Evidence of heavy-element ashes in thermonuclear X-ray bursts with photospheric superexpansion

Abstract

A small subset of thermonuclear X-ray bursts on neutron stars exhibit such a strong photospheric expansion that for a few seconds the photosphere is located at a radius r ph > ∼ 10 3 km. Such "superexpansions" imply a large and rapid energy release, a feature characteristic of pure He burst models. Previous calculations have shown that during a pure He burst, the freshly synthesized heavy-element ashes of burning can be ejected in a strong radiative wind and produce significant spectral absorption features. We search the burst data catalogs and literature and find 32 superexpansion bursts, 24 of which were detected with BeppoSAX and three with RXTE at high time resolution. We find that these bursts exhibit the following interesting features: (1) At least 31 are from (candidate) ultracompact X-ray binaries in which the neutron star accretes hydrogen-deficient fuel, suggesting that these bursts indeed ignite in a helium-rich layer. (2) In two of the RXTE bursts we detect strong absorption edges during the expansion phase. The edge energies and depths are consistent with the H-like or He-like edge of iron-peak elements with abundances > ∼ 100 times solar, suggesting that we are seeing the exposed ashes of nuclear burning. (3) The superexpansion phase is always followed by a moderate expansion phase during which r ph ∼ 30 km and the luminosity is near the Eddington limit. (4) The decay time of the bursts, τ decay , ranges from short (≈10 s) to intermediate ( > ∼ 10 3 s). However, despite the large range of τ decay , the duration of the superexpansion is always a few seconds, independent of τ decay . By contrast, the duration of the moderate expansion is always of order τ decay . (5) The photospheric radii r ph during the moderate expansion phase are much smaller than steady state wind models predict. We show that this may be further indication that the wind contains highly non-solar abundances of heavy elements.

Characterizing the Evolving X-Ray Spectral Features During a Superburst From 4u 1636-536

“…That superburst was exceptionally bright and it reached the Eddington limit (Strohmayer & Brown 2002;in 't Zand & Weinberg 2010;Keek 2012). If the enhanced cutoff power-law flux is due to Poynting-Robertson drag induced accretion, the effect is expected to be stronger for 4U 1820-30 than for 4U 1636-536.…”

“…The line as observed during the superburst is much stronger than if it were to be produced by reflection of the brightened cutoff power law and is therefore predominantly powered by reflected burst emission from the neutron star surface. Note that local spectral features have been detected with the PCA in bright X-ray bursts that were not well fit with a reflection model, but are better described by absorption edges thought to originate with heavy element ashes mixed into the neutron star photosphere (in 't Zand & Weinberg 2010). Therefore, despite the limited spectral resolution of the PCA, it is possible to distinguish reflection features from absorption features in the neutron star photosphere.…”

“…One of these superbursts, from 4U 1820-30 (Strohmayer & Brown 2002), was exceptionally powerful. It exhibits superexpansion during two phases of the superburst, and it has the longest period of moderate photospheric radius expansion (PRE) of any known X-ray burst (in 't Zand & Weinberg 2010;Keek 2012). Detailed spectral analysis found the presence of reflection features in the spectrum (Ballantyne & Strohmayer 2004), where part of the emission from the neutron star surface is thought to be reflected from the accretion disk.…”

Classical novae and type I X-ray bursts: Challenges for the 21st century

“…130 Recent, more encouraging results include the observation, albeit with limited spectral resolution, of absorption edges from Fe-peak elements in photospheric radius "superexpansion" bursts. 131 Higher spectral resolution observations of these superexpansion bursts (e.g., with Chandra 132 or XMM-Newton, 127 though an exposure time of ≈500 ks may be necessary) would be of great interest.…”

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

“…More recently, there have been reports of absorption edges being detected during X-ray bursts. 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).…”