IGR J17254-3257, a new bursting neutron star

TL;DR

This paper reports on the discovery and analysis of IGR J17254-3257, a neutron star X-ray burster exhibiting both short and long bursts, providing insights into nuclear burning regimes at low accretion rates.

Contribution

It presents the first detailed observational analysis of IGR J17254-3257, highlighting its unique burst properties and implications for nuclear burning processes at low accretion rates.

Findings

Detected both short and long X-ray bursts from the source.

Estimated the source distance to about 14.5 kpc.

Indicated the source is in a low accretion rate state with a hard spectrum.

Abstract

The study of the observational properties of uncommonly long bursts from low luminosity sources with extended decay times up to several tens of minutes 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. 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. An upper limit to its distance is estimated to about 14.5 kpc. The broad-band spectrum of the persistent emission in the 0.3-100 keV energy band obtained using contemporaneous INTEGRAL and XMM-Newton data indicates a bolometric flux of 1.1x10^-10 erg/cm2/s corresponding, at the canonical distance of 8 kpc, to a luminosity about 8.4x10^35 erg/s between 0.1-100 keV, which translates to a mean accretion rate of about 7x10^-11 solar masses per year. 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.