A. Jessner

The International Pulsar Timing Array project: using pulsars as a gravitational wave detector

The International Pulsar Timing Array project combines observations of pulsars from both northern and southern hemisphere observatories with the main aim of detecting ultra-low frequency (similar to 10(-9)-10(-8) Hz) gravitational waves. Here we introduce the project, review the methods used to search for gravitational waves emitted from coalescing supermassive binary black-hole systems in the centres of merging galaxies and discuss the status of the project.

The Characteristics of Millisecond Pulsar Emission. I. Spectra, Pulse Shapes, and the Beaming Fraction

The extreme physical conditions in millisecond pulsar magnetospheres, as well as an evolutionary history that differs from that of normal pulsars, raise the question whether these objects also differ in their radio emission properties. We have monitored a large sample of millisecond pulsars for a period of 3 yr using the 100 m Effelsberg radio telescope in order to compare the radio emission properties of these two pulsar populations. Our sample comprises a homogeneous data set of very high quality. With some notable exceptions, our findings suggest that the two groups of objects share many common properties. A comparison of the spectral indices between samples of normal and millisecond pulsars demonstrates that millisecond pulsar spectra are not significantly different from those of normal pulsars. This is contrary to what has previously been thought. There is evidence, however, that millisecond pulsars are slightly less luminous and less efficient radio emitters than normal pulsars. We confirm recent suggestions that a diversity exists among the luminosities of millisecond pulsars, with the isolated millisecond pulsars being less luminous than the binary millisecond pulsars, implying that the different evolutionary history has an influence on the emission properties. There are indications that old millisecond pulsars exhibit somewhat flatter spectra than the presumably younger ones. We present evidence that, contrary to common belief, the millisecond pulsar profiles are only marginally more complex than those found among the normal pulsar population. Moreover, the development of the profiles with frequency is rather slow, suggesting very compact magnetospheres. The profile development seems to anticorrelate with the companion mass and the spin period, again suggesting that the amount of mass transfer in a binary system might directly influence the emission properties. The angular radius of radio beams of millisecond pulsars does not follow the scaling predicted from a canonical pulsar model applicable for normal pulsars. Instead, they are systematically smaller, supporting the concept of a critical rotational period below which such a scaling ceases to exist. The smaller inferred luminosity and narrower emission beams will need to be considered in future calculations of the birthrate of the Galactic population.

A 2.1 M☉ Pulsar Measured by Relativistic Orbital Decay

PSR J0751+1807 is a millisecond pulsar in a circular 6 hr binary system with a helium white dwarf secondary. Through high-precision pulse timing measurements with the Arecibo and Effelsberg radio telescopes, we have detected the decay of its orbit due to emission of gravitational radiation. This is the first detection of the relativistic orbital decay of a low-mass, circular binary pulsar system. The measured rate of change in orbital period, corrected for acceleration biases, is = (-6.4 ± 0.9) × 10-14. Interpreted in the context of general relativity, and combined with measurement of Shapiro delay, it implies a pulsar mass of 2.1 ± 0.2 M☉, the most massive pulsar measured. This adds to the emerging trend toward relatively high neutron star masses in neutron star-white dwarf binaries. In addition, there is some evidence for an inverse correlation between pulsar mass and orbital period in these systems. We consider alternatives to the general relativistic analysis of the data, and we use the pulsar timing data to place limits on violations of the strong equivalence principle.

Placing Limits on the Stochastic Gravitational-Wave Background Using European Pulsar Timing Array Data

The paper ‘Placing limits on the stochastic gravitational-wave background using European Pulsar Timing Array data’ was published in Mon. Not. R. Astron. Soc. 414, 3117–3128 (2011).

