Context.
The gravitational strong equivalence principle (SEP) is a cornerstone of the general theory of relativity (GR). Hence, testing the validity of SEP is of great importance when confronting GR, ...or its alternatives, with experimental data. Pulsars that are orbited by white dwarf companions provide an excellent laboratory, where the extreme difference in binding energy between neutron stars and white dwarfs allows for precision tests of the SEP via the technique of radio pulsar timing.
Aims.
To date, the best limit on the validity of SEP under strong-field conditions was obtained with a unique pulsar in a triple stellar system, PSR J0337+1715. We report here on an improvement of this test using an independent data set acquired over a period of 6 years with the Nançay radio telescope. The improvements arise from a uniformly sampled data set, a theoretical analysis, and a treatment that fixes some short-comings in the previously published results, leading to better precision and reliability of the test.
Methods.
In contrast to the previously published test, we use a different long-term timing data set, developed a new timing model and an independent numerical integration of the motion of the system, and determined the masses and orbital parameters with a different methodology that treats the parameter Δ, describing a possible strong-field SEP violation, identically to all other parameters.
Results.
We obtain a violation parameter Δ = ( + 0.5 ± 1.8) × 10
−6
at 95% confidence level, which is compatible with and improves upon the previous study by 30%. This result is statistics-limited and avoids limitation by systematics as previously encountered. We find evidence for red noise in the pulsar spin frequency, which is responsible for up to 10% of the reported uncertainty. We use the improved limit on SEP violation to place constraints on a class of well-studied scalar-tensor theories, in particular we find
ω
BD
> 140 000 for the Brans-Dicke parameter. The conservative limits presented here fully take into account current uncertainties in the equation for state of neutron-star matter.
Formation of Double Neutron Star Systems Tauris, T. M.; Kramer, M.; Freire, P. C. C. ...
The Astrophysical journal,
09/2017, Letnik:
846, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Double neutron star (DNS) systems represent extreme physical objects and the endpoint of an exotic journey of stellar evolution and binary interactions. Large numbers of DNS systems and their mergers ...are anticipated to be discovered using the Square Kilometre Array searching for radio pulsars, and the high-frequency gravitational wave detectors (LIGO/VIRGO), respectively. Here we discuss all key properties of DNS systems, as well as selection effects, and combine the latest observational data with new theoretical progress on various physical processes with the aim of advancing our knowledge on their formation. We examine key interactions of their progenitor systems and evaluate their accretion history during the high-mass X-ray binary stage, the common envelope phase, and the subsequent Case BB mass transfer, and argue that the first-formed NSs have accreted at most . We investigate DNS masses, spins, and velocities, and in particular correlations between spin period, orbital period, and eccentricity. Numerous Monte Carlo simulations of the second supernova (SN) events are performed to extrapolate pre-SN stellar properties and probe the explosions. All known close-orbit DNS systems are consistent with ultra-stripped exploding stars. Although their resulting NS kicks are often small, we demonstrate a large spread in kick magnitudes that may, in general, depend on the past interaction history of the exploding star and thus correlate with the NS mass. We analyze and discuss NS kick directions based on our SN simulations. Finally, we discuss the terminal evolution of close-orbit DNS systems until they merge and possibly produce a short γ-ray burst.
Abstract
We report the discovery of PSR J1757−1854, a 21.5-ms pulsar in a highly-eccentric, 4.4-h orbit with a neutron star (NS) companion. PSR J1757−1854 exhibits some of the most extreme ...relativistic parameters of any known pulsar, including the strongest relativistic effects due to gravitational-wave damping, with a merger time of 76 Myr. Following a 1.6-yr timing campaign, we have measured five post-Keplerian parameters, yielding the two component masses (mp = 1.3384(9) M⊙ and mc = 1.3946(9) M⊙) plus three tests of general relativity, which the theory passes. The larger mass of the NS companion provides important clues regarding the binary formation of PSR J1757−1854. With simulations suggesting 3-σ measurements of both the contribution of Lense–Thirring precession to the rate of change of the semimajor axis and the relativistic deformation of the orbit within ∼7–9 yr, PSR J1757−1854 stands out as a unique laboratory for new tests of gravitational theories.
