government or seed companies. This can be attained if the farmer groups are linked to organizations such as Association of Smallholder Seed Multiplication Action Group (ASSMAG). The project supported groups can get organized into Seed Multiplication Action Groups (SMAGS) in the districts or join the existing SMAGs and access marketing linkages that has already been developed. ASSMAG works collectively through groups at different levels and ASSMAG coordinates production trainings, seed certification activities, seed processing and seed marketing-related issues of all the member groups. This would be of benefit to the farmer groups in the project area. To achieve maximum benefits and ensure sustainable small scale seed groups the following needs to be taken into consideration: Training of farmers on group dynamics, production technologies and practices; Facilitation of governance issues for sustainability; coordinated effort on marketing; sourcing of foundation seed and distribution to farmers; organizing inspections and seed testing