ORCID Profile
0000-0002-1775-9692
Current Organisations
City University of Hong Kong
,
CNRS Délégation Bretagne et Pays de Loire
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Publisher: Oxford University Press (OUP)
Date: 19-02-2016
DOI: 10.1093/MNRAS/STW395
Publisher: Wiley
Date: 18-07-2018
Abstract: Exploring of new catalyst activation principle holds a key to unlock catalytic powers of cheap and earth-abundant materials for large-scale applications. In this regard, the vacancy defects have been proven to be effective to initiate catalytic active sites and endow high electrocatalytic activities. However, such electrocatalytically active defects reported to date have been mostly formed by anion vacancies. Herein, it is demonstrated for the first time that iron cation vacancies induce superb water splitting bifunctionality in alkaline media. A simple wet-chemistry method is developed to grow ultrathin feroxyhyte (δ-FeOOH) nanosheets with rich Fe vacancies on Ni foam substrate. The theoretical and experimental results confirm that, in contrast to anion vacancies, the formation of rich second neighboring Fe to Fe vacancies in δ-FeOOH nanosheets can create catalytic active centers for both hydrogen and oxygen evolution reactions. The atomic level insight into the new catalyst activation principle based on metal vacancies is adaptable for developing other transition metal electrocatalysts, including Fe-based ones.
Publisher: Oxford University Press (OUP)
Date: 16-06-2016
Publisher: American Astronomical Society
Date: 13-12-2017
Publisher: American Astronomical Society
Date: 28-03-2016
Publisher: Oxford University Press (OUP)
Date: 19-11-2015
Publisher: American Astronomical Society
Date: 10-08-2011
Publisher: Oxford University Press (OUP)
Date: 28-05-2015
Publisher: Oxford University Press (OUP)
Date: 07-04-2020
Abstract: We study the timing stability of three black widow pulsars (BWPs), both in terms of their long-term spin evolution and their shorter term orbital stability. The erratic timing behaviour and radio eclipses of the first two BWP systems discovered (PSRs B1957+20 and J2051−0827) were assumed to be representative for this class of pulsars. With several new black widow systems added to this population in the last decade, there are now several systems known that do not show these typical orbital variations or radio eclipses. We present timing solutions using 7–8 yr of observations from four of the European Pulsar Timing Array telescopes for PSRs J0023+0923, J2214+3000, and J2234+0944, and confirm that two of these systems do not show any significant orbital variability over our observing time span, both in terms of secular or orbital parameters. The third pulsar PSR J0023+0923 shows orbital variability and we discuss the implications for the timing solution. Our results from the long-term timing of these pulsars provide several new or improved parameters compared to earlier works. We discuss our results regarding the stability of these pulsars, and the stability of the class of BWPs in general, in the context of the binary parameters, and discuss the potential of the Roche lobe filling factor of the companion star being an indicator for stability of these systems.
Publisher: American Astronomical Society
Date: 29-07-2013
Publisher: American Astronomical Society
Date: 02-01-2013
Publisher: American Astronomical Society
Date: 05-2022
Abstract: Pulsar H α bow shocks provide rare opportunities to constrain the energetics and kinematics of the relativistic pulsar wind. We have acquired optical imaging and integral field unit spectroscopy of the bow shock of the millisecond pulsar PSR J1959+2048, measuring the shock symmetry axis at a position angle = 213.2 ± 0.°2 and showing that this slow nonradiative shock has a broad-to-narrow line component ratio I b / I n = 4. The data show that the pulsar’s velocity lies 2.°2 out of the plane of the sky. Coupled with a new fit for its timing proper motion, giving μ tot = 30.05 mas yr −1 and a Very Long Baseline Array (VLBA) interferometric parallax measurement giving d = 2.57 − 0.77 + 1.84 kpc (90% range), we have unusually complete information on the pulsar kinematics. The bow shock constraints on the wind momentum flux imply that, at the best-fit parallax distance, the pulsar moment of inertia must be very large and/or the H α efficiency at its modest shock velocity must be very high.
