3,523 research outputs found
Proffessor Dr.H.G. Schulze: Der Drosse Anreger.
Met die aftrede van prof. dr. H. G.Schulze met pensioen word ons blikterug gevoer na een van die persoonlikhedewat die Potchefstroomse Universiteitvir C.H.O. in sy wordingsjare gehelpdra en bou het
Constraining the neutron star equation of state using XMM-Newton
We have identified three possible ways in which future XMM-Newton
observations can provide significant constraints on the equation of state of
neutron stars. First, using a long observation of the neutron star X-ray
transient CenX-4 in quiescence one can use the RGS spectrum to constrain the
interstellar extinction to the source. This removes this parameter from the
X-ray spectral fitting of the pn and MOS spectra and allows us to investigate
whether the variability observed in the quiescent X-ray spectrum of this source
is due to variations in the soft thermal spectral component or variations in
the power law spectral component coupled with variations in N_H. This will test
whether the soft thermal spectral component can indeed be due to the hot
thermal glow of the neutron star. Potentially such an observation could also
reveal redshifted spectral lines from the neutron star surface. Second,
XMM-Newton observations of radius expansion type I X-ray bursts might reveal
redshifted absorption lines from the surface of the neutron star. Third,
XMM-Newton observations of eclipsing quiescent low-mass X-ray binaries provide
the eclipse duration. With this the system inclination can be determined
accurately. The inclination determined from the X-ray eclipse duration in
quiescence, the rotational velocity of the companion star and the
semi-amplitude of the radial velocity curve determined through optical
spectroscopy, yield the neutron star mass.Comment: 4 pages, 1 figure, proceedings of the XMM-Newton workshop, June 2007,
accepted for publication in A
Detection of a 1258 Hz high-amplitude kilohertz quasi-periodic oscillation in the ultra-compact X-ray binary 1A 1246-588
We have observed the ultra-compact low-mass X-ray binary (LMXB) 1A 1246-588
with the Rossi X-ray Timing Explorer (RXTE). In this manuscript we report the
discovery of a kilohertz quasi-periodic oscillation (QPO) in 1A 1246-588. The
kilohertz QPO was only detected when the source was in a soft high-flux state
reminiscent of the lower banana branch in atoll sources. Only one kilohertz QPO
peak is detected at a relatively high frequency of 1258+-2 Hz and at a single
trial significance of more than 7 sigma. Kilohertz QPOs with a higher frequency
have only been found on two occasions in 4U 0614+09. Furthermore, the frequency
is higher than that found for the lower kilohertz QPO in any source, strongly
suggesting that the QPO is the upper of the kilohertz QPO pair often found in
LMXBs. The full-width at half maximum is 25+-4 Hz, making the coherence the
highest found for an upper kilohertz QPO. From a distance estimate of ~6 kpc
from a radius expansion burst we derive that 1A 1246-588 is at a persistent
flux of ~0.2-0.3 per cent of the Eddington flux, hence 1A 1246-588 is one of
the weakest LMXBs for which a kilohertz QPO has been detected. The
root-mean-square (rms) amplitude in the 5-60 keV band is 27+-3 per cent, this
is the highest for any kilohertz QPO source so far, in line with the general
anti-correlation between source luminosity and rms amplitude of the kilohertz
QPO peak identified before. Using the X-ray spectral information we produce a
colour-colour diagram. The source behaviour in this diagram provides further
evidence for the atoll nature of the source.Comment: 4 pages, 3 figures, accepted for publication in MNRA
Time-resolved X-Shooter spectra and RXTE light curves of the ultra-compact X-ray binary candidate 4U 0614+091
In this paper we present X-Shooter time resolved spectroscopy and RXTE PCA
light curves of the ultra-compact X-ray binary candidate 4U 0614+091. The
X-Shooter data are compared to the GMOS data analyzed previously by Nelemans et
al. (2004). We confirm the presence of C III and O II emission features at ~
4650 {\AA} and ~ 5000 {\AA}. The emission lines do not show evident Doppler
shifts that could be attributed to the motion of the donor star/hot spot around
the center of mass of the binary. We note a weak periodic signal in the
red-wing/blue-wing flux ratio of the emission feature at ~ 4650 {\AA}. The
signal occurs at P = 30.23 +/- 0.03 min in the X-Shooter and at P = 30.468 +/-
0.006 min in the GMOS spectra when the source was in the low/hard state. Due to
aliasing effects the period in the GMOS and X-Shooter data could well be the
same. We deem it likely that the orbital period is thus close to 30 min,
however, as several photometric periods have been reported for this source in
the literature already, further confirmation of the 30 min period is warranted.
We compare the surface area of the donor star and the disc of 4U 0614+091 with
the surface area of the donor star and the disc in typical hydrogen-rich
low-mass X-ray binaries and the class of AM Canum Venaticorum stars and argue
that the optical emission in 4U 0614+091 is likely dominated by the disc
emission. Additionally, we search for periodic signals in all the publicly
available RXTE PCA light curves of 4U 0614+091 which could be associated with
the orbital period of this source. A modulation at the orbital period with an
amplitude of ~ 10% such as those that have been found in other ultra-compact
X-ray binaries (4U 0513-40, 4U 1820-30) is not present in 4U 0614+091.Comment: Accepted for publication in MNRAS, 11 pages, 7 figure
On the stratification of a class of specially structured matrices
We consider specially structured matrices representing optimization problems with quadratic objective functions and (finitely many) affine linear equality constraints in an n-dimensional Euclidean space. The class of all such matrices will be subdivided into subsets ['strata'], reflecting the features of the underlying optimization problems. From a differential-topological point of view, this subdivision turns out to be very satisfactory: Our strata are smooth manifolds, constituting a so-called Whitney Regular Stratification, and their dimensions can be explicitly determined. We indicate how, due to Thom's Transversality Theory, this setting leads to some fundamental results on smooth one-parameter families of linear-quadratic optimization problems with ( finitely many) equality and inequality constraints
Constraints on Thermal X-ray Radiation from SAX J1808.4-3658 and Implications for Neutron Star Neutrino Emission
Thermal X-ray radiation from neutron star soft X-ray transients in quiescence
provides the strongest constraints on the cooling rates of neutron stars, and
thus on the interior composition and properties of matter in the cores of
neutron stars. We analyze new (2006) and archival (2001) XMM-Newton
observations of the accreting millisecond pulsar SAX J1808.4-3658 in
quiescence, which provide the most stringent constraints to date. The X-ray
spectrum of SAX J1808.4-3658 in the 2006 observation is consistent with a
power-law of photon index 1.83\pm0.17, without requiring the presence of a
blackbody-like component from a neutron star atmosphere. Our 2006 observation
shows a slightly lower 0.5-10 keV X-ray luminosity, at a level of
68^{+15}_{-13}% that inferred from the 2001 observation. Simultaneous fitting
of all available XMM data allows a constraint on the quiescent neutron star
(0.01-10 keV) luminosity of L_{NS}<1.1*10^{31} erg/s. This limit excludes some
current models of neutrino emission mediated by pion condensates, and provides
further evidence for additional cooling processes, such as neutrino emission
via direct Urca processes involving nucleons and/or hyperons, in the cores of
massive neutron stars.Comment: 5 pages, 2 figures; slight revisions, accepted by Ap
- …