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Register a new userMultiple X-ray bursts and the model of a spreading layer of accreting matter over the neutron star surface
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S.A. Grebenev
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We report the detection during the JEM-X/INTEGRAL observations of several X-ray bursters of series of close type I X-ray bursts consisting of two or three events with a recurrence time much shorter than the characteristic (at the observed mean accretion rate) time of matter accumulation needed for a thermonuclear explosion to be initiated on the neutron star surface. We show that such series of bursts are naturally explained in the model of a spreading layer of accreting matter over the neutron star surface in the case of a sufficiently high ($dot{M}geq 1times 10^{-9} M_{odot} mbox{yr}^{-1}$) accretion rate (corresponding to a mean luminosity $L_{rm tot}geq 1times 10^{37} mbox{erg s}^{-1}$). The existence of triple bursts requires some refinement of the model - the importance of a central ring zone is shown. In the standard model of a spreading layer no infall of matter in this zone is believed to occur.
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The effects of a new triple-$alpha$ reaction rate (OKK rate) on the helium flash of a helium accreting neutron star in a binary system have been investigated. Since the ignition points determine the properties of a thermonuclear flash of type I X-ray bursts, we examine the cases of different accretion rates, $dM/dt (dot{M})$, of helium from $3times10^{-10} M_{odot} rm yr^{-1}$ to $3times10^{-8} M_{odot} rm yr^{-1}$, which could cover the observed accretion rates. We find that for the cases of low accretion rates, nuclear burnings are ignited at the helium layers of rather low densities. As a consequence, helium deflagration would be triggered for all cases of lower accretion rate than $dot{M}simeq 3times10^{-8} M_{odot} rm yr^{-1}$. We find that OKK rate could be barely consistent with the available observations of the X-ray bursts on the helium accreting neutron star. However this coincidence is found to depend on the properties of crustal heating and the neutron star model.We suggest that OKK rate would be reduced by a factor of $10^{2-3}$ for $10^8$ K in the range of the observational errors.
Hotspots on the surface of accreting neutron stars have been directly observed via pulsations in the lightcurves of X-ray pulsars. They are thought to occur due to magnetic channelling of the accreted fuel to the neutron star magnetic poles. Some X-ray pulsars exhibit burst oscillations during Type I thermonuclear X-ray bursts which are thought to be caused by asymmetries in the burning. In rapidly rotating neutron stars, it has been shown that the lower gravity at the equator can lead to preferential ignition of X-ray bursts at this location. These models, however, do not include the effect of accretion hotspots at the neutron star surface. There are two accreting neutron star sources in which burst oscillations have been observed to track exactly the neutron star spin period. We analyse whether this could be due to the X-ray bursts igniting at the magnetic pole of the neutron star, because of heating in the accreted layers under the hotspot causing ignition conditions to be reached earlier. We investigate heat transport in the accreted layers using a 2D model and study the prevalence of heating down to the ignition depth of X-ray bursts for different hotspot temperatures and sizes. We perform calculations for accretion at the pole and at the equator, and infer that ignition could occur away from the equator at the magnetic pole for hotspots with temperatures greater than $1times10^8$ K. However, current observations have not identified such high temperatures in accreting X-ray pulsars.
When the upper layer of an accreting neutron star experiences a thermonuclear runaway of helium and hydrogen, it exhibits an X-ray burst of a few keV with a cool-down phase of typically 1~minute. When there is a surplus of hydrogen, hydrogen fusion is expected to simmer during that same minute due to the rp process, which consists of rapid proton captures and slow beta-decays of proton-rich isotopes. We have analyzed the high-quality light curves of 1254 X-ray bursts, obtained with the Proportional Counter Array on the Rossi X-ray Timing Explorer between 1996 and 2012, to systematically study the cooling and rp process. This is a follow-up of a study on a selection of 37 bursts from systems that lack hydrogen and show only cooling during the bursts. We find that the bolometric light curves are well described by the combination of a power law and a one-sided Gaussian. The power-law decay index is between 1.3 and 2.1 and similar to that for the 37-bursts sample. There are individual bursters with a narrower range. The Gaussian is detected in half of all bursts, with a typical standard deviation of 50~s and a fluence ranging up to 60% of the total fluence. The Gaussian appears consistent with being due to the rp process. The Gaussian fluence fraction suggests that the layer where the rp process is active is underabundant in H by a factor of at least five with respect to cosmic abundances. Ninety-four percent of all bursts from ultracompact X-ray binaries lack the Gaussian component, and the remaining 6% are marginal detections. This is consistent with a hydrogen deficiency in these binaries. We find no clear correlation between the power law and Gaussian light-curve components.
The ultracompact low-mass X-ray binary 4U 1820-30 situated in the globular cluster NGC 6624 has an orbital period of only $approx$11.4 min which likely implies a white dwarf companion. The observed X-ray bursts demonstrate a photospheric radius expansion phase and therefore are believed to reach the Eddington luminosity allowing us to estimate the mass and the radius of the neutron star (NS) in this binary. Here we re-analyse all Rossi X-ray Timing Explorer observations of the system and confirm that almost all the bursts took place during the hard persistent state of the system. This allows us to use the recently developed direct cooling tail method to estimate the NS mass and radius. However, because of the very short, about a second, duration of the cooling tail phases that can be described by the theoretical atmosphere models, the obtained constraints on the NS radius are not very strict. Assuming a pure helium NS atmosphere we found that the NS radius is in the range 10-12 km, if the NS mass is below 1.7 $M_odot$, and in a wider range of 8-12 km for a higher 1.7-2.0 $M_odot$ NS mass. The method also constrains the distance to the system to be 6.5$pm$0.5 kpc, which is consistent with the distance to the cluster. For the solar composition atmosphere, the NS parameters are in strong contradiction with the generally accepted range of possible NS masses and radii.
The problem of disk accretion onto the surface of a neutron star with a weak magnetic field at a luminosity exceeding several percent of Eddington is reduced to the problem of the braking of a hypersonic flow with a velocity that is 0.4-0.5 of the speed of light above the base of the spreading layer -- a dense atmosphere made up of previously fallen matter. We show that turbulent braking in the Prandtl-Karman model with universally accepted coefficients for terrestrial conditions and laboratory experiments and a ladder of interacting gravity waves in a stratified quasi-exponential atmosphere at standard Richardson numbers lead to a spin-up of the massive zone that extends to the ocean made up of a plasma with degenerate electrons. Turbulent braking in the ocean at the boundary with the outer solid crust reduces the rotation velocity to the solid-body rotation velocity of the star. This situation should lead to strong heating of deep atmospheric layers and to the switch-off of the explosive helium burning mechanism. Obviously, a more efficient mechanism for the dissipation of a fast azimuthal flow in the atmosphere should operate in X-ray bursters. We show that a giant solitary gravity wave in the atmosphere can lead to energy dissipation and to a sharp decrease in azimuthal velocity in fairly rarefied atmospheric layers above the zone of explosive helium burning nuclear reactions. We discuss the reasons why this wave, that has no direct analog in the Earths atmosphere or ocean, appears and its stability. We pose the question as to whether neutron stars with massive atmospheres, spun up to high velocities by accreting matter from a disk, can exist among the observed Galactic X-ray sources.
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