IBIS discovers 30-day period in SFXT
During the past 6 years, the IBIS instrument on-board the INTEGRAL gamma-ray satellite has revolutionised our classical view of High Mass X-ray binaries (HMXB). Specifically, IBIS has discovered many new HMXBs hosting massive OB supergiant stars (the so-called Supergiant High Mass X-ray binaries, SGXB); in just a few years their population has been almost tripled thanks to the IBIS identification of two previously unrecognized subclasses of SGXBs: the highly obscured SGXBs and the Supergiant Fast X-ray Transients (SFXT).
SFXTs are characterised by fast but rare outbursts (i.e. a few are detected per year), and long periods of inactivity which are interspersed here and there with short flares spaced by irregular intervals of time.
Because of this, the identification of periodicity is a very challenging task which can be fullfilled only through long-term and continous monitoring using a sensitive instrument with a wide field-of-view.
Recent analyses of the history of SAX J1818.6-1703 have discovered a 30-day period in this supergiant binary system strongly indicative of a binary orbital period. The figure shows the hard X-ray flux detected by IBIS/ISGRI in the 18-60 keV energy band folded on the 30-day orbital period. It shows significant emission over a ~4 day period around periastron for the whole dataset (narrow line) and even when the two largest outbursts are ignored (thick line).
Further analysis of IBIS/ISGRI and Swift/BAT lightcurves show a high level of recurrence from the source, with several new outbursts detected, and significant emission from the majority of periaston passages. X-ray measurements made of the binary system prior to the discovery of the periodicity (shown by the arrows) were unfortunate to miss the emission, but give a strong upper limit on the quiescent flux from the system. The inset image shows one of the newly discovered outbursts, showing the fast transient activity that led to this source being characterised as a supergiant fast X-ray transient (SFXT). Several of the SFXT systems have now been shown to display periodic outbursts with a wide range of periods.
Further details on these results can be found in: A.J. Bird et al, MNRAS, 393, L11, 2008