5,203 research outputs found
Accretion-powered Millisecond Pulsar Outbursts
The population of accretion-powered millisecond pulsars has grown rapidly
over the last four years, with the discovery of six new examples to bring the
total sample to seven. While the first six discovered are transients active for
a few weeks every two or more years, the most recently-discovered source HETE
J1900.1-2455, has been active for more than 8 months. We summarise the
transient behaviour of the population to estimate long-term time-averaged
fluxes, and equate these fluxes to the expected mass transfer rate driven by
gravitational radiation in order to constrain the distances. We also estimate
an upper limit of 6 kpc to the distance of IGR J00291+5934 based on the
non-detection of bursts from this source.Comment: 5 pages, 2 figures, to be published in: "The Transient Milky Way: a
perspective for MIRAX", eds. F. D'Amico, J. Braga & R. Rothschild, AIP Conf.
Pro
On the consistency of neutron-star radius measurements from thermonuclear bursts
The radius of neutron stars can in principle be measured via the
normalisation of a blackbody fitted to the X-ray spectrum during thermonuclear
(type-I) X-ray bursts, although few previous studies have addressed the
reliability of such measurements. Here we examine the apparent radius in a
homogeneous sample of long, mixed H/He bursts from the low-mass X-ray binaries
GS 1826-24 and KS 1731-26. The measured blackbody normalisation (proportional
to the emitting area) in these bursts is constant over a period of up to 60s in
the burst tail, even though the flux (blackbody temperature) decreased by a
factor of 60-75% (30-40%). The typical rms variation in the mean normalisation
from burst to burst was 3-5%, although a variation of 17% was found between
bursts observed from GS 1826-24 in two epochs. A comparison of the
time-resolved spectroscopic measurements during bursts from the two epochs
shows that the normalisation evolves consistently through the burst rise and
peak, but subsequently increases further in the earlier epoch bursts. The
elevated normalisation values may arise from a change in the anisotropy of the
burst emission, or alternatively variations in the spectral correction factor,
f_c, of order 10%. Since burst samples observed from systems other than GS
1826-24 are more heterogeneous, we expect that systematic uncertainties of at
least 10% are likely to apply generally to measurements of neutron-star radii,
unless the effects described here can be corrected for.Comment: 9 pages, 6 figures; accepted by Ap
Discovery of pulsations in the X-ray transient 4U 1901+03
We describe observations of the 2003 outburst of the hard-spectrum X-ray
transient 4U 1901+03 with the Rossi X-ray Timing Explorer. The outburst was
first detected in 2003 February by the All-Sky Monitor, and reached a peak
2.5-25 keV flux of 8x10^-9 ergs/cm^2/s (around 240 mCrab). The only other known
outburst occurred 32.2 yr earlier, likely the longest presently known
recurrence time for any X-ray transient. Proportional Counter Array (PCA)
observations over the 5-month duration of the 2003 outburst revealed a 2.763 s
pulsar in a 22.58 d orbit. The detection of pulsations down to a flux of
3x10^-11 ergs/cm^2/s (2.5-25 keV), along with the inferred long-term accretion
rate of 8.1x10^-11 M_sun/yr (assuming a distance of 10 kpc) suggests that the
surface magnetic field strength is below ~5x10^11 G. The corresponding
cyclotron energy is thus below 4 keV, consistent with the non-detection of
resonance features at high energies. Although we could not unambiguously
identify the optical counterpart, the lack of a bright IR candidate within the
1' RXTE error circle rules out a supergiant mass donor. The neutron star in 4U
1901+03 probably accretes from the wind of a main-sequence O-B star, like most
other high-mass binary X-ray pulsars. The almost circular orbit e=0.036
confirms the system's membership in a growing class of wide, low-eccentricity
systems in which the neutron stars may have received much smaller kicks as a
result of their natal supernova explosions.Comment: 7 pages, 6 figures, accepted by ApJ. Very minor addition in response
to referee's comment; updated author affiliatio
A study of the new X-ray transient RXTE J2123-058 during its post-outburst state
We carried out I, R, V and B photometric observations of the neutron star X-ray binary RXTE J2123â058 shortly after the end of the X-ray outburst in mid-1998. We adopt the low-mass binary model to interpret our observations. After folding our data on the 0.24 821-d orbital period, and correcting for the steady brightness decline following the outburst, we observed sinusoidal oscillations with hints of ellipsoidal modulations which became progressively more evident. Our data also show that the decline in brightness was faster in the V band than in the R and I bands. This suggests both the cooling of an irradiation-heated secondary star and the fading of an accretion disc over the nights of our observations
The AdS/CFT Correspondence Conjecture and Topological Censorship
In gr-qc/9902061 it was shown that (n+1)-dimensional asymptotically
anti-de-Sitter spacetimes obeying natural causality conditions exhibit
topological censorship. We use this fact in this paper to derive in arbitrary
dimension relations between the topology of the timelike boundary-at-infinity,
\scri, and that of the spacetime interior to this boundary. We prove as a
simple corollary of topological censorship that any asymptotically anti-de
Sitter spacetime with a disconnected boundary-at-infinity necessarily contains
black hole horizons which screen the boundary components from each other. This
corollary may be viewed as a Lorentzian analog of the Witten and Yau result
hep-th/9910245, but is independent of the scalar curvature of \scri.
Furthermore, the topology of V', the Cauchy surface (as defined for
asymptotically anti-de Sitter spacetime with boundary-at-infinity) for regions
exterior to event horizons, is constrained by that of \scri. In this paper,
we prove a generalization of the homology results in gr-qc/9902061 in arbitrary
dimension, that H_{n-1}(V;Z)=Z^k where V is the closure of V' and k is the
number of boundaries interior to . As a consequence, V
does not contain any wormholes or other compact, non-simply connected
topological structures. Finally, for the case of n=2, we show that these
constraints and the onto homomorphism of the fundamental groups from which they
follow are sufficient to limit the topology of interior of V to either B^2 or
.Comment: Revtex, 20 page
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