Model Atmospheres for X-Ray Bursting Neutron Stars

Medin, Zach; Steinkirch, Marina von; Calder, A. C.; Fontes, Christopher J.; Fryer, Chris L.; Hungerford, Aimee L.

doi:10.3847/0004-637x/832/2/102

Cited by 12 publications

(13 citation statements)

References 69 publications

(153 reference statements)

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“…Applying such a correction would increase the inferred radius by a factor of f 2 c = (T c /T ef f ) 2 , where T c and T ef f are the color (measured) and effective temperatures, respectively, and f c is the so-called hardening factor. Based on model atmosphere calculations, f c is likely to fall within the range from ≈ 1.2 − 1.5 (Medin et al 2016;Suleimanov et al 2011), resulting in an increase in inferred radius by a factor of ≈ 1.4 − 2.2. However, given the other assumptions and uncertainties, the value is at least approximately consistent with expectations for surface emission from a neutron star.…”

Section: Properties Of Mhz Oscillationsmentioning

confidence: 99%

NICER Discovers mHz Oscillations in the “Clocked” Burster GS 1826−238

Strohmayer

Gendreau

Altamirano

et al. 2018

ApJ

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We report the discovery with the Neutron Star Interior Composition Explorer (NICER) of mHz X-ray brightness oscillations from the "clocked burster" GS 1826−238. NICER observed the source in the periods 2017 June 20 -29, July 11 -13, and September 9 -15, for a total useful exposure of 34 ks. Two consecutive dwells obtained on 2017 September 9 revealed highly significant oscillations at a frequency of 8 mHz. The fractional, sinusoidal modulation amplitude increases from 0.7% at 1 keV to ≈ 2% at 6 keV. Similar oscillations were also detected at lower significance in three additional dwells. The oscillation frequency and amplitude are consistent with those of mHz QPOs reported in other accreting neutron star systems. A thermonuclear X-ray burst was also observed on 2017 June 22. The burst properties and X-ray colors are both consistent with GS 1826 being in a soft spectral state during these observations, findings that are confirmed by ongoing monitoring with MAXI and SWIFT-BAT. Assuming that the mHz oscillations are associated with black body emission from the neutron star surface, modeling of the phase-resolved

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Section: Properties Of Mhz Oscillationsmentioning

confidence: 99%

NICER Discovers mHz Oscillations in the “Clocked” Burster GS 1826−238

Strohmayer

Gendreau

Altamirano

et al. 2018

ApJ

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“…This is thought to be the moment when the photosphere "touches down" on the NS surface at the end of the PRE. By knowing the distance to a source and associating the touchdown flux with the Eddington flux, it is possible to constrain the NS radius R. However, the measurements may be subject to considerable systematic errors (Boutloukos et al 2010;Steiner et al 2010;Suleimanov et al 2011;Miller 2013;Medin et al 2016;Miller & Lamb 2016). This is partly due to spectral modeling uncertainties, such as how the color-correction factor, which enters the fit to R, depends on luminosity and composition.…”

Section: Introductionmentioning

confidence: 99%

Super-Eddington Winds from Type I X-Ray Bursts

Weinberg

2018

ApJ

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We present hydrodynamic simulations of spherically symmetric super-Eddington winds from radiusexpansion type I X-ray bursts. Previous studies assumed a steady-state wind and treated the massloss rate as a free parameter. Using MESA, we follow the multi-zone time-dependent burning, the convective and radiative heating of the atmosphere during the burst rise, and the launch and evolution of the optically thick radiation-driven wind as the photosphere expands outward to radii r ph 100 km. We focus on neutron stars (NSs) accreting pure helium and study bursts over a range of ignition depths. We find that the wind ejects ≈ 0.2% of the accreted layer, nearly independent of ignition depth. This implies that ≈ 30% of the nuclear energy release is used to unbind matter from the NS surface. We show that ashes of nuclear burning are ejected in the wind and dominate the wind composition for bursts that ignite at column depths 10 9 g cm −2 . The ejecta are composed primarily of elements with mass numbers A > 40, which we find should imprint photoionization edges on the burst spectra. Evidence of heavy-element edges has been reported in the spectra of strong, radius-expansion bursts. We find that after ≈ 1 s the wind composition transitions from mostly light elements ( 4 He and 12 C), which sit at the top of the atmosphere, to mostly heavy elements (A > 40), which sit deeper down. This may explain why the photospheric radii of all superexpansion bursts show a transition after ≈ 1 s from a superexpansion (r ph > 10 3 km) to a moderate expansion (r ph ∼ 50 km).

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“…Swank et al 1977;Hoffman et al 1977), although the underlying physics is complex and only captured by appropriate NS atmosphere models (e.g. London et al 1986;Lapidus et al 1986; see Suleimanov et al 2011bSuleimanov et al , 2012Nättilä et al 2015;Medin et al 2016 for recent computations). In some cases rapid variability on millisecond time scales, so-called burst oscillations, are detected during X-ray bursts and these can be used to infer the spin period of the NS (Strohmayer et al 1996; see Strohmayer and Bildsten 2006; Watts 2012 for reviews).…”

Section: Type-i X-ray Burstsmentioning

confidence: 99%

Accretion Disks and Coronae in the X-Ray Flashlight

et al. 2017

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Plasma accreted onto the surface of a neutron star can ignite due to unstable thermonuclear burning and produce a bright flash of X-ray emission called a Type-I X-ray burst. Such events are very common; thousands have been observed to date from over a hundred accreting neutron stars. The intense, often Eddington-limited, radiation generated in these thermonuclear explosions can have a discernible effect on the surrounding accretion flow that consists of an accretion disk and a hot electron corona. Type-I X-ray bursts can there- fore serve as direct, repeating probes of the internal dynamics of the accretion process. In this work we review and interpret the observational evidence for the impact that Type-I X-ray bursts have on accretion disks and coronae. We also provide an outlook of how to make further progress in this research field with prospective experiments and analysis techniques, and by exploiting the technical capabilities of the new and concept X-ray missions ASTROSAT, NICER, Insight-HXMT, eXTP, and STROBE-X.

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Model Atmospheres for X-Ray Bursting Neutron Stars

Cited by 12 publications

References 69 publications

NICER Discovers mHz Oscillations in the “Clocked” Burster GS 1826−238

NICER Discovers mHz Oscillations in the “Clocked” Burster GS 1826−238

Super-Eddington Winds from Type I X-Ray Bursts

Accretion Disks and Coronae in the X-Ray Flashlight

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