arXiv subjects

Paulo C. C. Freire

Publications and source records attributed to Paulo C. C. Freire.

At least 55 records · Page 3Linked to original sources

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

Pulse Profiles and Polarization of Terzan 5 Pulsars

Terzan 5 is a rich globular cluster within the galactic bulge that contains 39 known millisecond pulsars, the largest known population of any globular cluster. The Terzan 5 pulsars are faint, so that individual observations of most of the pulsars have too little signal-to-noise (S/N) to measure reliable flux density or polarization information. We combined over 5.2\,days of archival data, at each of 1.5\,GHz and 2.0\,GHz, taken with the Green Bank Telescope over the past 11\,years. We created high S/N profiles for 32 of the pulsars and determined precise rotation measures (RMs) for 28 of them. We used the RMs, and the known pulsar positions and dispersion measures (DMs), to map the projected parallel component of the Galactic magnetic field toward the cluster. The $\langle B_{||}\rangle$ shows a rough gradient of $\sim$6\,nG/arcsec ($\sim$160\,nG/parsec), or fractionally, a change of $\sim$20$\%$ in the right ascension direction across the cluster, implying Galactic magnetic field variability at sub-parsec scales. We also measured average flux densities $S_ν$ for the pulsars, ranging from $\sim$10\,$μ$Jy to $\sim$2\,mJy, and an average spectral index $α= -1.35$, where $S_ν\propto ν^α)$. This spectral index is flatter than most known pulsars, likely a selection effect due to the high frequencies used in pulsar searches to mitigate dispersion and scattering. The inferred pulsar luminosity function is roughly power-law, with slope $(d\log N)/(d\log L) = -1$ at the high-luminosity end. At the low-luminosity end, there are incompleteness effects implying that Terzan 5 contains many more pulsars to be found.

astro-ph.HE↗

arXiv2022

Tests of gravity theories with pulsar timing

Over the last few years, a set of new results from pulsar timing has introduced much tighter constraints on violations of the strong equivalence principle (SEP), either via a direct verification of the universality of free fall for a pulsar in a triple star system, or from tests of the nature of gravitational waves, in particular a search for dipolar gravitational wave emission in a variety of binary pulsars with different masses. No deviations from the SEP have been detected in our experiments. These results introduce some of the most stringent constraints on several classes of alternative theories of gravity and complement recent results from the ground-based gravitational wave detectors.

gr-qc↗

arXiv2022

Closing a spontaneous-scalarization window with binary pulsars

Benefitting from the unequaled precision of the pulsar timing technique, binary pulsars are important testbeds of gravity theories, providing some of the tightest bounds on alternative theories of gravity. One class of well-motivated alternative gravity theories, the scalar-tensor gravity, predict large deviations from general relativity for neutron stars through a nonperturbative phenomenon known as spontaneous scalarization. This effect, which cannot be tested in the Solar System, can now be tightly constrained using the latest results from the timing of a set of 7 binary pulsars (PSRs J0348+0432, J0737$-$3039A, J1012+5307, J1738+0333, J1909$-$3744, J1913+1102, and J2222$-$0137), especially with the updated parameters of PSRs J0737$-$3039A, J1913+1102, and J2222$-$0137. Using new timing results, we constrain the neutron star's effective scalar coupling, which describes how strongly neutron stars couple to the scalar field, to a level of $\left|α_{\rm A}\right| \lesssim 6 \times 10^{-3}$ in a Bayesian analysis. Our analysis is thorough, in the sense that our results apply to all neutron star masses and all reasonable equations of state of dense matters, in the full relevant parameter space. It excludes the possibility of spontaneous scalarization of neutron stars, at least within a class of scalar-tensor gravity theories.

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arXiv2021

New searches for continuous gravitational waves from seven fast pulsars

We conduct searches for continuous gravitational waves from seven pulsars, that have not been targeted in continuous wave searches of Advanced LIGO data before. We target emission at exactly twice the rotation frequency of the pulsars and in a small band around such frequency. The former search assumes that the gravitational wave quadrupole is changing phase-locked with the rotation of the pulsar. The search over a range of frequencies allows for differential rotation between the component emitting the radio signal and the component emitting the gravitational waves, for example the crust or magnetosphere versus the core. Timing solutions derived from the Arecibo 327-MHz Drift-Scan Pulsar Survey (AO327) observations are used. No evidence of a signal is found and upper limits are set on the gravitational wave amplitude. For one of the pulsars we probe gravitational wave intrinsic amplitudes just a factor of 3.8 higher than the spin-down limit, assuming a canonical moment of inertia of $10^{38}$ kg m$^2$. Our tightest ellipticity constraint is $1.5 \times 10^{-8}$, which is a value well within the range of what a neutron star crust could support.

