PSR J0437-4715 Research Articles

DETECTION OF X-RAY EMISSION FROM THE VERY OLD PULSAR J0108-1431

PSR J0108-1431 is a nearby, 170 Myr old, very faint radio near the pulsar death line in the P- diagram. We observed the field with the Chandra X-ray Observatory and detected a point source (53 counts in a 30 ks exposure; energy flux (9 ± 2) × 10–15 erg cm–2 s–1 in the 0.3-8 keV band) close to the radio position. Based on the large X-ray/optical flux ratio at the X-ray source position, we conclude that the source is the X-ray counterpart of PSR J0108-1431. The spectrum can be described by a power-law model with photon index Γ ≈ 2.2 and luminosity L 0.3-8 keV ≈ 2 × 1028 d 2 130 erg s–1, or by a blackbody model with temperature kT ≈ 0.28 keV and bolometric luminosity L bol ≈ 1.3 × 1028 d 2 130 erg s–1, for a plausible hydrogen column density N H = 7.3 × 1019 cm–2 (d 130 = d/130 pc). The converts ~0.4% of its spin-down power into X-ray luminosity, i.e., its X-ray efficiency is higher than for most younger pulsars. From the comparison of the X-ray position with the previously measured radio positions, we estimated the proper motion of 0.2 arcsec yr–1 (V ⊥ ≈ 130d 130 km s–1), in the south-southeast direction.

Thermal X‐Rays from Millisecond Pulsars: Constraining the Fundamental Properties of Neutron Stars

Abridged) We model the X-ray properties of millisecond pulsars (MSPs) by considering hot spot emission from a weakly magnetized rotating neutron star (NS) covered by an optically-thick hydrogen atmosphere. We investigate the limitations of using the thermal X-ray pulse profiles of MSPs to constrain the mass-to-radius ($M/R$) ratio of the underlying NS. The accuracy is strongly dependent on the viewing angle and magnetic inclination. For certain systems, the accuracy is ultimately limited only by photon statistics implying that future X-ray observatories could, in principle, achieve constraints on $M/R$ and hence the NS equation of state to better than $\sim$5%. We demonstrate that valuable information regarding the basic properties of the NS can be extracted even from X-ray data of fairly limited photon statistics through modeling of archival spectroscopic and timing observations of the nearby isolated PSRs J0030+0451 and J2124--3358. The X-ray emission from these pulsars is consistent with the presence of a hydrogen atmosphere and a dipolar magnetic field configuration, in agreement with previous findings for PSR J0437--4715. For both MSPs, the favorable geometry allows us to place interesting limits on the allowed $M/R$ of NSs. Assuming 1.4 M$_{\odot}$, the stellar radius is constrained to be $R > 9.4$ km and $R > 7.8$ km (68% confidence) for PSRs J0030+0451 and J2124--3358, respectively. We explore the prospects of using future observatories such as \textit{Constellation-X} and \textit{XEUS} to conduct blind X-ray timing searches for MSPs not detectable at radio wavelengths due to unfavorable viewing geometry. Using the observational constraints on the pulsar obliquities we are also able to place strong constraints on the magnetic field evolution model proposed by Ruderman.

Extremely High Precision VLBI Astrometry of PSR J0437-4715 and Implications for Theories of Gravity

Using the recently upgraded Long Baseline Array, we have measured the trigonometric parallax of PSR J0437-4715 to better than 1% precision, the most precise pulsar distance determination made to date. Comparing this VLBI distance measurement to the kinematic distance obtained from pulsar timing, which is calculated from the pulsar's proper motion and apparent rate of change of orbital period, gives a precise limit on the unmodeled relative acceleration between the Solar System and PSR J0437-4715, which can be used in a variety of applications. Firstly, it shows that Newton's gravitational constant G is stable with time (\dot{G}/G = (-5 +- 26) x 10^{-13} yr^{-1}, 95% confidence). Secondly, if a stochastic gravitational wave background existed at the currently quoted limit, this null result would fail ~50% of the time. Thirdly, it excludes Jupiter-mass planets within 226 AU of the Sun in 50% of the sky (95% confidence). Finally, the ~1% agreement of the parallax and orbital period derivative distances provides a fundamental confirmation of the parallax distance method upon which all astronomical distances are based.

