The ultra-compact binary 4U 1850-087 observed with INTEGRAL: hard X-ray emission from an X-ray burster

¹ INAF-IASF, Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via E.Bassini 15, 20133 Milano, Italy e-mail: sidoli@iasf-milano.inaf.it
² INAF-IASF, Istituto di Astrofisica Spaziale e Fisica Cosmica Roma, via del Fosso del Cavaliere 100, 00133 Roma, Italy

Received: 5 June 2006
Accepted: 2 September 2006

Abstract

Context.The X-ray burster 4U 1850-087, located in the Galactic globular cluster NGC 6712, is an ultracompact binary (orbital period ~21 min), likely harbouring a degenerate companion.

Aims.The source has been observed at soft γ-rays several times with the INTEGRAL satellite, during the monitoring of the Galactic plane, with an unprecedented exposure time. We analysed all available INTEGRAL observations, with the main aim of studying the long-term behaviour of this Galactic bulge X-ray burster.

Methods.The spectral results are based on the systematic analysis of all INTEGRAL observations covering the source position performed between March 2003 and November 2005.

Results.The source X-ray emission is hard and is observed, for the first time, up to 100 keV. A broad-band spectrum obtained combining the INTEGRAL spectrum together with a quasi-simultaneous XMM–Newton observation performed in September 2003 is well modeled with a disk-blackbody emission (with an inner disk temperature of ~0.8 keV) together with a power-law (with a photon index, Γ, of 2.1). The 2-100 keV luminosity is  erg s^-1 (assuming a distance of 6.8 kpc).

Conclusions.INTEGRAL observations reveal for the first time that this X-ray burster displays a very hard X-ray spectrum, with a cut-off at energies higher than 100 keV, and that the source spends most of the time in this low luminosity and hard state. Indeed, a previous BeppoSAX observation in April 1997 observed high energy emission from 4U 1850-087 only up to 50 keV.