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arXiv2020

Precise mass measurements for the double neutron star system J1829+2456

PSR J1829+2456 is a radio pulsar in a relativistic binary system with another neutron star. It has a rotational period of 41 ms and a mildly eccentric ($e = 0.14$) 28-hr orbit. We have continued its observations with the Arecibo radio telescope and have now measured the individual neutron star masses of this system. The pulsar and companion masses are $1.306\,\pm\,0.007\,M_{\odot}$ and $1.299\,\pm\,0.007\,M_{\odot}$ (2$σ$ - 95% confidence, unless stated otherwise), respectively. We have also measured the proper motion for this system and used it to estimate a space velocity of 49$^{+77}_{-30}$ km s$^{-1}$ with respect to the local standard of rest. The relatively low values for companion mass, space velocity and orbital eccentricity in this system make it similar to other double neutron star systems in which the second-formed neutron star is thought to have formed in a low-kick, low mass-loss, symmetric supernova.

astro-ph.SR↗

arXiv2020

PSR J1641+3627F: a low-mass He white dwarf orbiting a possible high-mass neutron star in the globular cluster M13

We report on the discovery of the companion star to the millisecond pulsar J1631+3627F in the globular cluster M13. By means of a combination of optical and near-UV high-resolution observations obtained with the Hubble Space Telescope, we identified the counterpart at the radio source position. Its location in the color-magnitude diagrams reveals that the companion star is a faint (V \sim 24.3) He-core white dwarf. We compared the observed companion magnitudes with those predicted by state-of-the-art binary evolution models and found out that it has a mass of 0.23 \pm 0.03 Msun, a radius of 0.033^+0.004_-0.005 Rsun and a surface temperature of 11500^+1900_-1300 K. Combining the companion mass with the pulsar mass function is not enough to determine the orbital inclination and the neutron star mass; however, the last two quantities become correlated: we found that either the system is observed at a low inclination angle, or the neutron star is massive. In fact, assuming that binaries are randomly aligned with respect to the observer line of sight, there is a \sim 70% of probability that this system hosts a neutron star more massive than 1.6 Msun. In fact, the maximum and median mass of the neutron star, corresponding to orbital inclination angles of 90 deg and 60 deg, are M_NS,max = 3.1 \pm 0.6 Msun and M_NS,med = 2.4 \pm 0.5 Msun, respectively. On the other hand, assuming also an empirical neutron star mass probability distribution, we found that this system could host a neutron star with a mass of 1.5 \pm 0.1 Msun if orbiting with a low-inclination angle around 40 deg.

astro-ph.HE↗

arXiv2019

Upgraded Giant Metrewave Radio Telescope timing of NGC 1851A: a possible millisecond pulsar-neutron star system

(abridged) In this work we present the results of one year of upgraded Giant Metrewave Radio Telescope timing measurements of PSR~J0514$-$4002A, a 4.99-ms pulsar in a 18.8-day, eccentric ($e \, =\, 0.89$) orbit with a massive companion located in the globular cluster NGC~1851. Combining these data with earlier Green Bank Telescope data, we greatly improve the precision of the rate of advance of periastron, $\dotω \, = \, 0.0129592(16)\, °\, \rm yr^{-1}$ which, assuming the validity of general relativity, results in a much refined measurement of the total mass of the binary, $M_{\rm tot} \, = \, 2.4730(6) \, M_{\odot}$. Additionally, we measure the Einstein delay parameter, $γ\,, = \, 0.0216(9) \, \rm s$. Furthermore, we measure the proper motion of the system ($μ_α \, = \, 5.19(22)$ and $μ_δ = -0.56(25)\rm~mas ~ yr^{-1}$), which is not only important for analyzing its motion in the cluster, but is also essential for a proper interpretation of $γ$, given the latter parameter's correlation with the variation of the projected semi-major axis. The measurements of $γ$ and the proper motion enable a separation of the system component masses: we obtain a pulsar mass of $M_{p} \, = \, 1.25^{+0.05}_{-0.06} \, M_{\odot}$ and a companion mass of $M_{c} \, = \, 1.22^{+0.06}_{-0.05} \, M_{\odot}$. This raises the possibility that the companion is also a neutron star. Searches for radio pulsations from the companion have thus far been unsuccessful, hence we cannot confirm the latter hypothesis. The low mass of this millisecond pulsar - one of the lowest ever measured for such objects - clearly indicates that the recycling process can be achieved with a relatively small amount of mass transfer.

