Millisecond Radio Pulsars Research Articles

Population Syntheses of Millisecond Pulsars from the Galactic Disk and Bulge

Abstract We present the results of a population synthesis of radio and γ-ray millisecond pulsars (MSPs) from the galactic disk (GD). Using 92 radio MSPs detected in 13 radio surveys and 54 Fermi MSPs detected as point sources in the first point source catalog, we establish six free parameters corresponding to the overall factor and the exponents of the period and period derivative dependence for each of the radio and γ-ray empirical luminosity models. We test three high-energy emission models described by the two-pole caustic slot-gap, outer-gap, and pair-starved polar-cap geometries. The simulated distributions of pulsar properties adequately describe the distributions of detected MSPs from the GD. We explore the γ-ray emission from groups of MSPs in globular clusters and in the galactic bulge. The simulation predicts reasonable numbers of Fermi MSPs detected in the other point source catalogs and anticipates a bright future for Fermi observations of MSPs, expecting a total of ≈170 MSP detections from the GD within 10 years. Our numbers of simulated MSPs in globular clusters are in agreement with those derived from Fermi detections. The simulation predicts that about 11,000 MSPs in the galactic bulge are required to explain the γ-ray galactic center excess.

The minimum magnetic field of millisecond pulsars calculated according to accretion: application to the X-ray neutron star SAX J1808.4–3658 in a low-mass X-ray binary

Based on the model of the accretion-induced magnetic field decay of a neutron star (NS), millisecond pulsars (MSPs) will obtain their minimum magnetic field when the NS magnetosphere radius shrinks to the stellar surface during the binary accretion phase. We find that this minimummagnetic field is related to the accretion rate Mdot as Bmin ~2.0*10^7 G( Mdot/Mdot_min)^1/2, where Mdot_min = 4.6*10^15 g/s is the average minimum accretion rate required for MSP formation and is constrained by the long-term accretion time, which corresponds to the companion lifetime, being less than the Hubble time. The value of Bmin is consistent with that of observed radio MSPs and accreting MSPs in low-mass X-ray binaries, which can be found the illustrated case of the minimum and present field strength of SAX J1808.4-3658. The prediction of the minimum magnetic field of MSPs would be the lowest field strength of NSs in the Universe, which could constrain the evolution mechanism of the magnetic field of accreting NSs.

Universality of free fall from the orbital motion of a pulsar in a stellar triple system

Einstein's theory of gravity-the general theory of relativity1-is based on the universality of free fall, which specifies that all objects accelerate identically in an external gravitational field. In contrast to almost all alternative theories of gravity2, the strong equivalence principle of general relativity requires universality of free fall to apply even to bodies with strong self-gravity. Direct tests of this principle using Solar System bodies3,4 are limited by the weak self-gravity of the bodies, and tests using pulsar-white-dwarf binaries5,6 have been limited by the weak gravitational pull of the Milky Way. PSRJ0337+1715 is a hierarchical system of three stars (a stellar triple system) in which a binary consisting of a millisecond radio pulsar and a white dwarf in a 1.6-day orbit is itself in a 327-day orbit with another white dwarf. This system permits a test that compares how the gravitational pull of the outer white dwarf affects the pulsar, which has strong self-gravity, and the inner white dwarf. Here we report that the accelerations of the pulsar and its nearby white-dwarf companion differ fractionally by no more than 2.6×10-6. For a rough comparison, our limit on the strong-field Nordtvedt parameter, which measures violation of the universality of free fall, is a factor of ten smaller than that obtained from (weak-field) Solar System tests3,4 and a factor of almost a thousand smaller than that obtained from other strong-field tests5,6.

