301 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
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
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