Letter to the Editor
IGR J17254-3257, a new bursting neutron star
Danish National Space Center, Technical University of Denmark, Juliane Maries Vej 30, 2100 Copenhagen, Denmark e-mail: email@example.com
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 ISOC, ESA/ESAC, Urb. Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
5 INTEGRAL Science Data Centre, Chemin d'Écogia 16, 1290 Versoix, Switzerland
6 Observatoire de Genève, Université de Genève, 51 ch. des Maillettes, 1290 Sauverny, Switzerland
7 Kavli Institute for Theoretical Physics, Kohn Hall, University of California, Santa Barbara, CA 93106, USA
8 Science Payload and Advance Concepts Office, ESA-ESTEC, Postbus 299, 2200 AG, Noordwijk, The Netherlands
Accepted: 3 May 2007
Aims.The study of the observational properties of uncommonly long bursts from low luminosity sources is important when investigating the transition from a hydrogen - rich bursting regime to a pure helium regime and from helium burning to carbon burning as predicted by current burst theories. On a few occasions X-ray bursts have been observed with extended decay times up to several tens of minutes, intermediate between usual type I X-ray bursts and so-called superbursts.
Methods.IGR J17254-3257 is a recently discovered X-ray burster of which only two bursts have been recorded: an ordinary short type I X-ray burst, and a 15 min long burst. The properties of the X-ray bursts observed from IGR J17254-3257 are investigated. The broad-band spectrum of the persistent emission in the 0.3–100 keV energy band is studied using contemporaneous INTEGRAL and XMM-Newton data.
Results.A refined position of IGR J17254-3257 is given and an upper limit to its distance is estimated to about 14.5 kpc. The persistent bolometric flux of 1.110-10 erg cm-2 s-1 corresponds, at the canonical distance of 8 kpc, to 8.41035 erg s-1 between 0.1–100 keV, which translates to a mean accretion rate of about 710-11 yr-1.
Conclusions.The low X-ray persistent luminosity of IGR J17254-3257 seems to indicate the source may be in a state of low accretion rate usually associated with a hard spectrum in the X-ray range. The nuclear burning regime may be intermediate between pure He and mixed H/He burning. The long burst is the result of the accumulation of a thick He layer, while the short one is a prematurate H-triggered He burning burst at a slightly lower accretion rate.
Key words: binaries: close / stars: individual: IGR J17254-3257 (1RXS J172525.5-325717) / stars: neutron / X-rays: bursts
© ESO, 2007