Accretion and propeller torque in the spin-down phase of neutron stars: The case of transitional millisecond pulsar PSR J1023+0038

ABSTRACT The spin-down rate of PSR J1023+0038, one of the three confirmed transitional millisecond pulsars, was measured in both radio pulsar (radio millisecond pulsar, RMSP) and X-ray pulsar (low-mass X-ray binary, LMXB) states. The spin-down rate in the LMXB state is only about 27 per cent greater than the spin-down rate in the RMSP state. The inner disc radius, rin, obtained recently by Ertan for the propeller phase, which is close to the co-rotation radius, rco, and insensitive to the mass-flow rate, can explain the observed torques together with the X-ray luminosities, Lx. The X-ray pulsar and radio pulsar states correspond to accretion with spin-down (weak-propeller) and strong-propeller situations, respectively. A several times increase in the disc mass-flow rate takes the source from the strong propeller with a low Lx to the weak propeller with a higher Lx powered by accretion on to the star. The resultant decrease in rin increases the magnetic torque slightly, explaining the observed small increase in the spin-down rate. We have found that the spin-up torque exerted by accreting material is much smaller than the magnetic spin-down torque exerted by the disc in the LMXB state.

A Deep X-ray Survey of the globular cluster Omega Centauri

We identify 233 X-ray sources, of which 95 are new, in a 222 ks exposure of Omega Centauri with the Chandra X-ray Observatory's ACIS-I detector. The limiting unabsorbed flux in the core is $f_x$ (0.5$-$6.0 keV) $\simeq$ 3 $\times$ 10$^{-16}$ erg s$^{-1}$ cm$^{-2}$ ($L_X$ $\simeq$ 1 $\times$ 10$^{30}$ erg s$^{-1}$ at 5.2 kpc). We estimate that ~$60\pm 20$ of these are cluster members, of which ~30 lie within the core ($r_c$ $=$ 155 arcsec), and another ~30 between 1$-$2 core radii. We identify four new optical counterparts, for a total of 45 likely identifications. Probable cluster members include 18 cataclysmic variables (CVs) and CV candidates, one quiescent low-mass X-ray binary, four variable stars, and five stars that are either associated with w Cen's anomalous red giant branch, or are sub-subgiants. We estimate that the cluster contains $40\pm 10$ CVs with $L_X$ $>$ 10$^{31}$ erg s$^{-1}$, confirming that CVs are underabundant in w Cen relative to the field. Intrinsic absorption is required to fit X-ray spectra of six of the nine brightest CVs, suggesting magnetic CVs, or high-inclination systems. Though no radio millisecond pulsars (MSPs) are currently known in w Cen, more than 30 unidentified sources have luminosities and X-ray colours like those of MSPs found in other globular clusters; these could be responsible for the Fermi-detected gamma-ray emission from the cluster. Finally, we identify a CH star as the counterpart to the second-brightest X-ray source in the cluster and argue that it is a symbiotic star. This is the first such giant/white dwarf binary to be identified in a globular cluster.

Noise Budget and Interstellar Medium Mitigation Advances in the NANOGrav Pulsar Timing Array

Gravitational wave (GW) detection with pulsar timing arrays (PTAs) requires accurate noise characterization. The noise of our Galactic-scale GW detector has been systematically evaluated by the Noise Budget and Interstellar Medium Mitigation working groups within the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration. Intrinsically, individual radio millisecond pulsars (MSPs) used by NANOGrav can have some degree of achromatic red spin noise, as well as white noise due to pulse phase jitter. Along any given line-of-sight, the ionized interstellar medium contributes chromatic noise through dispersion measure (DM) variations, interstellar scintillation, and scattering. These effects contain both red and white components. In the future, with wideband receivers, the effects of frequency-dependent DM will become important. Having anticipated and measured these diverse sources of detector noise, the NANOGrav PTA remains well-poised to detect low-frequency GWs.

Conquering systematics in the timing of the pulsar triple system J0337+1715: Towards a unique and robust test of the strong equivalence principle

PSR J0337+1715 is a millisecond radio pulsar in a hierarchical stellar triple system containing two white dwarfs. The pulsar orbits the inner white dwarf every 1.6 days. In turn, this inner binary system orbits the outer white dwarf every 327 days. The gravitational influence of the outer white dwarf strongly accelerates the inner binary, making this system an excellent laboratory in which to test the strong equivalence principle (SEP) of general relativity – especially because the neutron star has significant gravitational self-binding energy. This system has been intensively monitored using three radio telescopes: Arecibo, Green Bank and Westerbork. Using the more than 25000 pulse times of arrival (TOAs) collected to date, we have modeled the system using direct 3-body numerical integration. Here we present our efforts to quantify the effects of systematics in the TOAs and timing residuals, which can limit the precision to which we can test the SEP in this system. In this work we describe Fourier-based techniques that we apply to the residuals in order to isolate the effects of systematics that could masquerade as an SEP violation. We also demonstrate that tidal effects are insignificant in the modeling.

