Photospheric radius expansion in superburst precursors from neutron stars

TL;DR

This paper presents the first detailed spectral analysis of superburst precursors from neutron stars, revealing photospheric radius expansion and suggesting a detonation origin triggered by shock heating.

Contribution

It introduces a novel time-resolved spectroscopy method for superburst precursors, providing evidence for photospheric radius expansion and proposing a shock-induced detonation initiation.

Findings

Detection of photospheric radius expansion in superburst precursor

Precursor energy exceeds that of typical short bursts

Evidence supporting shock heating and detonation initiation

Abstract

Thermonuclear runaway burning of carbon is in rare cases observed from accreting neutron stars as day-long X-ray flares called superbursts. In the few cases where the onset is observed, superbursts exhibit a short precursor burst at the start. In each instance, however, the data was of insufficient quality for spectral analysis of the precursor. Using data from the propane anti-coincidence detector of the PCA instrument on RXTE, we perform the first detailed time resolved spectroscopy of precursors. For a superburst from 4U 1820-30 we demonstrate the presence of photospheric radius expansion. We find the precursor to be 1.4-2 times more energetic than other short bursts from this source, indicating that the burning of accreted helium is insufficient to explain the full precursor. Shock heating would be able to account for the lacking energy. We argue that this precursor is a strong indication that the superburst starts as a detonation, and that a shock induces the precursor. Furthermore, we employ our technique to study the superexpansion phase of the same superburst in greater detail.