Hardcore bosons in a zig-zag optical superlattice

We study a system of hard-core bosons at half-filling in a one-dimensional optical superlattice. The bosons are allowed to hop to nearest and next-nearest neighbor sites producing a zig-zag geometry and we obtain the ground state phase diagram as a function of microscopic parameters using the finite-size density matrix renormalization group (FS-DMRG) method. Depending on the sign of the next-nearest neighbor hopping and the strength of the superlattice potential the system exhibits three different phases, namely the bond-order (BO) solid, the superlattice induced Mott insulator (SLMI) and the superfluid (SF) phase. When the signs of both hopping amplitudes are the same (the "unfrustrated" case), the system undergoes a transition from the SF to the SLMI at a non-zero value of the superlattice potential. On the other hand, when the two amplitudes differ in sign (the "frustrated" case), the SF is unstable to switching on a superlattice potential and also exists only up to a finite value of the next nearest neighbor hopping. This part of the phase diagram is dominated by the BO phase which breaks translation symmetry spontaneously even in the absence of the superlattice potential and can thus be characterized by a bond order parameter. The transition from BO to SLMI appears to be first order.Comment: 6 pages, 11 figure

IXAE observations of the X-ray pulsar XTE J1946+274

We report results from two observations of the transient binary X-ray pulsar XTE J1946+274 with the Indian X-ray Astronomy Experiment (IXAE) made in September 18-30 1999 and June 28-July 7 2000. The pulsar has a ~80 days orbital period during which the X-ray intensity varies almost sinusoidally between intensity levels of 5 and 50 mCrab. The mid-time of the two observations with the IXAE are separated by an orbital phase of 0.56 and were in the decaying and rising parts of its orbital modulation, respectively. During both the observations, strong pulsations with 15.8 s period are detected in the 2-6 and 6-18 keV energy bands of IXAE. The pulse profiles in both the observations are double peaked and identical with a similar pulse fractions of ~30% in the two energy bands. The pulse periods and its local derivatives are obtained from the IXAE observations. The known pulse period history shows small deviations from an otherwise nearly constant spin-up trend of the pulsar since its discovery. If the measured local period derivatives are a sum of a nearly constant spin-up of the pulsar and effect of its orbital motion, it suggests that the orbit of this Be X-ray binary is eccentric. The RXTE-ASM light curve shows a continuous detection of 10 orbital modulation of the source since its discovery

Luminosity dependent changes in the X-ray pulse profile of the transient pulsar Cepheus X-4 during its declining phase of the 1997 outburst

The transient X-ray pulsar Cepheus X-4 underwent its latest outburst in 1997, during July-August, which lasted for about 30 days. The Pointed mode Proportional Counters (PPCs) of the Indian X-ray Astronomy Experiment (IXAE) on board IRS-P3 satellite observed the source in its declining phase during 1997 July 28 to July 30. The timing analysis of the data confirms the 66 seconds pulsation of the neutron star. The X-ray pulse profile obtained in two energy bands between 2 and 18 keV, shows energy dependent variations. The pulse profile obtained by us in the declining phase of the outburst when the X-ray luminosity of the source was about 6x1035 ergs s-1, is distinctly different from the one observed with the RXTE in the earlier phase of the outburst. It is found that near the end of the outburst, the relative strength of the two pulses of the double-pulse profile got reversed and the inter-pulse became more dominant compared to the main pulse. The observations can be interpreted in terms of a luminosity dependent emission profile of the pulsar, where depending on the pulsar geometry with respect to line of sight, one of the emission patterns, either a pencil-beam or a fan-beam, becomes more dominant. This is due to the changes in the pulsar magneto-sphere below a specific luminosity, which may cause relative changes in accretion process onto the two poles of the neutron star

Design and analysis of fractional factorial experiments from the viewpoint of computational algebraic statistics

We give an expository review of applications of computational algebraic statistics to design and analysis of fractional factorial experiments based on our recent works. For the purpose of design, the techniques of Gr\"obner bases and indicator functions allow us to treat fractional factorial designs without distinction between regular designs and non-regular designs. For the purpose of analysis of data from fractional factorial designs, the techniques of Markov bases allow us to handle discrete observations. Thus the approach of computational algebraic statistics greatly enlarges the scope of fractional factorial designs.Comment: 16 page

Large Area X-ray Proportional Counter (LAXPC) in Orbit Performance : Calibration, background, analysis software

The Large Area X-ray Proportional Counter (LAXPC) instrument on-board AstroSat has three nominally identical detectors for timing and spectral studies in the energy range of 3--80 keV. The performance of these detectors during the five years after the launch of AstroSat is described. Currently, only one of the detector is working nominally. The variation in pressure, energy resolution, gain and background with time are discussed. The capabilities and limitations of the instrument are described. A brief account of available analysis software is also provided.Comment: Accepted for publication in JA

Distorted cyclotron line profile in Cep X-4 as observed by NuSTAR

We present spectral analysis of NuSTAR and Swift observations of Cep X-4 during its outburst in 2014. We observed the source once during the peak of the outburst and once during the decay, finding good agreement in the spectral shape between the observations. We describe the continuum using a powerlaw with a Fermi-Dirac cutoff at high energies. Cep X-4 has a very strong cyclotron resonant scattering feature (CRSF) around 30 keV. A simple absorption-like line with a Gaussian optical depth or a pseudo-Lorentzian profile both fail to describe the shape of the CRSF accurately, leaving significant deviations at the red side of the line. We characterize this asymmetry with a second absorption feature around 19 keV. The line energy of the CRSF, which is not influenced by the addition of this feature, shows a small but significant positive luminosity dependence. With luminosities between (1-6)e36 erg/s, Cep X-4 is below the theoretical limit where such a correlation is expected. This behavior is similar to Vela X-1 and we discuss parallels between the two systems.Comment: 6 pages, 4 figure, accepted for publication in ApJ letter

Multi-objective evolutionary algorithms for the risk-return trade-off in bank loan management

Abstract Multi-Criteria Decision Making is an increasingly accepted tool for decision making in management. In this work, we highlight the application of a novel Multi-Objective Evolutionary Algorithm, NSGA-II to the risk-return trade-off for a bank loan portfolio manager. The manager of a bank operating in a competitive environment faces the standard goal of maximizing shareholder wealth. Specifically, this attempts to maximize the net worth of the bank, which in turn involves maximizing the net interest margin of the bank (among other factors, such as non-interest income). At the same time, there are significant regulatory constraints placed on the bank, such as the maintenance of adequate capital, interest-rate risk exposure, etc. The Genetic Algorithm based technique used here obtains an approximation to the set of Pareto-optimal solutions which increases the decision flexibility available to the bank manager and provides a visualization tool for one of the tradeoffs involved. The algorithm is also computationally efficient and is contrasted with a traditional multi-objective function -the epsilon-constraint method