The Spin Distribution of Fast-spinning Neutron Stars in Low-mass X-Ray Binaries: Evidence for Two Subpopulations

Abstract We study the current sample of rapidly rotating neutron stars in both accreting and non-accreting binaries in order to determine whether the spin distribution of accreting neutron stars in low-mass X-ray binaries (LMXBs) can be reconciled with current accretion torque models. We perform a statistical analysis of the spin distributions and show that there is evidence for two subpopulations among LMXBs, one at a relatively low spin frequency, with an average of and a broad spread, and a peaked population at higher frequency with an average spin frequency of . We show that the two subpopulations are separated by a cut-point at a frequency of . We also show that the spin frequency of radio millisecond pulsars (RMSPs) does not follow a log-normal distribution and shows no evidence for the existence of distinct subpopulations. We discuss the uncertainties of different accretion models and speculate that either the accreting neutron star cut-point marks the onset of gravitational waves as an efficient mechanism to remove angular momentum or some of the neutron stars in the fast subpopulation do not evolve into RMSPs.

State Switching of the X-Ray/Radio Transitional Millisecond Pulsar

Abstract PSR J1023+0038 is a known transitional pulsar, which switches between the accretion-powered X-ray pulsar state and the rotation-powered radio millisecond pulsar (MSP) state. Here, we study the conditions for its state transition by ascribing ∼GHz radio pulse emission to the plasma cutoff oscillation frequency at the magnetosphere, which depends on the plasma density or accretion rate. With the accretion X-ray luminosity declining fromhigh to low state (e.g., erg s−1 to erg s−1), the magnetosphere boundary expands to the light cylinder, where the plasma density is low enough to permit the emission of radio waves at the several-GHz frequency band. Thus, for a sufficiently low accretion-rate state ( erg s−1), the radio emission at the several-GHz band is not blocked by the plasma, and the radio pulsar can be detected in the favored “observational windows,” i.e., the L band (1–2 GHz), S band (2–4 GHz), and C band (4–8 GHz). Furthermore, to evaluate the penetration characteristics of the outgoing radio wave of low frequency, we calculate its skin depth, which is found to be a couple of centimeters for the low X-ray luminosity of erg s−1, a scale that is much shorter than the light cylinder radius of about 100 km for MSPs.

Are Gravitational Waves Spinning Down PSR J1023+0038?

The pulsar J1023+0038 rotates with a frequency ν≈592 Hz and has been observed to transition between a radio state, during which it is visible as a millisecond radio pulsar, and a low-mass x-ray binary (LMXB) state, during which accretion powered x-ray pulsations are visible. Timing during the two phases reveals that during the LMXB phase the neutron star is spinning down at a rate of ν[over ˙]≈-3×10^{-15} Hz/s, which is approximately 27% faster than the rate measured during the radio phase, ν[over ˙]≈-2.4×10^{-15} Hz/s, and is at odds with the predictions of accretion models. We suggest that the increase in spin-down rate is compatible with gravitational wave emission, particularly with the creation of a "mountain" during the accretion phase. We show that asymmetries in pycnonuclear reaction rates in the crust can lead to a large enough mass quadrupole to explain the observed spin-down rate, which thus far has no other self-consistent explanation. We also suggest two observational tests of this scenario, involving radio timing at the onset of the next millisecond radio pulsar phase, when the mountain should dissipate, and accurate timing during the next LMXB phase to track the increase in torque as the mountain builds up. Another possibility is that an unstable r mode with an amplitude α≈5×10^{-8} may be present in the system.

