arXiv subjects

Yashwant Gupta

Publications and source records attributed to Yashwant Gupta.

At least 37 records · Page 2Linked to original sources

arXiv2016

Detection of polarized quasi-periodic microstructure emission in millisecond pulsars

Microstructure emission, involving short time scale, often quasi-periodic, intensity fluctuations in subpulse emission, is well known in normal period pulsars. In this letter, we present the first detections of quasi-periodic microstructure emission from millisecond pulsars (MSPs), from Giant Metrewave Radio Telescope (GMRT) observations of two MSPs at 325 and 610 MHz. Similar to the characteristics of microstructure observed in normal period pulsars, we find that these features are often highly polarized, and exhibit quasi-periodic behavior on top of broader subpulse emission, with periods of the order of a few $μ$s. By measuring their widths and periodicities from single pulse intensity profiles and their autocorrelation functions, we extend the microstructure timescale - rotation period relationship by more than an order of magnitude down to rotation periods $\sim$ 5 ms, and find it to be consistent with the relationship derived earlier for normal pulsars. The similarity of behavior is remarkable, given the significantly different physical properties of MSPs and normal period pulsars, and rules out several previous speculations about the possible different characteristics of microstructure in MSP radio emission. We discuss the possible reasons for the non-detection of these features in previous high time resolution MSP studies along with the physical implications of our results, both in terms of a geometric beam sweeping model and temporal modulation model for micropulse production.

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arXiv2016

Neutron Star Physics in the Square Kilometer Array Era : An Indian Perspective

It is an exceptionally opportune time for Astrophysics when a number of next-generation mega-instruments are poised to observe the universe across the entire electromagnetic spectrum with unprecedented data quality. The Square Kilometre Array (SKA) is undoubtedly one of the major components of this scenario. In particular, the SKA is expected to discover tens of thousands of new neutron stars giving a major fillip to a wide range of scientific investigations. India has a sizeable community of scientists working on different aspects of neutron star physics with immediate access to both the uGMRT (an SKA pathfinder) and the recently launched X-ray observatory Astrosat. The current interests of the community largely centre around studies of - a) the generation of neutron stars and the SNe connection}, b) the neutron star population and evolutionary pathways}, c) the evolution of neutron stars in binaries and the magnetic fields}, d) the neutron star equation of state}, e) the radio pulsar emission mechanism}, and, f) the radio pulsars as probes of gravitational physics}. Most of these studies are the main goals of the SKA first phase, which is likely to be operational in the next four years. This article summarises the science goals of the Indian neutron star community in the SKA era, with significant focus on coordinated efforts among the SKA and other existing/upcoming instruments.

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arXiv2016

A Cyber Infrastructure for the SKA Telescope Manager

The Square Kilometre Array Telescope Manager (SKA TM) will be responsible for assisting the SKA Operations and Observation Management, carrying out System diagnosis and collecting Monitoring & Control data from the SKA sub-systems and components. To provide adequate compute resources, scalability, operation continuity and high availability, as well as strict Quality of Service, the TM cyber-infrastructure (embodied in the Local Infrastructure - LINFRA) consists of COTS hardware and infrastructural software (for example: server monitoring software, host operating system, virtualization software, device firmware), providing a specially tailored Infrastructure as a Service (IaaS) and Platform as a Service (PaaS) solution. The TM infrastructure provides services in the form of computational power, software defined networking, power, storage abstractions, and high level, state of the art IaaS and PaaS management interfaces. This cyber platform will be tailored to each of the two SKA Phase 1 telescopes (SKA_MID in South Africa and SKA_LOW in Australia) instances, each presenting different computational and storage infrastructures and conditioned by location. This cyber platform will provide a compute model enabling TM to manage the deployment and execution of its multiple components (observation scheduler, proposal submission tools, M&C components, Forensic tools and several Databases, etc). In this sense, the TM LINFRA is primarily focused towards the provision of isolated instances, mostly resorting to virtualization technologies, while defaulting to bare hardware if specifically required due to performance, security, availability, or other requirement.

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arXiv2015

A Real-time Coherent Dedispersion Pipeline for the Giant Metrewave Radio Telescope

A fully real-time coherent dedispersion system has been developed for the pulsar back-end at the Giant Metrewave Radio Telescope (GMRT). The dedispersion pipeline uses the single phased array voltage beam produced by the existing GMRT software back-end (GSB) to produce coherently dedispersed intensity output in real time, for the currently operational bandwidths of 16 MHz and 32 MHz. Provision has also been made to coherently dedisperse voltage beam data from observations recorded on disk. We discuss the design and implementation of the real-time coherent dedispersion system, describing the steps carried out to optimise the performance of the pipeline. Presently functioning on an Intel Xeon X5550 CPU equipped with a NVIDIA Tesla C2075 GPU, the pipeline allows dispersion free, high time resolution data to be obtained in real-time. We illustrate the significant improvements over the existing incoherent dedispersion system at the GMRT, and present some preliminary results obtained from studies of pulsars using this system, demonstrating its potential as a useful tool for low frequency pulsar observations. We describe the salient features of our implementation, comparing it with other recently developed real-time coherent dedispersion systems. This implementation of a real-time coherent dedispersion pipeline for a large, low frequency array instrument like the GMRT, will enable long-term observing programs using coherent dedispersion to be carried out routinely at the observatory. We also outline the possible improvements for such a pipeline, including prospects for the upgraded GMRT which will have bandwidths about ten times larger than at present.

