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Register a new userCooling of Accretion Disc Coronae by Type I X-ray Bursts
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Understanding the persistent emission is crucial for studying type I X-ray bursts, which provide insight into neutron star properties. Although accretion disc coronae appear to be common in many accreting systems, their fundamental properties remain insufficiently understood. Recent work suggests that Type I X-ray bursts from accreting neutron stars provide an opportunity to probe the characteristics of coronae. Several studies have observed hard X-ray shortages from the accretion disk during an X-ray burst implying strong coronal cooling by burst photons. Here, we use the plasma emission code EQPAIR to study the impact of X-ray bursts on coronae, and how the coronal and burst properties affect the coronal electron temperatures and emitted spectra. Assuming a constant accretion rate during the burst, our simulations show that soft photons can cool coronal electrons by a factor of $gtrsim 10$ and cause a reduction of emission in the $30$-$50$ keV band to $lesssim 1%$ of the pre-burst emission. This hard X-ray drop is intensified when the coronal optical depth and aspect ratio is increased. In contrast, depending on the properties of the burst and corona, the emission in the $8$-$24$ keV band can either increase, by a factor of $gtrsim20$, or decrease, down to $lesssim 1%$ of the pre-burst emission. An increasing accretion rate during the X-ray burst reduces the coronal cooling effects and the electron temperature drop can be mitigated by $gtrsim60%$. These results indicate that changes of the hard X-ray flux during an X-ray burst probe the geometrical properties of the corona.
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We perform a set of numerical experiments studying the interaction of Type I X-ray bursts with thin, Shakura-Sunyaev type accretion discs. Careful observations of X-ray spectra during such bursts have hinted at changes occurring in the inner regions of the disc. We now clearly demonstrate a number of key effects that take place simultaneously, including: evidence for weak, radiation-driven outflows along the surface of the disc; significant levels of Poynting-Robertson (PR) drag, leading to enhanced accretion; and prominent heating in the disc, which increases the height, while lowering the density and optical depth. The PR drag causes the inner edge of the disc to retreat from the neutron star surface toward larger radii and then recover on the timescale of the burst. We conclude that the rich interaction of an X-ray burst with the surrounding disc provides a novel way to study the physics of accretion onto compact objects.
Accreting neutron stars (NS) can exhibit high frequency modulations in their lightcurves during thermonuclear X-ray bursts, known as burst oscillations. The frequencies can be offset from the spin frequency of the NS by several Hz, and can drift by 1-3 Hz. One possible explanation is a mode in the bursting ocean, the frequency of which would decrease (in the rotating frame) as the burst cools, hence explaining the drifts. Most burst oscillations have been observed during H/He triggered bursts, however there has been one observation of oscillations during a superburst; hours long Type I X-ray bursts caused by unstable carbon burning deeper in the ocean. This paper calculates the frequency evolution of an oceanic r-mode during a superburst. The rotating frame frequency varies during the burst from 4-14 Hz, and is sensitive to the background parameters, in particular the temperature of the ocean and ignition depth. This calculation is compared to the superburst oscillations observed on 4U-1636-536. The predicted mode frequencies ($sim$ 10 Hz) would require a spin frequency of $sim$ 592 Hz to match observations; 6 Hz higher than the spin inferred from an oceanic r-mode model for the H/He triggered burst oscillations. This model also over-predicts the frequency drift during the superburst by 90 %.
Type I X-ray bursts are thermonuclear explosions on the neutron star (NS) surface by mass accretion from a companion star. Observation of X-ray bursts provides valuable information on X-ray binary systems, e.g., binary parameters, the chemical composition of accreted matter, and the nuclear equation of state (EOS) of NSs. There have been several theoretical studies to constrain the physics of X-ray bursters. However, they were mainly focused on the burning layers above the NS surface. The effects of the EOS and the heating and cooling processes inside the NS are still unknown. In this study, we calculated a series of X-ray bursts using a general relativistic stellar-evolution code with several NS EOSs. We compared the X-ray burst models with the burst parameters of a clocked burster associated with GS 1826-24. We found a monotonic correlation between the NS radius and the light-curve profile. A larger radius shows a higher recurrence time and a large peak luminosity. In contrast, the dependence of light curves on the NS mass becomes more complicated, where the neutrino cooling suppress the efficiency of nuclear ignition. We also constrained the EOS and mass of GS~1826-24, i.e., stiffer EOSs, corresponding to larger NS radii, are unpreffered due to a too high peak luminosity. The EOS and the cooling and heating of NSs are important to discuss the theoretical and observational properties of X-ray bursts.
The neutron-star X-ray transient XTE J1701-462 was observed for $sim$3 Ms with xte during its 2006-2007 outburst. Here we report on the discovery of three type-I X-ray bursts from XTE J1701-462. They occurred as the source was in transition from the typical Z-source behavior to the typical atoll-source behavior, at $sim10%$ of the Eddington luminosity. The first burst was detected in the Z-source flaring branch; the second in the vertex between the flaring and normal branches; and the third in the atoll-source soft state. The detection of the burst in the flaring branch cast doubts on earlier speculations that the flaring branch is due to unstable nuclear burning of accreted matter. The last two of the three bursts show photospheric radius expansion, from which we estimate the distance to the source to be 8.8 kpc with a 15% uncertainty. No significant burst oscillations in the range 30 to 4000 Hz were found during these three bursts.
Type I X-ray bursts are thermonuclear explosions that occur in the envelopes of accreting neutron stars. Detailed observations of these phenomena have prompted numerous studies in theoretical astrophysics and experimental nuclear physics since their discovery over 35 years ago. In this review, we begin by discussing key observational features of these phenomena that may be sensitive to the particular patterns of nucleosynthesis from the associated thermonuclear burning. We then summarize efforts to model type I X-ray bursts, with emphasis on determining the nuclear physics processes involved throughout these bursts. We discuss and evaluate limitations in the models, particularly with regard to key uncertainties in the nuclear physics input. Finally, we examine recent, relevant experimental measurements and outline future prospects to improve our understanding of these unique environments from observational, theoretical and experimental perspectives.
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