The 26.3-h orbit and multiwavelength properties of the ‘redback’ millisecond pulsar PSR J1306–40

Abstract We present the discovery of the variable optical and X-ray counterparts to the radio millisecond pulsar (MSP) PSR J1306–40, recently discovered by Keane et al. We find that both the optical and X-ray fluxes are modulated with the same period, which allows us to measure for the first time the orbital period Porb = 1.097 16[6] d. The optical properties are consistent with a main-sequence companion with spectral type G to mid K and, together with the X-ray luminosity (8.8 × 1031 erg s−1 in the 0.5–10 keV band, for a distance of 1.2 kpc), confirm the redback classification of this pulsar. Our results establish the binary nature of PSR J1306–40, which has the longest Porb among all known compact binary MSPs in the Galactic disc. We briefly discuss these findings in the context of irradiation and intrabinary shock emission in compact binary MSPs.

A Millisecond Pulsar Discovery in a Survey of Unidentified Fermi γ-Ray Sources with LOFAR

Abstract Using LOFAR, we have performed a very-low-frequency (115−155 MHz) radio survey for millisecond pulsars (MSPs). The survey targeted 52 unidentified Fermi γ-ray sources. Employing a combination of coherent and incoherent dedispersion, we have mitigated the dispersive effects of the interstellar medium while maintaining sensitivity to fast-spinning pulsars. Toward 3FGL J1553.1+5437 we have found PSR J1552+5437, the first MSP to be discovered (through its pulsations) at a radio frequency <200 MHz. PSR J1552+5437 is an isolated MSP with a 2.43 ms spin period and a dispersion measure of 22.9 pc cm−3. The pulsar has a very steep radio spectral index ( 2.8 ± 0.4). We obtain a phase-connected timing solution combining the 0.74 years of radio observations with γ-ray photon arrival times covering 7.5 years of Fermi observations. We find that the radio and γ-ray pulse profiles of PSR J1552+5437 appear to be nearly aligned. The very steep spectrum of PSR J1552+5437, along with other recent discoveries, hints at a population of radio MSPs that have been missed in surveys using higher observing frequencies. Detecting such steep spectrum sources is important for mapping the population of MSPs down to the shortest spin periods, understanding their emission in comparison to slow pulsars, and quantifying the prospects for future surveys with low-frequency radio telescopes like SKA-Low and its precursors.

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 (47 Tuc) from 2000, 2002 and 2014–2015 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 arcsec) 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. 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 fitted by blackbody 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 × 10−9 and constrain the shape of the r-mode instability window.

X-rays from Radio Millisecond Pulsars

AbstractThe Galactic population of rotation-powered (aka radio) millisecond pulsars (MSPs) exhibits diverse X-ray properties. Energetic MSPs show pulsed non-thermal radiation from their magnetospheres. Eclipsing binary MSPs predominantly have X-ray emission from a pulsar wind driven intra-binary shock. Typical radio MSPs emit X-rays from their heated magnetic polar caps. These thermally emitting MSPs offer the opportunity to place interesting constraints on the long sought after dense matter equation of state, making them important targets of investigation of the recently deployed Neutron Star Interior Composition Explorer (NICER) X-ray mission.

Conquering systematics in the timing of the pulsar triple system J0337+1715: Towards a unique and robust test of the strong equivalence principle

AbstractPSR J0337+1715 is a millisecond radio pulsar in a hierarchical stellar triple system with two white dwarfs. This system is a unique and excellent laboratory in which to test the strong equivalence principle (SEP) of general relativity. An initial SEP-violation test was performed using direct 3-body numerical integration of the orbit in order to model the more than 25000 pulse times of arrival (TOAs) from three radio telescopes: Arecibo, Green Bank and Westerbork. In this work I present our efforts to quantify the effects of systematics in the TOAs and timing residuals, which limit the precision of an SEP test. In particular, we apply Fourier-based techniques to the timing residuals in order to isolate the effects of systematics that can masquerade as an SEP violation.