In this work, we report the discovery and characterization of PSR J1411+2551, a new binary pulsar discovered in the Arecibo 327 MHz Drift Pulsar Survey. Our timing observations of the radio pulsar in ...the system span a period of about 2.5 years. This timing campaign allowed a precise measurement of its spin period (62.4 ms) and its derivative (9.6 0.7) × 10−20 s s−1; from these, we derive a characteristic age of >9.1 Gyr and a surface magnetic field strength of <2.6 × 109 G. These numbers indicate that this pulsar was mildly recycled by accretion of matter from the progenitor of the companion star. The system has an eccentric (e = 0.17) 2.61 day orbit. This eccentricity allows a highly significant measurement of the rate of advance of periastron, . Assuming general relativity accurately describes the orbital motion, this implies a total system mass M = 2.538 0.022 M . The minimum companion mass is 0.92 M and the maximum pulsar mass is 1.62 M . The large companion mass and the orbital eccentricity suggest that PSR J1411+2551 is a double neutron star system; the lightest known to date including the DNS merger GW170817. Furthermore, the relatively low orbital eccentricity and small proper motion limits suggest that the second supernova had a relatively small associated kick; this and the low system mass suggest that it was an ultra-stripped supernova.
The anticipated discovery of a pulsar in orbit with a black hole is expected to provide a unique laboratory for black hole physics and gravity. In this context, the next generation of radio ...telescopes, like the Five-hundred-meter Aperture Spherical radio Telescope (FAST) and the Square Kilometre Array (SKA), with their unprecedented sensitivity, will play a key role. In this paper, we investigate the capability of future radio telescopes to probe the space–time of a black hole and test gravity theories by timing a pulsar orbiting a stellar-mass black hole (SBH). Based on mock data simulations, we show that a few years of timing observations of a sufficiently compact pulsar–SBH (PSR–SBH) system with future radio telescopes would allow precise measurements of the black hole mass and spin. A measurement precision of 1 per cent can be expected for the spin. Measuring the quadrupole moment of the black hole, needed to test general relativity's (GR's) no-hair theorem, requires extreme system configurations with compact orbits and a large SBH mass. Additionally, we show that a PSR–SBH system can lead to greatly improved constraints on alternative gravity theories even if they predict black holes (practically) identical to GR's. This is demonstrated for a specific class of scalar–tensor theories. Finally, we investigate the requirements for searching for PSR–SBH systems. It is shown that the high sensitivity of the next generation of radio telescopes is key for discovering compact PSR–SBH systems, as it will allow for sufficiently short survey integration times.
PSR J1903+0327, a millisecond pulsar in an eccentric (e= 0.44) 95-d orbit with an ∼1 M⊙ companion poses a challenge to our understanding of stellar evolution in binary and multiple-star systems. Here ...we describe optical and radio observations which rule out most of the scenarios proposed to explain formation of this system. Radio timing measurements of three post-Keplerian effects yield the most precise measurement of the mass of a millisecond pulsar to date: 1.667 ± 0.021 solar masses (99.7 per cent confidence limit). This rules out some equations of state for superdense matter; furthermore, it is consistent with the spin-up of the pulsar by mass accretion, as suggested by its short spin period and low magnetic field. Optical spectroscopy of a proposed main-sequence counterpart shows that its orbital motion mirrors the pulsar's 95-d orbit; being therefore its binary companion. This finding rules out a previously suggested scenario which proposes that the system is presently a hierarchical triple. Conventional binary evolution scenarios predict that, after recycling a neutron star into a millisecond pulsar, the binary companion should become a white dwarf and its orbit should be nearly circular. This suggests that if PSR J1903+0327 was recycled, its present companion was not responsible for it. The optical detection also provides a measurement of the systemic radial velocity of the binary; this and the proper motion measured from pulsar timing allow the determination of the systemic 3D velocity in the Galaxy. We find that the system is always within 270 pc of the plane of the Galaxy, but always more than 3 kpc away from the Galactic Centre. Thus an exchange interaction in a dense stellar environment (like a globular cluster or the Galactic Centre) is not likely to be the origin of this system. We suggest that after the supernova that formed it, the neutron star was in a tight orbit with a main-sequence star and the present companion was a tertiary farther out. The neutron star then accreted matter from its evolving inner companion, forming a millisecond pulsar. The inner companion then disappeared, either due to a chaotic three-body interaction with the outer star (caused by the expansion of the inner orbit that necessarily results from mass transfer), or in the case of a very compact inner system, due to ablation/accretion by the newly formed millisecond pulsar. We discuss in detail the possible evolution of such a system before the supernova.
