arXiv subjects

P. Weltevrede

Publications and source records attributed to P. Weltevrede.

At least 55 records · Page 3Linked to original sources

arXiv2020

The LOFAR Tied-Array All-Sky Survey: Timing of 21 pulsars including the first binary pulsar discovered with LOFAR

We report on the multi-frequency timing observations of 21 pulsars discovered in the LOFAR Tied-Array All-Sky Survey (LOTAAS). The timing data were taken at central frequencies of 149 MHz (LOFAR) as well as 334 and 1532 MHz (Lovell Telecope). The sample of pulsars includes 20 isolated pulsars and the first binary pulsar discovered by the survey, PSR J1658$+$3630. We modelled the timing properties of the pulsars, which showed that they have, on average, larger characteristic ages. We present the pulse profiles of the pulsars across the three observing bands, where PSR J1643$+$1338 showed profile evolution that appears not to be well-described by the radius-to-frequency-mapping model. Furthermore, we modelled the spectra of the pulsars across the same observing bands, using a simple power law, and found an average spectral index of $-1.9 \pm 0.5$. Amongst the pulsars studied here, PSR J1657$+$3304 showed large flux density variations of a factor of 10 over 300 days, as well as mode changing and nulling on timescales of a few minutes. We modelled the rotational and orbital properties of PSR J1658$+$3630, which has a spin period of 33 ms in a binary orbit of 3.0 days with a companion of minimum mass of 0.87$M_{\odot}$, likely a Carbon-Oxygen or Oxygen-Neon-Magnesium type white dwarf. PSR J1658$+$3630 has a dispersion measure of 3.0 pc cm$^{-3}$, making it possibly one of the closest binary pulsars known.

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arXiv2019

The LOFAR Tied-Array All-Sky Survey (LOTAAS): Characterization of 20 pulsar discoveries and their single-pulse behavior

We are using the LOw-Frequency ARray (LOFAR) to perform the LOFAR Tied-Array All-Sky (LOTAAS) survey for pulsars and fast transients. Here we present the astrometric and rotational parameters of 20 pulsars discovered as part of LOTAAS. These pulsars have regularly been observed with LOFAR at 149 MHz and the Lovell telescope at 1532 MHz, supplemented by some observations with the Lovell telescope at 334 MHz and the Nancay Radio Telescope at 1484 MHz. Timing models are calculated for the 20 pulsars, some of which are among the slowest-spinning pulsars known. PSR J1236-0159 rotates with a period P ~ 3.6 s, while 5 additional pulsars show P > 2 s. Also, the spin-down rates Pdot are, on average, low, with PSR J0815+4611 showing Pdot ~ 4E-18. Some of the pulse profiles, generically single-peaked, present complex shapes evolving with frequency. Multi-frequency flux measurements show that these pulsars have generically relatively steep spectra but exceptions are present, with values ranging between ~ -4 and -1. Among the pulsar sample, a large fraction shows large single-pulse variability, with 4 pulsars being undetectable more than 15% of the time and one tentatively classified as a Rotating Radio Transient. Two single-peaked pulsars show drifting sub-pulses.

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arXiv2019

A LOFAR census of non-recycled pulsars: extending below 80 MHz

We present the results from the low-frequency (40--78 MHz) extension of the first LOFAR pulsar census of non-recycled pulsars. We have used the Low-Band Antennas of the LOFAR core stations to observe 87 pulsars out of 158 that have been detected previously with the High-Band Antennas. Forty-three pulsars have been detected and we present here their flux densities and flux-calibrated profiles. Seventeen of these pulsars have not been, to our knowledge, detected before at such low frequencies. We re-calculate the spectral indices using the new low-frequency flux density measurements from the LOFAR census and discuss the prospects of studying pulsars at the very low frequencies with the current and upcoming facilities, such as NenuFAR.

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arXiv2019

Timing of young radio pulsars I: Timing noise, periodic modulation and proper motion

The smooth spin-down of young pulsars is perturbed by two non-deterministic phenomenon, glitches and timing noise. Although the timing noise provides insights into nuclear and plasma physics at extreme densities, it acts as a barrier to high-precision pulsar timing experiments. An improved methodology based on Bayesian inference is developed to simultaneously model the stochastic and deterministic parameters for a sample of 85 high-$\dot{E}$ radio pulsars observed for $\sim$ 10 years with the 64-m Parkes radio telescope. Timing noise is known to be a red process and we develop a parametrization based on the red-noise amplitude ($A_{\rm red}$) and spectral index ($β$). We measure the median $A_{\rm red}$ to be $-10.4^{+1.8}_{-1.7}$ yr$^{3/2}$ and $β$ to be $-5.2^{+3.0}_{-3.8}$ and show that the strength of timing noise scales proportionally to $ν^{1}|\dotν|^{-0.6\pm0.1}$, where $ν$ is the spin frequency of the pulsar and $\dotν$ its spin-down rate. Finally, we measure significant braking indices for 19 pulsars, proper motions for two pulsars and discuss the presence of periodic modulation in the arrival times of five pulsars.

