Letter to the Editor

A superburst from GX 3+1

SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands

Corresponding author: erikk@sron.nl

Received: 23 November 2001
Accepted: 18 December 2001

Abstract

I found one long X-ray flare from the X-ray burster GX 3+1 in almost 6 years of observations with the RXTE All Sky Monitor (ASM). The event had a peak flux of about 1.1 Crab (1.5–12 keV), lasted between 4.4 and 16.2 hours and exhibited a fluence of more than about 510⁴¹ erg for a source distance of 5 kpc. During the exponential-like decay, with an exponential decay time of 1.6 hours, spectral softening is seen. The total ASM effective exposure time on GX 3+1 is estimated to be around a year. The flare bears all the characteristics of the recently discovered so-called superbursts in other X-ray burst sources.