PSR J0437-4715 Research Articles

Bursting emission from PSR B0611+22

Over the past decade it has become apparent that a class of `bursting pulsars' exist with the discovery of PSR J1752+2359 and PSR J1938+2213. In these pulsars, a sharp increase in the emission is observed that then tends to systematically drop-off from pulse-to-pulse. In this paper we describe the discovery of such a relationship in high-sensitivity observations of the young (characteristic age of 90,000 yrs) 0.33 s pulsar B0611+22 at both 327 MHz and 1400 MHz with the Arecibo radio telescope. While Nowakowski previously showed that B0611+22 has mode-switching properties, the data presented here show, for the first time, that this pulsar emits bursts with characteristic time-scales of several hundred seconds. At 327 MHz, the pulsar shows steady behaviour in one emission mode which is enhanced by bursting emission slightly offset in pulse phase from this steady emission. Contrastingly at 1400 MHz, the two modes appear to behave in a competing operation while still offset in phase. Using a fluctuation spectrum analysis, we also investigate each mode independently for sub-pulse drifting. Neither emission mode (i.e. during bursts or persistent emission) shows the presence of the drifting sub-pulse phenomenon. The bursting phenomena seen here appears to be a hybrid between bursting seen in other pulsars and the bistable profile illumination behaviour reported in two other pulsars by Rankin et al. Further examples of this cross-frequency behaviour are required, as this phenomenon may be quite common among the pulsar population.

Statistical Study of Neutron Star Glitches

Neutron star glitches; spanning a period of 42 years of pulsar timing were studied. These glitches are from Radio, X-ray, Anomalous X-ray and Milliseconds Pulsars. Radio Pulsars dominates the glitch events, contributing 87% of the glitches. Pulsars of characteristic age bracket 103 to 105 yrs dominated the glitch events, at a rate of 5.2 glitches per year per pulsar. Pulsar of the above age bracket exhibits large size glitches compared to others. A large frequency spin-up (△v) is generally associated with large frequency derivative jump (△v ̇). The distribution of the glitch magnitude (△v/v) is bimodal reaffirming dual glitch mechanism, but that of spin-up (△v) is tending towards multi-modal. Moreover, glitches in Vela pulsar and PSR J0537-6910 showed strong elasticity of the objects, suggesting that the interiors of these objects are in thermal equilibrium. Glitches from PSR J1740-3015 and PSR J1341-6220 appeared to occur in groups, suggesting that their interior fluid is switching between two phases. We discussed the glitch activity of young pulsars in terms of vortex creep model.

Braking PSR J1734–3333 with a possible fall-back disk

The very small braking index of PSR J1734–3333, n = 0.9 ± 0.2, challenges the current theories of braking mechanisms in pulsars. We present a possible interpretation that this pulsar is surrounded by a fall-back disk and braked by it. A modified braking torque is proposed based on the competition between the magnetic energy density of the pulsar and the kinetic energy density of the fall-back disk. With this torque, a self-similar disk can fit all the observed parameters of PSR J1734–3333 with natural initial values of parameters. In this regime, the star will evolve to the region having anomalous X-ray pulsars and soft gamma repeaters in the P – Ṗ diagram in about 20 000 years and stay there for a very long time. The mass of the disk around PSR J1734–3333 in our model is about 10M⊕, similar to the observed mass of the disk around AXP 4U 0142+61.