astro-ph.HE↗

arXiv2018

An algorithm for determining the rotation count of pulsars

We present here a simple, systematic method for determining the correct global rotation count of a radio pulsar; an essential step for the derivation of an accurate phase-coherent ephemeris. We then build on this method by developing a new algorithm for determining the global rotational count for pulsars with sparse timing data sets. This makes it possible to obtain phase-coherent ephemerides for pulsars for which this has been impossible until now. As an example, we do this for PSR J0024-7205aa, an extremely faint MSP recently discovered in the globular cluster 47 Tucanae. This algorithm has the potential to significantly reduce the number of observations and the amount of telescope time needed to follow up on new pulsar discoveries.

astro-ph.IM↗

arXiv2017

Chandra studies of the globular cluster 47 Tucanae: A deeper X-ray source catalogue, five new X-ray counterparts to millisecond radio pulsars, and new constraints to r-mode instability window

We combined Chandra ACIS observations of the globular cluster 47 Tucanae (hereafter, 47 Tuc) from 2000, 2002, and 2014-15 to create a deeper X-ray source list, and study some of the faint radio millisecond pulsars (MSPs) present in this cluster. We have detected 370 X-ray sources within the half-mass radius (2$'$.79) of the cluster, 81 of which are newly identified, by including new data and using improved source detection techniques. The majority of the newly identified sources are in the crowded core region, indicating cluster membership. We associate five of the new X-ray sources with chromospherically active BY Dra or W UMa variables identified by Albrow et al. (2001). We present alternative positions derived from two methods, centroiding and image reconstruction, for faint, crowded sources. We are able to extract X-ray spectra of the recently discovered MSPs 47 Tuc aa, 47 Tuc ab, the newly timed MSP 47 Tuc Z, and the newly resolved MSPs 47 Tuc S and 47 Tuc F. Generally, they are well fit by black body or neutron star atmosphere models, with temperatures, luminosities and emitting radii similar to those of other known MSPs in 47 Tuc, though 47 Tuc aa and 47 Tuc ab reach lower X-ray luminosities. We limit X-ray emission from the full surface of the rapidly spinning (542 Hz) MSP 47 Tuc aa, and use this limit to put an upper bound for amplitude of r-mode oscillations in this pulsar as $α<2.5\times 10^{-9}$ and constrain the shape of the r-mode instability window.

astro-ph.HE↗

arXiv2017

A Massive-born Neutron Star with a Massive White Dwarf Companion

We report on the results of a 4-year timing campaign of PSR~J2222$-0137$, a 2.44-day binary pulsar with a massive white dwarf (WD) companion, with the Nançay, Effelsberg and Lovell radio telescopes. Using the Shapiro delay for this system, we find a pulsar mass $m_{p}=1.76,\pm\,0.06,M_\odot$ and a WD mass $m_{c}\,=\,1.293\,\pm\,0.025\, M_\odot$. We also measure the rate of advance of periastron for this system, which is marginally consistent with the GR prediction for these masses. The short lifetime of the massive WD progenitor star led to a rapid X-ray binary phase with little ($< \, 10^{-2} \, M_\odot$) mass accretion onto the neutron star (NS); hence, the current pulsar mass is, within uncertainties, its birth mass; the largest measured to date. We discuss the discrepancy with previous mass measurements for this system; we conclude that the measurements presented here are likely to be more accurate. Finally, we highlight the usefulness of this system for testing alternative theories of gravity by tightly constraining the presence of dipolar radiation. This is of particular importance for certain aspects of strong-field gravity, like spontaneous scalarization, since the mass of PSR~J2222$-0137$ puts that system into a poorly tested parameter range.

astro-ph.HE↗

arXiv2016

An eccentric binary millisecond pulsar with a helium white dwarf companion in the Galactic Field

