arXiv subjects

Jayanta Roy

Publications and source records attributed to Jayanta Roy.

52 records · Page 3Linked to original sources

arXiv2020

The Polarization Convention of the uGMRT in Band 4

We present an experiment performed to understand the polarization convention adopted for band 4 (550--900 MHz) of the upgraded Giant Metrewave Radio Telescope (uGMRT). For that we observed the pulsar B1702--19 in this band, both in interferometry and pulsar modes, and compare the results with its already known Stokes $I$, $Q$, $U$, $V$ profiles obtained with the Lovell telescope. We find that the results obtained from interferometry and pulsar modes of the uGMRT agree with each other. However, although the Stokes $U$ profile obtained with the uGMRT match with that obtained by the Lovell telescope, Stokes $Q$ and $V$ do not. This can be explained if the $X$ and $Y$ dipoles in this band, from which $R$ and $L$ are derived, are swapped w.r.t. the IAU convention. The swapping makes $RR^*$ and $LL^*$ of the uGMRT band 4 to be $LL^*$ and $RR^*$ respectively according to the IEEE convention. This implies that if we need to compare polarization measurements obtained in band 4 of the uGMRT with telescopes like Lovell, Parkes, Very Large Array etc. (all follow IEEE convention for defining right and left hand circular polarization), we must interchange $RR^*$ and $LL^*$, and change the sign of Stokes $Q$ for the uGMRT data. Note that this is the current convention for uGMRT band 4, and is likely to change in future once the swapping of the dipoles is taken care of. Once it is done, it will be notified in another technical report.

astro-ph.IM↗

arXiv2019

High Velocity HI Jet-like Feature Towards the SNR Candidate G351.7-1.2

We present the HI 21 cm spectral line and continuum observations of the Galactic supernova remnant candidate G351.7-1.2 using the upgraded Giant Metrewave Radio Telescope. Strong absorption features are observed towards the HII regions in the star forming complex associated with G351.7-1.2. Along with HI emission towards the outer periphery of the supernova remnant shell, we distinguish a high velocity jet-like feature in the velocity range +40 km/s to +52 km/s in HI. This unusual and highly collimated feature, with a projected length of ~7 pc and an opening angle of 14.4 deg, is located towards the interior of the radio shell. This is the first report of a such a well collimated HI jet-like emission. The peculiar location and the detection of a gamma-ray source towards the central peak of this HI jet suggests its plausible association with the supernova remnant candidate.

astro-ph.GA↗

arXiv2018

Post-correlation beamformer for time-domain studies of pulsars and transients

We present a detailed analysis of post-correlation beamforming (i.e. beamforming which involves only phased sums of the correlation of the voltages of different antennas in an array), and compare it with the traditionally used incoherent and phased beamforming techniques. Using data from the GMRT we show that post-correlation beamformation results in a many-folds increase in the signal-to-noise for periodic signals from pulsars and several order of magnitude reduction in the number of false triggers from single pulse events like fast radio bursts (FRBs). This difference arises primarily because the post-correlation beam contains less red-noise, as well as less radio frequency interference. The post-correlation beam can also be more easily calibrated than the incoherent or phased array beams. We also discuss two different modes of post-correlation beamformation, viz. (1) by subtracting the incoherent beam from the coherent beam and (2) by phased addition of the visibilities. The computational costs for both these beamformation techniques as well as their suitability for studies of pulsars and FRBs are discussed. Techniques discussed here would be of interest for all upcoming surveys with interferometric arrays. Finally, we describe a time-domain survey with the GMRT using the post-correlation beamformation as a case study. We find that post-correlation beamforming will improve the current GMRT time-domain survey sensitivity by ~ 2 times for pulsars with periods of few 100s of millisecond and by many-folds for even slower pulsars, making it one of the most sensitive surveys for pulsars and FRBs at low and mid radio frequencies.

astro-ph.IM↗

arXiv2018

Time-domain Astronomy with the GMRT: uGMRT to eGMRT

The upgraded GMRT (uGMRT) with its unprecedented sensitivity and high figure of merit, is expected to result in the discovery of a large population of pulsars including pulsars of previously unknown type. In the phase-2 of the GMRT High Resolution Southern Sky (GHRSS) survey with the uGMRT we will reach 1/4th of sensitivity of the SKA Phase-1. In this paper we highlight the salient features of the survey of pulsars and fast transients with the uGMRT highlighting its discovery potential. The extended GMRT (eGMRT) equipped with wide field- of-view, increased collecting area will have unprecedented sensitivity in time-domain astronomy, reaching close to SKA-Phase1. Many fold increase in the number of elements, increase in the baseline length and addition of phased array feed will make eGMRT an excellent instrument for the survey of pulsars and transients with a promise of detecting large variety of pulsars and fast radio bursts.

