arXiv subjects

N. Wex

Publications and source records attributed to N. Wex.

52 records · Page 3Linked to original sources

arXiv2010

A white dwarf companion to the relativistic pulsar PSR J1141-6545

Pulsars with compact companions in close eccentric orbits are unique laboratories for testing general relativity and alternative theories of gravity. Moreover, they are excellent targets for future gravitational wave experiments like LISA and they are also highly important for understanding the equation of state of super-dense matter and the evolution of massive binaries. Here we report on optical observations of the 1.02 M_sun companion to the pulsar PSR J1141-6545. We detect an optical counterpart with apparent magnitudes V=25.08(11) and R=24.38(14), consistent with the timing position of the pulsar. We demonstrate that our results are in agreement with a white dwarf companion. However the latter is redder than expected and the inferred values are not consistent with the theoretical cooling tracks, preventing us from deriving the exact age. Our results confirm the importance of the PSR J1141-6545 system for gravitational experiments.

astro-ph.SR↗

arXiv2010

Generic Gravity Tests with the Double Pulsar

Presently the double pulsar allows for the measurement of six post-Keplerian parameters. In addition, its double-line nature gives access to the projected semi-major axes of both orbits. We use this wealth of information to pose some very general restrictions on a wide class of conservative and semi-conservative theories of gravity.

gr-qc↗

arXiv2009

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 Nançay. All the timing parameters 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 $e<8.4\times 10^{-7}$ 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 system's 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 $\dot{P}_{b} = 5(1)\times 10^{-14}$ 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 $\dot{G}$.

astro-ph.GA↗

arXiv2009

A new technique for timing the double pulsar system

In 2004, McLaughlin et al. discovered a phenomenon in the radio emission of PSR J0737-3039B (B) that resembles drifting sub-pulses. The repeat rate of the sub-pulses is equal to the spin frequency of PSR J0737-3039A (A); this led to the suggestion that they are caused by incidence upon B's magnetosphere of electromagnetic radiation from A. Here we describe a geometrical model which predicts the delay of B's sub-pulses relative to A's radio pulses. We show that measuring these delays is equivalent to tracking A's rotation from the point of view of an hypothetical observer located near B. This has three main astrophysical applications: (a) to determine the sense of rotation of A relative to its orbital plane; (b) to estimate where in B's magnetosphere the radio sub-pulses are modulated and (c) to provide an independent estimate of the mass ratio of A and B. The latter might improve existing tests of gravitational theories using this system.

astro-ph.IM↗

arXiv2007

A characteristic observable signature of preferred frame effects in relativistic binary pulsars

In this paper we develop a consistent, phenomenological methodology to measure preferred-frame effects (PFEs) in binary pulsars that exhibit a high rate of periastron advance. We show that in these systems the existence of a preferred frame for gravity leads to an observable characteristic `signature' in the timing data, which uniquely identifies this effect. We expand the standard Damour-Deruelle timing formula to incorporate this `signature' and show how this new PFE timing model can be used to either measure or constrain the parameters related to a violation of the local Lorentz invariance of gravity in the strong internal fields of neutron stars. In particular, we demonstrate that in the presence of PFEs we expect a set of the new timing parameters to have a unique relationship that can be measured and tested incontrovertibly. This new methodology is applied to the Double Pulsar, which turns out to be the ideal test system for this kind of experiments.The currently available dataset allows us only to study the impact of PFEs on the orbital precession rate, d omega/dt, providing limits that are, at the moment, clearly less stringent than existing limits on PFE strong-field parameters. However, simulations show that the constraints improve fast in the coming years, allowing us to study all new PFE timing parameters and to check for the unique relationship between them. Finally, we show how a combination of several suitable systems in a "PFE antenna array", expected to be availabe for instance with the Square-Kilometre-Array (SKA), provides full sensitivity to possible violations of local Lorentz invariance in strong gravitational fields in all directions of the sky. This PFE antenna array may eventually allow us to determine the direction of a preferred frame should it exist.

astro-ph↗

arXiv2001

Precision Timing Measurements of PSR J1012+5307

We present results and applications of high precision timing measurements of the millisecond pulsar J1012+5307. Combining our radio observations with results based on optical observations, we derive complete 3-D velocity information for this system. Correcting for Doppler effects, we derive the intrinsic spin parameters of this pulsar and a characteristic age of 8.6 +/- 1.9 Gyr. Our upper limit for the orbital eccentricity of only 8 * 10^-7 (68% C.L.) is the smallest ever measured for a binary system. We demonstrate that this makes the pulsar an ideal laboratory to test certain aspects of alternative theories of gravitation. Our precise measurements suggest deviations from a simple pulsar spin-down timing model, which are consistent with timing noise and the extrapolation of the known behaviour of slowly rotating pulsars.