Low-mass white dwarfs (LMWDs) are believed to be exclusive products of binary evolution, as the Universe is not yet old enough to produce them from single stars. Because of the strong tidal forces operating during the binary interaction phase, the remnant host systems observed today are expected to have negligible eccentricities. Here, we report on the first unambiguous identification of a LMWD in an eccentric (e=0.13) orbit with a millisecond pulsar, which directly contradicts this picture. We use our spectra and radio-timing solution (derived elsewhere) to infer the WD temperature T_eff = 8600 +/- 190 K) and 3D systemic velocity (179.5 km\s). We also place model-independent constraints on the WD radius (R_WD = 0.024+/- 0.004/0.002 R_sun) and surface gravity (log g = 7.11 +/- 0.08/0.16 dex). The WD and kinematic properties are consistent with the expectations for low-mass X-ray binary evolution and disfavour a three-body formation channel. In the case of the high eccentricity being the result of a spontaneous phase transition, we infer a mass of 1.6 M_sun for the progenitor of the pulsar, which is too low for the quark-nova mechanism proposed by Jiang et al. (2015). Similarly, the scenario of Freire & Tauris (2014), in which a WD collapses onto a neutron star via an rotationally-delayed accretion-induced collapse, requires both a high-mass differentially rotating progenitor and a significant momentum kick at birth under our constraints. Contrarily, we find that eccentricity pumping via interaction with a transient circumbinary disk is consistent with all inferred properties. Finally, we report tentative evidence for pulsations which, if confirmed, would transform the star into an unprecedented laboratory for WD physics and stellar convection.

astro-ph.SR↗

arXiv2016

The millisecond pulsar mass distribution: Evidence for bimodality and constraints on the maximum neutron star mass

The mass function of neutron stars (NSs) contains information about the late evolution of massive stars, the supernova explosion mechanism, and the equation-of-state of cold, nuclear matter beyond the nuclear saturation density. A number of recent NS mass measurements in binary millisecond pulsar (MSP) systems increase the fraction of massive NSs (with $M > 1.8$ M$_{\odot}$) to $\sim 20\% $ of the observed population. In light of these results, we employ a Bayesian framework to revisit the MSP mass distribution. We find that a single Gaussian model does not sufficiently describe the observed population. We test alternative empirical models and infer that the MSP mass distribution is strongly asymmetric. The diversity in spin and orbital properties of high-mass NSs suggests that this is most likely not a result of the recycling process, but rather reflects differences in the NS birth masses. The asymmetry is best accounted for by a bimodal distribution with a low mass component centred at $1.393_{-0.029}^{+0.031}$ M$_{\odot}$ and dispersed by $0.064_{-0.025}^{+0.064}$ M$_{\odot}$, and a high-mass component with a mean of $1.807_{-0.132}^{+0.081}$ and a dispersion of $0.177_{-0.072}^{+0.115}$ M$_{\odot}$. We also establish a lower limit of $M_{max} \ge 2.018$ M$_{\odot}$ at 98% C.L. for the maximum NS mass, from the absence of a high-mass truncation in the observed masses. Using our inferred model, we find that the measurement of 350 MSP masses, expected after the conclusion of pulsar surveys with the Square-Kilometre Array, can result in a precise localization of a maximum mass up to 2.15 M$_{\odot}$, with a 5% accuracy. Finally, we identify possible massive NSs within the known pulsar population and discuss birth masses of MSPs.

astro-ph.HE↗

arXiv2015

Testing General Relativity with Present and Future Astrophysical Observations

One century after its formulation, Einstein's general relativity has made remarkable predictions and turned out to be compatible with all experimental tests. Most of these tests probe the theory in the weak-field regime, and there are theoretical and experimental reasons to believe that general relativity should be modified when gravitational fields are strong and spacetime curvature is large. The best astrophysical laboratories to probe strong-field gravity are black holes and neutron stars, whether isolated or in binary systems. We review the motivations to consider extensions of general relativity. We present a (necessarily incomplete) catalog of modified theories of gravity for which strong-field predictions have been computed and contrasted to Einstein's theory, and we summarize our current understanding of the structure and dynamics of compact objects in these theories. We discuss current bounds on modified gravity from binary pulsar and cosmological observations, and we highlight the potential of future gravitational wave measurements to inform us on the behavior of gravity in the strong-field regime.