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arXiv2019

Spin frequency evolution and pulse profile variations of the recently re-activated radio magnetar XTE J1810-197

After spending almost a decade in a radio-quiet state, the Anomalous X-ray Pulsar XTE J1810-197 turned back on in early December 2018. We have observed this radio magnetar at 1.5 GHz with ~daily cadence since the first detection of radio re-activation on 8 December 2018. In this paper, we report on the current timing properties of XTE J1810-197 and find that the magnitude of the spin frequency derivative has increased by a factor of 2.6 over our 48-day data set. We compare our results with the spin-down evolution reported during its previous active phase in the radio band. We also present total intensity pulse profiles at five different observing frequencies between 1.5 and 8.4 GHz, collected with the Lovell and the Effelsberg telescopes. The profile evolution in our data set is less erratic than what was reported during the previous active phase, and can be seen varying smoothly between observations. Profiles observed immediately after the outburst show the presence of at least five cycles of a very stable ~50-ms periodicity in the main pulse component that lasts for at least tens of days. This remarkable structure is seen across the full range of observing frequencies.

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arXiv2019

The GMRT High Resolution Southern Sky Survey for pulsars and transients -II. New discoveries, timing and polarization properties

We have been conducting the GMRT High Resolution Southern Sky (GHRSS) survey for the last four years and have discovered 18 pulsars to date. The GHRSS survey is an off-Galactic-plane survey at 322 MHz in a region of the sky (declination range -40 degrees to -54 degrees) complementary to other ongoing low-frequency surveys. In this paper we report the discovery of three pulsars, PSRs J1239-48, J1516-43 and J1726-52. We also present timing solutions for three pulsars previously discovered with the GHRSS survey: PSR J2144-5237, a millisecond pulsar with a period P=5 ms in a 10 day orbit around a < 0.18 Msun companion; PSR J1516-43, a mildly recycled P=36 ms pulsar in a 228 day orbit with a companion of mass ~0.4 Msun; and the P=320 ms PSR J0514-4408 which we show is a source of pulsed $γ$-ray emission. We also report radio polarimetric observations of three of the GHRSS discoveries, PSRs J0418-4154, J0514-4408 and J2144-5237.

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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.

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arXiv2019

Low-frequency Faraday rotation measures towards pulsars using LOFAR: probing the 3-D Galactic halo magnetic field

We determined Faraday rotation measures (RMs) towards 137 pulsars in the northern sky, using Low-Frequency Array (LOFAR) observations at 110-190 MHz. This low-frequency RM catalogue, the largest to date, improves the precision of existing RM measurements on average by a factor of 20 - due to the low frequency and wide bandwidth of the data, aided by the RM synthesis method. We report RMs towards 25 pulsars for the first time. The RMs were corrected for ionospheric Faraday rotation to increase the accuracy of our catalogue to approximately 0.1 rad m$^{\rm -2}$. The ionospheric RM correction is currently the largest contributor to the measurement uncertainty. In addition, we find that the Faraday dispersion functions towards pulsars are extremely Faraday thin - mostly less than 0.001 rad m$^{\rm -2}$. We use these new precise RM measurements (in combination with existing RMs, dispersion measures, and distance estimates) to estimate the scale height of the Galactic halo magnetic field: 2.0$\pm$0.3 kpc for Galactic quadrants I and II above and below the Galactic plane (we also evaluate the scale height for these regions individually). Overall, our initial low-frequency catalogue provides valuable information about the 3-D structure of the Galactic magnetic field.