PULSE BROADENING MEASUREMENTS FROM THE GALACTIC CENTER PULSAR J1745-2900

We present temporal scattering measurements of single pulses and average profiles of PSR J1745--2900, a magnetar recently discovered only 3 arcsec away from Sagittarius A* (Sgr A*), from 1.2 - 18.95 GHz using the Effelsberg 100-m Radio Telescope, the Nan\c{c}ay Decimetric Radio Telescope, and the Jodrell Bank Lovell Telescope. Single pulse analysis shows that the integrated pulse profile above 2 GHz is dominated by pulse jitter, while below 2 GHz the pulse profile shape is dominated by scattering. The high dispersion measure and rotation measure of the magnetar suggest that it is close to Sgr A* (within ~0.1 pc). This is the first object in the GC with both pulse broadening and angular broadening measurements. We measure a pulse broadening spectral index of alpha = -3.8 +/- 0.2 and a pulse broadening time scale at 1 GHz of tau_GHz = 1.3 +/- 0.2 s, which is several orders of magnitude lower than the scattering predicted by the NE2001 model (Cordes and Lazio 2002). If this scattering timescale is representative of the GC as a whole, then previous surveys should have detected many pulsars. The lack of detections implies either our understanding of scattering in the GC is incomplete or there are fewer pulsars in the GC than previously predicted. Given that magnetars are a rare class of radio pulsar, we believe that there are many canonical and millisecond pulsars in the GC, and not surprisingly, scattering regions in the GC have complex spatial structures.

Pulsar Timing Residuals Induced by Gravitational Waves from Single Non-evolving Supermassive Black Hole Binaries with Elliptical Orbits

The pulsar timing residuals induced by gravitational waves from non-evolving single binary sources with general elliptical orbits are analyzed. For different orbital eccentricities, the timing residuals present different properties. The standard deviations of the timing residuals induced by a fixed gravitational wave source are calculated for different values of the eccentricity. We also analyze the timing residuals of PSR J0437-4715 induced by one of the best known single gravitational wave sources, the supermassive black hole binary in the blazar OJ287.

Towards a theory of extremely intermittent pulsars

We investigate whether there may be one or many companions orbiting at close distance to the light cylinder around the extremely intermittent pulsars PSR B1931+24 and PSR J1841-0500. These pulsars, behaving in a standard way when they are active, also "switch off" for durations of several days, during which their magnetospheric activity is interrupted or reduced. We constrained our analysis on eight fundamental properties of PSR B1931+24 that summarise the observations. We considered that the disruption/activation of the magnetospheric activity would be caused by the direct interaction of the star with the Alfv\'en wings emanating from the companions. We also considered the recurrence period of 70 days to be the period of precession of the periastron of the companions orbit. We analysed in which way the time scale of the "on/off" pseudo-cycle would be conditioned by the precession of the periastron and not by the orbital time scale, and we derived a set of orbital constraints that we solved. We then compared the model, based on PSR 1931+24, with the known properties of PSR 1841+0500. We conclude that PSR B1931+24 may be surrounded at a close distance to the star by a stream of small bodies of kilometric or sub-kilometric sizes that could originate from the tidal disruption of a body of moderate size that fell at a close distance to the neutron star on an initially very eccentric orbit. This scenario is also compatible with the properties of PSR J1841-0500, although the properties of PSR J1841-0500 are, by now, less constrained. These results raise new questions. Why are the asteroids not yet evaporated ? What kind of interaction can explain the disruption of the magnetospheric activity ? These questions are the object of two papers in preparation that will complete the present analysis.

X-RAY OBSERVATIONS OF HIGH-B RADIO PULSARS

The study of high-magnetic-field pulsars is important for examining the relationships between radio pulsars, magnetars, and X-ray-isolated neutron stars (XINSs). Here we report on X-ray observations of three such high-magnetic-field radio pulsars. We first present the results of a deep XMM-Newton observation of PSR J1734-3333, taken to follow up on its initial detection in 2009. The pulsar's spectrum is well fit by a blackbody with a temperature of 300 +/- 60 eV, with bolometric luminosity L_bb = 2.0(+2.2 -0.7)e+32 erg/s = 0.0036E_dot for a distance of 6.1 kpc. We detect no X-ray pulsations from the source, setting a 1 sigma upper limit on the pulsed fraction of 60% in the 0.5-3 keV band. We compare PSR J1734-3333 to other rotation-powered pulsars of similar age and find that it is significantly hotter, supporting the hypothesis that the magnetic field affects the observed thermal properties of pulsars. We also report on XMM-Newton and Chandra observations of PSRs B1845-19 and J1001-5939. We do not detect either pulsar, setting 3 sigma upper limits on their blackbody temperatures of 48 and 56 eV, respectively. Despite the similarities in rotational properties, these sources are significantly cooler than all but one of the XINSs, which we attribute to the two groups having been born with different magnetic fields and hence evolving differently.