The Characteristics of Millisecond Pulsar Emission. II. Polarimetry

We have made polarimetric monitoring observations of most of the millisecond pulsars visible from the northern hemisphere at 1410 MHz over a period of 3 yr. Their emission properties are presented here and compared with those of normal pulsars. Although we demonstrated in Paper I that millisecond pulsars exhibit the same flux density spectra and a similar profile complexity, the results presented here suggest that millisecond pulsar profiles do not comply with the predictions of classification schemes based on normal pulsars. The frequency development of a large number of millisecond pulsar profiles is abnormal when compared with the development seen in normal pulsars. Moreover, the polarization characteristics suggest that millisecond-pulsar magnetospheres might not simply represent scaled versions of the magnetospheres of normal pulsars, supporting the results of Paper I. However, phenomena such as mode-changing activity in both intensity and polarization are recognized here for the first time (e.g., for J1730-2304). This suggests that while the basic emission mechanism remains insensitive to rotational period, the conditions that regulate the radio emission in the canonical pulsar model might be satisfied at different regions in millisecond pulsar magnetospheres. At least three types of model have been proposed to describe the millisecond pulsar magnetospheres, ranging from distorted magnetic field configurations resulting from the recycled nature of these sources to traditional polar-cap emission and emission from outer gaps. A comparison of the predictions of these models with observations suggests that individual cases are better explained by different processes. However, we show that millisecond pulsars can be grouped according to common emission properties, a grouping that awaits verification with future multifrequency observations.

The Characteristics of Millisecond Pulsar Emission. III. From Low to High Frequencies

In this paper we present the first observations of a large sample of millisecond pulsars at frequencies of 2.7 GHz (11 cm) and 4.9 GHz (6 cm). For almost all sources, these represent the first 11 cm observations ever. The new measurements more than double the number of millisecond pulsars studied at 6 cm. Our new flux measurements extend the known spectra for millisecond pulsars to the highest frequencies to date. The coverage of more than a decade of the radio spectrum allows us for the first time to search for spectral breaks, as so often observed in normal pulsars around 1 GHz. The results suggest that, unlike normal pulsars, millisecond pulsar spectra can be largely described by a single power law. We align the observed millisecond pulsar profiles with data from lower frequencies to search for indications of disturbed magnetic fields, and attempt to resolve questions that have been raised in recent literature. Deviations from a dipolar magnetic field structure are not evident, and absolute timing across the wide frequency range with a single dispersion measure is possible. We seem to observe mainly unfilled emission beams, which must originate from a very compact region. The existence of nondipolar field components therefore cannot be excluded. A compact emission region is also suggested by a remarkably constant profile width or component separation over a very wide frequency range. This observed difference from the emission properties of normal pulsars is highly significant. For a few sources, polarization data at 2.7 and 4.9 GHz could also be obtained that indicate that despite the typically larger degree of polarization at lower frequencies, millisecond pulsars are weakly polarized or even unpolarized at frequencies above 3 GHz. The simultaneous decrease in degree of polarization and the constant profile width thus question proposals that link depolarization and decreasing profile width for normal pulsars to the same propagation effect (i.e., birefringence). Comparing the properties of core and conal-like profile components to those of normal pulsars, we find less significant patterns in their spectral evolution for the population of millisecond pulsars. Hence, we suggest that core and conal emission may be created by the same emission process. Given the small change in profile width, the indicated depolarization of the radiation, and the possible simple flux density spectra, MSP emission properties tend to resemble those of normal pulsars, only shifted toward higher frequencies.

High-precision timing of 42 millisecond pulsars with the European Pulsar Timing Array