Correlations in the properties of static and rapidly rotating compact stars

Correlations in the properties of the static compact stars (CSs) and the ones rotating with the highest observed frequency of 1122 Hz are studied using a large set of equations of state (EOSs). These EOSs span various approaches and their chemical composition varies from the nucleons to hyperons and quarks in {beta}-equilibrium. It is found that the properties of static CS, like the maximum gravitational mass M{sub max}{sup stat} and radius R{sub 1.4}{sup stat} corresponding to the canonical mass and supramassive or nonsupramassive nature of the CS rotating at 1122 Hz are strongly correlated. In particular, only those EOSs yield the CS rotating at 1122 Hz to be nonsupramassive for which ((M{sub max}{sup stat}/M{sub {center_dot}})){sup 1/2}((10 km/R{sub 1.4}{sup stat})){sup 3/2} is greater than unity. A suitable parametric form which can be used to split the M{sub max}{sup stat}-R{sub 1.4}{sup stat} plane into the regions of different supramassive nature of the CS rotating at 1122 Hz is presented. Currently measured maximum gravitational mass 1.76M{sub {center_dot}} of PSR J0437-4715 suggests that the CS rotating at 1122 Hz can be nonsupramassive provided R{sub 1.4}{sup stat}{<=}12.4 km.

Precision Timing of PSR J0437−4715: An Accurate Pulsar Distance, a High Pulsar Mass, and a Limit on the Variation of Newton’s Gravitational Constant

Analysis of ten years of high-precision timing data on the millisecond pulsar PSR J0437-4715 has resulted in a model-independent kinematic distance based on an apparent orbital period derivative, Pbdot, determined at the 1.5% level of precision (Dk = 157.0 +/- 2.4 pc), making it one of the most accurate stellar distance estimates published to date. The discrepancy between this measurement and a previously published parallax distance estimate is attributed to errors in the DE200 Solar System ephemerides. The precise measurement of Pbdot allows a limit on the variation of Newton's gravitational constant, |Gdot/G| < 23 x 10^{-12} 1/yr. We also constrain any anomalous acceleration along the line of sight to the pulsar to |a(Sun)/c| < 1.5 x 10^{-18} 1/s at 95% confidence, and derive a pulsar mass, m(psr) = 1.76 +/- 0.20 M, one of the highest estimates so far obtained.

Avalanche Dynamics of Radio Pulsar Glitches

We test statistically the hypothesis that radio pulsar glitches result from an avalanche process, in which angular momentum is transferred erratically from the flywheel-like superfluid in the star to the slowly decelerating, solid crust via spatially connected chains of local, impulsive, threshold-activated events, so that the system fluctuates around a self-organized critical state. Analysis of the glitch population (currently 285 events from 101 pulsars) demonstrates that the size distribution in individual pulsars is consistent with being scale invariant, as expected for an avalanche process. The measured power-law exponents fall in the range -0.13 ⩽ a⩽ 2.4, with a ≈ 1.2 for the youngest pulsars. The waiting-time distribution is consistent with being exponential in seven out of nine pulsars where it can be measured reliably, after adjusting for observational limits on the minimum waiting time, as for a constant-rate Poisson process. PSR J0537–6910 and PSR J0835–4510 are the exceptions; their waiting-time distributions show evidence of quasi-periodicity. In each object, stationarity requires that the rate λ equal −/⟨Δν⟩, where is the angular acceleration of the crust, ⟨ Δ ν ⟩ is the mean glitch size, and is the relative angular acceleration of the crust and superfluid. Measurements yield ⩽ 7 × 10−5 for PSR J0358+5413 and ⩽ 1 (trivially) for the other eight objects, which have a < 2. There is no evidence that λ changes monotonically with spin-down age. The rate distribution itself is fitted reasonably well by an exponential for λ ⩾ 0.25 yr−1, with ⟨ λ ⟩ = 1.3+ 0.7−0.6 yr−1. For λ < 0.25 yr−1 the exact form is unknown; the exponential overestimates the number of glitching pulsars observed at low λ, where the limited total observation time exercises a selection bias. In order to reproduce the aggregate waiting-time distribution of the glitch population as a whole, the fraction of pulsars with λ > 0.25 yr−1 must exceed ~70%.