HIGH-FIDELITY RADIO ASTRONOMICAL POLARIMETRY USING A MILLISECOND PULSAR AS A POLARIZED REFERENCE SOURCE

A new method of polarimetric calibration is presented in which the instrumental response is derived from regular observations of PSR J0437-4715 based on the assumption that the mean polarized emission from this millisecond pulsar remains constant over time. The technique is applicable to any experiment in which high-fidelity polarimetry is required over long time scales; it is demonstrated by calibrating 7.2 years of high-precision timing observations of PSR J1022+1001 made at the Parkes Observatory. Application of the new technique followed by arrival time estimation using matrix template matching yields post-fit residuals with an uncertainty-weighted standard deviation of 880 ns, two times smaller than that of arrival time residuals obtained via conventional methods of calibration and arrival time estimation. The precision achieved by this experiment yields the first significant measurements of the secular variation of the projected semi-major axis, the precession of periastron, and the Shapiro delay; it also places PSR J1022+1001 among the ten best pulsars regularly observed as part of the Parkes Pulsar Timing Array (PPTA) project. It is shown that the timing accuracy of a large fraction of the pulsars in the PPTA is currently limited by the systematic timing error due to instrumental polarization artifacts. More importantly, long-term variations of systematic error are correlated between different pulsars, which adversely affects the primary objectives of any pulsar timing array experiment. These limitations may be overcome by adopting the techniques presented in this work, which relax the demand for instrumental polarization purity and thereby have the potential to reduce the development cost of next-generation telescopes such as the Square Kilometre Array.

RADIO AND GAMMA-RAY PULSED EMISSION FROM MILLISECOND PULSARS

Pulsed gamma-ray emission from millisecond pulsars (MSPs) has been detected by the sensitive Fermi, which sheds light on studies of the emission region and mechanism. In particular, the specific patterns of radio and gamma-ray emission from PSR J0101-6422 challenge the popular pulsar models, e.g. outer gap and two-pole caustic (TPC) models. Using the three dimension (3D) annular gap model, we have jointly simulated radio and gamma-ray light curves for three representative MSPs (PSR J0034-0534, PSR J0101-6422 and PSR J0437-4715) with distinct radio phase lags and present the best simulated results for these MSPs, particularly for PSR J0101-6422 with complex radio and gamma-ray pulse profiles and for PSR J0437-4715 with a radio interpulse. It is found that both the gamma-ray and radio emission originate from the annular gap region located in only one magnetic pole, and the radio emission region is not primarily lower than the gamma-ray region in most cases. In addition, the annular gap model with a small magnetic inclination angle instead of "orthogonal rotator" can account for MSPs' radio interpulse with a large phase separation from the main pulse. The annular gap model is a self-consistent model not only for young pulsars but also MSPs, and multi-wavelength light curves can be fundamentally explained by this model.

Pulsar searches of Fermi unassociated sources with the Effelsberg telescope

Using the 100-m Effelsberg radio telescope operating at 1.36 GHz, we have performed a targeted radio pulsar survey of 289 unassociated γ-ray sources discovered by the Large Area Telescope (LAT) aboard the Fermi satellite and published in the 1FGL catalogue (Abdo et al. 2010a). This survey resulted in the discovery of millisecond pulsar J1745+1017, which resides in a short-period binary system with a low-mass companion, |$M_{c,\text{min}} \sim 0.0137\,\mathrm{M}_{\odot }$|⁠, indicative of ‘black widow’ type systems. A 2-yr timing campaign has produced a refined radio ephemeris, accurate enough to allow for phase-folding of the LAT photons, resulting in the detection of a dual-peaked γ-ray light curve, proving that PSR J1745+1017 is the source responsible for the γ-ray emission seen in 1FGL J1745.5+1018 (2FGL J1745.6+1015; Nolan et al. 2012). We find the γ-ray spectrum of PSR J1745+1017 to be well modelled by an exponentially cut-off power law with cut-off energy 3.2 GeV and photon index 1.6. The observed sources are known to contain a further 10 newly discovered pulsars which were undetected in this survey. Our radio observations of these sources are discussed and in all cases limiting flux densities are calculated. The reasons behind the seemingly low yield of discoveries are also discussed.

