Famous Brazilian educational and social theorist Paulo Freire presents his ideas on the importance of community solidarity in moving toward social justice in schools and society. In a set of talks ...and interviews shortly before his death, Freire addresses issues not often highlighted in his work, such as globalization, post-modern fatalism, and the qualities of educators for the 21st century. His illuminating comments are supplemented with commentaries by other well-known scholars, such as Ana Maria Araujo Freire, Walter de Oliveira, Norman Denzin, Henry Giroux, and Donaldo Macedo.
We summarize our current knowledge of neutron-star masses and radii. Recent instrumentation and computational advances have resulted in a rapid increase in the discovery rate and precise timing of ...radio pulsars in binaries in the past few years, leading to a large number of mass measurements. These discoveries show that the neutron-star mass distribution is much wider than previously thought, with three known pulsars now firmly in the 1.9-2.0-M
mass range. For radii, large, high-quality data sets from X-ray satellites as well as significant progress in theoretical modeling led to considerable progress in the measurements, placing them in the 10-11.5-km range and shrinking their uncertainties, owing to a better understanding of the sources of systematic errors. The combination of the massive-neutron-star discoveries, the tighter radius measurements, and improved laboratory constraints of the properties of dense matter has already made a substantial impact on our understanding of the composition and bulk properties of cold nuclear matter at densities higher than that of the atomic nucleus, a major unsolved problem in modern physics.
A Massive Pulsar in a Compact Relativistic Binary Antoniadis, John; Freire, Paulo C. C.; Wex, Norbert ...
Science (American Association for the Advancement of Science),
04/2013, Letnik:
340, Številka:
6131
Journal Article
Recenzirano
Odprti dostop
Many physically motivated extensions to general relativity (GR) predict substantial deviations in the properties of spacetime surrounding massive neutron stars. We report the measurement of a 2.01 ± ...0.04 solar mass (M⊙) pulsar in a 2.46-hour orbit with a 0.172 ± 0.003 M⊙ white dwarf. The high pulsar mass and the compact orbit make this system a sensitive laboratory of a previously untested strong-field gravity regime. Thus far, the observed orbital decay agrees with GR, supporting its validity even for the extreme conditions present in the system. The resulting constraints on deviations support the use of GR-based templates for ground-based gravitational wave detectors. Additionally, the system strengthens recent constraints on the properties of dense matter and provides insight to binary stellar astrophysics and pulsar recycling.
We report the results of a 10-year timing campaign on PSR J1738+0333, a 5.85-ms pulsar in a low-eccentricity 8.5-h orbit with a low-mass white dwarf companion. We obtained 17 376 pulse times of ...arrival with a stated uncertainty smaller than
s and weighted residual rms of
s. The large number and precision of these measurements allow highly significant estimates of the proper motion μα, δ= (+7.037 ± 0.005, +5.073 ± 0.012) mas yr−1, parallax π
x
= (0.68 ± 0.05) mas and a measurement of the apparent orbital decay,
(all 1σ uncertainties). The measurements of μα, δ and π
x
allow for a precise subtraction of the kinematic contribution to the observed orbital decay; this results in a significant measurement of the intrinsic orbital decay:
. This is consistent with the orbital decay from the emission of gravitational waves predicted by general relativity,
, i.e. general relativity passes the test represented by the orbital decay of this system. This agreement introduces a tight upper limit on dipolar gravitational wave emission, a prediction of most alternative theories of gravity for asymmetric binary systems such as this. We use this limit to derive the most stringent constraints ever on a wide class of gravity theories, where gravity involves a scalar-field contribution. When considering general scalar-tensor theories of gravity, our new bounds are more stringent than the best current Solar system limits over most of the parameter space, and constrain the matter-scalar coupling constant
to be below the 10−5 level. For the special case of the Jordan-Fierz-Brans-Dicke, we obtain the 1σ bound
, which is within a factor of 2 of the Cassini limit. We also use our limit on dipolar gravitational wave emission to constrain a wide class of theories of gravity which are based on a generalization of Bekenstein's Tensor-Vector-Scalar gravity, a relativistic formulation of modified Newtonian dynamics.
