Accretion Disks and Coronae in the X-Ray Flashlight

Abstract

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.

NICER Observes a Secondary Peak in the Decay of a Thermonuclear Burst from 4U 1608–52

“…Thus, the variation in f a likely represents a net outcome from the atmosphere as well as a possible contribution from the varying accretion flow during the burst. Our current understanding hardly allows us to segregate these effects from the spectrum due to degeneracy in theoretical modeling (Worpel et al 2015;Degenaar et al 2018). Using the above method, we obtain an improved fit with χ 2 (ν) = 577(505), 624(551) and 616(576) for the first, second and third time-segments, respectively.…”

“…It is commonly assumed that the "persistent" emission from the accretion process remains constant during the burst. However, recent studies suggest that the irradiation from bursts can modify the persistent continuum (Chen et al 2012;in't Zand et al 2013;Worpel et al 2013Worpel et al , 2015Degenaar et al 2018;Keek et al 2018b). These effects can be interpreted as reprocessing/reflection from the disk (Ballantyne 2004), changes in the accretion flow rate through Poynting-Robertson drag (Walker 1992), or cooling of the corona (Ji et al 2014b).…”

Observatory science with eXTP

“…• How do bursts impact and influence the surrounding accretion flow; can bursts yield insights into the underlying accretion physics? X-ray bursts can deliver a powerful impulse to the surrounding accretion disk (e.g., review by Degenaar et al, 2018). The response of the disk to the bursts could include (i) an increase in the accretion rate due to Poynting-Robertson drag, (ii) radiative driven outflows, (iii) an increasing scale height due to X-ray heating or (iv) a combination of all these effects (Ballantyne & Everett, 2005).…”

Searching for the most powerful thermonuclear X-ray bursts with the Neil Gehrels Swift Observatory

“…Nine literature cases are known (eight are listed in Degenaar et al 2018; an additional case is from A1246-58). We provide here some details of seven of these cases that are not among the Swift cases.…”

“…Some long bursts have very strong flux fluctuations with respect to the typical power-law decay of the burst (in 't Zand et al 2014bZand et al , 2017, with amplitudes above and below the cooling curve of order 50% on timescales of 1 s to 1 min. So far, nine such bursts have been reported (van Paradijs et al 1990;Strohmayer & Brown 2002;in 't Zand et al 2005;Molkov et al 2005;in 't Zand et al 2008in 't Zand et al , 2011Palmer 2011;; Barrière et al 2015) as measured with the instruments Ginga-LAC, INTEGRAL-JEM-X, BeppoSAX-WFC, RXTE-PCA, Swift-XRT, and NuSTAR (for a recent overview, see Degenaar et al 2018). The common characteristics are that the fluctuations are at least 10% and that they occur with some delay (order one minute) after the Eddington-limited phase.…”