arXiv subjects

Bhaswati Bhattacharyya

Publications and source records attributed to Bhaswati Bhattacharyya.

43 records · Page 3Linked to original sources

arXiv2014

Distribution of population averaged observables in stochastic gene expression

Observation of phenotypic diversity in a population of genetically identical cells is often linked to the stochastic nature of chemical reactions involved in gene regulatory networks. We investigate the distribution of population averaged gene expression levels as a function of population, or sample, size for several stochastic gene expression models to find out to what extent population averaged quantities reflect the underlying mechanism of gene expression. We consider three basic gene regulation networks corresponding to transcription with and without gene state switching and translation. Using analytical expressions for the probability generating function of observables and Large Deviation Theory, we calculate the distribution and first two moments of the population averaged mRNA and protein levels as a function of model parameters, population size and number of measurements contained in a data set. We validate our results using stochastic simulations also report exact results on the asymptotic properties of population averages which show qualitative differences among different models.

q-bio.MN↗

arXiv2013

Coherently dedispersed gated imaging of millisecond pulsars

Motivated by the need for rapid localisation of newly discovered faint millisecond pulsars (MSPs) we have developed a coherently dedispersed gating correlator. This gating correlator accounts for the orbital motions of MSPs in binaries while folding the visibilities with best-fit topocentric rotational model derived from periodicity search in simultaneously generated beamformer output. Unique applications of the gating correlator for sensitive interferometric studies of MSPs are illustrated using the Giant Metrewave Radio Telescope (GMRT) interferometric array. We could unambiguously localise five newly discovered Fermi MSPs in the on-off gated image plane with an accuracy of +-1". Immediate knowledge of such precise position allows the use of sensitive coherent beams of array telescopes for follow-up timing observations, which substantially reduces the use of telescope time (~ 20X for the GMRT). In addition, precise a-priori astrometric position reduces the effect of large covariances in timing fit (with discovery position, pulsar period derivative and unknown binary model), which in-turn accelerates the convergence to initial timing model. For example, while fitting with precise a-priori position (+-1"), timing model converges in about 100 days, accounting the effect of covariance between position and pulsar period derivative. Moreover, such accurate positions allows for rapid identification of pulsar counterpart at other wave-bands. We also report a new methodology of in-beam phase calibration using the on-off gated image of the target pulsar, which provides the optimal sensitivity of the coherent array removing the possible temporal and spacial decoherences.

astro-ph.IM↗

arXiv2012

A multi-pixel beamformer using an interferometric array and its application towards localisations of newly discovered pulsars

We have developed a multi-pixel beamformer technique, which can be used for enhancing the capabilities for studying pulsars using an interferometric array. Using the Giant Metrewave Radio Telescope (GMRT), we illustrate the application of this efficient technique, which combines the enhanced sensitivity of a coherent array beamformer with the wide field-of-view seen by an incoherent array beamformer. Multi-pixel beamformer algorithm is implemented using the recorded base-band data. With the optimisations in multi-pixelisation described in this paper, it is now possible to form 16 directed beams in real-time. We discuss a special application of this technique, where we use continuum imaging followed by the multi-pixel beamformer to obtain the precise locations of newly discovered millisecond pulsars with the GMRT. Accurate positions measured with single observations enable highly sensitive follow-up studies using coherent array beamformer and rapid follow up at higher radio frequencies and other wavelengths. Normally, such accurate positions can only be obtained from a long-term pulsar timing program. The multi-pixel beamformer technique can also be used for highly sensitive targeted pulsar searches in extended supernova remnants. In addition this method can provide optimal performance for the large scale pulsar surveys using multi-element arrays.