gr-qc↗

arXiv2014

Testing Gravity with Pulsars in the SKA Era

The Square Kilometre Array (SKA) will use pulsars to enable precise measurements of strong gravity effects in pulsar systems, which yield tests of gravitational theories that cannot be carried out anywhere else. The Galactic census of pulsars will discover dozens of relativistic pulsar systems, possibly including pulsar -- black hole binaries which can be used to test the "cosmic censorship conjecture" and the "no-hair theorem". Also, the SKA's remarkable sensitivity will vastly improve the timing precision of millisecond pulsars, allowing probes of potential deviations from general relativity (GR). Aspects of gravitation to be explored include tests of strong equivalence principles, gravitational dipole radiation, extra field components of gravitation, gravitomagnetism, and spacetime symmetries.

astro-ph.HE↗

arXiv2014

Direct formation of millisecond pulsars from rotationally delayed accretion-induced collapse of massive white dwarfs

Millisecond pulsars (MSPs) are believed to be old neutron stars, formed via Type Ib/c core-collapse supernovae, which have subsequently been spun up to high rotation rates via accretion from a companion star in a highly circularised low-mass X-ray binary. The recent discoveries of Galactic field binary MSPs in eccentric orbits, and mass functions compatible with that expected for helium white dwarf companions, PSR J2234+06 and PSR J1946+3417, therefore challenge this picture. Here we present a hypothesis for producing this new class of systems, where the MSPs are formed directly from a rotationally-delayed accretion-induced collapse of a super-Chandrasekhar mass white dwarf. We compute the orbital properties of the MSPs formed in such events and demonstrate that our hypothesis can reproduce the observed eccentricities, masses and orbital periods of the white dwarfs, as well as forecasting the pulsar masses and velocities. Finally, we compare this hypothesis to a triple star scenario.

astro-ph.SR↗

arXiv2013

On the disruption of pulsar and X-ray binaries in globular clusters

The stellar encounter rate Gamma has been shown to be strongly correlated with the number of X-ray binaries in clusters and also to the number of radio pulsars. However, the pulsar populations in different clusters show remarkably different characteristics: in some GCs the population is dominated by binary systems, in others by single pulsars and exotic systems that result from exchange encounters. In this paper, we describe a second dynamical parameter for globular clusters, the encounter rate for a single binary, gamma. We find that this parameter provides a good characterization of the differences between the pulsar populations of different globular clusters. The higher gamma is for any particular globular cluster the more isolated pulsars and products of exchange interactions are observed. Furthermore, we also find that slow and "young" pulsars are found almost exclusively in clusters with a high gamma; this suggests that these kinds of objects are formed by the disruption of X-ray binaries, thus halting the recycling of a previously dead neutron star. We discuss the implications of this for the nature of young pulsars and for the formation of neutron stars in globular clusters.

astro-ph.SR↗

arXiv2013

The Northern High Time Resolution Universe Pulsar Survey I: Setup and initial discoveries

We report on the setup and initial discoveries of the Northern High Time Resolution Universe survey for pulsars and fast transients, the first major pulsar survey conducted with the 100-m Effelsberg radio telescope and the first in 20 years to observe the whole northern sky at high radio frequencies. Using a newly developed 7-beam receiver system combined with a state-of-the-art polyphase filterbank, we record an effective bandwidth of 240 MHz in 410 channels centred on 1.36 GHz with a time resolution of 54 $μ$s. Such fine time and frequency resolution increases our sensitivity to millisecond pulsars and fast transients, especially deep inside the Galaxy, where previous surveys have been limited due to intra-channel dispersive smearing. To optimise observing time, the survey is split into three integration regimes dependent on Galactic latitude, with 1500-s, 180-s and 90-s integrations for latitude ranges $|b|<3.5^{\circ}$, $|b|<15^{\circ}$ and $|b|>15^{\circ}$, respectively. The survey has so far resulted in the discovery of 15 radio pulsars, including a pulsar with a characteristic age of $\sim18$ kyr, {PSR J2004+3429}, and a highly eccentric, binary millisecond pulsar, {PSR J1946+3417}. All newly discovered pulsars are timed using the 76-m Lovell radio telescope at the Jodrell Bank Observatory and the Effelsberg radio telescope. We present timing solutions for all newly discovered pulsars and discuss potential supernova remnant associations for {PSR J2004+3429}.

astro-ph.HE↗