Constraints on Neutron Star Properties from X‐Ray Observations of Millisecond Pulsars

We present a model of thermal X-ray emission from hot spots on the surface of a rotating compact star with an unmagnetized light-element atmosphere. An application to ROSAT, Chandra, and XMMNewton X-ray observations of the nearest known rotation-powered millisecond pulsar (MSP) PSR J0437–4715 reveals that the thermal emission from this pulsar is fully consistent with such a model, enabling constraints on important properties of the underlying neutron star. We confirm that the observed thermal X-ray pulsations from J0437–4715 are incompatible with blackbody emission and require the presence of an optically thick, light element (most likely hydrogen) atmosphere on the neutron star surface. The morphology of the X-ray pulse profile is consistent with a global dipole configuration of the pulsar magnetic field but suggests an off-center magnetic axis, with a displacement of 0.8 −3 km from the stellar center. For an assumed mass of 1.4 M⊙, the model restricts the allowed stellar radii to R = 6.8 − 13.8 km (90% confidence) and R > 6.7 km (99.9% confidence), which is consistent with standard NS equations of state and rules out an ultracompact star smaller than its photon sphere. Deeper spectroscopic and timing observations of this and other nearby radio MSPs with current and future X-ray facilities (Constellation-X and XEUS) can provide further insight into the fundamental properties of neutron stars. Subject headings: pulsars: general — pulsars: individual (PSR J0437–4715) — stars: neutron — X-rays: stars — gravitation — relativity

Contributions from nearby pulsars to the local cosmic ray electron spectrum

PSR J0437-4715 is one of the closest millisecond pulsars (MSPs) to Earth, lying at a distance of ∼140 pc. This pulsar has a characteristic age of 4.9 Gyr and a relatively low spindown power of ∼10 34 ergs/s. During its rather long lifetime, a large fraction of the energy output has been in the form of multi-TeV electrons. In this paper, we investigate the possible contribution of this nearby MSP to the local interstellar electron spectrum (LIS). The old age of the system justifies a steady-state evaluation of the contribution from this pulsar to the LIS. We calculate the electron spectrum at the light cylinder in the framework of a General Relativistic polar cap (PC) model, and use this as an injection spectrum in a diffusion model. The younger Geminga pulsar is also very close to Earth and warrants investigation. A steady-state approach is however no longer justified, so we use an impulsive injection model. We will present results of a study of the contribution from these pulsars to the cosmic ray (CR) LIS. Our calculations show that pulsars like Geminga can make a non-negligible contribution to the LIS.

VLT/NACOobservations of the high-magnetic field radio pulsar PSR J1119-6127

Context. Recent radio observations have unveiled the existence of a number of radio pulsars with spin-down derived magnetic fields in the magnetar range. However, their observational properties appear to be more similar to those of the classical radio pulsars than to the magnetars’s ones. Aims. To shed light on this puzzle we first have to determine whether the spin-down derived magnetic field values for these radio pulsars are indeed representative of the actual neutron star magnetic field or if they are polluted, e.g. by the effects of a torque from a fallback disk. Methods. To investigate this possibility, we have performed deep IR (J,H,Ks bands) observations of one of these high magnetic field radio pulsars (PSR J1119–6127) with the ESO VLT to search for IR emission which can be associated with a disk. Results. No IR emission is detected from the pulsar position down to J ∼ 24, H ∼ 23 and Ks ∼ 22. Conclusions. By comparing our flux upper limits with the predictions of fallback disk models, we have found that we can only exclude the presence of a disk with accretion rate u M > 3 × 10 16 gs −1 . This lower limit cannot rule out the presence of a substantial disk torque on the pulsar, which would then lead to overestimate the value of the magnetic field inferred from P and u P .W e have also compared the upper limit on the IR luminosity of PSR J1119–6127 with the IR luminosities of rotation-powered pulsars and magnetars. We found that, while magnetars are intrinsically more efficient IR emitters than rotation-powered pulsars, possibly because of their higher magnetic field, the relatively low IR emission efficiency of PSR J1119–6127 suggests that it is more similar to the latters than to the former.

