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

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&

Ultracompact X-ray Binaries in Globular Clusters: Variability of the Optical Counterpart of X1832-330 in NGC 6652

Evidence is emerging that the luminous X-ray sources in the cores of globular clusters may often consist of, or perhaps even as a class be dominated by, ultracompact (P < 1 hr) binary stars. To the two such systems already known, in NGC 6624 and NGC 6712, we now add evidence for two more. We detect large amplitude variability in the candidate optical counterpart for the X-ray source in the core of NGC 6652. Although the available observations are relatively brief, the existing Hubble Space Telescope data indicate a strong 43.6 min periodic modulation of the visible flux of semi-amplitude 30%. Further, although the orbital period of the source in NGC 1851 is not yet explicitly measured, we demonstrate that previous correlations of optical luminosity with X-ray luminosity and accretion disk size, strengthened by recent data, strongly imply that the period of that system is also less than 1 hr. Thus currently there is evidence that 4 of the 7 globular cluster X-ray sources with constrained periods are ultracompact, a fraction far greater than that found in X-ray binaries the field.Comment: 10 pages including 2 figures and 1 table. Accepted for publication in The Astrophysical Journal Letter

Coupling Between Periodic and Aperiodic Variability in SAX J1808.4-3658

We detect a significant broadening in the wings of the 401 Hz peak in the power spectrum of the accreting millisecond binary pulsar SAX J1808.4-3658. This feature is consistent with the convolution of the red noise present in the power spectrum with the harmonic line. We conclude that the flux modulated by the spin period shows aperiodic variability similar to the red noise in the overall flux, suggesting such variability also originates at the magnetic caps close to the neutron star surface. This is analogous to the results found in some longer period, higher magnetic field pulsators in high mass X-ray binaries.Comment: 16 pages, 3 figures, to be published in The Astrophysical Journa