10 research outputs found
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
Nan\c{c}ay. All the timing parameters have been improved from the previously
published values, most by an order of magnitude. In addition, a parallax
measurement of 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 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 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 .Comment: accepted for publication in MNRAS, 11 pages, 5 figures, 2 table
Cosmological Models of Gamma-Ray Bursts
We review models of cosmological gamma-ray bursts (GRBs). The statistical and
-ray transparency issues are summarized. Neutron-star and black-hole
merger scenarios are described and estimates of merger rates are summarized. We
review the simple fireball models for GRBs and the recent work on non-simple
fireballs. Alternative cosmological models, including models where GRBs are
analogs of active galactic nuclei and where they are produced by high-field,
short period pulsars, are also mentioned. The value of neutrino astronomy to
solve the GRB puzzle is briefly reviewed.Comment: 12 pages, no figures, uuencoded compressed postscript file. Invited
review to appear in the proceedings of the 29th ESLAB Symposium "Towards the
Source of Gamma-Ray Bursts," Noordwijk, Netherlands, 25-27 April, 199
Gamma-Ray Burst Energy Spectra: Theoretical Models, Old and New
The modelling of gamma-ray burst (GRB) spectra has considerable potential for
increasing the understanding of these enigmatic sources. A diversity of ideas
and analyses has been generated over the last two decades to explain line
features and continuum shapes, encompassing both older galactic neutron star
and ``new age'' cosmological source models. This paper reviews some of the
highlights of these studies, discussing the merits and limitations of various
ideas, and in particular their compatibility with the observational data. The
first focus will be on continuum models for GRBs, which include optically thin
synchrotron emission and resonant Compton upscattering near galactic neutron
stars, while the synchrotron and non-magnetic inverse Compton scattering
mechanisms are prominent in the less well-developed cosmological scenarios.
Line formation scenarios will then be discussed, in particular the scattering
model for producing cyclotron features, which remains the only viable
explanation for the Ginga observations of double lines. Absorption-like line
production in cosmological burst models is generally difficult, though
interesting notions such as femtolensing interference patterns have been
proffered.Comment: 8 pages with no figures, as a compressed, uuencoded, Postscript file.
Invited review, to appear in Astrophysics and Space Science as part of the
proceedings of the 29th ESLAB Symposium ``Towards the Source of Gamma-Ray
Bursts'' held in Noordwijk, 199
The origin and abundances of the chemical elements revisited
The basic scheme of nucleosynthesis (building of heavy elements from light ones) has held up very well since it was first proposed more than 30 years ago by E.M. Burbidge, G.R. Burbidge, A.G.W. Cameron, W.A. Fowler, and F. Hoyle. Significant advances in the intervening years include (a) observations of elemental and a few isotopic ratios in many more extrasolar-system sites, including metal-poor dwarf irregular galaxies, where very little has happened, and supernovae and their remnants, where a great deal has happened, (b) recognition of the early universe as good for making all the elements up to helium, (c) resolution of heavy element burning in stars into separate carbon, neon, oxygen, and silicon burning, with fine tuning of the resulting abundances by explosive nucleosynthesis in outgoing supernova shock waves, (d) clarification of the role of Type I supernovae, (e) concordance between elements produced in short-lived and long-lived stars with those that increased quickly and slowly over the history of the galaxy, and (f) calibration of calculations of the evolution and explosion of massive stars against the detailed observations of SN 1987A. The discussion presupposes a reader (a) with some prior knowledge of astronomy at the level of recognizing what is meant by an A star and an AGB star and (b) with at least a mild interest in how we got to where we currently are. © 1991 Springer-Verlag