Publisher: Oxford University Press (OUP)
Date: 20-04-2011
Publisher: Oxford University Press (OUP)
Date: 28-07-2014
Publisher: Oxford University Press (OUP)
Date: 02-03-2016
DOI: 10.1093/MNRAS/STW483
Publisher: Oxford University Press (OUP)
Date: 11-08-2014
Publisher: Oxford University Press (OUP)
Date: 12-05-2016
Publisher: Springer Science and Business Media LLC
Date: 14-08-2013
DOI: 10.1038/NATURE12499
Abstract: Earth's nearest candidate supermassive black hole lies at the centre of the Milky Way. Its electromagnetic emission is thought to be powered by radiatively inefficient accretion of gas from its environment, which is a standard mode of energy supply for most galactic nuclei. X-ray measurements have already resolved a tenuous hot gas component from which the black hole can be fed. The magnetization of the gas, however, which is a crucial parameter determining the structure of the accretion flow, remains unknown. Strong magnetic fields can influence the dynamics of accretion, remove angular momentum from the infalling gas, expel matter through relativistic jets and lead to synchrotron emission such as that previously observed. Here we report multi-frequency radio measurements of a newly discovered pulsar close to the Galactic Centre and show that the pulsar's unusually large Faraday rotation (the rotation of the plane of polarization of the emission in the presence of an external magnetic field) indicates that there is a dynamically important magnetic field near the black hole. If this field is accreted down to the event horizon it provides enough magnetic flux to explain the observed emission--from radio to X-ray wavelengths--from the black hole.
Publisher: Oxford University Press (OUP)
Date: 18-07-2016
Publisher: EDP Sciences
Date: 02-2022
DOI: 10.1051/0004-6361/202141121
Abstract: Context. Extracting precise pulse times of arrival (TOAs) and their uncertainties is the first and most fundamental step in high-precision pulsar timing. In the classical method, TOAs are derived from total intensity pulse profiles of pulsars via cross-correlation with an idealised 1D template of that profile. While a number of results have been presented in the literature that rely on the ever increasing sensitivity of these pulsar timing experiments, there is no consensus on the most reliable methods for creating TOAs, and, more importantly, on the associated TOA uncertainties for each scheme. Aims. We present a comprehensive comparison of TOA determination practices. We focus on creating timing templates, TOA determination methods, and the most useful TOA bandwidth. The aim is to present a possible approach towards TOA optimisation, the (partial) identification of an optimal TOA-creation scheme, and the demonstration of optimisation differences between pulsars and data sets. Methods. We compared the values of data-derived template profiles with analytic profiles and evaluated the three most commonly used template-matching methods. Finally, we studied the relation between timing precision and TOA bandwidth to identify any potential breaks in this relation. As a practical demonstration, we applied our selected methods to European Pulsar Timing Array data on three test pulsars, PSRs J0218+4232, J1713+0747, and J2145−0750. Results. Our demonstration shows that data-derived and smoothed templates are typically preferred to some more commonly applied alternatives. The template-matching method called Fourier domain with Markov chain Monte Carlo is generally superior to or competitive with other methods. While the optimal TOA bandwidth is strongly dependent on pulsar brightness, telescope sensitivity, and scintillation properties, some significant frequency averaging seems required for the data we investigated.
Publisher: Oxford University Press (OUP)
Date: 21-12-2012
DOI: 10.1093/MNRAS/STS449
Publisher: American Astronomical Society
Date: 10-2020
Abstract: We report the discovery of 1.97 ms period gamma-ray pulsations from the 75 minute orbital-period binary pulsar now named PSR J1653−0158. The associated Fermi Large Area Telescope gamma-ray source 4FGL J1653.6−0158 has long been expected to harbor a binary millisecond pulsar. Despite the pulsar-like gamma-ray spectrum and candidate optical/X-ray associations—whose periodic brightness modulations suggested an orbit—no radio pulsations had been found in many searches. The pulsar was discovered by directly searching the gamma-ray data using the GPU-accelerated Einstein@Home distributed volunteer computing system. The multidimensional parameter space was bounded by positional and orbital constraints obtained from the optical counterpart. More sensitive analyses of archival and new radio data using knowledge of the pulsar timing solution yield very stringent upper limits on radio emission. Any radio emission is thus either exceptionally weak, or eclipsed for a large fraction of the time. The pulsar has one of the three lowest inferred surface magnetic-field strengths of any known pulsar with B surf ≈ 4 × 10 7 G. The resulting mass function, combined with models of the companion star’s optical light curve and spectra, suggests a pulsar mass ≳2 M ⊙ . The companion is lightweight with mass ∼0.01 M ⊙ , and the orbital period is the shortest known for any rotation-powered binary pulsar. This discovery demonstrates the Fermi Large Area Telescope's potential to discover extreme pulsars that would otherwise remain undetected.
