Superexpansion as a possible probe of accretion in 4U 1820-30

Abstract

The ultracompact X-ray binary 4U 1820-30 is well known for its ≈170-d superorbital modulation in X-ray flux and spectrum, and the exclusiveness of bursting behavior to the low hard "island" state. In May−June 2009, there was an exceptionally long 51-d low state. This state was well covered by X-ray observations and 12 bursts were detected, 9 with the high-throughput RXTE. We investigate the character of these X-ray bursts and find an interesting change in their photospheric expansion behavior. At the lowest inferred mass accretion rates, this expansion becomes very large in 4 bursts and reaches the so-called superexpansion regime. We speculate that this is due to the geometry of the inner accretion flow being spherical and a decreasing accretion rate: when the flow geometry nearest to the neutron star is spherical and the accretion rate is low, the ram pressure of the accretion disk may become too low to counteract that of the photospheric expansion. In effect, this may provide a novel means to probe the accretion flow. Additionally, we observe a peculiar effect: the well-known cessation of X-ray bursts in the high state is too quick to be consistent with a transition to stable helium burning. We suggest an alternative explanation, that the cessation is due to the introduction of a non-nuclear heat source in the neutron star ocean.

Thermonuclear Burst Observations for Model Comparisons: A Reference Sample

“…For each sequence, we observed a pair of bursts within a few hours of each other, and a third burst some days earlier or later. We ultimately rejected one of these sequences, in 2009 May (MJD 54980; see also in’t Zand et al 2012) because there were significant variations in the persistent flux (and spectral shape) over the interval, and it was not possible to be confident about the recurrence time for the close pair of bursts, due to data gaps between them. This was not the case for the remaining train, in 1997 May (MJD 50572), and so we retained this set of bursts.…”

NICER Discovers Spectral Lines during Photospheric Radius Expansion Bursts from 4U 1820−30: Evidence for Burst-driven Winds

“…in't Zand & Weinberg (2010) reported evidence from RXTE/PCA data for edges during so-called "superexpansion" bursts from 4U 0614+091, 4U 1722−30, and 4U 1820−30. These are bursts with powerful photospheric radius expansion (PRE) phases, that very likely drive outflows (in't Zand et al 2012;Yu & Weinberg 2018). The edge energies were in the 5 -12 keV range, and were detected after the strong, initial expansion phase.…”