135 research outputs found
Production of case studies of the delivery of skills for learning, skills for life and skills for work
This report summarises the main themes to emerge from a study to highlight good practice in delivering practical, applied or vocational learning provision for all pupils
Chandra observations of the accretion-driven millisecond X-ray pulsars XTE J0929-314 and XTE J1751-305 in quiescence
(Abridge) We observed the accreting millisecond X-ray pulsars XTE J0929-314
and XTE J1751-305 in their quiescent states using Chandra. From XTE J0929-314
we detected 22 photons (0.3-8 keV) in 24.4 ksec, resulting in a count rate of 9
x 10^{-4} c/s. The small number of photons detected did not allow for a
detailed spectral analysis, but we can demonstrate that the spectrum is harder
than simple thermal emission which is what is usually presumed to arise from a
cooling neutron star that has been heated during the outbursts. Assuming a
power-law model for the spectrum, we obtain a power-law index of ~1.8 and an
unabsorbed flux of 6 x 10^{-15} ergs/s/cm^2 (0.5-10 keV), resulting in a
luminosity of 7 x 10^{31} (d/10 kpc)^2 ergs/s, with d in kpc. No thermal
component could be detected; such a component contributed at most 30% to the
0.5-10 keV flux. Variability in the count rate of XTE J0929-314 was observed at
the 95% confidence level. We did not conclusively detect XTE J1751-305 in our
43 ksec observation, with 0.5-10 keV flux upper limits between 0.2 and 2.7 x
10^{-14} ergs/s/cm^2 depending on assumed spectral shape, resulting in
luminosity upper limits of 0.2 - 2 x 10^{32} (d/8 kpc)^2 ergs/s. We compare our
results with those obtained for other neutron-star X-ray transients in their
quiescent state. Using simple accretion disk physics in combination with our
measured quiescent luminosity of XTE J0929-314 and the luminosity upper limits
of XTE J1751-305, and the known spin frequency of the neutron stars, we could
constrain the magnetic field of the neutron stars in XTE J0929-314 and XTE
J1751-305 to be less than 3 x 10^9 (d/10 kpc) and 3 - 7 x 10^8 (d/8 kpc) Gauss
(depending on assumed spectral shape of the quiescent spectrum), respectively.Comment: Accepted for publication in ApJ, 29 September 2004. Added spectral
variability search for the data of XTE J0929-314 and added the non-detection
with Chandra of XTE J1751-30
Monitoring Chandra observations of the quasi-persistent neutron-star X-ray transient MXB 1659-29 in quiescence: the cooling curve of the heated neutron-star crust
We have observed the quasi-persistent neutron-star X-ray transient and
eclipsing binary MXB 1659-29 in quiescence on three occasions with Chandra. The
purpose of our observations was to monitor the quiescent behavior of the source
after its last prolonged (~2.5 years) outburst which ended in September 2001.
The X-ray spectra of the source are consistent with thermal radiation from the
neutron-star surface. We found that the bolometric flux of the source decreased
by a factor of 7-9 over the time-span of 1.5 years between our first and last
Chandra observations. The effective temperature also decreased, by a factor of
1.6-1.7. The decrease in time of the bolometric flux and effective temperature
can be described using exponential decay functions, with e-folding times of 0.7
and ~3 years, respectively. Our results are consistent with the hypothesis that
we observed a cooling neutron-star crust which was heated considerably during
the prolonged accretion event and which is still out of thermal equilibrium
with the neutron-star core. We could only determine upper-limits for any
luminosity contribution due to the thermal state of the neutron-star core. The
rapid cooling of the neutron-star crust implies that it has a large thermal
conductivity. Our results also suggest that enhanced cooling processes are
present in the neutron-star core.Comment: Accepted for publication in ApJ Letters: March 19, 200
A Chandra observation of the long-duration X-ray transient KS 1731-260 in quiescence: too cold a neutron star?
