Letter to the Editor

A superburst from 4U 1254-69

¹ SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
² Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
³ ESA-ESTEC, Science Ops. & Data Systems Div., SCI-SDG, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
⁴ Dept. of Physics and Astronomy, University of Southampton, Hampshire SO17 1BJ, UK

Corresponding author: J. J. M. in 't Zand, jeanz@sron.nl

Received: 9 September 2003
Accepted: 9 October 2003

Abstract

We report the detection with the BeppoSAX Wide Field Cameras of a superburst from 4U 1254-69. The superburst is preceded by a normal type-I X-ray burst, has a decay time that is the longest of all eight superbursts detected so far and a peak luminosity that is the lowest. Like for the other seven superbursts, the origin is a well-known type-I X-ray burster with a persistent luminosity level close to one tenth of the Eddington limit. Based on WFC data of all persistently bright X-ray bursters, the average rate of superbursts is per year per persistently bright X-ray burster. Some systems may have higher superburst rates. For all superbursters, we present evidence for a pure helium layer which is burnt in an unstable as well as a stable manner.