Publisher: EDP Sciences
Date: 10-2023
Publisher: EDP Sciences
Date: 11-2022
DOI: 10.1051/0004-6361/202244171
Abstract: Context. The large number of ongoing surveys for pulsars and transients at various radio observatories is motivated by the science obtained from these sources. Timing and polarisation analysis of relativistic binaries can place strong constraints on theories of gravity. The observation of a growing number of millisecond pulsars (MSPs) spread over the celestial sphere may allow the detection of a stochastic gravitational wave background arising from supermassive black hole binaries. A more complete s le of young pulsars improves our knowledge of neutron star birth and evolution. Transients such as fast radio bursts can serve to probe the intergalactic medium. Aims. The SPAN512 pulsar survey covers intermediate Galactic latitudes using the L -band receiver of the Nançay Radio Telescope (NRT). The survey covers 224 sq. deg. of the sky for a total exposure time of 2200 h. Population syntheses predict the discovery of 3 to 19 new normal pulsars and a few MSPs. Methods. We present detailed modelling of the NRT beam with its L -band receiver and its sensitivity which we used to precisely assess the expected survey yield. We used the flexible Pulsar Arecibo L -band Feed Array data processing pipeline to search the 47 TB of SPAN512 data for pulsars and transients. Results. The SPAN512 survey discovered two new MSPs and one new middle-aged pulsar. We focus on the analysis of the 2.4-ms spin period pulsar J2205+6012 for which we also report the detection of gamma-ray pulsations. Its narrow pulse width (35µs at an observing frequency of 2.55 GHz) allows for sub-microsecond timing precision over 8 yr, with exciting prospects for pulsar timing array programs.
Publisher: EDP Sciences
Date: 10-2023
Publisher: EDP Sciences
Date: 28-08-2023
Publisher: EDP Sciences
Date: 10-2023
Publisher: American Astronomical Society
Date: 07-04-2017
Publisher: American Astronomical Society
Date: 07-2021
Publisher: American Astronomical Society
Date: 19-09-2014
Publisher: American Astronomical Society
Date: 09-2021
Publisher: Oxford University Press (OUP)
Date: 06-01-2023
Abstract: More than 100 millisecond pulsars (MSPs) have been discovered in radio observations of gamma-ray sources detected by the Fermi Large Area Telescope (LAT), but hundreds of pulsar-like sources remain unidentified. Here, we present the first results from the targeted survey of Fermi-LAT sources being performed by the Transients and Pulsars with MeerKAT (TRAPUM) Large Survey Project. We observed 79 sources identified as possible gamma-ray pulsar candidates by a Random Forest classification of unassociated sources from the 4FGL catalogue. Each source was observed for 10 min on two separate epochs using MeerKAT’s L-band receiver (856–1712 MHz), with typical pulsed flux density sensitivities of $\\sim 100\\, \\mu$Jy. Nine new MSPs were discovered, eight of which are in binary systems, including two eclipsing redbacks and one system, PSR J1526−2744, that appears to have a white dwarf companion in an unusually compact 5 h orbit. We obtained phase-connected timing solutions for two of these MSPs, enabling the detection of gamma-ray pulsations in the Fermi-LAT data. A follow-up search for continuous gravitational waves from PSR J1526−2744 in Advanced LIGO data using the resulting Fermi-LAT timing ephemeris yielded no detection, but sets an upper limit on the neutron star ellipticity of 2.45 × 10−8. We also detected X-ray emission from the redback PSR J1803−6707 in data from the first eROSITA all-sky survey, likely due to emission from an intrabinary shock.
Publisher: Elsevier BV
Date: 10-2017
Publisher: American Astronomical Society
Date: 06-09-2012
Publisher: Oxford University Press (OUP)
Date: 25-01-2013
DOI: 10.1093/MNRAS/STS657
Publisher: Oxford University Press (OUP)
Date: 28-04-2011
Publisher: Oxford University Press (OUP)
Date: 22-08-2012
Publisher: Oxford University Press (OUP)
Date: 31-08-2015
Publisher: Oxford University Press (OUP)
Date: 07-03-2016
DOI: 10.1093/MNRAS/STW179
Publisher: EDP Sciences
Date: 10-2014
Publisher: Oxford University Press (OUP)
Date: 23-10-2019
Abstract: 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 Nançay 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 $\\dot{P}$ are, on average, low, with PSR J0815+4611 showing $\\dot{P} \\sim 4\\times 10^{-18}$. 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 s le, 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.
