arXiv subjects

V. Venkatraman Krishnan

Publications and source records attributed to V. Venkatraman Krishnan.

At least 73 records · Page 4Linked to original sources

arXiv2021

Eight new millisecond pulsars from the first MeerKAT globular cluster census

We have used the central 44 antennas of the new 64-dish MeerKAT radio telescope array to conduct a deep search for new pulsars in the core of nine globular clusters. This has led to the discovery of eight new millisecond pulsars in six different clusters. Two new binaries, 47 Tuc ac and 47 Tuc ad, are eclipsing "spiders", featuring compact orbits ($\lesssim 0.32$ days), very low-mass companions and regular occultations of their pulsed emission. The other three new binary pulsars (NGC 6624G, M62G, and Ter 5 an) are in wider ($> 0.7$ days) orbits, with companions that are likely to be white dwarfs or neutron stars. NGC 6624G has a large eccentricity of $e\simeq 0.38$, which enabled us to detect the rate of advance of periastron. This suggests that the system is massive, with a total mass of $M{\rm tot} = 2.65 \pm 0.07$ M$_{\odot}$. Likewise, for Ter 5 an, with $e \simeq 0.0066$, we obtain $M{\rm tot}= 2.97 \pm 0.52$ M$_{\odot}$. The other three new discoveries (NGC 6522D, NGC 6624H and NGC 6752F) are faint isolated pulsars. Finally, we have used the whole MeerKAT array and synthesized 288 beams, covering an area of $\sim2$ arcmin in radius around the center of NGC 6624. This has allowed us to localize many of the pulsars in the cluster, demonstrating the beamforming capabilities of the TRAPUM software backend and paving the way for the upcoming MeerKAT globular cluster pulsar survey.

astro-ph.HE↗

arXiv2021

Measurements of pulse jitter and single-pulse variability in millisecond pulsars using MeerKAT

Using the state-of-the-art SKA precursor, the MeerKAT radio telescope, we explore the limits to precision pulsar timing of millisecond pulsars achievable due to pulse stochasticity (jitter). We report new jitter measurements in 15 of the 29 pulsars in our sample and find that the levels of jitter can vary dramatically between them. For some, like the 2.2~ms pulsar PSR J2241--5236, we measure an implied jitter of just $\sim$ 4~ns/hr, while others like the 3.9~ms PSR J0636--3044 are limited to $\sim$ 100 ns/hr. While it is well known that jitter plays a central role to limiting the precision measurements of arrival times for high signal-to-noise ratio observations, its role in the measurement of dispersion measure (DM) has not been reported, particularly in broad-band observations. Using the exceptional sensitivity of MeerKAT, we explored this on the bright millisecond pulsar PSR J0437--4715 by exploring the DM of literally every pulse. We found that the derived single pulse DMs vary by typically 0.0085 cm$^{-3}$ pc from the mean, and that the best DM estimate is limited by the differential pulse jitter across the band. We postulate that all millisecond pulsars will have their own limit on DM precision which can only be overcome with longer integrations. Using high-time resolution filterbank data of 9 $μ$s, we also present a statistical analysis of single pulse phenomenology. Finally, we discuss optimization strategies for the MeerKAT pulsar timing program and its role in the context of the International Pulsar Timing Array (IPTA).

astro-ph.HE↗

arXiv2020

The Thousand-Pulsar-Array programme on MeerKAT II: observing strategy for pulsar monitoring with subarrays

The Thousand Pulsar Array (TPA) project currently monitors about 500 pulsars with the sensitive MeerKAT radio telescope by using subarrays to observe multiple sources simultaneously. Here we define the adopted observing strategy, which guarantees that each target is observed long enough to obtain a high fidelity pulse profile, thereby reaching a sufficient precision of a simple pulse shape parameter. This precision is estimated from the contribution of the system noise of the telescope, and the pulse-to-pulse variability of each pulsar, which we quantify under some simplifying assumptions. We test the assumptions and choice of model parameters using data from the MeerKAT 64-dish array, Lovell and Parkes telescopes. We demonstrate that the observing times derived from our method produce high fidelity pulse profiles that meet the needs of the TPA in studying pulse shape variability and pulsar timing. Our method can also be used to compare strategies for observing large numbers of pulsars with telescopes capable of forming multiple subarray configurations. We find that using two 32-dish MeerKAT subarrays is the most efficient strategy for the TPA project. We also find that the ability to observe in different array configurations will become increasingly important for large observing programmes using the Square Kilometre Array telescope.