PSR J1119–6127 and the X-ray emission from high magnetic field radio pulsars

The existence of radio pulsars having inferred magnetic fields in the magnetar regime suggests that possible transition objects could be found in the radio pulsar population. The discovery of such an object would contribute greatly to our understanding of neutron star physics. Here we report on unusual X-ray emission detected from the radio pulsar PSR J1119–6127 using XMM–Newton. The pulsar has a characteristic age of 1,700 yrs and inferred surface dipole magnetic field strength of 4.1×1013 G. In the 0.5–2.0 keV range, the emission shows a single, narrow pulse with an unusually high pulsed fraction of ∼70%. No pulsations are detected in the 2.0–10.0 keV range, where we derive an upper limit at the 99% level for the pulsed fraction of 28%. The pulsed emission is well described by a thermal blackbody model with a high temperature of ∼2.4×106 K. While no unambiguous signature of magnetar-like emission has been found in high-magnetic-field radio pulsars, the X-ray characteristics of PSR J1119–6127 require alternate models from those of conventional thermal emission from neutron stars. In addition, PSR J1119–6127 is now the radio pulsar with the smallest characteristic age from which thermal X-ray emission has been detected.

Internal heating and thermal emission from old neutron stars

The equilibrium composition of neutron star matter is achieved through weak interactions (direct and inverse beta decays), which proceed on relatively long time scales. If the density of a matter element is perturbed, it will relax to the new chemical equilibrium through non-equilibrium reactions, which produce entropy that is partly released through neutrino emission, while a similar fraction heats the matter and is eventually radiated as thermal photons. We examined two possible mechanisms causing such density perturbations: (1) the reduction in centrifugal force caused by spin-down (particularly in millisecond pulsars), leading to rotochemical heating, and (2) a hypothetical time-variation of the gravitational constant, as predicted by some theories of gravity and current cosmological models, leading to gravitochemical heating. If only slow weak interactions are allowed in the neutron star (modified Urca reactions, with or without Cooper pairing), rotochemical heating can account for the observed ultraviolet emission from the closest millisecond pulsar, PSR J0437-4715, which also provides a constraint on |dG/dt| of the same order as the best available in the literature.

Thermal evolution of rotating hybrid stars

As a neutron star spins down, the nuclear matter is continuously converted into quark matter due to the core density increase, and then latent heat is released. We have investigated the thermal evolution of neutron stars undergoing such deconfinement phase transition. We have taken into account the conversion in the frame of the general theory of relativity. The released energy has been estimated as a function of changed rate of deconfinement baryon number. The numerical solutions to the cooling equation are seen to be very different from those without the heating effect. The results show that neutron stars may be heated to higher temperatures which is well matched with pulsar's data despite the onset of fast cooling in neutron stars with quark matter cores. It is also found that the heating effect has a magnetic field strength dependence. This feature could be particularly interesting for high temperatures of low-field millisecond pulsars at a later stage. The high temperature could fit the observed temperature for PSR J0437-4715.

