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

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.

A&A class LateX file, 5 pages, 4 postscript figures. Accepted in Astronomy & Astrophysics Letters. One co-author added, modified abstract and improved conclusions

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• The Multi-INstrument Burst ARchive (MINBAR)
• Evidence for enhanced persistent emission during sub-Eddington thermonuclear bursts
• Sedimentation and Type I X-ray Bursts at Low Accretion Rates
• Intermediate long X-ray bursts from the ultra-compact binary source SLX 1737-282
• NuSTAR Observations of the Accreting Atolls GX 3+1, 4U 1702-429, 4U 0614+091, and 4U 1746-371
• Reaction Rate and Composition Dependence of the Stability of Thermonuclear Burning on Accreting Neutron Stars
• The X-ray spectrum of the bursting atoll source 4U~1728-34 observed with INTEGRAL
• Restless quiescence: thermonuclear flashes between transient X-ray outbursts
• Study of the reflection spectrum of the accreting neutron star GX 3+1 using XMM-Newton and INTEGRAL
• GX~3+1: the stability of spectral index as a function of mass accretion rate
• Long tails on thermonuclear X-ray bursts from neutron stars: a signature of inward heating?
• Interstellar dust along the line of sight of GX 3+1
• The Galactic LMXB Population and the Galactic Centre Region
• IGR J17254-3257, a new bursting neutron star
• A catalogue of unusually long thermonuclear bursts on neutron stars
• The new intermediate long bursting source XTE J1701-407
• Swift J1734.5-3027: a new long type-I X-ray bursting source
• Spectral evolution of bright NS LMXBs with INTEGRAL: an application of the thermal plus bulk Comptonization model
• Sample of LMXBs in the Galactic bulge. I. Optical and near-infrared constraints from the Virtual Observatory
• Astrophysics in 2006
• A catalog of intermediate duration Type I X-ray bursts observed with the INTEGRAL satellite