astro-ph.HE↗

arXiv2020

The Thousand-Pulsar-Array programme on MeerKAT IV: Polarisation properties of young, energetic pulsars

We present observations of 35 high spin-down energy radio pulsars using the MeerKAT telescope. Polarisation profiles and associated parameters are also presented. We derive the geometry for a selection of pulsars which show interpulse emission. We point out that, in several cases, these radio pulsars should also be seen in $γ$-rays but that improved radio timing is required to aid the high-energy detection. We discuss the relationship between the width of the radio profile and its high-energy detectability. Finally, we reflect on the correlation between the spin-down energy and the radio polarisation fraction and the implications this may have for $γ$-ray emission.

astro-ph.HE↗

arXiv2020

Giant pulses from J1823-3021A observed with the MeerKAT telescope

The millisecond pulsar J1823-3021A is a very active giant pulse emitter in the globular cluster NGC 6624. New observations with the MeerKAT radio telescope have revealed 14350 giant pulses over 5 hours of integration time, with an average wait time of about 1 second between giant pulses. The giant pulses occur in phases compatible with the ordinary radio emission, follow a power-law distribution with an index of -2.63 $\pm$ 0.02 and contribute 4 percent of the total integrated flux. The spectral index of the giant pulses follows a Gaussian distribution centered around -1.9 with a standard deviation of 0.6 and is on average flatter than the integrated emission, which has a spectral index of -2.81 $\pm$ 0.02. The waiting times between the GPs are accurately described by a Poissonian distribution, suggesting that the time of occurrence of a GP is independent from the times of occurrence of other GPs. 76 GPs show multiple peaks within the same rotation, a rate that is also compatible with the mutual independence of the GP times of occurrence. We studied the polarization properties of the giant pulses finding, on average, linear polarization only at the 1 percent level and circular polarization at the 3 percent level, similar to the polarization percentages of the total integrated emission. In 4 cases it was possible to measure the RM of the GPs which are highly variable and, in two cases, is inconsistent with the mean RM of the total integrated pulsar signal.

astro-ph.HE↗

arXiv2020

The SUrvey for Pulsars and Extragalactic Radio Bursts V: Recent Discoveries and Full Timing Solutions

The SUrvey for Pulsars and Extragalactic Radio Bursts ran from 2014 April to 2019 August, covering a large fraction of the southern hemisphere at mid- to high-galactic latitudes, and consisting of 9-minute pointings taken with the 20-cm multibeam receiver on the Parkes Radio Telescope. Data up to 2017 September 21 have been searched using standard Fourier techniques, single-pulse searches, and Fast Folding Algorithm searches. We present 19 new discoveries, bringing the total to 27 discoveries in the programme, and we report the results of follow-up timing observations at Parkes for 26 of these pulsars, including the millisecond pulsar PSR J1421-4409; the faint, highly-modulated, slow pulsar PSR J1646-1910; and the nulling pulsar PSR J1337-4441. We present new timing solutions for 23 pulsars, and we report flux densities, modulation indices, and polarization properties.

astro-ph.HE↗

arXiv2020

The MeerKAT Telescope as a Pulsar Facility: System verification and early science results from MeerTime

