Evidence for Accretion Rate Change During Type I X-Ray Bursts

Wörpel, H.; Galloway, D. K.; Price, Daniel J.

doi:10.1088/0004-637x/772/2/94

Cited by 132 publications

(221 citation statements)

References 84 publications

(68 reference statements)

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“…Both models provide good and stable fits. Like Worpel et al (2013), we found that f a increases during the peak phase (up to 20 or so, with typical values around 5), and remains significantly above 1 over the 20-s interval following the burst. An example of f a variations for three bursts is shown in Fig.…”

Section: Comparison With Burst Parameterssupporting

confidence: 74%

Probing X-ray burst – accretion disk interaction in low mass X-ray binaries through kilohertz quasiperiodic oscillations

Peille

Olive

Barret

2014

A&A

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The intense radiation flux of Type I X-ray bursts is expected to interact with the accretion flow around neutron stars. High frequency quasiperiodic oscillations (kHz QPOs), observed at frequencies matching orbital frequencies at tens of gravitational radii, offer a unique probe of the innermost disk regions. In this paper, we follow the lower kHz QPOs, in response to Type I X-ray bursts, in two prototypical QPO sources, namely 4U 1636-536 and 4U 1608-522, as observed by the Proportional Counter Array of the Rossi X-ray Timing Explorer. We have selected a sample of 15 bursts for which the kHz QPO frequency can be tracked on timescales commensurable with the burst durations (tens of seconds). We find evidence that the QPOs are affected for over ∼200 s during one exceptionally long burst and ∼100 s during two others (although at a less significant level), while the burst emission has already decayed to a level that would enable the pre-burst QPO to be detected. On the other hand, for most of our burst-kHz QPO sample, we show that the QPO is detected as soon as the statistics allow and in the best cases, we are able to set an upper limit of ∼20 s on the recovery time of the QPO. This diversity of behavior cannot be related to differences in burst peak luminosity. We discuss these results in the framework of recent findings that accretion onto the neutron star may be enhanced during Type I X-ray bursts. The subsequent disk depletion could explain the disappearance of the QPO for ∼100 s, as possibly observed in two events. However, alternative scenarios would have to be invoked for explaining the short recovery timescales inferred from most bursts. Heating of the innermost disk regions would be a possibility, although we cannot exclude that the burst does not affect the QPO emission at all. Clearly the combination of fast timing and spectral information of Type I X-ray bursts holds great potential in the study of the dynamics of the inner accretion flow around neutron stars. However, as we show, breakthrough observations will require a timing instrument providing at least ten times the effective area of the RXTE/PCA.

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Section: Comparison With Burst Parameterssupporting

confidence: 74%

Probing X-ray burst – accretion disk interaction in low mass X-ray binaries through kilohertz quasiperiodic oscillations

Peille

Olive

Barret

2014

A&A

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“…This works well in practice, because the burst emission is much stronger than the persistent emission, at least around the burst peak (see also the discussion in Kuulkers et al 2002). Recently, however, observations have shown that the persistent component does evolve during the burst (in 't Zand et al 2013;Worpel et al 2013;Ji et al 2014;Keek et al 2014a). Furthermore, as the reflection spectrum resembles that of the neutron star's flux (Ballantyne 2004), they are typically not distinguished.…”

Section: Introductionmentioning

confidence: 74%

“…Moreover, recently indications have been found that anisotropies may even evolve during a single X-ray burst. Worpel et al (2013) found that the observed persistent flux briefly increases during bursts, which they measure as a multiplicative factor, f a (see also in 't Zand et al 2013;Worpel et al 2015). The anisotropy parameter p 1 x -has a similar effect on the observed flux to f a .…”

Section: Variability In Persistent and Burst Fluxmentioning

confidence: 86%

“…This is confirmed by the 2001 superburst from 4U1636-536, where the observed increase in persistent emission was not directly correlated with the evolution of the reflection fraction (Keek et al 2014a(Keek et al , 2014b. PoyntingRobertson drag has been suggested to cause a temporary increase in the persistent flux (Ballantyne & Everett 2005;Worpel et al 2013). This would evacuate the inner disk, and potentially change the anisotropy factors during a burst.…”

