arXiv subjects

Michael Kramer

Publications and source records attributed to Michael Kramer.

At least 55 records · Page 3Linked to original sources

arXiv2024

PSR J1227$-$6208 and its massive white dwarf companion: pulsar emission analysis, timing update and mass measurements

PSR J1227$-$6208 is a 34.53-ms recycled pulsar with a massive companion. This system has long been suspected to belong to the emerging class of massive recycled pulsar-ONeMg white dwarf systems such as PSR J2222$-$0137, PSR J1528$-$3146 and J1439$-$5501. Here we present an updated emission and timing analysis with more than 11 years of combined Parkes and MeerKAT data, including 19 hours of high-frequency data from the newly installed MeerKAT S-band receivers. We measure a scattering timescale of 1.22 ms at 1 GHz with a flat scattering index 3.33<$β$<3.62, and a mean flux density of 0.53-0.62 mJy at 1 GHz with a steep spectral index 2.06<$α$<2.35. Around 15% of the emission is linearly and circularly polarised, but the polarisation angle does not follow the rotating vector model. Thanks to the sensitivity of MeerKAT, we successfully measure a rate of periastron advance of 0.0171(11) deg/yr, and a Shapiro delay with an orthometric amplitude of 3.6$\pm$0.5 $μ$s and an orthometric shape of 0.85$\pm$0.05. The main source of uncertainty in our timing analysis is chromatic correlated dispersion measure noise, which we model as a power law in the Fourier space thanks to the large frequency coverage provided by the Parkes UWL receiver. Assuming general relativity and accounting for the measurements across all the implemented timing noise models, the total mass, companion mass, pulsar mass and inclination angle are constrained at 2.3<Mt/$M_\odot$<3.2, 1.21<Mc/$M_\odot$<1.47, 1.16<Mp/$M_\odot$<1.69 and 77.5<i/deg<80.3. We also constrain the longitude of ascending node to either 266$\pm$78 deg or 86$\pm$78 deg. We argue against a neutron star nature of the companion based on the very low orbital eccentric of the system (e=1.15e-3), and instead classify the companion of PSR J1227-6208 as a rare, massive ONeMg white dwarf close to the Chandrasekhar limit.

astro-ph.HE↗

arXiv2024

The Relativistic Spin Precession in the Compact Double Neutron Star System PSR~J1946+2052

We observe systematic profile changes in the visible pulsar of the compact double neutron star system PSR~J1946+2052 using observations with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The interpulse of PSR~J1946+2052 changed from single-peak to double-peak shape from 2018 to 2021. We attribute this evolution as the result of the relativistic spin precession of the pulsar. With the high sensitivity of FAST, we also measure significant polarization for the first time, allowing us to model this with the precessional rotating vector model. Assuming, to the first order, a circular hollow-cone-like emission beam pattern and taking the validity of general relativity, we derive the binary's orbital inclination angle (${63^\circ}^{+5^\circ}_{-3^\circ}$) and pulsar's spin geometry. Pulsar's spin vector and the orbital angular momentum vector are found to be only slightly misaligned (${0.21^\circ}^{+0.28^\circ}_{-0.10^\circ}$).The quoted uncertainties do not reflect the systematic uncertainties introduced by our model assumptions. By simulating future observations of profile and polarization evolution, we estimate that we could constrain the precession rate within a $43\%$ uncertainty in 9 years. Hence, we suggest that the system's profile evolution could be combined with precise pulsar timing to test general relativity in the future.

astro-ph.HE↗

arXiv2024

Radio Pulse Profile Evolution of Magnetar Swift J1818.0-1607

The shape and polarisation properties of the radio pulse profiles of radio-loud magnetars provide a unique opportunity to investigate their magnetospheric properties. Gaussian Process Regression analysis was used to investigate the variation in the total intensity shape of the radio pulse profiles of the magnetar Swift J1818.0-1607. The observed profile shape was found to evolve through three modes between MJDs 59104 and 59365. The times at which these transitions occurred coincided with changes in the amplitude of modulations in the spin-down rate. The amount of linear and circular polarisation was also found to vary significantly with time. Lomb-Scargle periodogram analysis of the spin-down rate revealed three possibly harmonically related frequencies. This could point to the magnetar experiencing seismic activity. However, no profile features exhibited significant periodicity, suggesting no simple correlations between the profile variability and fluctuations of the spin-down on shorter timescales within the modes. Overall, this implies the mode changes seen are a result of local magnetospheric changes, with other theories, such as precession, less able to explain these observations.

