Searches for radio transients

Exploration of the transient Universe is an exciting and fast-emerging area within radio astronomy. Known transient phenomena range in time scales from sub-nanoseconds to years or longer, thus spanning a huge range in time domain and hinting a rich diversity in their underlying physical processes. Transient phenomena are likely locations of explosive or dynamic events and they offer tremendous potential to uncover new physics and astrophysics. A number of upcoming next-generation radio facilities and recent advances in computing and instrumentation have provided a much needed impetus for this field which has remained a relatively uncharted territory for the past several decades. In this paper we focus mainly on the class of phenomena that occur on very short time scales (i.e. from $\sim$ milliseconds to $\sim$ nanoseconds), known as {\it fast transients}, the detections of which involve considerable signal processing and data management challenges, given the high time and frequency resolutions required in their explorations, the role of propagation effects to be considered and a multitude of deleterious effects due to radio frequency interference. We will describe the techniques, strategies and challenges involved in their detections and review the world-wide efforts currently under way, both through scientific discoveries enabled by the ongoing large-scale surveys at Parkes and Arecibo, as well as technical developments involving the exploratory use of multi-element array instruments such as VLBA and GMRT. Such developments will undoubtedly provide valuable inputs as next-generation arrays such as LOFAR and ASKAP are designed and commissioned. With their wider fields of view and higher sensitivities, these instruments, and eventually the SKA, hold great potential to revolutionise this relatively nascent field, thereby opening up exciting new science avenues in astrophysics.Comment: To appear in the special issue of the Bulletin of the Astronomical Society of India on Transients at different wavelengths, eds D.J. Saikia and D.A. Green. 21 pages, 5 figures. http://www.ncra.tifr.res.in/~bas

Phase separation in transparent liquid-liquid miscibility gap systems

A program to be carried out on transparent liquid-phase miscibility gap materials was developed for the purpose of acquiring additional insight into the separation process occurring in these systems. The transparency feature allows the reaction to be viewed directly through light scattering and holographic methods

A CLEAN-based Method for Deconvolving Interstellar Pulse Broadening from Radio Pulses

Multipath propagation in the interstellar medium distorts radio pulses, an effect predominant for distant pulsars observed at low frequencies. Typically, broadened pulses are analyzed to determine the amount of propagation-induced pulse broadening, but with little interest in determining the undistorted pulse shapes. In this paper we develop and apply a method that recovers both the intrinsic pulse shape and the pulse broadening function that describes the scattering of an impulse. The method resembles the CLEAN algorithm used in synthesis imaging applications, although we search for the best pulse broadening function, and perform a true deconvolution to recover intrinsic pulse structre. As figures of merit to optimize the deconvolution, we use the positivity and symmetry of the deconvolved result along with the mean square residual and the number of points below a given threshold. Our method makes no prior assumptions about the intrinsic pulse shape and can be used for a range of scattering functions for the interstellar medium. It can therefore be applied to a wider variety of measured pulse shapes and degrees of scattering than the previous approaches. We apply the technique to both simulated data and data from Arecibo observations.Comment: 9 pages, 6 figures, Accepted for publication in the Astrophysical Journa

Plate on Plate Technique of Minimally Invasive Percutaneous Plate Osteosynthesis in Distal Tibial Fractures, an Easy and Inexpensive Method of Fracture Fixation

Background: Plate on plate technique can lessen operative time and patient morbidity. Objectives: This study aimed to evaluate the outcomes of minimally invasive percutaneous plate osteosynthesis (MIPPO) using plate on plate technique of locking plate fixation for closed fractures of distal tibia in a prospective study. Patients and Methods: Twenty-five patients with distal tibial fractures were treated by MIPPO using locking plate by plate on plate technique. Preoperative variables including age of patient, mode of trauma, type of fracture and soft tissue status were recorded for each patient. Perioperative variables included surgical time and radiation exposure. Postoperative variables included wound status, time to union, return to activity and the American orthopaedic foot and ankle score (AOFAS). Results: All the fractures had united at one year. The average time to union was 16.8 weeks. There were two cases of superficial infection and two cases of deep infection, which required removal of hardware after the fracture was united. The average AO foot and ankle score was 83.6 in our study population. Conclusions: MIPPO using locking plate by plate on plate technique was a safe, effective, inexpensive and easily reproducible method for the treatment of distal tibial fractures in properly selected patients, which minimized operative time and soft tissue morbidity

Secondary periodicities of microbursts of TeV gamma rays from the Crab pulsar

Observations were made during the past several years on the Crab pulsar using the Ooty atmospheric Cerenkov array with the aim of detecting possible emission of ultra high energy gamma rays by the pulsar. During the course of these observations, it was found that the Crab pulsar emits TeV gamma rays in bursts of short duration. The microbursts of TeV gamma rays from the Crab pulsar, which were seen in the data of at least three years, also reveal interesting secondary periodicities. It was noticed at first that some bursts could be connected with the others that occurred during the same night or during the next two nights with integral number of cycles of periods 43 + or - 1 minute. Ten possible periods in the vicinity of 43 minutes were determined for all the combinations of bursts for each year. The best values of periods thus obtained were different from year to year. But when, instead of the real time, the number of Crab cycles elapsed between the bursts was used as the unit of time, two values of burst periods - 77460 and 77770 Crab cycles - were found to be significant in the data of at least two years. A Monte Carlo simulation using 1500 trial periods chosen randomly within + or - 5 minutes of the original burst period did not reveal any value of the period as significant