We describe system verification tests and early science results from the pulsar processor (PTUSE) developed for the newly-commissioned 64-dish SARAO MeerKAT radio telescope in South Africa. MeerKAT is a high-gain (~2.8 K/Jy) low-system temperature (~18 K at 20cm) radio array that currently operates from 580-1670 MHz and can produce tied-array beams suitable for pulsar observations. This paper presents results from the MeerTime Large Survey Project and commissioning tests with PTUSE. Highlights include observations of the double pulsar J0737-3039A, pulse profiles from 34 millisecond pulsars from a single 2.5h observation of the Globular cluster Terzan 5, the rotation measure of Ter5O, a 420-sigma giant pulse from the Large Magellanic Cloud pulsar PSR J0540-6919, and nulling identified in the slow pulsar PSR J0633-2015. One of the key design specifications for MeerKAT was absolute timing errors of less than 5 ns using their novel precise time system. Our timing of two bright millisecond pulsars confirm that MeerKAT delivers exceptional timing. PSR J2241-5236 exhibits a jitter limit of <4 ns per hour whilst timing of PSR J1909-3744 over almost 11 months yields an rms residual of 66 ns with only 4 min integrations. Our results confirm that the MeerKAT is an exceptional pulsar telescope. The array can be split into four separate sub-arrays to time over 1000 pulsars per day and the future deployment of S-band (1750-3500 MHz) receivers will further enhance its capabilities.

astro-ph.IM↗

arXiv2020

The Thousand-Pulsar-Array programme on MeerKAT I: Science objectives and first results

We report here on initial results from the Thousand Pulsar Array (TPA) programme, part of the Large Survey Project "MeerTime" on the MeerKAT telescope. The interferometer is used in tied-array mode in the band from 856 to 1712~MHz, and the wide band coupled with the large collecting area and low receiver temperature make it an excellent telescope for the study of radio pulsars. The TPA is a 5 year project which aims to observe (a) more than 1000 pulsars to obtain high-fidelity pulse profiles, (b) some 500 of these pulsars over multiple epochs, (c) long sequences of single-pulse trains from several hundred pulsars. The scientific outcomes from the programme will include determination of pulsar geometries, the location of the radio emission within the pulsar magnetosphere, the connection between the magnetosphere and the crust and core of the star, tighter constraints on the nature of the radio emission itself as well as interstellar medium studies. First results presented here include updated dispersion measures, 26 pulsars with Faraday rotation measures derived for the first time and a description of interesting emission phenomena observed thus far.

astro-ph.HE↗

arXiv2020

Lense-Thirring frame dragging induced by a fast-rotating white dwarf in a binary pulsar system

Radio pulsars in short-period eccentric binary orbits can be used to study both gravitational dynamics and binary evolution. The binary system containing PSR J1141$-$6545 includes a massive white dwarf (WD) companion that formed before the gravitationally bound young radio pulsar. We observe a temporal evolution of the orbital inclination of this pulsar that we infer is caused by a combination of a Newtonian quadrupole moment and Lense-Thirring precession of the orbit resulting from rapid rotation of the WD. Lense-Thirring precession, an effect of relativistic frame-dragging, is a prediction of general relativity. This detection is consistent with the evolutionary scenario in which the WD accreted matter from the pulsar progenitor, spinning up the WD to a period $< 200$ seconds.

astro-ph.HE↗

arXiv2019

Polarization studies of Rotating Radio Transients

We study the polarization properties of 22 known rotating radio transients (RRATs) with the 64-m Parkes radio telescope and present the Faraday rotation measures (RMs) for the 17 with linearly polarized flux exceeding the off-pulse noise by 3$σ$. Each RM was estimated using a brute-force search over trial RMs that spanned the maximum measurable range $\pm1.18 \times 10^5 \, \mathrm{rad \, m^2}$ (in steps of 1 $\mathrm{rad \, m^2}$), followed by an iterative refinement algorithm. The measured RRAT RMs are in the range |RM| $\sim 1$ to $\sim 950$ rad m$^{-2}$ with an average linear polarization fraction of $\sim 40$ per cent. Individual single pulses are observed to be up to 100 per cent linearly polarized. The RMs of the RRATs and the corresponding inferred average magnetic fields (parallel to the line-of-sight and weighted by the free electron density) are observed to be consistent with the Galactic plane pulsar population. Faraday rotation analyses are typically performed on accumulated pulsar data, for which hundreds to thousands of pulses have been integrated, rather than on individual pulses. Therefore, we verified the iterative refinement algorithm by performing Monte Carlo simulations of artificial single pulses over a wide range of S/N and RM. At and above a S/N of 17 in linearly polarized flux, the iterative refinement recovers the simulated RM value 100 per cent of the time with a typical mean uncertainty of $\sim5$ rad m$^{-2}$. The method described and validated here has also been successfully used to determine reliable RMs of several fast radio bursts (FRBs) discovered at Parkes.