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arXiv2018

LOFAR discovery of a 23.5-second radio pulsar

We present the discovery of PSR J0250+5854, a radio pulsar with a spin period of 23.5 s. This is the slowest-spinning radio pulsar known. PSR J0250+5854 was discovered by the LOFAR Tied-Array All-Sky Survey (LOTAAS), an all-Northern-sky survey for pulsars and fast transients at a central observing frequency of 135 MHz. We subsequently detected pulsations from the pulsar in the interferometric images of the LOFAR Two-metre Sky Survey, allowing for sub-arcsecond localization. This, along with a pre-discovery detection 2 years prior, allowed us to measure the spin-period derivative to be $\dot{P}=2.7 \times 10^{-14}$ s s$^{-1}$. The observed spin period derivative of PSR J0250+5854 indicates a surface magnetic field strength, characteristic age and spin-down luminosity of $2.6 \times 10^{13}$G, $13.7$ Myr and $8.2 \times 10^{28}$ erg s$^{-1}$ respectively, for a dipolar magnetic field configuration. This also places the pulsar beyond the conventional pulsar death line, where radio emission is expected to cease. The spin period of PSR J0250+5854 is similar to those of the high-energy-emitting magnetars and X-ray dim isolated neutron stars (XDINSs). However, the pulsar was not detected by the Swift/XRT in the energy band of 0.3-10 keV, placing a bolometric luminosity limit of $1.5 \times 10^{32}$ erg s$^{-1}$ for an assumed $N_{\rm H}=1.35\times10^{21}$ cm$^{-2}$ and a temperature of 85 eV (typical of XDINSs). We discuss the implications of the discovery for models of the pulsar death line as well as the prospect of finding more similarly long-period pulsars, including the advantages provided by LOTAAS for this.

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arXiv2018

Peculiar Spin Frequency and Radio Profile Evolution of PSR J1119$-$6127 Following Magnetar-like X-ray Bursts

We present the spin frequency and profile evolution of the radio pulsar J1119$-$6127 following magnetar-like X-ray bursts from the system in 2016 July. Using data from the Parkes radio telescope, we observe a smooth and fast spin-down process subsequent to the X-ray bursts resulting in a net change in the pulsar rotational frequency of $Δν\approx-4\times10^{-4}$\,Hz. During the transition, a net spin-down rate increase of $Δ\dotν\approx-1\times10^{-10}$\,Hz\,s$^{-1}$ is observed, followed by a return of $\dotν$ to its original value. In addition, the radio pulsations disappeared after the X-ray bursts and reappeared about two weeks later with the flux density at 1.4\,GHz increased by a factor of five. The flux density then decreased and undershot the normal flux density followed by a slow recovery back to normal. The pulsar's integrated profile underwent dramatic and short-term changes in total intensity, polarization and position angle. Despite the complex evolution, we observe correlations between the spin-down rate, pulse profile shape and radio flux density. Strong single pulses have been detected after the X-ray bursts with their energy distributions evolving with time. The peculiar but smooth spin frequency evolution of PSR~J1119$-$6127 accompanied by systematic pulse profile and flux density changes are most likely to be a result of either reconfiguration of the surface magnetic fields or particle winds triggered by the X-ray bursts. The recovery of spin-down rate and pulse profile to normal provides us the best case to study the connection between high magnetic-field pulsars and magnetars.

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arXiv2018

The largest glitch observed in the Crab pulsar

We have observed a large glitch in the Crab pulsar (PSR B0531+21). The glitch occurred around MJD 58064 (2017 November 8) when the pulsar underwent an increase in the rotation rate of $Δν= 1.530 \times 10^{-5}$ Hz, corresponding to a fractional increase of $Δν/ ν= 0.516 \times 10^{-6}$ making this event the largest glitch ever observed in this source. Due to our high-cadence and long-dwell time observations of the Crab pulsar we are able to partially resolve a fraction of the total spin-up of the star. This delayed spin-up occurred over a timescale of $\sim$1.7 days and is similar to the behaviour seen in the 1989 and 1996 large Crab pulsar glitches. The spin-down rate also increased at the glitch epoch by $Δ\dotν / \dotν = 7 \times 10^{-3}$. In addition to being the largest such event observed in the Crab, the glitch occurred after the longest period of glitch inactivity since at least 1984 and we discuss a possible relationship between glitch size and waiting time. No changes to the shape of the pulse profile were observed near the glitch epoch at 610 MHz or 1520 MHz, nor did we identify any changes in the X-ray flux from the pulsar. The long-term recovery from the glitch continues to progress as $\dotν$ slowly rises towards pre-glitch values. In line with other large Crab glitches, we expect there to be a persistent change to $\dotν$. We continue to monitor the long-term recovery with frequent, high quality observations.

