Intermediate long X-ray bursts from the ultra-compact binary candidate SLX 1737-282

Abstract

Aims: The low persistent flux X-ray burster source SLX 1737-282 is classified as an ultra-compact binary candidate. We compare the data on SLX 1737-282 with the other similar objects and attempt to derive constraints on the physical processes responsible for the formation of intermediate long bursts.
Methods: Up to now only four bursts, all with duration between ≃15{-}30 min, have been recorded for SLX 1737-282. The properties of three of these intermediate long X-ray bursts observed by INTEGRAL are investigated and compared to other burster sources. The broadband spectrum of the persistent emission in the 3-100 keV energy band is studied with the INTEGRAL data.
Results: The persistent emission is measured to be 0.5% Eddington luminosity. From the photospheric radius expansion observed during at least one burst we derive the source distance at 7.3 kpc assuming a pure helium atmosphere. The observed intermediate long burst properties from SLX 1737-282 are consistent with helium ignition at the column depth of 5-8 × 10⁹ g cm^-2 and a burst energy release of 10⁴¹ erg. The apparent recurrence time of ≃86 days between the intermediate long bursts from SLX 1737-282 suggests a regime of unstable burning of a thick, pure helium layer slowly accreted from a helium donor star.