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 $\rho$ or 'heartbeat' variability and the $\theta$ 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

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

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

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$c$ to 0.3$c$ 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 ($y_{\rm ign}$ = 10$^{10}$ g cm$^{-2}$). The detonation would possibly require a faster nuclear reaction chain, such as bypassing the alpha-capture on $^{12}$C with the much faster $^{12}$C(p,$\gamma$)$^{13}$N($\alpha$,p)$^{16}$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$^8$ g cm$^{-2}$.Comment: 11 pages, 6 figures, accepted by Astronomy and Astrophysic

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