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arXiv2018

A Long-term study of three rotating radio transients

We present the longest-term timing study so far of three Rotating Radio Transients (RRATs) - J1819-1458, J1840-1419 and J1913+1330 - performed using the Lovell, Parkes and Green Bank telescopes over the past decade. We study long-term and short- term variations of the pulse emission rate from these RRATs and report a marginal indication of a long-term increase in pulse detection rate over time for PSR J1819-1458 and J1913+1330. For PSR J1913+1330, we also observe a two orders of magnitude variation in the observed pulse detection rates across individual epochs, which may constrain the models explaining the origin of RRAT pulses. PSR J1913+1330 is also observed to exhibit a weak persistent emission mode. We investigate the post-glitch timing properties of J1819-1458 (the only RRAT for which glitches are observed) and discuss the implications for possible glitch models. Its post-glitch over-recovery of the frequency derivative is magnetar-like and similar behaviour is only observed for two other pulsars, both of which have relatively high magnetic field strengths. Following the over-recovery we also observe that some fraction of the pre-glitch frequency derivative is gradually recovered.

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arXiv2018

MeerTime - the MeerKAT Key Science Program on Pulsar Timing

The MeerKAT telescope represents an outstanding opportunity for radio pulsar timing science with its unique combination of a large collecting area and aperture efficiency (effective area $\sim$7500 m$^2$), system temperature ($T<20$K), high slew speeds (1-2 deg/s), large bandwidths (770 MHz at 20cm wavelengths), southern hemisphere location (latitude $\sim -30^\circ$) and ability to form up to four sub-arrays. The MeerTime project is a five-year program on the MeerKAT array by an international consortium that will regularly time over 1000 radio pulsars to perform tests of relativistic gravity, search for the gravitational wave signature induced by supermassive black hole binaries in the timing residuals of millisecond pulsars, explore the interiors of neutron stars through a pulsar glitch monitoring programme, explore the origin and evolution of binary pulsars, monitor the swarms of pulsars that inhabit globular clusters and monitor radio magnetars. In addition to these primary programmes, over 1000 pulsars will have their arrival times monitored and the data made immediately public. The MeerTime pulsar backend comprises two server-class machines each of which possess four Graphics Processing Units. Up to four pulsars can be coherently dedispersed simultaneously up to dispersion measures of over 1000 pc cm$^{-3}$. All data will be provided in psrfits format. The MeerTime backend will be capable of producing coherently dedispersed filterbank data for timing multiple pulsars in the cores of globular clusters that is useful for pulsar searches of tied array beams. All MeerTime data will ultimately be made available for public use, and any published results will include the arrival times and profiles used in the results.

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arXiv2018

Simultaneous 13 cm/3 cm single-pulse observations of PSR B0329+54

We have investigated the mode-changing properties of PSR B0329+54 using 31 epochs of simultaneous 13 cm/3 cm single-pulse observations obtained with Shanghai Tian Ma 65 m telescope. The pulsar was found in the abnormal emission mode 17 times, accounting for ~13% of the 41.6 hours total observation time. Single pulse analyses indicate that mode changes took place simultaneously at 13 cm/3 cm within a few rotational periods. We detected occasional bright and narrow pulses whose peak flux densities were 10 times higher than that of the integrated profile in both bands. At 3 cm, about 0.66% and 0.27% of single pulses were bright in the normal mode and abnormal mode respectively, but at 13 cm the occurrence rate was only about 0.007%. We divided the pulsar radiation window into three components (C1, C2 and C3) corresponding to the main peaks of the integrated profile. The bright pulses preferentially occurred at pulse phases corresponding to the peaks of C2 and C3. Fluctuation spectra showed that C2 had excess red noise in the normal mode, but broad quasi-periodic features with central frequencies around 0.12 cycles/period in the abnormal mode. At 3 cm, C3 had a stronger quasi-periodic modulation centered around 0.06 cycles/period in the abnormal mode. Although there were some asymmetries in the two-dimensional fluctuation spectra, we found no clear evidence for systematic subpulse drifting. Consistent with previous low-frequency observations, we found a very low nulling probability for B0329+54 with upper limits of 0.13% and 1.68% at 13 cm/3 cm respectively.

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arXiv2018

Low-frequency pulse profile variation in PSR B2217+47: evidence for echoes from the interstellar medium

We have observed a complex and continuous change in the integrated pulse profile of PSR B2217+47, manifested as additional components trailing the main peak. These transient components are detected over 6 years at $150$ MHz using the LOw Frequency ARray (LOFAR), but they are not seen in contemporaneous Lovell observations at $1.5$ GHz. We argue that propagation effects in the ionized interstellar medium (IISM) are the most likely cause. The putative structures in the IISM causing the profile variation are roughly half-way between the pulsar and the Earth and have transverse radii $R \sim 30$ AU. We consider different models for the structures. Under the assumption of spherical symmetry, their implied average electron density is $\overline{n}_e \sim 100$ cm$^{-3}$. Since PSR B2217+47 is more than an order of magnitude brighter than the average pulsar population visible to LOFAR, similar profile variations would not have been identified in most pulsars, suggesting that subtle profile variations in low-frequency profiles might be more common than we have observed to date. Systematic studies of these variations at low frequencies can provide a new tool to investigate the proprieties of the IISM and the limits to the precision of pulsar timing.