astro-ph.HE↗

arXiv2019

The UTMOST Survey for Magnetars, Intermittent pulsars, RRATs and FRBs I: System description and overview

We describe the ongoing `Survey for Magnetars, Intermittent pulsars, Rotating radio transients and Fast radio bursts' (SMIRF), performed using the newly refurbished UTMOST telescope. SMIRF repeatedly sweeps the southern Galactic plane performing real-time periodicity and single-pulse searches, and is the first survey of its kind carried out with an interferometer. SMIRF is facilitated by a robotic scheduler which is capable of fully autonomous commensal operations. We report on the SMIRF observational parameters, the data analysis methods, the survey's sensitivities to pulsars, techniques to mitigate radio frequency interference and present some early survey results. UTMOST's wide field of view permits a full sweep of the Galactic plane to be performed every fortnight, two orders of magnitude faster than previous surveys. In the six months of operations from January to June 2018, we have performed $\sim 10$ sweeps of the Galactic plane with SMIRF. Notable blind re-detections include the magnetar PSR J1622$-$4950, the RRAT PSR J0941$-$3942 and the eclipsing pulsar PSR J1748$-$2446A. We also report the discovery of a new pulsar, PSR J1705$-$54. Our follow-up of this pulsar with the UTMOST and Parkes telescopes at an average flux limit of $\leq 20$ mJy and $\leq 0.16$ mJy respectively, categorizes this as an intermittent pulsar with a high nulling fraction of $< 0.002$

astro-ph.IM↗

arXiv2019

Five new real-time detections of Fast Radio Bursts with UTMOST

We detail a new fast radio burst (FRB) survey with the Molonglo Radio Telescope, in which six FRBs were detected between June 2017 and December 2018. By using a real-time FRB detection system, we captured raw voltages for five of the six events, which allowed for coherent dedispersion and very high time resolution (10.24 $μ$s) studies of the bursts. Five of the FRBs show temporal broadening consistent with interstellar and/or intergalactic scattering, with scattering timescales ranging from 0.16 to 29.1 ms. One burst, FRB181017, shows remarkable temporal structure, with 3 peaks each separated by 1 ms. We searched for phase-coherence between the leading and trailing peaks and found none, ruling out lensing scenarios. Based on this survey, we calculate an all-sky rate at 843 MHz of $98^{+59}_{-39}$ events sky$^{-1}$ day$^{-1}$ to a fluence limit of 8 Jy-ms: a factor of 7 below the rates estimated from the Parkes and ASKAP telescopes at 1.4 GHz assuming the ASKAP-derived spectral index $α=-1.6$ ($F_ν\proptoν^α$). Our results suggest that FRB spectra may turn over below 1 GHz. Optical, radio and X-ray followup has been made for most of the reported bursts, with no associated transients found. No repeat bursts were found in the survey.

astro-ph.HE↗

arXiv2019

Modelling annual and orbital variations in the scintillation of the relativistic binary PSR J1141$-$6545

We have observed the relativistic binary pulsar PSR J1141$-$6545 over a period of $\sim$6 years using the Parkes 64 m radio telescope, with a focus on modelling the diffractive intensity scintillations to improve the accuracy of the astrometric timing model. The long-term scintillation, which shows orbital and annual variations, allows us to measure parameters that are difficult to measure with pulsar timing alone. These include: the orbital inclination $i$; the longitude of the ascending node $Ω$; and the pulsar system transverse velocity. We use the annual variations to resolve the previous ambiguity in the sense of the inclination angle. Using the correct sense, and a prior probability distribution given by a constraint from pulsar timing ($i=73\pm3^\circ$), we find $Ω=24.8\pm1.8^\circ$ and we estimate the pulsar distance to be $D=10^{+4}_{-3}$ kpc. This then gives us an estimate of this pulsar's proper motion of $μ_α\cosδ=2.9\pm1.0$ mas yr$^{-1}$ in right ascension and $μ_δ=1.8\pm0.6$ mas yr$^{-1}$ in declination. Finally, we obtain measurements of the spatial structure of the interstellar electron density fluctuations, including: the spatial scale and anisotropy of the diffraction pattern; the distribution of scattering material along the line of sight; and spatial variation in the strength of turbulence from epoch to epoch. We find that the scattering is dominated by a thin screen at a distance of $(0.724\pm0.008)D$, with an anisotropy axial ratio $A_{\rm r} = 2.14\pm0.11$.