We report on timing, flux density, and polarimetric observations of the transient magnetar and 5.54 s radio pulsar XTE J1810-197 using the GBT, Nancay, and Parkes radio telescopes beginning in early 2006, until its sudden disappearance as a radio source in late 2008. Repeated observations through 2016 have not detected radio pulsations again. The torque on the neutron star, as inferred from its rotation frequency derivative f-dot, decreased in an unsteady manner by a factor of 3 in the first year of radio monitoring. In contrast, during its final year as a detectable radio source, the torque decreased steadily by only 9%. The period-averaged flux density, after decreasing by a factor of 20 during the first 10 months of radio monitoring, remained steady in the next 22 months, at an average of 0.7+/-0.3 mJy at 1.4 GHz, while still showing day-to-day fluctuations by factors of a few. There is evidence that during this last phase of radio activity the magnetar had a steep radio spectrum, in contrast to earlier behavior. There was no secular decrease that presaged its radio demise. During this time the pulse profile continued to display large variations, and polarimetry indicates that the magnetic geometry remained consistent with that of earlier times. We supplement these results with X-ray timing of the pulsar from its outburst in 2003 up to 2014. For the first 4 years, XTE J1810-197 experienced non-monotonic excursions in f-dot by at least a factor of 8. But since 2007, its f-dot has remained relatively stable near its minimum observed value. The only apparent event in the X-ray record that is possibly contemporaneous with the radio shut-down is a decrease of ~20% in the hot-spot flux in 2008-2009, to a stable, minimum value. However, the permanence of the high-amplitude, thermal X-ray pulse, even after the radio demise, implies continuing magnetar activity.

Multi-telescope timing of PSR J1518+4904

Context. PSR J1518+4904 is one of only 9 known double neutron star systems. These systems are highly valuable for measuring the masses of neutron stars, measuring the effects of gravity, and testing gravitational theories. Aims. We determine an improved timing solution for a mildly relativistic double neutron star system, combining data from multiple telescopes. We set better constraints on relativistic parameters and the separate masses of the system, and discuss the evolution of PSR J1518+4904 in the context of other double neutron star systems. Methods. PSR J1518+4904 has been regularly observed for more than 10 years by the European Pulsar Timing Array (EPTA) network using the Westerbork, Jodrell Bank, Effelsberg and Nancay radio telescopes. The data were analysed using the updated timing software tempo2. Results. We have improved the timing solution for this double neutron star system. The periastron advance has been refined and a significant detection of proper motion is presented. It is not likely that more post-Keplerian parameters, with which the individual neutron star masses and the inclination angle of the system can be determined separately, can be measured in the near future. Conclusions. Using a combination of the high-quality data sets present in the EPTA collaboration, extended with the original ,

The European Pulsar Timing Array:current efforts and a LEAP toward the future

The European Pulsar Timing Array (EPTA) is a multi-institutional, multi-telescope collaboration, with the goal of using high-precision pulsar timing to directly detect gravitational waves. In this paper we discuss the EPTA member telescopes, current achieved timing precision and near-future goals. We report a preliminary upper limit to the amplitude of a gravitational wave background. We also discuss the Large European Array for Pulsars, in which the five major European telescopes involved in pulsar timing will be combined to provide a coherent array that will give similar sensitivity to the Arecibo radio telescope, and larger sky coverage.

Generic tests of the existence of the gravitational dipole radiation and the variation of the gravitational constant

We present results from the high precision timing analysis of the pulsar-white dwarf (WD) binary PSR J1012+5307 using 15 years of multi-telescope data. Observations were performed regularly by the European Pulsar Timing Array (EPTA) network, consisting of Effelsberg, Jodrell Bank, Westerbork and Nancay. All the timing param- eters have been improved from the previously published values, most by an order of magnitude. In addition, a parallax measurement of π = 1.2(3)mas is obtained for the first time for PSR J1012+5307, being consistent with the optical estimation from the WD companion. Combining improved 3D velocity information and models for the Galactic potential the complete evolutionary Galactic path of the system is obtained. A new intrinsic eccentricity upper limit of e < 8.4× 10 7 is acquired, one of the smallest calculated for a binary system and a measurement of the variation of the projected semi-major axis also constrains the systems orbital orientation for the first time. It is shown that PSR J1012+5307 is an ideal laboratory for testing alternative theories of gravity. The measurement of the change of the orbital period of the system of u Pb = 5(1)× 10 14 is used to set an upper limit on the dipole gravitational wave emission that is valid for a wide class of alternative theories of gravity. Moreover, it is shown that in combination with other binary pulsars PSR J1012+5307 is an ideal system to provide self-consistent, generic limits, based only on millisecond pulsar data, for the dipole radiation and the variation of the gravitational constant u G.