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End of outburst of IGR J11215-5952; possible pulsations

ATel #773; David M. Smith, Nathan Bezayiff (U. C. Santa Cruz), and Ignacio Negueruela (U. Alicante)
on 24 Mar 2006; 19:58 UT
Credential Certification: David M. Smith (dsmith@ssl.berkeley.edu)

Subjects: X-ray, Binary, Transient, Pulsar

Referred to by ATel #: 997

The recent outburst of IGR J11215-5952, predicted by the ephemeris of Sidoli et al. (2006, astro-ph/0603081) and observed with the Rossi X-ray Timing Explorer (RXTE) (Smith, Bezayiff and Negueruela 2006, ATEL #766) has apparently ended. The first and last observations that showed significant activity began and ended on 22:09UT March 16 and 21:48UT March 18, respectively. Each observation lasted about 1000 s, and three per day were taken from March 15 through March 18. Four later pointings were taken at longer intervals to make certain the outburst had ended. The lower and upper limits to the total duration of the outburst are 2.0 and 3.4 dy. This is similar to the 2004 observation reported by Sidoli et al. 2006. The lightcurve of the outburst is shown in Figure 1 at the link below, including the time at which the Swift X-ray telescope observed the field and found that the companion is indeed the supergiant HD 306414 (Steeghs, Torres and Jonker 2006, ATEL #768) as suggested by Negueruela, Smith and Chaty (ATEL #470).

Several of the pointings show variability which suggests a pulsation period of approximately 195 +/- 10 s. It is clearest in the brightest pointing; that lightcurve, with 1-second binning, is shown in Figure 2 at the link below. We have not been able to connect the observations coherently in phase. An x-ray pulsar with an orbital period of 329 dy and a spin period of 195 s would lie neatly among the Be/NS binary systems on the Corbet diagram correlating spin and orbital periods (Corbet 1986, MNRAS 220, 1047) and far from the supergiant systems. The Swift result clearly associates this transient with a supergiant, however, and the fast, highly variable outburst is consistent with other supergiant fast x-ray transients (SFXTs), not Be/NS systems.

The spectra were hard and changed little through the outburst. A power law, a power law with exponential cutoff (Sidoli et al. 2006), and a thermal bremsstrahlung spectrum all give acceptable fits. The absorption column seemed to drop after the first pointing in outburst, from (11 +/- 3)e22 per cm2 to a level about 1/3 to 1/2 of that during subsequent outburst intervals. Note, however, that the absorption is difficult to measure due to the need to subtract the soft Galactic diffuse emission from the spectrum. The spectrum from the third pre-outburst pointing, which had the lowest flux, was used as background.

We thank Jean Swank and Rudy Wijnands for discussions about the apparent pulsation in the source.

Erratum: The paper of Sidoli et al. 2006, which has been accepted by Astronomy and Astrophysics Letters, was given an incorrect citation in ATEL #766. Its correct listing on the arXiv.org preprint server is astro-ph/0603081, not astro-ph/0203081.

FIGURES: Outburst lightcurve and apparent pulsations in the brightest pointing