Radius-expansion burst spectra from 4U 1728-34: an ultracompact binary?

TL;DR

This study searched for photoionisation edges in X-ray burst spectra from 4U 1728-34, finding none but setting upper limits, and provided evidence suggesting the system is an ultracompact binary with hydrogen-deficient accretion.

Contribution

First search for photoionisation edges in 4U 1728-34's burst spectra, and discovery of a 10.77 min periodic signal indicating an ultracompact binary.

Findings

No discrete spectral features detected, upper limits consistent with predictions.

Detected a 10.77 min periodic signal, likely orbital, indicating an ultracompact binary.

Burst properties suggest hydrogen-deficient accretion from an evolved donor.

Abstract

Recent theoretical and observational studies have shown that ashes from thermonuclear burning may be ejected during radius-expansion bursts, giving rise to photoionisation edges in the X-ray spectra. We report a search for such features in Chandra spectra observed from the low-mass X-ray binary 4U 1728-34. We analysed the spectra from four radius-expansion bursts detected in 2006 July, and two in 2002 March, but found no evidence for discrete features. We estimate upper limits for the equivalent widths of edges of a few hundred eV, which for the moderate temperatures observed during the bursts, are comparable with the predictions. During the 2006 July observation 4U 1728-34 exhibited weak, unusually frequent bursts (separated by <2 hr in some cases), with profiles and alpha-values characteristic of hydrogen-poor fuel. Recurrence times as short as those measured are insufficient to exhaust the accreted hydrogen at solar composition, suggesting that the source accretes hydrogen deficient fuel, for example from an evolved donor. The detection for the first time of a 10.77 min periodic signal in the persistent intensity, perhaps arising from orbital modulation, supports this explanation, and suggests that this system is an ultracompact binary similar to 4U 1820-30.