Section: Variability In Persistent and Burst Fluxmentioning

confidence: 94%

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Anisotropy of X-Ray Bursts From Neutron Stars With Concave Accretion Disks

Keek

2016

ApJ

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Emission from neutron stars and accretion disks in low-mass X-ray binaries is anisotropic. The non-spherical shape of the disk as well as blocking of the neutron star by the disk make the observed flux dependent on the inclination angle of the disk with respect to the line of sight. This is of importance for the interpretation of thermonuclear X-ray bursts from neutron stars. Because part of the X-ray burst is reflected off the disk, the observed burst flux depends on the anisotropies for both direct emission from the neutron star and reflection off the disk. This influences measurements of source distance, mass accretion rate, and constraints on the neutron star's equation of state. Previous predictions of the anisotropy factors assumed a geometrically flat disk. Detailed observations of two so-called superbursts allowed for the direct and the reflected burst fluxes to each be measured separately. The reflection fraction was much higher than what the anisotropies of a flat disk can account for. We create numerical models to calculate the anisotropy factors for different disk shapes, including concave disks. We present the anisotropy factors of the direct and reflected burst fluxes separately, as well as the anisotropy of the persistent flux. Reflection fractions substantially larger than unity are produced in the case where the inner accretion disk increases steeply in height, such that part of the star is blocked from view. Such a geometry could possibly be induced by the X-ray burst if X-ray heating causes the inner disk to puff up.

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“…Although Worpel et al (2013) suggest that the persistent emission may increase during bursts, the statistics of the JEM-X spectra are not sensitive enough to include this effect. Our net-burst spectra are described well by a black-body component modified by the interstellar absorption, which is dominated by photoelectric absorption.…”

Section: Burst Spectral Analysismentioning

confidence: 99%

New insights into the quasi-periodic X-ray burster GS 0836–429

Aranzana

Sánchez–Fernández

Kuulkers

2016

A&A

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GS 0836-429 is a neutron star X-ray transient that displays Type-I X-ray bursts. In 2003 and 2004 it experienced two outbursts in X-rays. We present here an analysis of the system's bursting properties during these outbursts. We studied the evolution of the 2003-2004 outbursts in soft X-rays using RXTE (2.5-12 keV; ASM) and in hard X-rays with INTEGRAL (17-80 keV, IBIS/ISGRI). Using data from the JEM-X monitor onboard INTEGRAL, we studied the bursting properties of the source. We detected 61 Type-I X-ray bursts during the 2004 outburst and confirm that the source displayed a quasi-periodic burst recurrence time of about 2.3 h. We improve the characterisation of the fuel composition, as well as the description of the typical burst durations and fluences. We estimate the average value of α to be 49 ± 3, which describes the ratio of the gravitational energy released between bursts to the nuclear energy released in an X-ray burst. Both this value and the observed burst profiles indicate a regime of a mixed He/H runaway triggered by unstable helium ignition. In addition, we report the detection of four series of double bursts, with burst recurrence times of ≤20 min. The secondary bursts are always shorter and less energetic than the primary and typical bursts from the source. The measured recurrence time in double bursts is too short to allow the accretion of enough fresh material, which is needed to trigger a Type-I X-ray burst. This suggests the presence of leftover, unburned material from the preceding burst, which gets ignited on a time scale of minutes. The energies and time scales of the secondary bursts suggest a lower fraction of hydrogen compared to that estimated for the primary bursts. The persistent emission was roughly constant during the period when the Type I X-ray bursts were detected. We derive an average accretion rate during our observations ofṁ ∼ 8%ṁ Edd . The spectrum of the persistent emission during these observations can be fit with a non-thermal component, indicative for the source to be in a hard state when the INTEGRAL observations were performed.

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Evidence for Accretion Rate Change During Type I X-Ray Bursts

Cited by 132 publications

References 84 publications

Probing X-ray burst – accretion disk interaction in low mass X-ray binaries through kilohertz quasiperiodic oscillations

Probing X-ray burst – accretion disk interaction in low mass X-ray binaries through kilohertz quasiperiodic oscillations

Anisotropy of X-Ray Bursts From Neutron Stars With Concave Accretion Disks

New insights into the quasi-periodic X-ray burster GS 0836–429

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