astro-ph↗

arXiv2000

The 1997 periastron passage of the binary pulsar PSR B1259-63

We report here on multifrequency radio observations of the pulsed emission from PSR B1259-63 around the time of the closest approach to its B2e companion star. There was a general increase in the pulsar's dispersion measure and scatter broadening, and a decrease in the flux density towards periastron although changes in these parameters were seen on timescales as short as minutes. The pulsed emission disappeared 16 days prior to periastron and remained undetectable until 16 days after periastron. The observations are used to determine the parameters of the wind from the Be star. We show that a simple model, in which the wind density varies with radius as r^-2, provides a good fit to the data. The wind is clumpy with size scales <= 10^10 cm, densities of 10^6 cm^-3 and a velocity of 2000 km/s at a distance of 20 - 50 stellar radii. We find a correlation between dispersion measure variations and the pulse scattering times, suggesting that the same electrons are responsible for both effects.

astro-ph↗

arXiv2000

A 1400-MHz pilot search for young pulsars

We have used the Effelsberg 100-m radio telescope to conduct a 1400-MHz ($λ$ 21-cm) search for young and rapidly rotating radio pulsars along a 2 deg$^2$ strip of the northern Galactic plane defined by $28^{\circ} \leq l \leq 30^{\circ}$ and $|b|\leq0.5^{\circ}$. This region lies close to the Scutum spiral arm which is already known to contain a number of radio and X-ray pulsars. The search was nominally sensitive to pulsars with 1400-MHz flux densities above 0.3 mJy; this represents a threefold improvement in sensitivity over all previous searches of this region of the Galaxy. Four new long-period pulsars were discovered as a result of this survey. All three previously known pulsars in this region were also detected. The four new pulsars are relatively young ($<$ 0.5 Myr), weak ($<1$ mJy) sources with dispersion measures in the range 180--910 cm$^{-3}$ pc. None of the newly-discovered pulsars are associated with catalogued supernova remnants.

astro-ph↗

arXiv2000

Small-eccentricity binary pulsars and relativistic gravity

Small-eccentricity binary pulsars with white dwarf companions provide excellent test laboratories for various effects predicted by alternative theories of gravity, in particular tests for the emission of gravitational dipole radiation and the existence of gravitational Stark effects. We will present new limits to these effects. The statistical analysis presented here, for the first time, takes appropriately care of selection effects.

gr-qc↗

arXiv2000

Geodetic Precession and the Binary Pulsar B1913+16

A change of the component separation in the profiles of the binary pulsar PSR B1913+16 has been observed for the first time (Kramer 1998) as expected by geodetic precession. In this work we extend the previous work by accounting for recent data from the Effelsberg 100-m telescope and Arecibo Observatory and testing model predictions. We demonstrate how the new information will provide additional information on the solutions of the system geometry.

astro-ph↗

arXiv1999

Constraints on Supernova Kicks from the Double Neutron Star System PSR B1913+16

We use recent information on geodetic precession of the binary pulsar B1913+16 along with measurements of its orbital parameters and proper motion to derive new constraints on the immediate progenitor of this double neutron star system. As part of our analysis we model the motion of the binary in the Galaxy after the second supernova explosion, and we derive constraints on the unknown radial velocity. We also obtain limits on the magnitude and direction of the kick velocity imparted to the pulsar companion during the second supernova explosion. We consider the complete set of possible cases, depending on the kinematic age of the system and the 180 degree ambiguity in the pulsar spin orientation. Most interestingly, we find that the natal kick must have been directed almost perpendicular to the spin axis of the neutron star progenitor, independent of the specific pre-supernova configuration. Such a tight constraint on the kick direction has important implications for the physical mechanism responsible for the kick.

astro-ph↗

arXiv1999

Frame dragging in black hole-pulsar binaries

The discovery of frame-dragging effects in binary pulsar timing experiments requires a compact companion with sufficiently large spin. A pulsar orbiting a fast rotating black hole could provide an appropriate test system. In this paper we address questions concerning the identification of a black hole companion in such a system, the measurability of the frame dragging caused by the rotation of the black hole, and the measurability of the quadrupole moment, which would prove the presence of a Kerr black hole.