THE NEAREST MILLISECOND PULSAR REVISITED WITHXMM-NEWTON: IMPROVED MASS-RADIUS CONSTRAINTS FOR PSR J0437-4715

I present an analysis of the deepest X-ray exposure of a radio millisecond pulsar (MSP) to date, an X-ray Multi Mirror-Newton European Photon Imaging Camera spectroscopic and timing observation of the nearest known MSP, PSR J0437--4715. The timing data clearly reveal a secondary broad X-ray pulse offset from the main pulse by $\sim$0.55 in rotational phase. In the context of a model of surface thermal emission from the hot polar caps of the neutron star, this can be plausibly explained by a magnetic dipole field that is significantly displaced from the stellar center. Such an offset, if commonplace in MSPs, has important implications for studies of the pulsar population, high energy pulsed emission, and the pulsar contribution to cosmic ray positrons. The continuum emission shows evidence for at least three thermal components, with the hottest radiation most likely originating from the hot magnetic polar caps and the cooler emission from the bulk of the surface. I present pulse phase-resolved X-ray spectroscopy of PSR J0437--4715, which for the first time, properly accounts for the system geometry of a radio pulsar. Such an approach is essential for unbiased measurements of the temperatures and emission areas of polar cap radiation from pulsars. Detailed modelling of the thermal pulses, including relativistic and atmospheric effects, provides a constraint on the redshift-corrected neutron star radius of R>11.1 km (at 3 sigma conf.) for the current radio timing mass measurement of 1.76 M_sun. This limit favors "stiff'" equations of state.

Deep hard X-ray survey of the Large Magellanic Cloud

Results of the deep survey of the Large Magellanic Cloud (LMC), performed with the INTEGRAL observatory, are presented. The long exposure (~7 Ms) allowed us to detect twenty one sources in this sky region: ten belonging to the LMC itself (7 HMXBs, 2 PSRs, 1 LMXB), six of extragalactic origin and three belonging to other galaxies from the Local Group - the Milky Way (2 sources) and Small Magellanic Cloud (1 source). Four new hard X-ray sources of these 21 ones were discovered during the survey in addition to IGR J05414-6858 reported earlier; two of them were identified with extragalactic objects. We report also for the first time the detection of a hard X-ray emission from the Crab-like pulsar PSR J0537-6910 and identification of the hard X-ray source IGR J05305-6559 with the high-mass X-ray binary EXO 053109-6609.

RADIO AND X-RAY OBSERVATIONS OF THE INTERMITTENT PULSAR J1832+0029

We report on radio and X-ray observations of PSR 1832+0029, a 533-ms radio pulsar discovered in the Parkes Multibeam Pulsar Survey. From radio observations taken with the Parkes, Lovell and Arecibo telescopes, we show that this pulsar exhibits two spindown states akin to PSRs B1931+24 reported by Kramer et al. and J1841-0500 reported by Camilo et al. Unlike PSR B1931+24, which switches between "on" and "off" states on a 30--40 day time-scale, PSR 1832+0029 is similar to PSR J1841-0500 in that it spends a much longer period of time in the off-state. So far, we have fully sampled two off-states. The first one lasted between 560 and 640 days and the second one lasted between 810 and 835 days. From our radio timing observations, the ratio of on/off spindown rates is $1.77 \pm 0.03$. Chandra observations carried out during both the on- and off-states of this pulsar failed to detect any emission. Our results challenge but do not rule out models involving accretion onto the neutron star from a low-mass stellar companion. In spite of the small number of intermittent pulsars currently known, difficulties in discovering them and in quantifying their behavior imply that their total population could be substantial.