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arXiv2018

The Low-Frequency Radio Eclipses of the Black Widow Pulsar J1810+1744

We have observed and analysed the eclipses of the black widow pulsar J1810+1744 at low radio frequencies. Using LOw-Frequency ARray (LOFAR) and Westerbork Synthesis Radio Telescope observations between 2011--2015 we have measured variations in flux density, dispersion measure and scattering around eclipses. High-time-resolution, simultaneous beamformed and interferometric imaging LOFAR observations show concurrent disappearance of pulsations and total flux from the source during the eclipses, with a $3σ$ upper limit of 36 mJy ($<10\%$ of the pulsar's averaged out-of-eclipse flux density). The dispersion measure variations are highly asymmetric, suggesting a tail of material swept back due to orbital motion. The egress deviations are variable on timescales shorter than the 3.6 hr orbital period and are indicative of a clumpy medium. Additional pulse broadening detected during egress is typically $<20\%$ of the pulsar's spin period, showing no evidence of scattering the pulses beyond detectability in the beamformed data. The eclipses, lasting $\sim13\%$ of the orbit at 149 MHz, are shown to be frequency-dependent with total duration scaling as $\proptoν^{-0.41\pm0.03}$. The results are discussed in the context of the physical parameters of the system, and an examination of eclipse mechanisms reveals cyclotron-synchrotron absorption as the most likely primary cause, although non-linear scattering mechanisms cannot be quantitatively ruled out. The inferred mass loss rate is a similar order-of-magnitude to the mean rate required to fully evaporate the companion in a Hubble time.

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arXiv2018

Resolving discrete pulsar spin-down states with current and future instrumentation

An understanding of pulsar timing noise offers the potential to improve the timing precision of a large number of pulsars as well as facilitating our understanding of pulsar magnetospheres. For some sources, timing noise is attributable to a pulsar switching between two different spin-down rates $(\dotν)$. Such transitions may be common but difficult to resolve using current techniques. In this work, we use simulations of $\dotν$-variable pulsars to investigate the likelihood of resolving individual $\dotν$ transitions. We inject step-changes in the value of $\dotν$ with a wide range of amplitudes and switching timescales. We then attempt to redetect these transitions using standard pulsar timing techniques. The pulse arrival-time precision and the observing cadence are varied. Limits on $\dotν$ detectability based on the effects such transitions have on the timing residuals are derived. With the typical cadences and timing precision of current timing programs, we find we are insensitive to a large region of $Δ\dotν$ parameter space which encompasses small, short timescale switches. We find, where the rotation and emission states are correlated, that using changes to the pulse shape to estimate $\dotν$ transition epochs, can improve detectability in certain scenarios. The effects of cadence on $Δ\dotν$ detectability are discussed and we make comparisons with a known population of intermittent and mode-switching pulsars. We conclude that for short timescale, small switches, cadence should not be compromised when new generations of ultra-sensitive radio telescopes are online.

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arXiv2016

Variability, polarimetry, and timing properties of single pulses from PSR J1713+0747 using the Large European Array for Pulsars

Single pulses preserve information about the pulsar radio emission and propagation in the pulsar magnetosphere, and understanding the behaviour of their variability is essential for estimating the fundamental limit on the achievable pulsar timing precision. Here we report the findings of our analysis of single pulses from PSR J1713+0747 with data collected by the Large European Array for Pulsars (LEAP). We present statistical studies of the pulse properties that include distributions of their energy, phase and width. Two modes of systematic sub-pulse drifting have been detected, with a periodicity of 7 and 3 pulse periods. The two modes appear at different ranges of pulse longitude but overlap under the main peak of the integrated profile. No evidence for pulse micro-structure is seen with a time resolution down to 140 ns. In addition, we show that the fractional polarisation of single pulses increases with their pulse peak flux density. By mapping the probability density of linear polarisation position angle with pulse longitude, we reveal the existence of two orthogonal polarisation modes. Finally, we find that the resulting phase jitter of integrated profiles caused by single pulse variability can be described by a Gaussian probability distribution only when at least 100 pulses are used for integration. Pulses of different flux densities and widths contribute approximately equally to the phase jitter, and no improvement on timing precision is achieved by using a sub-set of pulses with a specific range of flux density or width.

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