astro-ph.HE↗

arXiv2019

Relativistic spin precession in the binary PSR J1141$-$6545

PSR J1141$-$6545 is a precessing binary pulsar that has the rare potential to reveal the two-dimensional structure of a non-recycled pulsar emission cone. It has undergone $\sim 25 °$ of relativistic spin precession in the $\sim18$ years since its discovery. In this paper, we present a detailed Bayesian analysis of the precessional evolution of the width of the total intensity profile, to understand the changes to the line-of-sight impact angle ($β$) of the pulsar using four different physically motivated prior distribution models. Although we cannot statistically differentiate between the models with confidence, the temporal evolution of the linear and circular polarisations strongly argue that our line-of-sight crossed the magnetic pole around MJD 54000 and that only two models remain viable. For both these models, it appears likely that the pulsar will precess out of our line-of-sight in the next $3-5$ years, assuming a simple beam geometry. Marginalising over $β$ suggests that the pulsar is a near-orthogonal rotator and provides the first polarization-independent estimate of the scale factor ($\mathbb{A}$) that relates the pulsar beam opening angle ($ρ$) to its rotational period ($P$) as $ρ= \mathbb{A}P^{-0.5}$ : we find it to be $> 6 \rm~deg~s^{0.5}$ at 1.4 GHz with 99\% confidence. If all pulsars emit from opposite poles of a dipolar magnetic field with comparable brightness, we might expect to see evidence of an interpulse arising in PSR J1141$-$6545, unless the emission is patchy.

astro-ph.HE↗

arXiv2018

Hunting for radio emission from the intermittent pulsar J1107-5907 at low frequencies

The rare intermittent pulsars pose some of the most challenging questions surrounding the pulsar emission mechanism, but typically have relatively minimal low-frequency ($\lesssim$ 300 MHz) coverage. We present the first low-frequency detection of the intermittent pulsar J1107-5907 with the Murchison Widefield Array (MWA) at 154 MHz and the simultaneous detection from the recently upgraded Molonglo Observatory Synthesis Telescope (UTMOST) at 835 MHz, as part of an on-going observing campaign. During a 30-minute simultaneous observation, we detected the pulsar in its bright emission state for approximately 15 minutes, where 86 and 283 pulses were detected above a signal-to-noise threshold of 6 with the MWA and UTMOST, respectively. Of the detected pulses, 51 had counterparts at both frequencies and exhibited steep spectral indices for both the bright main pulse component and the precursor component. We find that the bright state pulse energy distribution is best parameterised by a log-normal distribution at both frequencies, contrary to previous results which suggested a power law distribution. Further low-frequency observations are required in order to explore in detail aspects such as pulse-to-pulse variability, intensity modulations and to better constrain the signal propagation effects due to the interstellar medium and intermittency characteristics at these frequencies. The spectral index, extended profile emission covering a large fraction of pulse longitude, and the broadband intermittency of PSR J1107-5907 suggests that future low-frequency pulsar searches, for instance those planned with SKA-Low, will be in an excellent position to find and investigate new pulsars of this type.