astro-ph.HE↗

arXiv2024

A pulsar in a binary with a compact object in the mass gap between neutron stars and black holes

Among the compact objects observed in gravitational wave merger events a few have masses in the gap between the most massive neutron stars (NSs) and least massive black holes (BHs) known. Their nature and the formation of their merging binaries are not well understood. We report on pulsar timing observations using the Karoo Array Telescope (MeerKAT) of PSR J0514-4002E, an eccentric binary millisecond pulsar in the globular cluster NGC 1851 with a total binary mass of $3.887 \pm 0.004$ solar masses. The companion to the pulsar is a compact object and its mass (between $2.09$ and $2.71$ solar masses, 95% confidence interval) is in the mass gap, so it either is a very massive NS or a low-mass BH. We propose the companion was formed by a merger between two earlier NSs.

astro-ph.HE↗

arXiv2024

A Gaussian-processes approach to fitting for time-variable spherical solar wind in pulsar timing data

Propagation effects are one of the main sources of noise in high-precision pulsar timing. For pulsars below an ecliptic latitude of $5^\circ$, the ionised plasma in the solar wind can introduce dispersive delays of order 100 microseconds around solar conjunction at an observing frequency of 300 MHz. A common approach to mitigate this assumes a spherical solar wind with a time-constant amplitude. However, this has been shown to be insufficient to describe the solar wind. We present a linear, Gaussian-process piecewise Bayesian approach to fit a spherical solar wind of time-variable amplitude, which has been implemented in the pulsar software run_enterprise. Through simulations, we find that the current EPTA+InPTA data combination is not sensitive to such variations; however, solar wind variations will become important in the near future with the addition of new InPTA data and data collected with the low-frequency LOFAR telescope. We also compare our results for different high-precision timing datasets (EPTA+InPTA, PPTA, and LOFAR) of three millisecond pulsars (J0030$+$0451, J1022$+$1001, J2145$-$0450), and find that the solar-wind amplitudes are generally consistent for any individual pulsar, but they can vary from pulsar to pulsar. Finally, we compare our results with those of an independent method on the same LOFAR data of the three millisecond pulsars. We find that differences between the results of the two methods can be mainly attributed to the modelling of dispersion variations in the interstellar medium, rather than the solar wind modelling.

astro-ph.HE↗

arXiv2024

MeerKAT Pulsar Timing Array parallaxes and proper motions

We have determined positions, proper motions, and parallaxes of $77$ millisecond pulsars (MSPs) from $\sim3$ years of MeerKAT radio telescope observations. Our timing and noise analyses enable us to measure $35$ significant parallaxes ($12$ of them for the first time) and $69$ significant proper motions. Eight pulsars near the ecliptic have an accurate proper motion in ecliptic longitude only. PSR~J0955$-$6150 has a good upper limit on its very small proper motion ($<$0.4 mas yr$^{-1}$). We used pulsars with accurate parallaxes to study the MSP velocities. This yields $39$ MSP transverse velocities, and combined with MSPs in the literature (excluding those in Globular Clusters) we analyse $66$ MSPs in total. We find that MSPs have, on average, much lower velocities than normal pulsars, with a mean transverse velocity of only $78(8)$ km s$^{-1}$ (MSPs) compared with $246(21)$ km s$^{-1}$ (normal pulsars). We found no statistical differences between the velocity distributions of isolated and binary millisecond pulsars. From Galactocentric cylindrical velocities of the MSPs, we derive 3-D velocity dispersions of $σ_ρ$, $σ_ϕ$, $σ_{z}$ = $63(11)$, $48(8)$, $19(3)$ km s$^{-1}$. We measure a mean asymmetric drift with amplitude $38(11)$ km s$^{-1}$, consistent with expectation for MSPs, given their velocity dispersions and ages. The MSP velocity distribution is consistent with binary evolution models that predict very few MSPs with velocities $>300$ km s$^{-1}$ and a mild anticorrelation of transverse velocity with orbital period.