A decade of transitional millisecond pulsars

AbstractTransitional millisecond pulsars (tMSPs), which are systems that harbor a pulsar in the throes of the recycling process, have emerged as a new source class since the discovery of the first such system a decade ago. These systems switch between accretion-powered low-mass X-ray binary (LMXB) and rotation-powered radio millisecond pulsar (RMSP) states, and provide exciting avenues to understand the physical processes that spin-up neutron stars to millisecond periods. During the last decade, three tMSPs, as well as a candidate source, have been extensively probed using systematic, multi-wavelength campaigns. Here we review the observational highlights from these campaigns and our general understanding of tMSPs.

Millisecond Pulsars, their Evolution and Applications

Millisecond pulsars (MSPs) are short-period pulsars that are distinguished from "normal" pulsars, not only by their short period, but also by their very small spin-down rates and high probability of being in a binary system. These properties are consistent with MSPs having a different evolutionary history to normal pulsars, viz., neutron-star formation in an evolving binary system and spin-up due to accretion from the binary companion. Their very stable periods make MSPs nearly ideal probes of a wide variety of astrophysical phenomena. For example, they have been used to detect planets around pulsars, to test the accuracy of gravitational theories, to set limits on the low-frequency gravitational-wave background in the Universe, and to establish pulsar-based timescales that rival the best atomic-clock timescales in long-term stability. MSPs also provide a window into stellar and binary evolution, often suggesting exotic pathways to the observed systems. The X-ray accretion-powered MSPs, and especially those that transition between an accreting X-ray MSP and a non-accreting radio MSP, give important insight into the physics of accretion on to highly magnetised neutron stars.

Millisecond radio pulsars with known masses: Parameter values and equation of state models

The recent fast growth of a population of millisecond pulsars with precisely measured mass provides an excellent opportunity to characterize these compact stars at an unprecedented level. This is because the stellar parameter values can be accurately computed for known mass and spin rate and an assumed equation of state (EoS) model. For each of the 16 such pulsars and for a set of EoS models from nucleonic, hyperonic, strange quark matter and hybrid classes, we numerically compute fast spinning stable stellar parameter values considering the full effect of general relativity. This first detailed catalogue of the computed parameter values of observed millisecond pulsars provides a testbed to probe the physics of compact stars, including their formation, evolution and EoS. We estimate uncertainties on these computed values from the uncertainty of the measured mass, which could be useful to quantitatively constrain EoS models. We note that the largest value of the central density ρc in our catalogue is ∼5.8 times the nuclear saturation density ρsat, which is much less than the expected maximum value 13ρsat. We argue that the ρc-values of at most a small fraction of compact stars could be much larger than 5.8ρsat. Besides, we find that the constraints on EoS models from accurate radius measurements could be significantly biased for some of our pulsars, if stellar spinning configurations are not used to compute the theoretical radius values.

X-ray bounds on the r-mode amplitude in millisecond pulsars

r-mode astroseismology provides a unique way to study the internal composition of compact stars. Due to their precise timing, recycled millisecond radio pulsars present a particularly promising class of sources. Although their thermal properties are still poorly constrained, X-ray data is very useful for astroseismology since r-modes could strongly heat a star. Using known and new upper bounds on the temperatures and luminosities of several non-accreting millisecond radio pulsars we derive bounds on the r-mode amplitude as low as $\alpha\lesssim10^{-8}$ and discuss the impact on scenarios for their internal composition.

EVOLUTION OF LOW-MASS X-RAY BINARIES: THE EFFECT OF DONOR EVAPORATION

ABSTRACT Millisecond pulsars (MSPs) are thought to originate from low-mass X-ray binaries (LMXBs). The discovery of eclipsing radio MSPs, including redbacks and black widows, indicates that evaporation of the donor star by the MSP’s irradiation takes place during the LMXB evolution. In this work, we investigate the effect of donor evaporation on the secular evolution of LMXBs, considering different evaporation efficiencies and related angular momentum loss. We find that for widening LMXBs, the donor star leaves a less massive white dwarf than without evaporation; for contracting systems, evaporation can speed up the evolution, resulting in dynamically unstable mass transfer and possibly the formation of isolated MSPs.