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$c$ 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

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 J17480$-$2446. 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&