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 (firstname.lastname@example.org)
Subjects: X-ray, Request for Observations, Binary, Transient
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.