1,598 research outputs found
Operad profiles of Nijenhuis structures
Recently S. Merkulov established a new link between differential geometry and
homological algebra by giving descriptions of several differential geometric
structures in terms of algebraic operads and props. In particular he described
Nijenhuis structures as corresponding to representations of the cobar
construction on the Koszul dual of a certain quadratic operad. In this paper we
prove, using the PBW-basis method of E. Hoffbeck, that the operad governing
Nijenhuis structures is Koszul, thereby showing that Nijenhuis structures
correspond to representations of the minimal resolution of this operad. We also
construct an operad such that representations of its minimal resolution in a
vector space V are in one-to-one correspondence with pairs of compatible
Nijenhuis structures on the formal manifold associated to V.Comment: 16 page
Future Probes of the Neutron Star Equation of State Using X-ray Bursts
Observations with NASA's Rossi X-ray Timing Explorer (RXTE) have resulted in
the discovery of fast (200 - 600 Hz), coherent X-ray intensity oscillations
(hereafter, "burst oscillations") during thermonuclear X-ray bursts from 12 low
mass X-ray binaries (LMXBs). It is now beyond doubt that these oscillations
result from spin modulation of the thermonuclear burst flux from the neutron
star surface. Among the new timing phenomena revealed by RXTE the burst
oscillations are perhaps the best understood, in the sense that many of their
properties can be explained in the framework of this relatively simple model.
Because of this, detailed modelling of burst oscillations can be an extremely
powerful probe of neutron star structure, and thus the equation of state (EOS)
of supra-nuclear density matter. The new discoveries have spurred much new
theoretical work on thermonuclear burning and propagation on neutron stars, so
that in the near future it is not unreasonable to think that detailed physical
models of the time dependent flux from burning neutron stars will be available
for comparison with the observed pulse profiles from a future, large collecting
area X-ray timing observatory. In addition, recent high resolution burst
spectroscopy with XMM/Newton suggests the presence of redshifted absorption
lines from the neutron star surface during bursts. This leads to the
possibility of using large area, high spectral resolution measurements of X-ray
bursts as a precise probe of neutron star structure. In this work I will
explore the precision with which constraints on neutron star structure, and
hence the dense matter EOS, can be made with the implementation of such
programs.Comment: 8 pages, 7 figures, AIP conference proceedings format. Contribution
to "X-ray Timing 2003: Rossi and Beyond." meeting held in Cambridge, MA,
November, 200
Precision X-ray Timing of RX J0806.3+1527 with CHANDRA: Evidence for Gravitational Radiation from an Ultracompact Binary
RX J0806.3+1527 is a candidate double degenerate binary with possibly the
shortest known orbital period. The source shows an 100% X-ray intensity
modulation at the putative orbital frequency of 3.11 mHz (321.5 s). If the
system is a detached, ultracompact binary gravitational radiation should drive
spin-up with a magnitude of ~10-16 Hz/s. Here we describe the results of the
first phase coherent X-ray monitoring campaign on RX J0806.3+1527 with Chandra.
We obtained a total of 70 ksec of exposure in 6 epochs logarithmically spaced
over 320 days. These data conclusively show that the X-ray frequency is
increasing at a rate of 3.77 +- 0.8 x 10-16 Hz/s. Using our new ephemeris we
are able to phase up all the earlier Chandra and ROSAT data and show they are
consistent with a rate of 3.63 +- 0.06 x 10-16 Hz/s over the past decade. This
value appears consistent with that recently derived by Israel et al. largely
from monitoring of the optical modulation, and is in rough agreement with the
solutions reported initially by Hakala et al., based on ground-based optical
observations. The large spin-up is consistent with gravitational radiation
losses driving the evolution. An intermediate polar (IP) scenario where the
observed X-ray period is the spin period of an accreting white dwarf appears
less tenable. If the ultracompact scenario is correct, then the X-ray flux
cannot be powered by stable accretion which would drive the components apart,
suggesting a new type of energy source (perhaps electromagnetic) may power the
X-ray flux.Comment: 23 pages, 9 figures, AASTeX, accepted for publication in the
Astrophysical Journa
Millisecond Time Variations of X-Ray Binaries
The Rossi X-Ray Timing Explorer (RXTE) has found that the neutron stars in
low-mass X-ray binaries exhibit oscillations in the range 300-1200 Hz.
Persistent emission may exhibit one or both of two features. In bursts a nearly
coherent pulsation is seen, which may be the rotation period of the neutron
star. For some the frequency equals the difference between the two higher
frequencies, suggesting a beat frequency model, but in others it is twice the
difference. Similar maximum frequencies suggests that it corresponds to the
Kepler orbit frequency at the minimum stable orbit or the neutron star surface,
either of which would determine the neutron star masses, radii and equation of
state. Theories of accretion onto black holes predict a quasi-periodic
oscillation (QPO) related to the inner accretion disk. The two microquasar
black hole candidates (BHCs) have exhibited candidates for this or related
frequencies.Comment: 4 pages, to be published in the proceedings of IAU Symposium 188: The
Hot Univers
Discovery of a 115 Day Orbital Period in the Ultraluminous X-ray Source NGC 5408 X-1
We report the detection of a 115 day periodicity in SWIFT/XRT monitoring data
from the ultraluminous X-ray source (ULX) NGC 5408 X-1. Our ongoing campaign
samples its X-ray flux approximately twice weekly and has now achieved a
temporal baseline of ~485 days. Periodogram analysis reveals a significant
periodicity with a period of 115.5 +- 4 days. The modulation is detected with a
significance of 3.2 e-4. The fractional modulation amplitude decreases with
increasing energy, ranging from 0.13 above 1 keV to 0.24 below 1 keV. The shape
of the profile evolves as well, becoming less sharply peaked at higher
energies. The periodogram analysis is consistent with a periodic process,
however, continued monitoring is required to confirm the coherent nature of the
modulation. Spectral analysis indicates that NGC 5408 X-1 can reach 0.3 - 10
keV luminosities of ~2 e40 ergs/s. We suggest that, like the 62 day period of
the ULX in M82 (X41.4+60), the periodicity detected in NGC 5408 X-1 represents
the orbital period of the black hole binary containing the ULX. If this is true
then the secondary can only be a giant or supergiant star.Comment: Accepted for Publication in the Astrophysical Journal Letter
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