After more than a decade of actively accreting at about a tenth of the
Eddington critical mass accretion rate, the neutron-star X-ray transient KS
1731-260 returned to quiescence in early 2001. We present a Chandra/ACIS-S
observation taken several months after this transition. We detected the source
at an unabsorbed flux of ~2 x 10^{-13} erg/cm^2/s (0.5-10 keV). For a distance
of 7 kpc, this results in a 0.5-10 keV luminosity of ~1 x 10^{33} erg/s and a
bolometric luminosity approximately twice that. This quiescent luminosity is
very similar to that of the other quiescent neutron star systems. However, if
this luminosity is due to the cooling of the neutron star, this low luminosity
may indicate that the source spends at least several hundreds of years in
quiescence in between outbursts for the neutron star to cool. If true, then it
might be the first such X-ray transient to be identified and a class of
hundreds of similar systems may be present in the Galaxy. Alternatively,
enhanced neutrino cooling could occur in the core of the neutron star which
would cool the star more rapidly. However, in that case the neutron star in KS
1731-260 would be more massive than those in the prototypical neutron star
transients (e.g., Aql X-1 or 4U 1608-52).Comment: Accepted for publicaton in ApJ letters, 13 September 200
Literature Review of Teacher Education for the Twenty-First Century
This literature review was commissioned by the Education Analytical Services Division ( EASD) of the Scottish Government and forms part of a larger programme of work contributing to the Review of Teacher Education in Scotland ( RTES). The RTES is examining how the current system of educating teachers equips teachers throughout their professional career to respond to curriculum change and meet the needs of pupils in the 21st century. The overall aim of this literature review is 'to understand the contribution that teacher education can make to the quality and effectiveness of the educational experience and wider personal development of young people, drawing on effective practice in Scotland and elsewhere'
Non-Detection of Gravitationally Redshifted Absorption Lines in the X-ray Burst Spectra of GS 1826-24
During a 200 ks observation with the XMM-Newton Reflection Grating
Spectrometer, we detected 16 type-I X-ray bursts from GS 1826-24. We combined
the burst spectra in an attempt to measure the gravitational redshifts from the
surface of the neutron star. We divided the composite GS 1826-24 burst spectrum
into three groups based on the blackbody temperature during the bursts. The
spectra do not show any obvious discrete absorption lines. We compare our
observations with those of EXO 0748-676.Comment: 4 pages, 4 figures; accepted for publication in ApJ
XMM-Newton observations of the black hole X-ray transient XTE J1650-500 in quiescence
We report the result of an XMM-Newton observation of the black-hole X-ray
transient XTE J1650-500 in quiescence. The source was not detected and we set
upper limits on the 0.5-10 keV luminosity of 0.9e31-1.0e31 erg/s (for a newly
derived distance of 2.6 kpc). These limits are in line with the quiescent
luminosities of black-hole X-ray binaries with similar orbital periods (~7-8
hr)Comment: 3 pages. Accepted for publication in MNRA
The burst behavior of the eclipsing low-mass X-ray binary MXB 1659-298
We present a detailed study of the correlations between the burst properties
and the inferred mass accretion rate for the X-ray transient MXB 1659-298. The
bursts which exhibited oscillations were observed when the source was at
relatively high mass accretion rate, similar to what has been seen for other
sources. However, due to the limited number of observations at lower mass
accretion rates, no bursts were observed at such accretion rates and it is
still possible that when MXB 1659-298 accretes at such low mass accretion
rates, bursts can occur which might still exhibit burst oscillations. No clear
correlations were found between the different burst properties and accretion
rate, in contrast to what has been found for KS 1731-260 and 4U 1728-34, but
similar to what has been reported for Aql X-1. However, this lack of
correlation for MXB 1659-298 and Aql X-1 might be due to the limited range of
mass accretion rate observed for those sources compared to KS 1731-260 and 4U
1728-34.Comment: Accepted for publication in ApJ, 18 October 200
- âŠ