Swift/XRT observes the fifth outburst of the periodic supergiant fast X-ray transient IGR J11215-5952
Context: The hard X-ray transient source IGR J11215-5952 was discovered in April 2005 with INTEGRAL and is a confirmed member of the new class of high mass X-ray binaries, the supergiant fast X-ray transients (SFXTs). Archival INTEGRAL data and RXTE observations have shown that the outbursts occur with a periodicity of ~330 days. Thus, IGR J11215-5952 is the first SFXT displaying periodic outbursts, possibly related to the orbital period.
Aims: We performed a target of opportunity observation with Swift with the main aim of monitoring the source behaviour around the time of the fifth outburst, expected on 2007 Feb. 9.
Methods: The source field was observed with Swift twice a day (2 ks/day) starting from 2007 Feb. 4 until the fifth outburst, and then for ~5 ks a day afterwards, during a monitoring campaign that lasted 23 days for a total on-source exposure of ~73 ks. This is the most complete monitoring campaign of an outburst from an SFXT.
Results: The spectrum during the brightest flares is described well by an absorbed power law with a photon index of 1 and NH ∼1×1022 cm-2. A 1-10 keV peak luminosity of ~1036 erg s-1 was derived (assuming 6.2 kpc, the distance of the optical counterpart).
Conclusions: These Swift observations are a unique data-set for an outburst of an SFXT, thanks to the combination of sensitivity and time coverage, and they allowed a study of IGR J11215-5952 from outburst onset to almost quiescence. We find that the accretion phase lasts longer than previously thought on the basis of lower-sensitivity instruments observing only the brightest flares. The observed phenomenology is consistent with a smoothly increasing flux triggered at the periastron passage in a wide eccentric orbit with many flares superimposed, possibly due to episodic or inhomogeneous accretion.
Astronomy and Astrophysics
- Pub Date:
- July 2007
- X-rays: stars;
- X-rays: individual: IGR J11215-5952;
- Accepted for Publication in Astronomy and Astrophysics Letters. 5 pages, 3 figures, 2 tables