astro-ph.HE↗

arXiv2023

Quasi-periodic sub-pulse structure as a unifying feature for radio-emitting neutron stars

Magnetars are highly-magnetised rotating neutron stars that are predominantly observed as high-energy sources. Six of this class of neutron star are known to also emit radio emission, and magnetars are, thus, a favoured model for the origin for at least some of the Fast Radio Bursts (FRBs). If magnetars, or neutron stars in general, are indeed responsible, sharp empirical constraints on the mechanism producing radio emission are required. Here we report on the detection of polarised quasi-periodic sub-structure in the emission of all well-studied radio-detected magnetars. A correlation previously seen, relating sub-structure in pulsed emission of radio emitting neutron stars to their rotational period, is extended, and shown to now span more than six of orders of magnitude in pulse period. This behaviour is not only seen in magnetars but in members of all classes of radio-emitting rotating neutron stars, regardless of their evolutionary history, their power source or their inferred magnetic field strength. If magnetars are responsible for FRBs, it supports the idea of being able to infer underlying periods from sub-burst timescales in FRBs.

astro-ph.HE↗

arXiv2023

FRB 20210405I: a nearby Fast Radio Burst localised to sub-arcsecond precision with MeerKAT

We present the first sub-arcsecond localised Fast Radio Burst (FRB) detected using MeerKAT. FRB 20210405I was detected in the incoherent beam using the MeerTRAP pipeline on 2021 April 05 with a signal to noise ratio of 140.8 and a dispersion measure of 565.17 pc cm$^{-3}$. It was detected while MeerTRAP was observing commensally with the ThunderKAT large survey project, and was sufficiently bright that we could use the ThunderKAT 8s images to localise the FRB. Two different models of the dispersion measure in the Milky Way and halo suggest that the source is either right at the edge of the Galaxy, or outside. This highlights the uncertainty in the Milky Way dispersion measure models, particularly in the Galactic Plane, and the uncertainty of Milky Way halo models. Further investigation and modelling of these uncertainties will be facilitated by future detections and localisations of nearby FRBs. We use the combined localisation, dispersion measure, scattering, specific luminosity and chance coincidence probability information to find that the origin is most likely extra-galactic and identify the likely host galaxy of the FRB: 2MASS J1701249$-$4932475. Using SALT spectroscopy and archival observations of the field, we find that the host is a disk/spiral galaxy at a redshift of $z=0.066$.

astro-ph.HE↗

arXiv2023

A search for pulsars around Sgr A* in the first Event Horizon Telescope dataset

The Event Horizon Telescope (EHT) observed in 2017 the supermassive black hole at the center of the Milky Way, Sagittarius A* (Sgr A*), at a frequency of 228.1 GHz ($λ$=1.3 mm). The fundamental physics tests that even a single pulsar orbiting Sgr A* would enable motivate searching for pulsars in EHT datasets. The high observing frequency means that pulsars - which typically exhibit steep emission spectra - are expected to be very faint. However, it also negates pulse scattering, an effect that could hinder pulsar detections in the Galactic Center. Additionally, magnetars or a secondary inverse Compton emission could be stronger at millimeter wavelengths than at lower frequencies. We present a search for pulsars close to Sgr A* using the data from the three most-sensitive stations in the EHT 2017 campaign: the Atacama Large Millimeter/submillimeter Array, the Large Millimeter Telescope and the IRAM 30 m Telescope. We apply three detection methods based on Fourier-domain analysis, the Fast-Folding-Algorithm and single pulse search targeting both pulsars and burst-like transient emission; using the simultaneity of the observations to confirm potential candidates. No new pulsars or significant bursts were found. Being the first pulsar search ever carried out at such high radio frequencies, we detail our analysis methods and give a detailed estimation of the sensitivity of the search. We conclude that the EHT 2017 observations are only sensitive to a small fraction ($\lesssim$2.2%) of the pulsars that may exist close to Sgr A*, motivating further searches for fainter pulsars in the region.

