SAX J1712.6-3739: a persistent hard X-ray source as monitored with INTEGRAL

Abstract

Context. The X-ray source SAX J1712.6-3739 is a very weak low mass X-ray binary discovered in 1999 with BeppoSAX and located in the Galactic Center. This region has been intensively investigated by the INTEGRAL satellite with an unprecedented exposure time, providing an unique opportunity to study the hard X-ray behavior also for weak objects. Aims. We analyzed all available INTEGRAL public and private Key-Program observations with the main aim of studying the long-term behavior 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 February 2003 and October 2006. SAX J1712.6-3739 did not show any flux variation during this period as well as compared to previous BeppoSAX observations. Hence, to better constrain the physical parameters we combined data from both instruments. Results. Long INTEGRAL monitoring reveals that this X-ray burster is a weak persistent source, displaying an X-ray spectrum extended to high energy and being most of the time in a low luminosity hard state. The broad-band spectrum is well modeled with a simple Comptonized model with a seed photon temperature of ∼0.5 keV and an electron temperature of -24 keV. The low mass accretion rate (∼2 × 10 -10 M sun /yr), the long bursts recurrence time, the small sizes of the region emitting the seed photons consisting of the inner disk radius and the high luminosity ratio in the 40-100 keV and 20-40 keV band, are all features common to the Ultra Compact source class. Conclusions. We report, for the first time, on the X-ray behavior of this source: observations with unprecedent wide energy band and sensitivity revealed this X-ray burster is a persistent source with a hard spectrum extending up to energies > 100 keV.