1,231 research outputs found
A Firm Upper Limit to the Radius of the Neutron Star in SAX J1808.4-3658
We show that observations of X-ray pulsing from SAX J1808.4-3658 place a firm
upper limit of 13.8 m^{1/3} km on the radius of the neutron star, where m is
its mass in solar units. The limit is independent of distance or assumptions
about the magnetospheric geometry, and could be significantly tightened by
observations of the pulsations in the near future. We discuss the implications
for the equation of state and the possible neutron star mass.Comment: (7 pages, 1 figure, accepted for publication in ApJ Letters
Relativistic outflow from two thermonuclear shell flashes on neutron stars
We study the exceptionally short (32-41 ms) precursors of two
intermediate-duration thermonuclear X-ray bursts observed with RXTE from the
neutron stars in 4U 0614+09 and 2S 0918-549. They exhibit photon fluxes that
surpass those at the Eddington limit later in the burst by factors of 2.6 to
3.1. We are able to explain both the short duration and the super-Eddington
flux by mildly relativistic outflow velocities of 0.1 to 0.3 subsequent
to the thermonuclear shell flashes on the neutron stars. These are the highest
velocities ever measured from any thermonuclear flash. The precursor rise times
are also exceptionally short: about 1 ms. This is inconsistent with predictions
for nuclear flames spreading laterally as deflagrations and suggests
detonations instead. This is the first time that a detonation is suggested for
such a shallow ignition column depth ( = 10 g cm).
The detonation would possibly require a faster nuclear reaction chain, such as
bypassing the alpha-capture on C with the much faster
C(p,)N(,p)O process previously proposed.
We confirm the possibility of a detonation, albeit only in the radial
direction, through the simulation of the nuclear burning with a large nuclear
network and at the appropriate ignition depth, although it remains to be seen
whether the Zel'dovich criterion is met. A detonation would also provide the
fast flame spreading over the surface of the neutron star to allow for the
short rise times. (...) As an alternative to the detonation scenario, we
speculate on the possibility that the whole neutron star surface burns almost
instantly in the auto-ignition regime. This is motivated by the presence of 150
ms precursors with 30 ms rise times in some superexpansion bursts from 4U
1820-30 at low ignition column depths of ~10 g cm.Comment: 11 pages, 6 figures, accepted by Astronomy and Astrophysic
A closer look at the X-ray transient XTE J1908+094: identification of two new near-infrared candidate counterparts
We had reported in Chaty, Mignani, Israel (2002) on the near-infrared (NIR)
identification of a possible counterpart to the black hole candidate XTE
J1908+094 obtained with the ESO/NTT. Here, we present new, follow-up, CFHT
adaptive optics observations of the XTE J1908+094 field, which resolved the
previously proposed counterpart in two objects separated by about 0.8".
Assuming that both objects are potential candidate counterparts, we derive that
the binary system is a low-mass system with a companion star which could be
either an intermediate/late type (A-K) main sequence star at a distance of 3-10
kpc, or a late-type (K) main sequence star at a distance of 1-3 kpc.
However, we show that the brighter of the two objects (J ~ 20.1, H ~ 18.7, K' ~
17.8) is more likely to be the real counterpart of the X-ray source. Its
position is more compatible with our astrometric solution, and colours and
magnitudes of the other object are not consistent with the lower limit of 3 kpc
derived independently from the peak bolometric flux of XTE J1908+094. Further
multi-wavelength observations of both candidate counterparts are crucial in
order to solve the pending identification.Comment: accepted for publication in MNRAS, 5 pages, 3 figure
Indications for a slow rotator in the Rapid Burster from its thermonuclear bursting behaviour
We perform time-resolved spectroscopy of all the type I bursts from the Rapid
Burster (MXB 1730-335) detected with the Rossi X-ray Timing Explorer. Type I
bursts are detected at high accretion rates, up to \sim 45% of the Eddington
luminosity. We find evidence that bursts lacking the canonical cooling in their
time-resolved spectra are, none the less, thermonuclear in nature. The type I
bursting rate keeps increasing with the persistent luminosity, well above the
threshold at which it is known to abruptly drop in other bursting low-mass
X-ray binaries. The only other known source in which the bursting rate keeps
increasing over such a large range of mass accretion rates is the 11 Hz pulsar
IGR J174802446. This may indicate a similarly slow spin for the neutron star
in the Rapid Burster
IGR J17254-3257, a new bursting neutron star
The study of the observational properties of uncommonly long bursts from low
luminosity sources with extended decay times up to several tens of minutes is
important when investigating the transition from a hydrogen-rich bursting
regime to a pure helium regime and from helium burning to carbon burning as
predicted by current burst theories. IGR J17254-3257 is a recently discovered
X-ray burster of which only two bursts have been recorded: an ordinary short
type I X-ray burst, and a 15 min long burst. An upper limit to its distance is
estimated to about 14.5 kpc. The broad-band spectrum of the persistent emission
in the 0.3-100 keV energy band obtained using contemporaneous INTEGRAL and
XMM-Newton data indicates a bolometric flux of 1.1x10^-10 erg/cm2/s
corresponding, at the canonical distance of 8 kpc, to a luminosity about
8.4x10^35 erg/s between 0.1-100 keV, which translates to a mean accretion rate
of about 7x10^-11 solar masses per year. The low X-ray persistent luminosity of
IGR J17254-3257 seems to indicate the source may be in a state of low accretion
rate usually associated with a hard spectrum in the X-ray range. The nuclear
burning regime may be intermediate between pure He and mixed H/He burning. The
long burst is the result of the accumulation of a thick He layer, while the
short one is a prematurate H-triggered He burning burst at a slightly lower
accretion rate.Comment: 4 pages, 4 figures, 1 table; accepted for publication in A&A Letters.