astro-ph.HE↗

arXiv2023

Galactic Interstellar Scintillation Observed from Four Globular Cluster Pulsars by FAST

We report detections of scintillation arcs for pulsars in globular clusters M5, M13 and M15 for the first time using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). From observations of these arcs at multiple epochs, we infer that screen-like scattering medium exists at distances $4.1_{-0.3}^{+0.2}$ kpc, $6.7_{-0.2}^{+0.2}$ kpc and $1.3_{-1.0}^{+0.7}$ kpc from Earth in the directions of M5, M13 and M15, respectively. This means M5's and M13's scattering screens are located at $3.0_{-0.2}^{+0.1}$ kpc and $4.4_{-0.1}^{+0.1}$ kpc above the galactic plane, whereas, M15's is at $0.6_{-0.5}^{+0.3}$ kpc below the plane. We estimate the scintillation timescale and decorrelation bandwidth for each pulsar at each epoch using the one-dimensional auto-correlation in frequency and time of the dynamic spectra. We found that the boundary of the Local Bubble may have caused the scattering of M15, and detected the most distant off-plane scattering screens to date through pulsar scintillation, which provides evidence for understanding the medium circulation in the Milky Way.

astro-ph.GA↗

arXiv2023

The Third Fermi Large Area Telescope Catalog of Gamma-ray Pulsars

We present 294 pulsars found in GeV data from the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope. Another 33 millisecond pulsars (MSPs) discovered in deep radio searches of LAT sources will likely reveal pulsations once phase-connected rotation ephemerides are achieved. A further dozen optical and/or X-ray binary systems co-located with LAT sources also likely harbor gamma-ray MSPs. This catalog thus reports roughly 340 gamma-ray pulsars and candidates, 10% of all known pulsars, compared to $\leq 11$ known before Fermi. Half of the gamma-ray pulsars are young. Of these, the half that are undetected in radio have a broader Galactic latitude distribution than the young radio-loud pulsars. The others are MSPs, with 6 undetected in radio. Overall, >235 are bright enough above 50 MeV to fit the pulse profile, the energy spectrum, or both. For the common two-peaked profiles, the gamma-ray peak closest to the magnetic pole crossing generally has a softer spectrum. The spectral energy distributions tend to narrow as the spindown power $\dot E$ decreases to its observed minimum near $10^{33}$ erg s$^{-1}$, approaching the shape for synchrotron radiation from monoenergetic electrons. We calculate gamma-ray luminosities when distances are available. Our all-sky gamma-ray sensitivity map is useful for population syntheses. The electronic catalog version provides gamma-ray pulsar ephemerides, properties and fit results to guide and be compared with modeling results.

astro-ph.HE↗

arXiv2023

High-altitude Magnetospheric Emissions from Two Pulsars

We discover three new weak pulse components in two known pulsars, one in PSR J0304+1932 and two in PSR J1518+4904. These components are emitted about half way between the main emission beam and the interpulse beam (beam from the opposite pole). They are separated from their main pulse peak by $99^{\circ}\pm{3}^{\circ}$ for J0304+1932, $123^{\circ}.6\pm{0^{\circ}.7}$ (leading) and $93^{^{\circ}}\pm 0^{\circ}.4$ (trailing) for J1518+4904, respectively. Their peak-intensity ratios to main pulses are: $\sim$ 0.06% for J0304+1932, $\sim$ 0.17% and $\sim$ 0.83% for J1518+4904. We also analyzed flux fluctuation and profile variation of the emissions for two pulsars. The results show correlations between the weak pulses and their main pulses, indicating that these emissions come from the same pole. We estimated the emission altitude of these weak pulses and derived a height of about half of the pulsar's light-cylinder radius. These pulse components are a unique sample of high-altitude emissions from pulsars, and challenge the current pulsar emission models.