Publisher: American Astronomical Society
Date: 28-02-2013
Publisher: American Astronomical Society
Date: 09-2021
Publisher: EDP Sciences
Date: 08-2022
DOI: 10.1051/0004-6361/202142552
Abstract: Context. Interstellar scintillation analysis of pulsars allows us to probe the small-scale distribution and inhomogeneities of the ionized interstellar medium. From the frequency scale of scintillation, one can estimate the geometric time delays from multipath propagation, a source of (typically) unmodeled, correlated noise in pulsar timing. Interstellar scintillation analysis of well-timed pulsars is useful to quantify the effects of time delays and may lead to improved timing precision, enhancing the probability of detecting gravitational waves. Aims. Our priority is to present the data set and the basic measurements of scintillation parameters of pulsars, employing long-term scintillation observations carried out from 2011 January to 2020 August by the European Pulsar Timing Array radio telescopes in the 21-cm and 11-cm bands. Additionally, we aim to identify future possible lines of study using this long-term scintillation data set. Methods. The autocorrelation function of dynamic spectra has been used to estimate the scintillation bandwidth v d and scintillation timescale τ d . Results. We present the long-term time series of v d and τ d for 13 pulsars. Sanity checks and comparisons indicate that the scintillation parameters of our work and previously published works are mostly consistent. For two pulsars, PSRs J1857+0943 and J1939+2134, we were able to obtain measurements of the v d at both bands, which allowed us to derive the time series of frequency scaling indices with a mean and a standard deviation of 2.82 ± 1.95 and 3.18 ± 0.60, respectively. We found some interesting features which will be studied in more detail in subsequent papers in this series: (i) in the time series of PSR J1939+2134, where v d and τ d sharply decrease associated with a sharp increase in the dispersion measure (ii) PSR J0613-0200 and PSR J0636+5126 show a strong annual variation in the time series of the τ d and (iii) PSR J1939+2134 shows a weak anticorrelation between the scintillation timescale and the dispersion in Westerbork Synthesis Radio Telescope data.
Publisher: EDP Sciences
Date: 08-2021
DOI: 10.1051/0004-6361/202039339
Abstract: Context. NenuFAR (New extension in Nançay upgrading LOFAR) is a new radio telescope developed and built on the site of the Nançay Radio Observatory. It is designed to observe the largely unexplored frequency window from 10 to 85 MHz, offering a high sensitivity across its full bandwidth. NenuFAR has started its “early science” operation in July 2019, with 58% of its final collecting area. Aims. Pulsars are one of the major phenomena utilized in the scientific exploitation of this frequency range and represent an important challenge in terms of instrumentation. Designing instrumentation at these frequencies is complicated by the need to compensate for the effects of both the interstellar medium and the ionosphere on the observed signal. We have designed a dedicated backend and developed a complete pulsar observation and data analysis pipeline, which we describe in detail in the present paper, together with first science results illustrating the ersity of the pulsar observing modes. Methods. Our real-time pipeline LUPPI (Low frequency Ultimate Pulsar Processing Instrumentation) is able to cope with a high data rate and provide real-time coherent de-dispersion down to the lowest frequencies reached by NenuFAR (10 MHz). The full backend functionality is described, as the available pulsar observing modes (folded, single-pulse, waveform, and dynamic spectrum). Results. We also present some of the early science results of NenuFAR on pulsars: the detection of 12 millisecond pulsars (eight of which are detected for the first time below 100 MHz) a high-frequency resolution mapping of the PSR B1919+21 emission profile and a detailed observation of single-pulse substructures from PSR B0809+74 down to 16 MHz the high rate of giant-pulse emission from the Crab pulsar detected at 68.7 MHz (43 events per minute) and the illustration of the very good timing performance of the instrumentation, which allows us to study dispersion measure variations in great detail.
Publisher: American Astronomical Society
Date: 10-12-2013
Publisher: American Physical Society (APS)
Date: 22-07-2015
Publisher: American Astronomical Society
Date: 14-04-2011
Publisher: American Physical Society (APS)
Date: 13-12-2021
Publisher: American Astronomical Society
Date: 06-09-2012
Publisher: American Astronomical Society
Date: 23-12-2010
Publisher: American Astronomical Society
Date: 17-08-2011
Publisher: Oxford University Press (OUP)
Date: 15-02-2016
DOI: 10.1093/MNRAS/STW347
No related grants have been discovered for Ismaël Cognard.