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arXiv2013

A simulation calibrated limit on the HI power spectrum from the GMRT Epoch of Reionization experiment

The GMRT Epoch of Reionization (EoR) experiment is an ongoing effort to measure the power spectrum from neutral hydrogen at high redshift. We have previously reported an upper limit of (70 mK)^2 at wavenumbers of k=0.65 h/Mpc using a basic piecewise-linear foreground subtraction. In this paper we explore the use of a singular value decomposition to remove foregrounds with fewer assumptions about the foreground structure. Using this method we also quantify, for the first time, the signal loss due to the foreground filter and present new power spectra adjusted for this loss, providing a revised measurement of a 2-sigma upper limit at (248 mK)^2 for k=0.50 h/Mpc. While this revised limit is larger than previously reported, we believe it to be more robust and still represents the best current constraints on reionization at z=8.6.

astro-ph.CO↗

arXiv2012

Tracking dispersion measure variations of timing array pulsars with the GMRT

We present the results from nearly three years of monitoring of the variations in dispersion measure (DM) along the line-of-sight to 11 millisecond pulsars using the Giant Metrewave Radio Telescope (GMRT). These results demonstrate accuracies of single epoch DM estimates of the order of 5x10^(-4) cm^(-3) pc. A preliminary comparison with the Parkes Pulsar Timing Array (PPTA) data shows that the measured DM fluctuations are comparable. We show effects of DM variations due to the solar wind and solar corona and compare with the existing models.

astro-ph.HE↗

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.

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arXiv2012

Observations of four glitches in the young pulsar J1833-1034 and study of its glitch activity

We present the results from timing observations with the GMRT of the young pulsar J1833-1034, in the galactic supernova remnant G21.5-0.9. We detect the presence of 4 glitches in this pulsar over a period of 5.5 years, making it one of a set of pulsars that show fairly frequent glitches. The glitch amplitudes, characterized by the fractional change of the rotational frequency, range from 1 \times 10^-9 to 7 \times 10^-9, with no evidence for any appreciable relaxation of the rotational frequency after the glitches. The fractional changes observed in the frequency derivative are of the order of 10-5 . We show conclusively that, in spite of having significant timing noise, the sudden irregularities like glitches detected in this pulsar can not be modeled as smooth timing noise. Our timing solution also provides a stable estimate of the second derivative of the pulsar spin-down model, and a plausible value for the braking index of 1.857, which, like the value for other such young pulsars, is much less than the canonical value of 3.0. PSR J1833-1034 appears to belong to a class of pulsars exhibiting fairly frequent occurrence of low amplitude glitches. This is further supported by an estimate of the glitch activity parameter, Ag = 1.53 \times 10^-15 s^-2, which is found to be significantly lower than the trend of glitch activity versus characteristic age (or spin frequency derivative) that a majority of the glitching pulsars follow. We present evidence for a class of such young pulsars, including the Crab, where higher internal temperature of the neutron star could be responsible for the nature of the observed glitch activity.

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arXiv2011

Diffractive and refractive timescales at 4.8 GHz in PSR B0329+54

We present the results of flux density monitoring of PSR B0329+54 at the frequency of 4.8 GHz using the 32-meter TCfA radiotelescope. The observations were conducted between 2002 and 2005. The main goal of the project was to find interstellar scintillation (ISS) parameters for the pulsar at the frequency at which it was never studied in detail. To achieve this the 20 observing sessions consisted of 3-minute integrations which on average lasted 24 hours. Flux density time series obtained for each session were analysed using structure functions. For some of the individual sessions as well as for the general average structure function we were able to identify two distinctive timescales present, the timescales of diffractive and refractive scintillations. To the best of our knowledge, this is the first case when both scintillation timescales, t_DISS=42.7 minutes and t_RISS=305 minutes, were observed simultaneously in a uniform data set and estimated using the same method. The obtained values of the ISS parameters combined with the data found in the literature allowed us to study the frequency dependence of these parameters over a wide range of observing frequencies, which is crucial for understanding the ISM turbulence. We found that the Kolmogorov spectrum is not best suited for describing the density fluctuations of the ISM, and a power-law spectrum with beta =4 seems to fit better with our results. We were also able to estimate the transition frequency (transition from strong to weak scintillation regimes) as 10.1 GHz, much higher than was previously predicted. We were also able to estimate the strength of scattering parameter u=2.67$ and the Fresnel scale as 6.7x10^8 meters.

astro-ph.GA↗

arXiv2010

The GMRT-EoR Experiment: A new upper limit on the neutral hydrogen power spectrum at z~8.6