astro-ph.HE↗

arXiv2023

BlackHoleCam -- Testing general relativity with pulsars orbiting Sagittarius A*

BlackHoleCam is a project funded by a European Research Council Synergy Grant to build a complete astrophysical description of nearby supermassive black holes by using a combination of radio imaging, pulsar observations, stellar astrometry and general relativistic magneto-hydrodynamic models. BlackHoleCam scientists are active partners of the Event Horizon Telescope Consortium. In this talk I will discuss the use of pulsars orbiting Sagittarius A* for tests of General Relativity, the current difficulties in detecting such sources, recent results from the Galactic Centre magnetar PSR J1745-2900 and how BlackHoleCam aims to search for undiscovered pulsars in the Galactic Centre.

astro-ph.HE↗

arXiv2023

Pulsar Scintillation Studies with LOFAR: II. Dual-frequency scattering study of PSR J0826+2637 with LOFAR and NenuFAR

Interstellar scattering (ISS) of radio pulsar emission can be used as a probe of the ionised interstellar medium (IISM) and causes corruptions in pulsar timing experiments. Two types of ISS phenomena (intensity scintillation and pulse broadening) are caused by electron density fluctuations on small scales (< 0.01 AU). Theory predicts that these are related, and both have been widely employed to study the properties of the IISM. Larger scales ($\sim$1-100\,AU) cause measurable changes in dispersion and these can be correlated with ISS observations to estimate the fluctuation spectrum over a very wide scale range. IISM measurements can often be modeled by a homogeneous power-law spatial spectrum of electron density with the Kolmogorov ($-11/3$) spectral exponent. Here we aim to test the validity of using the Kolmogorov exponent with PSR~J0826+2637. We do so using observations of intensity scintillation, pulse broadening and dispersion variations across a wide fractional bandwidth (20 -- 180\,MHz). We present that the frequency dependence of the intensity scintillation in the high frequency band matches the expectations of a Kolmogorov spectral exponent but the pulse broadening in the low frequency band does not change as rapidly as predicted with this assumption. We show that this behavior is due to an inhomogeneity in the scattering region, specifically that the scattering is dominated by a region of transverse size $\sim$40\,AU. The power spectrum of the electron density, however, maintains the Kolmogorov spectral exponent from spatial scales of 5$\times10^{-6}$\,AU to $\sim$100\,AU.

astro-ph.HE↗

arXiv2023

Missing for 20 years: MeerKAT re-detects the elusive binary pulsar M30B

PSR J2140$-$2311B is a 13-ms pulsar discovered in 2001 in a 7.8-hour Green Bank Telescope (GBT) observation of the core-collapsed globular cluster M30 and predicted to be in a highly eccentric binary orbit. This pulsar has eluded detection since then, therefore its precise orbital parameters have remained a mystery until now. In this work, we present the confirmation of this pulsar using observations taken with the UHF receivers of the MeerKAT telescope as part of the TRAPUM Large Survey Project. Taking advantage of the beamforming capability of our backends, we have localized it, placing it $1.2(1)^\prime$ from the cluster centre. Our observations have enabled the determination of its orbit: it is highly eccentric ($e = 0.879$) with an orbital period of $6.2$ days. We also measured the rate of periastron advance, $\dotω = 0.078 \pm 0.002\, \rm deg \, yr^{-1}$. Assuming that this effect is fully relativistic, general relativity provides an estimate of the total mass of the system, $M_{\rm TOT} = 2.53 \pm 0.08$ M$_{\odot}$, consistent with the lightest double neutron star systems known. Combining this with the mass function of the system gives the pulsar and companion masses of $m_p < 1.43 \, \rm M_{\odot}$ and $m_c > 1.10 \, \rm M_{\odot}$ respectively. The massive, undetected companion could either be a massive WD or a NS. M30B likely formed as a result of a secondary exchange encounter. Future timing observations will allow the determination of a phase-coherent timing solution, vastly improving our uncertainty in $\dotω$ and likely enabling the detection of additional relativistic effects which will determine $m_p$ and $m_c$.