PSR J1841-0500: A RADIO PULSAR THAT MOSTLY IS NOT THERE

In a search for radio pulsations from the magnetar 1E 1841-045, we have discovered the unrelated pulsar J1841-0500, with rotation period P=0.9 s and characteristic age 0.4 Myr. One year after discovery with the Parkes telescope at 3 GHz, radio emission ceased from this bright pulsar. After 580 days, emission resumed as before. The P-dot during both on states is 250% of the average in the off state. PSR J1841-0500 is a second example of an extremely intermittent pulsar, although with a much longer off period and larger ratio of spin-down rates than PSR B1931+24. The new pulsar is hugely scattered by the ISM, with a fitted timescale referenced to 1 GHz of tau_1=2 s. Based on polarimetric observations at 5 GHz with the Green Bank Telescope, the intrinsic pulse profile has not obviously changed between the two on states observed so far, although relatively small variations cannot be excluded. The magnitude of its rotation measure is the largest known, RM=-3000 rad/m^2, and with a dispersion measure DM=532 pc/cc implies a large electron-weighted average magnetic field strength along the line of sight, 7 microG.

THE SPECTRUM OF THE RECYCLED PSR J0437–4715 AND ITS WHITE DWARF COMPANION

We present extensive spectral and photometric observations of the recycled pulsar/white-dwarf binary containing PSR J0437-4715, which we analyzed together with archival X-ray and gamma-ray data, to obtain the complete mid-infrared to gamma-ray spectrum. We first fit each part of the spectrum separately, and then the whole multi-wavelength spectrum. We find that the optical-infrared part of the spectrum is well fit by a cool white dwarf atmosphere model with pure hydrogen composition. The model atmosphere (Teff = 3950pm150K, log g=6.98pm0.15, R_WD=(1.9pm0.2)e9 cm) fits our spectral data remarkably well for the known mass and distance (M=0.25pm0.02Msun, d=156.3pm1.3pc), yielding the white dwarf age (tau=6.0pm0.5Gyr). In the UV, we find a spectral shape consistent with thermal emission from the bulk of the neutron star surface, with surface temperature between 1.25e5 and 3.5e5K. The temperature of the thermal spectrum suggests that some heating mechanism operates throughout the life of the neutron star. The temperature distribution on the neutron star surface is non-uniform. In the X-rays, we confirm the presence of a high-energy tail which is consistent with a continuation of the cut-off power-law component (Gamma=1.56pm0.01, Ecut=1.1pm0.2GeV) that is seen in gamma-rays and perhaps even extends to the near-UV.

A new time-domain estimation method for period of pulsars

The estimation of pulsar period is important for pulsar searching, observation and timing. In this paper, the frequency-domain method which is popular in periodicity searches is analyzed, and further more, a novel method in time-domain is proposed to estimate the cyclo-period of pulsars based on the second-order cyclo-stationary model of the radiation signal. Firstly, the discrete-time observing signal is blocked by assumed period which is alterable. Then, the correlation estimations and their sample variances are computed each as a function of blocking operator index, where the largest peaks display a periodic pattern, from which the cyclo-period can be obtained. Using this method, the period of PSR J0437-4715 with single pulse is estimated well based on original signal, and the period of PSR B1821-24 with double pulses is also estimated based on the simulated signal. The same work is done for the observed data from ATNF EPN pulsar database. The simulation results and the experimental results for many pulsars show that compared with the classical Fourier-transform method, this algorithm is non-sensitive to the noise, and it is effective even though in the case of low signal-to-noise ratio. And it is of low computation complexity compared with the bispectrum coherence method. This method possesses higher popularization value in real-time observation and processing for weak pulsar signal, though it does not work well in dealing with noncontinuous observing data.

