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Outburst of IGR J11215-5952 Observed with RXTE

ATel #766; David. M. Smith and Nathan Bezayiff (U. C. Santa Cruz), Ignacio Negueruela (U. Alicante)
on 18 Mar 2006; 03:57 UT
Distributed as an Instant Email Notice Transients
Credential Certification: David M. Smith (dsmith@ssl.berkeley.edu)

Subjects: X-ray, Request for Observations, Binary, Transient

Referred to by ATel #: 768, 773, 994, 995, 1151, 1444

Sidoli et al. (2006, astro-ph/0203081) reported that the hard x-ray transient IGR J11215-5952 appeared to have a recurrence period of ~330 days, based on three short outbursts observed by INTEGRAL, the most recent of which was actually the discovery observation (Lubinski et al., ATEL #469). This short outburst, and the presence in the INTEGRAL error circle of the blue supergiant HD 306414 (Negueruela et al. 2005, ATEL #470), suggested that this source is a member of the rapidly growing class of supergiant fast x-ray transients (SFXTs; Negueruela et al. 2005, astro-ph/0511088; Smith et al. 2006, ApJ 638, 974; Negueruela et al. 2006, ApJ 638, 982).

Based on this recurrence period, we suggested monitoring the source with the Rossi X-ray Timing Explorer (RXTE) during the period of March 15 to March 18, 2006, based on the expectation of the next outburst. Five 1-ksec pointings spanning the time from 00:38UT on March 15 to 12:09UT on March 16 show no activity. The next four pointings, ranging from 22:09UT on March 16 to 20:07UT on March 17, show the source to be in outburst. The first observation of activity is 328.7 days after the first activity seen by Lubinski et al., which began on April 22, 2005 at 06:02:25. Counting three periods from the start of first outburst found by Sidoli et al., the period is 329.0 days.

The RXTE/PCA data taken so far show three characteristics typical of SFXT outbursts. First, there is extreme variability: the flux at 13:43UT on March 17 is nearly back to background levels, while the first and last of the four active pointings are the brightest, and nearly equal. Second, the spectrum is hard: the initial pointing is fit well by a power law of photon index (-1.7 +/- 0.2) in the range 2.5-15 keV. In this restricted range, this could be consistent with either a black-hole hard state or an accreting pulsar spectrum. Finally, the best-fit hydrogen absorption column, (11 +/- 3)e22 per cm2, is higher than expected from Galactic absorption; this is also typical of SFXTs.

The peaks of the outburst are at about 2e-10 ergs/cm2/s (absorption removed) from 2-10 keV, corresponding to about 1e36 erg/s at 6.2 kpc (the distance to HD 306414 found by Masetti et al., astro-ph 0512399). This is about 1/3 of the luminosity from 5-100 keV given by Sidoli et al., and so roughly consistent with the other outbursts.

It is not yet known whether the outburst has ended. Another telegram will be posted at that time. In the meantime, prompt observations in the x-ray and at other wavelengths are encouraged. The best-fit position of the x-ray source, given by Sidoli et al., is (RA: 11 21 50.8, Dec: -59 52 48.3). The Tycho position of HD 306414 is (RA: 11 21 46.807, Dec: -59 51 47.93).

This is the first confirmation of a recurrence period in an SFXT candidate. If the proposed link between IGR J11215-5952 and HD 306414 holds up, this period suggests that the difference between SFXTs and persistently bright supergiant x-ray binaries like Cyg X-1 and Vela X-1 may be that the former have long period, elliptical orbits.

We thank RXTE project scientist Jean Swank and the RXTE schedulers for their rapid and positive response to this request.