astro-ph.HE↗

arXiv2022

The MeerKAT Pulsar Timing Array: First Data Release

We present the first 2.5 years of data from the MeerKAT Pulsar Timing Array (MPTA), part of MeerTime, a MeerKAT Large Survey Project. The MPTA aims to precisely measure pulse arrival times from an ensemble of 88 pulsars visible from the Southern Hemisphere, with the goal of contributing to the search, detection and study of nanohertz-frequency gravitational waves as part of the International Pulsar Timing Array. This project makes use of the MeerKAT telescope, and operates with a typical observing cadence of two weeks using the L-band receiver that records data from 856-1712 MHz. We provide a comprehensive description of the observing system, software, and pipelines used and developed for the MeerTime project. The data products made available as part of this data release are from the 78 pulsars that had at least $30$ observations between the start of the MeerTime programme in February 2019 and October 2021. These include both sub-banded and band-averaged arrival times, as well as the initial timing ephemerides, noise models, and the frequency-dependent standard templates (portraits) used to derive pulse arrival times. After accounting for detected noise processes in the data, the frequency-averaged residuals of $67$ of the pulsars achieved a root-mean-square residual precision of $< 1 μ\rm{s}$. We also present a novel recovery of the clock correction waveform solely from pulsar timing residuals, and an exploration into preliminary findings of interest to the international pulsar timing community. The arrival times, standards and full Stokes parameter calibrated pulsar timing archives are publicly available.

astro-ph.HE↗

arXiv2022

Searches for Shapiro delay in seven binary pulsars using the MeerKAT telescope

Precision timing of millisecond pulsars in binary systems enables observers to detect the relativistic Shapiro delay induced by space time curvature. When favourably aligned, this enables constraints to be placed on the component masses and system orientation. Here we present the results of timing campaigns on seven binary millisecond pulsars observed with the 64-antenna MeerKAT radio telescope that show evidence of Shapiro delay: PSRs~J0101$-$6422, J1101$-$6424, J1125$-$6014, J1514$-$4946, J1614$-$2230, J1732$-$5049, and J1909$-$3744. Evidence for Shapiro delay was found in all of the systems, and for three the orientations and data quality enabled strong constraints on their orbital inclinations and component masses. For PSRs~J1125$-$6014, J1614$-$2230 and J1909$-$3744, we determined pulsar masses to be $M_{\rm p} = 1.68\pm 0.17 \, {\rm M_{\odot}} $, $1.94\pm 0.03 \, {\rm M_{\odot}} $ and $1.45 \pm 0.03 \, {\rm M_{\odot}}$, and companion masses to be $M_{\rm c} = 0.33\pm 0.02 \, {\rm M_{\odot}} $, $0.495\pm 0.005 \, {\rm M_{\odot}} $ and $0.205 \pm 0.003 \, {\rm M_{\odot}}$, respectively. This provides the first independent confirmation of PSR~J1614$-$2230's mass, one of the highest known. The Shapiro delays measured for PSRs~J0101$-$6422, J1101$-$6424, J1514$-$4946, and J1732$-$5049 were only weak, and could not provide interesting component mass limits. Despite a large number of millisecond pulsars being routinely timed, relatively few have accurate masses via Shapiro delays. We use simulations to show that this is expected, and provide a formula for observers to assess how accurately a pulsar mass can be determined. We also discuss the observed correlation between pulsar companion masses and spin period, and the anti-correlation between recycled pulsar mass and their companion masses.

astro-ph.HE↗

arXiv2022

The Thousand-Pulsar-Array programme on MeerKAT XI: Application of the rotating vector model

In spite of the rich phenomenology of the polarization properties of radio pulsars, the rotating vector model (RVM) created 50 years ago remains the best method to determine the beam geometry of a pulsar. We apply the RVM to a sample of 854 radio pulsars observed with the MeerKAT telescope in order to draw conclusions about the population of pulsars as a whole. The main results are that (i) the geometrical interpretation of the position angle traverse is valid in the majority of the population, (ii) the pulsars for which the RVM fails tend to have a high fraction of circular polarization compared to linear polarization, (iii) emission heights obtained through both geometrical and relativistic methods show that the majority of pulsars must have emission heights less than 1000~km independent of spin period, (iv) orthogonal mode jumps are seen in the position angle traverse in about one third of the population. All these results are weakly dependent on the pulsar spin-down energy.

astro-ph.HE↗