1 reference (Cooper & Narayan, 2007) correcte
A new bursting X-ray transient: SAX J1750.8-2900
We have analysed in detail the discovery measurements of the X-ray burster
SAX J1750.8-2900 by the Wide Field Cameras on board BeppoSAX in spring 1997, at
a position ~1.2 degrees off the Galactic Centre. The source was in outburst on
March 13th when the first observation started and showed X-ray emission for ~ 2
weeks. A total of 9 bursts were detected, with peak intensities varying from ~
0.4 to 1.0 Crab in the 2-10 keV range. Most bursts showed a fast rise time (~
1s), an exponential decay profile with e-folding time of ~ 5s, spectral
softening during decay, and a spectrum which is consistent with few keV
blackbody radiation. These features identify them as type-I X-ray bursts of
thermonuclear origin. The presence of type-I bursts and the source position
close to the Galactic Centre favours the classification of this object as a
neutron star low mass X-ray binary. X-ray emission from SAX J1750.8-2900 was
not detected in the previous and subsequent Galactic bulge monitoring, and the
source was never seen bursting again.Comment: 13 pages, 3 Postscript figures, aaspp4 styl
Long tails on thermonuclear X-ray bursts from neutron stars: a signature of inward heating?
We report the discovery of one-hour long tails on the few-minutes long X-ray
bursts from the `clocked burster' GS 1826-24. We propose that the tails are due
to enduring thermal radiation from the neutron star envelope. The enduring
emission can be explained by cooling of deeper NS layers which were heated up
through inward conduction of heat produced in the thermonuclear shell flash
responsible for the burst. Similar, though somewhat shorter, tails are seen in
bursts from EXO 0748-676 and 4U 1728-34. Only a small amount of cooling is
detected in all these tails. This is either due to compton up scattering of the
tail photons or, more likely, to a NS that is already fairly hot due to other,
stable, nuclear processes.Comment: Accepted for publication in Astronomy & Astrophysics, 12 pages, 14
figure
The cooling rate of neutron stars after thermonuclear shell flashes
Thermonuclear shell flashes on neutron stars are detected as bright X-ray
bursts. Traditionally, their decay is modeled with an exponential function.
However, this is not what theory predicts. The expected functional form for
luminosities below the Eddington limit, at times when there is no significant
nuclear burning, is a power law. We tested the exponential and power-law
functional forms against the best data available: bursts measured with the
high-throughput Proportional Counter Array (PCA) on board the Rossi X-ray
Timing Explorer. We selected a sample of 35 'clean' and ordinary (i.e., shorter
than a few minutes) bursts from 14 different neutron stars that 1) show a large
dynamic range in luminosity, 2) are the least affected by disturbances by the
accretion disk and 3) lack prolonged nuclear burning through the rp-process. We
find indeed that for every burst a power law is a better description than an
exponential function. We also find that the decay index is steep, 1.8 on
average, and different for every burst. This may be explained by contributions
from degenerate electrons and photons to the specific heat capacity of the
ignited layer and by deviations from the Stefan-Boltzmann law due to changes in
the opacity with density and temperature. Detailed verification of this
explanation yields inconclusive results. While the values for the decay index
are consistent, changes of it with the burst time scale, as a proxy of ignition
depth, and with time are not supported by model calculations.Comment: 10 pages, 7 figures, recommended for publication in A&
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