We present a new upper limit to the 21cm power spectrum during the Epoch of Reionization (EoR) which constrains reionization models with an unheated IGM. The GMRT-EoR experiment is an ongoing effort to make a statistical detection of the power spectrum of 21cm neutral hydrogen emission at redshift z~9. Data from this redshift constrain models of the EoR, the end of the Dark Ages arising from the formation of the first bright UV sources, probably stars or mini-quasars. We present results from approximately 50 hours of observations at the Giant Metrewave Radio Telescope in India from December 2007. We describe radio frequency interference (RFI) localisation schemes which allow bright sources on the ground to be identified and physically removed in addition to automated flagging. Singular-value decomposition is used to remove remaining broadband RFI by identifying ground sources with large eigenvalues. Foregrounds are modelled using a piecewise linear filter and the power spectrum is measured using cross-correlations of foreground subtracted images.

astro-ph.CO↗

arXiv2010

A real-time software backend for the GMRT

The new era of software signal processing has a large impact on radio astronomy instrumentation. Our design and implementation of a 32 antennae, 33 MHz, dual polarization, fully real-time software backend for the GMRT, using only off-the-shelf components, is an example of this. We have built a correlator and a beamformer, using PCI-based ADC cards and a Linux cluster of 48 nodes with dual gigabit inter-node connectivity for real-time data transfer requirements. The highly optimized compute pipeline uses cache efficient, multi-threaded parallel code, with the aid of vectorized processing. This backend allows flexibility in final time and frequency resolutions, and the ability to implement algorithms for radio frequency interference rejection. Our approach has allowed relatively rapid development of a fairly sophisticated and flexible backend receiver system for the GMRT, which will greatly enhance the productivity of the telescope. In this paper we describe some of the first lights using this software processing pipeline. We believe this is the first instance of such a real-time observatory backend for an intermediate sized array like the GMRT.

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↗

arXiv2009

The GMRT EoR Experiment: Limits on Polarized Sky Brightness at 150 MHz

The GMRT reionization effort aims to map out the large scale structure of the Universe during the epoch of reionization (EoR). Removal of polarized Galactic emission is a difficult part of any 21 cm EoR program, and we present new upper limits to diffuse polarized foregrounds at 150 MHz. We find no high significance evidence of polarized emission in our observed field at mid galactic latitude (J2000 08h26m+26). We find an upper limit on the 2-dimensional angular power spectrum of diffuse polarized foregrounds of [l^2 C_l/(2 PI)]^{1/2}< 3K in frequency bins of width 1 MHz at 300 0.03 h/Mpc, k < 0.1 h/Mpc. This can be compared to the expected EoR signal in total intensity of [k^3 P(k)/ (2 PI^2) ]^{1/2} ~ 10 mK. We find polarized structure is substantially weaker than suggested by extrapolation from higher frequency observations, so the new low upper limits reported here reduce the anticipated impact of these foregrounds on EoR experiments. We discuss Faraday beam and depth depolarization models and compare predictions of these models to our data. We report on a new technique for polarization calibration using pulsars, as well as a new technique to remove broadband radio frequency interference. Our data indicate that, on the edges of the main beam at GMRT, polarization squint creates ~ 3% leakage of unpolarized power into polarized maps at zero rotation measure. Ionospheric rotation was largely stable during these solar minimum night time observations.

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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.

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arXiv2008

Multi-frequency integrated profiles of pulsars

We have observed a total of 67 pulsars at five frequencies ranging from 243 to 3100 MHz. Observations at the lower frequencies were made at the Giant Metre Wave Telescope in India and those at higher frequencies at the Parkes Telescope in Australia. We present profiles from 34 of the sample with the best signal to noise ratio and the least scattering. The general `rules' of pulsar profiles are seen in the data; profiles get narrower, the polarization fraction declines and outer components become more prominent as the frequency increases. Many counterexamples to these rules are also observed, and pulsars with complex profiles are especially prone to rule breaking. We hypothesise that the location of pulsar emission within the magnetosphere evolves with time as the the pulsar spins down. In highly energetic pulsars, the emission comes from a confined range of high altitudes, in the middle range of spin down energies the emission occurs over a wide range of altitudes whereas in pulsars with low spin-down energies it is confined to low down in the magnetosphere.

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arXiv2008

The GMRT Search for Reionization

We present an overview for the reionization search at GMRT. The forecast sensitivities are promising for an early detection. RFI mitigation has been successful. Several hundred hours of telescope time have already been invested in this ongoing effort, and analysis of the data is in progress.

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arXiv2007

The Eccentric Binary Millisecond Pulsar in NGC 1851

PSR J0514-4002A is a 5-ms pulsar is located in the globular cluster NGC 1851; it belongs to a highly eccentric (e = 0.888) binary system. It is one of the earliest known examples of a numerous and fast-growing class of eccentric binary MSPs recently discovered in globular clusters. Using the GBT, we have obtained a phase-coherent timing solution for the pulsar, which includes a measurement of the rate of advance of periastron: 0.01289(4) degrees per year, which if due completely to general relativity, implies a total system mass of 2.453(14) solar masses. We also derive m_p < 1.5 solar masses and m_c > 0.96 solar masses. The companion is likely to be a massive white dwarf star.

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