astro-ph.IM↗

arXiv2015

The GMRT High Resolution Southern Sky Survey for pulsars and transients -I. Survey description and initial discoveries

We are conducting a survey for pulsars and transients using the Giant Metrewave Radio Telescope (GMRT). The GMRT High Resolution Southern Sky (GHRSS) survey is an off-Galactic-plane (|b|>5) survey in the declination range -40 deg to -54 deg at 322 MHz. With the high time (up to 30.72 micro-sec) and frequency (up to 0.016275 MHz) resolution observing modes, the 5-sigma detection limit is 0.5 mJy for a 2 ms pulsar with 10% duty cycle at 322 MHz. Total GHRSS sky coverage of 2866 square-deg, will result from 1953 pointing, each covering 1.8 square-deg. The 10-sigma detection limit for a 5 milli-sec transient burst is 1.6 Jy for the GHRSS survey. In addition, the GHRSS survey can reveal transient events like rotating radio transients or fast radio bursts. With 35% of the survey completed (i.e. 1000 square-deg), we report the discovery of 10 pulsars, one of which is a millisecond pulsar (MSP), one of the highest pulsar per square degree discovery rates for any off-Galactic plane survey. We re-detected 23 known in-beam pulsars. Utilising the imaging capability of the GMRT we also localised 4 of the GHRSS pulsars (including the MSP) in the gated image plane within +/- 10 arc-second. We demonstrated rapid convergence in pulsar timing with a more precise position than is possible with single dish discoveries. We also exhibited that we can localise the brightest transient sources with simultaneously obtained lower time resolution imaging data, demonstrating a technique that may have application in the SKA.

astro-ph.SR↗

arXiv2015

The study of multi-frequency scattering of ten radio pulsars

We present the results of the multi-frequency scatter time measurements for ten radio pulsars that were relatively less studied in this regard. The observations were performed using the Giant Meterwave Radio Telescope at the observing frequencies of 150, 235, 325, 610 and 1060~MHz. The data we collected, in conjunction with the results from other frequencies published earlier, allowed us to estimate the scatter time frequency scaling indices for eight of these sources. For PSR J1852$-$0635 it occurred that its profile undergoes a strong evolution with frequency, which makes the scatter time measurements difficult to perform, and for PSR J1835$-$1020 we were able to obtain reliable pulse broadening estimates at only two frequencies. We used the eight frequency scaling indices to estimate both: the electron density fluctuation strengths along the respective lines-of-sight, and the standardized amount of scattering at the frequency of 1 GHz. Combining the new data with the results published earlier by Lewandowski et al., we revisited the scaling index versus the dispersion measure (DM) relation, and similarly to some of the earlier studies we show that the average value of the scaling index deviates from the theoretical predictions for large DM pulsars, however it reaches the magnitude claimed by Löhmer et al. only for pulsars with very large DMs ($>$650 pc cm$^{-3}$). We also investigated the dependence of the scattering strength indicators on the pulsar distance, DM, and the position of the source in the Milky Way Galaxy.

astro-ph.HE↗

arXiv2014

Discovery of PSR J1227-4853: A transition from a low-mass X-ray binary to a redback millisecond pulsar

XSS J12270-4859 is an X-ray binary associated with the Fermi LAT gamma-ray source 1FGL J1227.9-4852. In 2012 December, this source underwent a transition where the X-ray and optical luminosity dropped and the spectral signatures of an accretion disc disappeared. We report the discovery of a 1.69 millisecond pulsar (MSP), PSR J1227-4853, at a dispersion measure of 43.4 pc cm$^{-3}$ associated with this source, using the GMRT at 607 MHz. This demonstrates that, post-transition, the system hosts an active radio MSP. This is the third system after PSR J1023+0038 and PSR J1824-2452I showing evidence of state switching between radio MSP and low-mass X-ray binary (LMXB) states. We report timing observations of PSR J1227-4853 with the GMRT and Parkes, which give a precise determination of the rotational and orbital parameters of the system. The companion mass measurement of 0.17 to 0.46 M$_{sun}$ suggests that this is a redback system. PSR J1227-4853 is eclipsed for about 40% of its orbit at 607 MHz; with additional short-duration eclipses at all orbital phases. We also find that the pulsar is very energetic, with a spin-down luminosity of ~ 10$^{35}$ erg s$^{-1}$. We report simultaneous imaging and timing observations with the GMRT, which suggests that eclipses are caused by absorption, rather than dispersion smearing or scattering.

astro-ph.HE↗

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↗

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

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.

astro-ph.IM↗

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.

astro-ph.SR↗

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↗

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.

astro-ph↗

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.

astro-ph↗