40 research outputs found
Magnetars and pulsars: a missing link
There is growing evidence that soft gamma-ray repeaters (SGRs) and anomalous
X-ray pulsars (AXPs) are isolated neutron stars with superstrong magnetic
fields, i.e., magnetars, marking them a distinguished species from the
conventional species of spindown-powered isolated neutron stars, i.e., radio
pulsars. The current arguments in favor of the magnetar interpretation of
SGR/AXP phenomenology will be outlined, and the two energy sources in
magnetars, i.e. a magnetic dissipation energy and a spindown energy, will be
reviewed. I will then discuss a missing link between magnetars and pulsars,
i.e., lack of the observational evidence of the spindown-powered behaviors in
known magnetars. Some recent theoretical efforts in studying such behaviors
will be reviewed along with some predictions testable in the near future.Comment: Invited talk at the Sixth Pacific Rim Conference on Stellar
Astrophysics, a tribute to Helmut A. Abt, July 11-17, 2002, Xi'an. To appear
in the proceedings (eds. K. S. Cheng, K. C. Leung & T. P. Li
A dusty pinwheel nebula around the massive star WR 104
Wolf-Rayet (WR) stars are luminous massive blue stars thought to be immediate
precursors to the supernova terminating their brief lives. The existence of
dust shells around such stars has been enigmatic since their discovery some 30
years ago; the intense radiation field from the star should be inimical to dust
survival. Although dust-creation models, including those involving interacting
stellar winds from a companion star, have been put forward, high-resolution
observations are required to understand this phenomena. Here we present
resolved images of the dust outflow around Wolf-Rayet WR 104, obtained with
novel imaging techniques, revealing detail on scales corresponding to about 40
AU at the star. Our maps show that the dust forms a spatially confined stream
following precisely a linear (or Archimedian) spiral trajectory. Images taken
at two separate epochs show a clear rotation with a period of 220 +/- 30 days.
Taken together, these findings prove that a binary star is responsible for the
creation of the circumstellar dust, while the spiral plume makes WR 104 the
prototype of a new class of circumstellar nebulae unique to interacting wind
systems.Comment: 7 pages, 2 figures, Appearing in Nature (1999 April 08
The Supernova Gamma-Ray Burst Connection
The chief distinction between ordinary supernovae and long-soft gamma-ray
bursts (GRBs) is the degree of differential rotation in the inner several solar
masses when a massive star dies, and GRBs are rare mainly because of the
difficulty achieving the necessary high rotation rate. Models that do provide
the necessary angular momentum are discussed, with emphasis on a new single
star model whose rapid rotation leads to complete mixing on the main sequence
and avoids red giant formation. This channel of progenitor evolution also gives
a broader range of masses than previous models, and allows the copious
production of bursts outside of binaries and at high redshifts. However, even
the production of a bare helium core rotating nearly at break up is not, by
itself, a sufficient condition to make a gamma-ray burst. Wolf-Rayet mass loss
must be low, and will be low in regions of low metallicity. This suggests that
bursts at high redshift (low metallicity) will, on the average, be more
energetic, have more time structure, and last longer than bursts nearby. Every
burst consists of three components: a polar jet (~0.1 radian), high energy,
subrelativistic mass ejection (~1 radian), and low velocity equatorial mass
that can fall back after the initial explosion. The relative proportions of
these three components can give a diverse assortment of supernovae and high
energy transients whose properties may vary with redshift.Comment: 10 pages, to appear in AIP Conf. Proc. "Gamma Ray Bursts in the Swift
Era", Eds. S. S. Holt, N. Gehrels, J. Nouse
Neutron Stars in Teleparallel Gravity
In this paper we deal with neutron stars, which are described by a perfect
fluid model, in the context of the teleparallel equivalent of general
relativity. We use numerical simulations to find the relationship between the
angular momentum of the field and the angular momentum of the source. Such a
relation was established for each stable star reached by the numerical
simulation once the code is fed with an equation of state, the central energy
density and the ratio between polar and equatorial radii. We also find a regime
where linear relation between gravitational angular momentum and moment of
inertia (as well as angular velocity of the fluid) is valid. We give the
spatial distribution of the gravitational energy and show that it has a linear
dependence with the squared angular velocity of the source.Comment: 19 pages, 14 figures. arXiv admin note: text overlap with
arXiv:1206.331
Relativistic ejecta from XRF 060218 and the rate of cosmic explosions
Over the last decade, long-duration gamma-ray bursts (GRBs) including the
subclass of X-ray flashes (XRFs) have been revealed to be a rare variety of
Type Ibc supernova (SN). While all these events result from the death of
massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those
of ordinary Type Ibc SNe by many orders of magnitude. The essential physical
process that causes a dying star to produce a GRB or XRF, and not just an SN,
remains the crucial open question. Here we present radio and X-ray observations
of XRF 060218 (associated with SN 2006aj), the second nearest GRB identified
to-date, which allow us to measure its total energy and place it in the larger
context of cosmic explosions. We show that this event is 100 times less
energetic but ten times more common than cosmological GRBs. Moreover, it is
distinguished from ordinary Type Ibc SNe by the presence of 10^48 erg coupled
to mildly-relativistic ejecta, along with a central engine (an accretion-fed,
rapidly rotating compact source) which produces X-rays for weeks after the
explosion. This suggests that the production of relativistic ejecta is the key
physical distinction between GRBs/XRFs and ordinary SNe, while the nature of
the central engine (black hole or magnetar) may distinguish typical bursts from
low-luminosity, spherical events like XRF 060218.Comment: To appear in Nature on August 31 2006 (15 pages, 3 figures, 1 table,
including Supplementary Information
Non-thermal emission processes in massive binaries
In this paper, I present a general discussion of several astrophysical
processes likely to play a role in the production of non-thermal emission in
massive stars, with emphasis on massive binaries. Even though the discussion
will start in the radio domain where the non-thermal emission was first
detected, the census of physical processes involved in the non-thermal emission
from massive stars shows that many spectral domains are concerned, from the
radio to the very high energies.
First, the theoretical aspects of the non-thermal emission from early-type
stars will be addressed. The main topics that will be discussed are
respectively the physics of individual stellar winds and their interaction in
binary systems, the acceleration of relativistic electrons, the magnetic field
of massive stars, and finally the non-thermal emission processes relevant to
the case of massive stars. Second, this general qualitative discussion will be
followed by a more quantitative one, devoted to the most probable scenario
where non-thermal radio emitters are massive binaries. I will show how several
stellar, wind and orbital parameters can be combined in order to make some
semi-quantitative predictions on the high-energy counterpart to the non-thermal
emission detected in the radio domain.
These theoretical considerations will be followed by a census of results
obtained so far, and related to this topic... (see paper for full abstract)Comment: 47 pages, 5 postscript figures, accepted for publication in Astronomy
and Astrophysics Review. Astronomy and Astrophysics Review, in pres