Predicting the Starquakes in PSR J0537−6910

We report the results of more than seven years of monitoring of PSR J0537-6910, the 16 ms pulsar in the Large Magellanic Cloud, using data acquired with the Rossi X-ray Timing Explorer. During this campaign the pulsar experienced 22 sudden increases in frequency (glitches - 21 with increases of at least eight microHz) amounting to a total gain of over six parts per million of rotation frequency superposed on its gradual spindown of nu-dot = -2 x 10(exp -l0) Hz /s. The time interval from one glitch to the next obeys a strong linear correlation to the amplitude of the first glitch, with a mean slope of about 400 days per part per million (6.5 days per micro Hz), such that these intervals can be predicted to within a few days, an accuracy which has never before been seen in any other pulsar. There appears to be an upper limit of approximately 40 micro Hz for the size of glitches in all pulsars, with the 1999 April glitch of PSR J0537-6910 as the largest so far. The change of its spindown across the glitches, delta (nu-dot), appears to have the same hard lower limit of -1.5 x 10 (exp -13) Hz/s, as, again, that observed in all other pulsars. The spindown continues to increase in the long term, nu-dot = -10(exp -21) Hz / s(exp 2), and thus the timing age of PSR 505374910 (-0.5 nu nu-dot (exp -1) continues to decrease at a rate of nearly one year every year, consistent with movement of its magnetic moment away from its rotational axis by one radian every 10,000 years, or about one meter per year. PSR J0537-6910 was likely to have been born as a nearly-aligned rotator spinning at 75-80 Hz, with a absolute value of nu considerably smaller than its current value of 2x 10(exp -10) Hz per second. Its pulse profile consists of a single pulse which is found to be flat at its peak for at least 0.02 cycles. Glitch activity may grow exponentially with a timescale of 170 years nu nu-dot ((nu nu-dot)(sub crab))exp -l in all young pulsars.

Detection of Giant Pulses in PSR J1752+2359

We report the detection of Giant Pulses (GPs) in the pulsar PSR J1752+2359. The energy of the strongest GP exceeds the energy of the average pulse by a factor of 200, in which it stands out from all known pulsars with GPs. PSR J1752+2359 as well as the previously detected PSR B0031–07 and PSR B1112+50, belongs to the first group of pulsars found to have GPs without a high magnetic field at the light cylinder.

Constraining a Possible Time Variation of the Gravitational Constant through “Gravitochemical Heating” of Neutron Stars

A hypothetical time variation of the gravitational constant G would cause neutron star matter to depart from beta equilibrium, due to the changing hydrostatic equilibrium. This induces nonequilibrium beta processes, which release energy that is invested partly in neutrino emission and partly in internal heating. Eventually, the star arrives at a stationary state in which the temperature remains nearly constant, as the forcing through the change of G is balanced by the ongoing reactions. Using the surface temperature of the nearest millisecond pulsar, PSR J0437-4715, inferred from ultraviolet observations, we estimate two upper limits for this variation: (1) |.G/G|< 2 x 10(-10) yr(-1), if direct Urca reactions are allowed, and (2) |.G/G|< 4 x 10(-12) yr(-1), considering only modified Urca reactions. The latter is among the most restrictive obtained by other methods.

30 glitches in slow pulsars

We have analyzed 5.5 years of timing observations of 7 slowly rotating radio pulsars, made with the Westerbork Synthesis Radio Telescope. We present improved timing solutions and 30, mostly small, new glitches. Particularly interesting are our results on PSR J1814-1744, which is one of the pulsars with similar rotation parameters and magnetic field strength to the Anomalous X-ray Pulsars (AXPs). Although the high-B radio pulsars do not show X-ray emission, and no radio emission is detected for AXPs, the roughly similar glitch parameters provide us with another tool to compare these classes of neutron stars. Furthermore, we were able to detect glitches one to two orders of magnitude smaller than before, for example in our well-sampled observations of PSR B0355+54. We double the total number of known glitches in PSR B1737-30, and improve statistics on glitch sizes for this pulsar individually and pulsars in general. We detect no significant variations in dispersion measure for PSRs B1951+32 and B2224+65, two pulsars located in high-density surroundings. We discuss the effect of small glitches on timing noise, and show it is possible to resolve timing-noise looking structures in the residuals of PSR B1951+32 by using a set of small glitches.