astro-ph.IM↗

arXiv2010

Investigation of the unique nulling properties of PSR B0818-41

We report on the unique nulling properties of PSR B0818-41, using the GMRT at 325 and 610 MHz. We find following interesting behaviour just before and after the nulls: (i) The pulsar's intensity does not switch off abruptly at the null, but fades gradually, taking ~ 10 P1. Just after nulls intensity rises to a maximum over a short (less than one period) time scale. (ii) While the last active pulses before nulls are dimmer, the first few active pulses just after nulls outshine normal ones. This effect is very clear for inner region of pulsar profile, where mean intensity of last few active pulses just after nulls is ~ 2.8 times more than that for last active pulses just before nulls. (iii) There is a significant evolution of shape of the pulsar's profile, around nulls, especially at beginning of bursts: an enhanced bump of intensity in inner region, a change in ratio of strengths of the leading and trailing peaks towards a more symmetric profile, an increase in profile width of about 10%, and a shift of profile centre towards later longitudes. (iv) Just before nulls, the apparent drift rate becomes slower, transitioning to an almost phase stationary drift pattern. Further, when the pulsar comes out of null, the increased intensity is very often accompanied by what looks like a disturbed drift rate behaviour, which settles down to the regular drift pattern as the pulsar intensity returns to normal. Thus, we find some very specific and well correlated changes in the radio emission properties of PSR B0818-41 when the emission restarts after a null. These could imply that the phenomenon of nulling is associated with some kind of a "reset" of the pulsar radio emission engine. We also present plausible explanations for some of the observed behaviour, using the Partially Screened Gap model of the inner pulsar accelerator.

astro-ph.SR↗

arXiv2009

Wide profile drifting pulsars : an elegant way to probe pulsar magnetospheres

We present the results from a study of wide profile pulsars using high sensitivity multifrequency observations with the GMRT. Since the line of sight samples a large region of the polar cap in case of the wide profile pulsars, presence of simultaneous multiple drift regions is quite probable (as seen in PSR B0826-34 and PSR B0818-41). We solve the aliasing problem of PSR B0818-41 using the observed phase relationship of the drift regions, and determine its pattern rotation period P4 to be ~ 10s, which makes it the fastest known carousel. We find that, for all the pulsars showing drifting in multiple rings of emission, the drift pattern from the rings are phase locked. This can constraint the theoretical models of pulsar emission as it favors a pan magnetospeheric radiation mechanism.

astro-ph.GA↗

arXiv2008

Exploring the remarkable subpulse drift and polarization properties of PSR B0818-41

PSR B0818-41 is one of the few pulsars which show multiple drift regions having well defined phase relationship. In this paper we report new results from the multifrequency GMRT observations of this pulsar. Significant linear polarization is observed with depolarization at the edge of the profile, due to orthogonal polarization. Circular polarization changes sign near the middle of the pulse profile at 1060 MHz, but not observed at 325 and 610 MHz. Remarkable frequency evolution of polarization angle (PA) is observed. Based on the frequency evolution of average profile, PA swing and results from subpulse drifting, we propose two possible emission geometries, alpha~11 deg, beta~-5.4 deg and alpha~175.4 deg, beta~-6.9 deg. Simulation of the pulsar radiation pattern with both these geometries reproduces the observed features in the drift pattern quite well. In addition to the remarkable subpulse drifting observed at 325 MHz, we report subpulse drifting at 244 and 610 MHz. We observe changes of drift rates, transitions from negative to stationary and stationary to negative drift rates, with some connection with nulls. Though P3m is the same for all the drift regions, P2m and Delta(Phi) values are different for the inner and outer drift regions. The peak emission from the leading and the trailing outer regions are offset by ~ 9 P1. Utilising this information, we solve the aliasing problem and predict P4~ 10 s, which makes it the fastest known carousel. The drift pattern from the two rings are phase locked for PSR B0818-41. The same is found to be true for, all pulsars showing drifting in multiple rings of emission and puts constraints on the theoretical models of pulsar emission mechanism and, favors a pan magnetospeheric radiation mechanism.

astro-ph↗

arXiv2008

Determination of the orbital parameters of binary pulsars

We present a simple method for determination of the orbital parameters of binary pulsars, using data on the pulsar period at multiple observing epochs. This method uses the circular nature of the velocity space orbit of Keplerian motion and produces preliminary values based on two one dimensional searches. Preliminary orbital parameter values are then refined using a computationally efficient linear least square fit. This method works for random and sparse sampling of the binary orbit. We demonstrate the technique on (a) the highly eccentric binary pulsar PSR J0514-4002 (the first known pulsar in the globular cluster NGC 1851) and (b) 47 Tuc T, a binary pulsar with a nearly circular orbit.

astro-ph↗