The stellar encounter rate Γ has been shown to be strongly correlated with the number of X-ray binaries in globular clusters (GCs) and also to the number of radio pulsars. However, the pulsar ...populations in different GCs show remarkably different characteristics: in some GCs the population is dominated by binary systems, in others by single pulsars and exotic systems that result from exchange encounters. In this paper, we describe a second dynamical parameter for GCs, the encounter rate for a single binary, γ. We find that this parameter provides a good characterization of the differences between the pulsar populations of different GCs. The higher γ is for any particular GC, the more isolated pulsars and products of exchange interactions are observed. Furthermore, we also find that slow and “young” pulsars are found almost exclusively in clusters with a high γ; this suggests that these kinds of objects are formed by the disruption of X-ray binaries, thus halting the recycling of a previously dead neutron star. We discuss the implications of this for the nature of young pulsars and for the formation of neutron stars in GCs.
A Radio Pulsar Spinning at 716 Hz Jason W. T. Hessels; Ransom, Scott M.; Stairs, Ingrid H. ...
Science (American Association for the Advancement of Science),
03/2006, Letnik:
311, Številka:
5769
Journal Article
Recenzirano
Odprti dostop
We have discovered a 716-hertz eclipsing binary radio pulsar in the globular cluster Terzan 5 using the Green Bank Telescope. It is the fastest spinning neutron star found to date, breaking the ...24-year record held by the 642-hertz pulsar B1937+21. The difficulty in detecting this pulsar, because of its very low flux density and high eclipse fraction (~40% of the orbit), suggests that even faster spinning neutron stars exist. If the pulsar has a mass less than twice the mass of the Sun, then its radius must be constrained by the spin rate to be <16 kilometers. The short period of this pulsar also constrains models that suggest that gravitational radiation, through an r-mode (Rossby wave) instability, limits the maximum spin frequency of neutron stars.
Over the past decade, the discovery of three unique stellar populations and a large number of confirmed pulsars within the globular cluster Terzan 5 has raised questions over its classification. ...Using the long-term radio pulsar timing of ms pulsars in the cluster core, we provide new measurements of key physical properties of the system. As Terzan 5 is located within the galactic bulge, stellar crowding and reddening make optical and near-infrared observations difficult. Pulsar accelerations, however, allow us to study the intrinsic characteristics of the cluster independent of reddening and stellar crowding and probe the mass density profile without needing to quantify the mass-to-light ratio. Relating the spin and orbital periods of each pulsar to the acceleration predicted by a King model, we find a core density of × 106 pc−3, a core radius of pc, a pulsar density profile of , and a total mass of ( pc) 3.0 × 105 , assuming a cluster distance of 5.9 kpc. Using this information, we argue against Terzan 5 being a disrupted dwarf galaxy and discuss the possibility of it being a fragment of the Milky Way's proto-bulge. We also discuss whether low-mass pulsars were formed via electron-capture supernovae or exist in a core full of heavy white dwarfs and hard binaries. Finally, we provide an upper limit for the mass of a possible black hole at the core of the cluster of .
The Timing of Nine Globular Cluster Pulsars Lynch, Ryan S; Freire, Paulo C. C; Ransom, Scott M ...
The Astrophysical journal,
02/2012, Letnik:
745, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We have used the Robert C. Byrd Green Bank Telescope to time nine previously known pulsars without published timing solutions in the globular clusters (GCs) M62, NGC 6544, and NGC 6624. We have full ...timing solutions that measure the spin, astrometric, and (where applicable) binary parameters for six of these pulsars. The remaining three pulsars (reported here for the first time) were not detected enough to establish solutions. We also report our timing solutions for five pulsars with previously published solutions, and find good agreement with other authors, except for PSR J1701-3006B in M62. Gas in this system is probably responsible for the discrepancy in orbital parameters, and we have been able to measure a change in the orbital period over the course of our observations. Among the pulsars with new solutions we find several binary pulsars with very low mass companions (members of the so-called "black widow" class) and we are able to place constraints on the mass-to-light ratio in two clusters. We confirm that one of the pulsars in NGC 6624 is indeed a member of the rare class of non-recycled pulsars found in GCs. We have also measured the orbital precession and Shapiro delay for a relativistic binary in NGC 6544. If we assume that the orbital precession can be described entirely by general relativity, which is likely, we are able to measure the total system mass (2.57190(73) M sub(middot in circle)) and companion mass (1.2064(20) M sub(middot in circle)), from which we derive the orbital inclination (sin i = 0.9956(14)) and the pulsar mass (1.3655(21) M sub(middot in circle)), the most precise such measurement ever obtained for a millisecond pulsar. The companion is the most massive known around a fully recycled pulsar.
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