X-Rays from Radio Millisecond Pulsars: Comptonized Thermal Radiation

X-ray emission from many rotation-powered millisecond pulsars (MSPs) is observed to be of predominantly thermal nature. In PSR J0437--4715, the nearest MSP known, an additional faint power-law tail is observed above 2.5 keV, commonly attributed to non-thermal magnetospheric radiation. We propose that the hard emission in this and other similar MSPs is instead due to weak Comptonization of the thermal (blackbody or hydrogen atmosphere) polar cap emission by energetic electrons/positrons of small optical depth in the pulsar magnetosphere. This spectral model implies that all soft X-rays are of purely thermal origin, which has profound implications in the study of neutron star structure and fundamental pulsar physics.

Detection of giant radio pulses from the pulsar PSR B0656+14

Giant pulses (GPs) have been detected from the pulsar PSR B0656 + 14. A pulse that is more intense than the average pulse by a factor of 120 is encountered approximately once in 3000 observed periods of the pulsar. The peak flux density of the strongest pulse, 120 Jy, is a factor of 630 higher than that of the average pulse. The GP energy exceeds the energy of the average pulse by up to a factor of 110, which is comparable to that for other known pulsars with GPs, including the Crab pulsar and the millisecond pulsar PSR B1937+21. The giant pulses are a factor of ∼6 narrower than the average pulse and are clustered at the head of the average pulse. PSR B0656+14 along with PSR B0031-07, PSR B1112+50, and PSR J1752+2359 belong to a distinctive group of pulsars in which GPs have been detected without any extremely strong magnetic field on the light cylinder.

High-precision baseband timing of 15 millisecond pulsars

We describe extremely precise timing experiments performed on five solitary and 10 binary millisecond pulsars during the past 3 yr, with the Caltech Parkes Swinburne Recorder (CPSR2) coherent dedispersion system at the Parkes 64-m radio telescope. 12 of our sources have rms timing residuals below 1.5 μs and four are below 200 ns. The quality of our data allows us to measure eight parallaxes and nine proper motions, from which we conclude that models of galactic electron density still have limited predictive power for individual objects. We derive a mean transverse velocity of 87+31/−14 km s −1 for these pulsars, in good agreement with previous authors. We demonstrate that unless multifrequency observations are made, typical variations in dispersion measure (DM) could introduce an additional drift in arrival times of ∼1 μs per year at 20-cm wavelengths. Our high timing precision means that Shapiro delay can be used to constrain the inclination angles and component masses of all but two of the selected binary systems. The signature of annual orbital parallax is detected in the timing of PSR J0437−4715 and PSR J1713+0747, providing additional geometric constraints. The timing of PSR J1909−3744 is used to demonstrate that the DE405 ephemeris is a better model of the Solar system than the earlier DE200. In addition, we show that pulsar astrometric parameters measured using DE200 and DE405 often differ significantly. In order to use pulsars to search for a cosmological gravitational wave background, it is desirable to time them against each other to eliminate Earth-based time standards. We demonstrate that PSR J1909−3744 can be used as a reference against which we obtain a very small rms residual of 133 ns for PSR J1713+0747. Although the gain of the Parkes antenna is small compared to other telescopes involved in precision timing, we obtain some of the lowest rms residuals ever measured, highlighting the importance of good instrumentation such as CPSR2 and good analysis software.

Interstellar scintillation of PSR J0437–4715 on two scales

We sought to determine the scale of scintillation in the interstellar plasma of PSR J0437-4715. We used the Very Long Baseline Array to obtain scintillation amplitude and phase data, from dynamic spectra at 327 MHz. We observe two scales of scintillation of pulsar PSR J0437-4715, differing by more than an order of magnitude in scintillation bandwidth. The wider-bandwidth scale of scintillation that we observe indicates less scattering for this pulsar than for other nearby pulsars, except for PSR B0950+08.