astro-ph.HE↗

arXiv2018

The UTMOST pulsar timing programme I: overview and first results

We present an overview and the first results from a large-scale pulsar timing programme that is part of the UTMOST project at the refurbished Molonglo Observatory Synthesis Radio Telescope (MOST) near Canberra, Australia. We currently observe more than 400 mainly bright southern radio pulsars with up to daily cadences. For 205 (8 in binaries, 4 millisecond pulsars) we publish updated timing models, together with their flux densities, flux density variability, and pulse widths at 843 MHz, derived from observations spanning between 1.4 and 3 yr. In comparison with the ATNF pulsar catalogue, we improve the precision of the rotational and astrometric parameters for 123 pulsars, for 47 by at least an order of magnitude. The time spans between our measurements and those in the literature are up to 48 yr, which allows us to investigate their long-term spin-down history and to estimate proper motions for 60 pulsars, of which 24 are newly determined and most are major improvements. The results are consistent with interferometric measurements from the literature. A model with two Gaussian components centred at 139 and $463~\text{km} \: \text{s}^{-1}$ fits the transverse velocity distribution best. The pulse duty cycle distributions at 50 and 10 per cent maximum are best described by log-normal distributions with medians of 2.3 and 4.4 per cent, respectively. We discuss two pulsars that exhibit spin-down rate changes and drifting subpulses. Finally, we describe the autonomous observing system and the dynamic scheduler that has increased the observing efficiency by a factor of 2-3 in comparison with static scheduling.

astro-ph.HE↗

arXiv2018

FRB microstructure revealed by the real-time detection of FRB170827

We report a new Fast Radio Burst (FRB) discovered in real-time as part of the UTMOST project at the Molonglo Observatory Synthesis Radio Telescope (MOST). FRB170827 is the first detected with our low-latency ($< 24$ s), machine-learning-based FRB detection system. The FRB discovery was accompanied by the capture of voltage data at the native time and frequency resolution of the observing system, enabling coherent dedispersion and detailed off-line analysis, which have unveiled fine temporal and frequency structure. The dispersion measure (DM) of 176.80 $\pm$ 0.04 pc cm$^{-3}$, is the lowest of the FRB population. The Milky Way contribution along the line of sight is $\sim$ 40 pc cm$^{-3}$, leaving an excess DM of $\sim$ 145 pc cm$^{-3}$. The FRB has a fluence $>$ 20 $\pm$ 7 Jy ms, and is narrow, with a width of $\sim$ 400 $μ$s at 10$\%$ of its maximum amplitude. However, the burst shows three temporal components, the narrowest of which is $\sim$ 30 $μ$s, and a scattering timescale of $4.1 \pm 2.7$ $μ$s. The FRB shows spectral modulations on frequency scales of 1.5 MHz and 0.1 MHz. Both are prominent in the dynamic spectrum, which shows a very bright region of emission between 841 and 843 MHz, and weaker, patchy emission across the entire band. We show the fine spectral structure could arise in the FRB host galaxy, or its immediate vicinity.

astro-ph.HE↗

arXiv2018

The SUrvey for Pulsars and Extragalactic Radio Bursts III: Polarization properties of FRBs 160102 & 151230

We report on the polarization properties of two fast radio bursts (FRBs): 151230 and 160102 discovered in the SUrvey for Pulsars and Extragalactic Radio Bursts (SUPERB) at the Parkes radio telescope. FRB 151230 is observed to be 6 $\pm$ 11% circularly polarized and 35 $\pm$ 13 % linearly polarized with a rotation measure (RM) consistent with zero. Conversely, FRB 160102 is observed to have a circular polarization fraction of 30 $\pm$ 11 %, linear polarization fraction of 84 $\pm$ 15 % for RM = -221(6) rad m$^{-2}$ and the highest measured DM (2596.1 $\pm$ 0.3 pc cm$^{-3}$) for an FRB to date. We examine possible progenitor models for FRB 160102 in extragalactic, non-cosmological and cosmological scenarios. After accounting for the Galactic foreground contribution, we estimate the intrinsic RM to be -256(9) rad m$^{-2}$ in the low-redshift case and $\sim 2.4 \times 10^{2}$ rad m$^{-2}$ in the high-redshift case. We assess the relative likeliness of these scenarios and how each can be tested. We also place constraints on the scattering measure and study the impact of scattering on the signal's polarization position angle.

astro-ph.HE↗