astro-ph↗

arXiv1998

Frame dragging and other precessional effects in black hole-pulsar binaries

For radio pulsars in orbit with a compact companion, pulsar timing observations have proved to be a powerful tool for identifying the physical nature of the companion. Unfortunately, perhaps the most intriguing system where such a tool could be used, a pulsar in orbit with a black hole, has yet to be discovered. In this paper we give a detailed investigation of what one can learn about the black hole companion via timing observations of the pulsar. We present an analytic calculation for the propagation delay caused by the frame-dragging effect and show that it has the same functional behavior as the modulation of the observed rotational phase of the pulsar caused by the deflection of the radio signals in the gravitational field of the companion (bending delay). Thus, contrary to statements of other authors, the frame-dragging delay is unlikely to be separately measurable in pulsar binaries where the companion is a stellar mass black hole. We demonstrate, however, that the precession of the binary orbit caused by the relativistic spin-orbit coupling can lead to observable effects which can be used to set a lower limit to the black-hole spin, or possibly allow the determination of its magnitude and orientation. We give parameter estimates for two possible systems, a 10 solar mass black hole in orbit either with a young (~0.1 s) pulsar or with a millisecond pulsar. Finally, we discuss the measurability of the quadrupole moment of the rotating black hole companion which would test the presence of a Kerr black hole. As an interesting side result of our calculations, we can give a further argument why the companion of PSR J0045-7319 cannot be a Kerr black hole.

astro-ph↗

arXiv1998

Timing models for the long-orbital period binary pulsar PSR B1259-63

The pulsar PSR B1259-63 is in a highly eccentric 3.4-yr orbit with the Be star SS 2883. Timing observations of this pulsar, made over a 7-yr period using the Parkes 64-m radio telescope, cover two periastron passages, in 1990 August and 1994 January. The timing data cannot be fitted by the normal pulsar and Keplerian binary parameters. A timing solution including a (non-precessing) Keplerian orbit and timing noise (represented as a polynomial of fifth order in time) provide a satisfactory fit to the data. However, because the Be star probably has a significant quadrupole moment, we prefer to interpret the data by a combination of timing noise, dominated by a cubic phase term, and $\dotω$ and $\dot x$ terms. We show that the $\dotω$ and $\dot x$ are likely to be a result of a precessing orbit caused by the quadrupole moment of the tilted companion star. We further rule out a number of possible physical effects which could contribute to the timing data of PSR B1259-63 on a measurable level.

astro-ph↗

arXiv1998

A timing formula for main-sequence star binary pulsars

In binary radio pulsars with a main-sequence star companion, the spin-induced quadrupole moment of the companion gives rise to a precession of the binary orbit. As a first approximation one can model the secular evolution caused by this classical spin-orbit coupling by linear-in-time changes of the longitude of periastron and the projected semi-major axis of the pulsar orbit. This simple representation of the precession of the orbit neglects two important aspects of the orbital dynamics of a binary pulsar with an oblate companion. First, the quasiperiodic effects along the orbit, due to the anisotropic $1/r^3$ nature of the quadrupole potential. Secondly, the long-term secular evolution of the binary orbit which leads to an evolution of the longitude of periastron and the projected semi-major axis which is non-linear in time. In this paper a simple timing formula for binary radio pulsars with a main-sequence star companion is presented which models the short-term secular and most of the short-term periodic effects caused by the classical spin-orbit coupling. I also give extensions of the timing formula which account for long-term secular changes in the binary pulsar motion. It is shown that the short-term periodic effects are important for the timing observations of the binary pulsar PSR B1259--63. The long-term secular effects are likely to become important in the next few years of timing observations of the binary pulsar PSR J0045--7319. They could help to restrict or even determine the moments of inertia of the companion star and thus probe its internal structure. Finally, I reinvestigate the spin-orbit precession of the binary pulsar PSR J0045--7319 since the analysis given in the literature is based on an incorrect expression for the precession of the longitude of periastron.

astro-ph↗

arXiv1995

Microlensing of pulsar radiation in the Galactic Center

We investigate the possibility of identifying massive objects (lenses) in the Galactic Center region (GC) by means of pulsar timing. The well known intensity change due to microlensing is found to be less important. For typical stellar masses, the flux magnification can be significant only if the lens passes very close to the pulsar--Earth axis. We show that in the case of a pulsar the time varying travel-time delay, which is observable because of the pulsating nature of pulsar radiation, is a much more powerful tool to investigate mass distributions in the GC. We find that a travel-time delay is measurable even for rather large distances between the lens and the pulsar--Earth axis. The time varying travel-time delay can be used to determine the mass of the lens and the ratio of the transverse velocity to the minimum impact parameter. Based on Monte Carlo simulations of synthetic data sets we give the expected accuracies in the parameter determination for various masses. We argue that pulsars found behind the very center of our Galaxy would provide an excellent opportunity to test the mass distribution in the Galactic nucleus, and therefore to distinguish between a super massive black hole (SMBH) and a super-dense star cluster (SDSC) within the central 0.1 pc.

astro-ph↗