Letter to the Editor

Two-phase X-ray burst from GX 3+1 observed by INTEGRAL

¹ Danish National Space Center, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark e-mail: jerome@spacecenter.dk
² CEA Saclay, DSM/DAPNIA/Service d'Astrophysique (CNRS FRE 2591), 91191 Gif-sur-Yvette, France
³ Unité mixte de Recherche Astroparticule et Cosmologie, 11 place Berthelot, 75005 Paris, France
⁴ Dipartimento di Fisica, Università di Ferrara, via Saragat 1, 44100 Ferrara, Italy
⁵ Istituto di Astrofisica Spaziale e Fisica Cosmica CNR, via Gobetti 101, 40129 Bologna, Italy
⁶ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁷ Astronomical Institute, Utrecht University, PO Box 80 000, 3508 TA Utrecht, The Netherlands
⁸ ESA/ESAC, Urb. Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain

Received: 4 November 2005
Accepted: 4 February 2006

Abstract

INTEGRAL detected on August 31, 2004, an unusual thermonuclear X-ray burst from the low-mass X-ray binary GX 3+1. Its duration was 30 min, which is between the normal burst durations for this source (10 s) and the superburst observed in 1998 (several hours). We see emission up to 30 keV energy during the first few seconds of the burst where the bolometric peak luminosity approaches the Eddington limit. This peculiar burst is characterized by two distinct phases: an initial short spike of ~6 s consistent with being similar to a normal type I X-ray burst, followed by a remarkable extended decay of cooling emission. We discuss three alternative schemes to explain its twofold nature: 1) unstable burning of a hydrogen/helium layer involving an unusually large amount of hydrogen; 2) pure helium ignition at an unusually large depth (unlikely in the present case); and 3) limited carbon burning at an unusually shallow depth triggered by unstable helium ignition. Though none of these provide a satisfactory description of this uncommon event, the former one seems the most probable.