1,104 research outputs found
Achromatic late-time variability in thermonuclear X-ray bursts - an accretion disk disrupted by a nova-like shell?
An unusual Eddington-limited thermonuclear X-ray burst was detected from the
accreting neutron star in 2S 0918-549 with the Rossi X-ray Timing Explorer. The
burst commenced with a brief (40 ms) precursor and maintained near-Eddington
fluxes during the initial 77 s. These characteristics are indicative of a
nova-like expulsion of a shell from the neutron star surface. Starting 122 s
into the burst, the burst shows strong (87 +/- 1% peak-to-peak amplitude)
achromatic fluctuations for 60 s. We speculate that the fluctuations are due to
Thompson scattering by fully-ionized inhomogeneities in a resettling accretion
disk that was disrupted by the effects of super-Eddington fluxes. An expanding
shell may be the necessary prerequisite for the fluctuations.Comment: 7 pages, 4 figures. Submitted to A&
A population study of type II bursts in the Rapid Burster
Type II bursts are thought to arise from instabilities in the accretion flow
onto a neutron star in an X-ray binary. Despite having been known for almost 40
years, no model can yet satisfactorily account for all their properties. To
shed light on the nature of this phenomenon and provide a reference for future
theoretical work, we study the entire sample of Rossi X-ray Timing Explorer
data of type II bursts from the Rapid Burster (MXB 1730-335). We find that type
II bursts are Eddington-limited in flux, that a larger amount of energy goes in
the bursts than in the persistent emission, that type II bursts can be as short
as 0.130 s, and that the distribution of recurrence times drops abruptly below
15-18 s. We highlight the complicated feedback between type II bursts and the
NS surface thermonuclear explosions known as type I bursts, and between type II
bursts and the persistent emission. We review a number of models for type II
bursts. While no model can reproduce all the observed burst properties and
explain the source uniqueness, models involving a gating role for the magnetic
field come closest to matching the properties of our sample. The uniqueness of
the source may be explained by a special combination of magnetic field
strength, stellar spin period and alignment between the magnetic field and the
spin axis.Comment: Accepted 2015 February 12. Received 2015 February 10; in original
form 2014 December 1
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
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
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&
Identification of the optical and quiescent counterparts to the bright X-ray transient in NGC 6440
After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright
transient X-ray source turning on in August 2001, as noted with the RXTE
All-Sky Monitor. We carried out a short target of opportunity observation with
the Chandra X-ray Observatory and are able to associate the transient with the
brightest of 24 X-ray sources detected during quiescence in July 2000 with
Chandra. Furthermore, we securely identify the optical counterpart and
determine that the 1998 X-ray outburst in NGC 6440 was from the same object.
This is the first time that an optical counterpart to a transient in a globular
cluster is securely identified. Since the transient is a type I X-ray burster,
it is established that the compact accretor is a neutron star. Thus, this
transient provides an ideal case to study the quiescent emission in the optical
and X-ray of a transiently accreting neutron star while knowing the distance
and reddening accurately. One model that fits the quiescent spectrum is an
absorbed power law plus neutron star hydrogen atmosphere model. We find an
intrinsic neutron star radius of 17_{-12}^{+31} km and an unabsorbed bolometric
luminosity for the neutron star atmosphere of (2.1+/-0.8)E33 erg/s which is
consistent with predictions for a cooling neutron star.Comment: Accepted for publication in ApJ Letter
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