33,917 research outputs found
Discovery of GRS 1915+105 variability patterns in the Rapid Burster
We report the discovery of two new types of variability in the neutron star
low-mass X-ray binary MXB 1730-335 (the 'Rapid Burster'). In one observation in
1999, it exhibits a large-amplitude quasi-periodic oscillation with a period of
about 7 min. In another observation in 2008, it exhibits two 4-min long 75 per
cent deep dips 44 min apart. These two kinds of variability are very similar to
the so-called or 'heartbeat' variability and the variability,
respectively, seen in the black hole low-mass X-ray binaries GRS 1915+105 and
IGR J17091-3624. This shows that these types of behavior are unrelated to a
black hole nature of the accretor. Our findings also show that these kinds of
behaviour need not take place at near-Eddington accretion rates. We speculate
that they may rather be related to the presence of a relatively wide orbit with
an orbital period in excess of a few days and about the relation between these
instabilities and the type II bursts.Comment: Accepted for publication in MNRAS letter
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
Searching for the most powerful thermonuclear X-ray bursts with the Neil Gehrels Swift Observatory
We searched for thermonuclear X-ray bursts from Galactic neutron stars in all
event mode data of the Neil Gehrels Swift Observatory collected until March 31,
2018. In particular, we are interested in the intermediate-duration bursts
(shell flashes fueled by thick helium piles) with the ill-understood phenomenon
of strong flux fluctuations. Nine such bursts have been discussed in the
literature to date. Swift is particularly suitable for finding additional
examples. We find and list a total of 134 X-ray bursts; 44 are detected with
BAT only, 41 with XRT only, and 49 with both. Twenty-eight bursts involve
automatic slews. We find 12 intermediate-duration bursts, all detected in
observations involving automatic slews. Five show remarkably long
Eddington-limited phases in excess of 200 s. Five show fluctuations during the
decay phase; four of which are first discussed in the present study. We discuss
the general properties of the fluctuations, considering also 7 literature
cases. In general two types of fluctuations are observed: fast ones, with a
typical timescale of 1 s and up and downward fluctuations of up to 70%, and
slow ones, with a typical timescale of 1 min and only downward fluctuations of
up to 90%. The latter look like partial eclipses because the burst decay
remains visible in the residual emission. We revisit the interpretation of this
phenomenon in the context of the new data set and find that it has not changed
fundamentally despite the expanded data set. It is thought to be due to a
disturbance of the accretion disk by outflowing matter and photons, causing
obscuration and reflection due to Thompson scattering in an orbiting highly
ionized cloud or structure above or below the disk. We discuss in detail the
most pronounced burster SAX J1712.6-3739. One of the bursts from this source is
unusual in that it lasts longer than 5600 s, but does not appear to be a
superburst.Comment: Accepted for publication in Astronomy & Astrophysics, 29 pages, 12
figures. Version 2 has 3 bursts from IGR J17480-2446 re-identified to 2 from
Swift J174805.3-244637 and 1 from EXO 1745-24
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
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
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