Profile-shape stability and phase-jitter analyses of millisecond pulsars

Millisecond pulsars (MSPs) have been studied in detail since their discovery in 1982. The integrated pulse profiles of MSPs appear to be stable, which enables precision monitoring of the pulse times of arrival (TOAs). However, for individual pulses the shape and arrival phase can vary dramatically, which is known as pulse jitter. In this paper, we investigate the stability of integrated pulse profiles for 5 MSPs, and estimate the amount of jitter for PSR J0437-4715. We do not detect intrinsic profile shape variation based on integration times from ~10 to ~100 s with the provided instrumental sensitivity. For PSR J0437-4715 we calculate the jitter parameter to be f_J=0.067+-0.002, and demonstrate that the result is not significantly affected by instrumental TOA uncertainties. Jitter noise is also found to be independent of observing frequency and bandwidth around 1.4 GHz on frequency scales of <100 MHz, which supports the idea that pulses within narrow frequency scale are equally jittered. In addition, we point out that pulse jitter would limit TOA calculation for the timing observations with future telescopes like the Square Kilometre Array and the Five hundred metre Aperture Spherical Telescope. A quantitative understanding of pulse profile stability and the contribution of jitter would enable improved TOA calculations, which are essential for the ongoing endeavours in pulsar timing, such as the detection of the stochastic gravitational wave background.

THE VLT-FLAMES TARANTULA SURVEY: THE FASTEST ROTATING O-TYPE STAR AND SHORTEST PERIOD LMC PULSAR—REMNANTS OF A SUPERNOVA DISRUPTED BINARY?

We present a spectroscopic analysis of an extremely rapidly rotating late O-type star, VFTS102, observed during a spectroscopic survey of 30 Doradus. VFTS102 has a projected rotational velocity larger than 500\kms\ and probably as large as 600\kms; as such it would appear to be the most rapidly rotating massive star currently identified. Its radial velocity differs by 40\kms\ from the mean for 30 Doradus, suggesting that it is a runaway. VFTS102 lies 12 pcs from the X-ray pulsar PSR J0537-6910 in the tail of its X-ray diffuse emission. We suggest that these objects originated from a binary system with the rotational and radial velocities of VFTS102 resulting from mass transfer from the progenitor of PSR J0537-691 and the supernova explosion respectively.

THE BRAKING INDEX OF PSR J1734–3333 AND THE MAGNETAR POPULATION

PSR J1734−3333 is a radio pulsar rotating with a period P = 1.17 s and slowing down with a period derivative , the third largest among rotation-powered pulsars. These properties are midway between those of normal rotation-powered pulsars and magnetars, two populations of neutron stars that are notably different in their emission properties. Here we report on the measurement of the second period derivative of the rotation of PSR J1734−3333 and calculate a braking index n = 0.9 ± 0.2. This value is well below 3, the value expected for an electromagnetic braking due to a constant magnetic dipole, and indicates that this pulsar may soon have the rotational properties of a magnetar. While there are several mechanisms that could lead to such a low braking index, we discuss this observation, together with the properties exhibited by some other high- rotation-powered pulsars, and interpret it as evidence of a possible evolutionary route for magnetars through a radio-pulsar phase, supporting a unified description of the two classes of the object.

The impact of a stochastic gravitational-wave background on pulsar timing parameters

Gravitational waves are predicted by Einstein's theory of general relativity as well as other theories of gravity. The rotational stability of the fastest pulsars means that timing of an array of these objects can be used to detect and investigate gravitational waves. Simultaneously, however, pulsar timing is used to estimate spin period, period derivative, astrometric, and binary parameters. Here we calculate the effects that a stochastic background of gravitational waves has on pulsar timing parameters through the use of simulations and data from the millisecond pulsars PSR J0437--4715 and PSR J1713+0747. We show that the reported timing uncertainties become underestimated with increasing background amplitude by up to a factor of $\sim10$ for a stochastic gravitational-wave background amplitude of $A=5\times 10^{-15}$, where $A$ is the amplitude of the characteristic strain spectrum at one-year gravitational wave periods. We find evidence for prominent low-frequency spectral leakage in simulated data sets including a stochastic gravitational-wave background. We use these simulations along with independent Very Long Baseline Interferometry (VLBI) measurements of parallax to set a 2--sigma upper limit of $A\le9.1\times 10^{-14}$. We find that different supermassive black hole assembly scenarios do not have a significant effect on the calculated upper limits. We also test the effects that ultralow--frequency (10$^{-12}$--10$^{-9}$ Hz) gravitational waves have on binary pulsar parameter measurements and find that the corruption of these parameters is less than those due to $10^{-9}$--$10^{-7}$ Hz gravitational waves.