PSR J1738+0333 is one of the four millisecond pulsars known to be orbited by a white dwarf companion bright enough for optical spectroscopy. Of these, it has the shortest orbital period, making it ...especially interesting for a range of astrophysical and gravity related questions. We present a spectroscopic and photometric study of the white dwarf companion and infer its radial-velocity curve, effective temperature, surface gravity and luminosity. We find that the white dwarf has properties consistent with those of low-mass white dwarfs with thick hydrogen envelopes, and use the corresponding mass-radius relation to infer its mass;
M⊙. Combined with the mass ratio q= 8.1 ± 0.2 inferred from the radial velocities and the precise pulsar timing ephemeris, the neutron star mass is constrained to
M⊙. Contrary to expectations, the latter is only slightly above the Chandrasekhar limit. We find that, even if the birth mass of the neutron star was only 1.20 M⊙, more than 60 per cent of the matter that left the surface of the white dwarf progenitor escaped the system. The accurate determination of the component masses transforms this system in a laboratory for fundamental physics by constraining the orbital decay predicted by general relativity. Currently, the agreement is within 1σ of the observed decay. Further radio timing observations will allow precise tests of white dwarf models, assuming the validity of general relativity.
The discovery of radio pulsars in compact orbits around Sgr A* would allow an unprecedented and detailed investigation of the spacetime of this supermassive black hole. This paper shows that pulsar ...timing, including that of a single pulsar, has the potential to provide novel tests of general relativity, in particular its cosmic censorship conjecture and no-hair theorem for rotating black holes. These experiments can be performed by timing observations with 100 mu s precision, achievable with the Square Kilometre Array for a normal pulsar at frequency above 15 GHz. Based on the standard pulsar timing technique, we develop a method that allows the determination of the mass, spin, and quadrupole moment of Sgr A*, and provides a consistent covariance analysis of the measurement errors. Furthermore, we test this method in detailed mock data simulations. It seems likely that only for orbital periods below ~0.3 yr is there the possibility of having negligible external perturbations. For such orbits, we expect a ~10 super(-3) test of the frame dragging and a ~10 super(-2) test of the no-hair theorem within five years, if Sgr A* is spinning rapidly. Our method is also capable of identifying perturbations caused by distributed mass around Sgr A*, thus providing high confidence in these gravity tests. Our analysis is not affected by uncertainties in our knowledge of the distance to the Galactic center, R sub(0). A combination of pulsar timing with the astrometric results of stellar orbits would greatly improve the measurement precision of R sub(0).
Earth's nearest candidate supermassive black hole lies at the centre of the Milky Way. Its electromagnetic emission is thought to be powered by radiatively inefficient accretion of gas from its ...environment, which is a standard mode of energy supply for most galactic nuclei. X-ray measurements have already resolved a tenuous hot gas component from which the black hole can be fed. The magnetization of the gas, however, which is a crucial parameter determining the structure of the accretion flow, remains unknown. Strong magnetic fields can influence the dynamics of accretion, remove angular momentum from the infalling gas, expel matter through relativistic jets and lead to synchrotron emission such as that previously observed. Here we report multi-frequency radio measurements of a newly discovered pulsar close to the Galactic Centre and show that the pulsar's unusually large Faraday rotation (the rotation of the plane of polarization of the emission in the presence of an external magnetic field) indicates that there is a dynamically important magnetic field near the black hole. If this field is accreted down to the event horizon it provides enough magnetic flux to explain the observed emission--from radio to X-ray wavelengths--from the black hole.
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