The black hole candidate IGR J17091-3624 is still active
ATel #3913; D. Altamirano, R. Wijnands (UvA), T. Belloni and S. Motta (INAF)
on 6 Feb 2012; 15:18 UT
Credential Certification: Sara Elisa Motta (email@example.com)
Subjects: X-ray, Binary, Black Hole, Transient
In February 2011 it was reported that the black hole candidate IGR
J17091--3624 exhibited a new outburst (using Swift/BAT, see Atel
#3144). During this outburst IGR J17091-3624 was followed up by
multi-wavelength observations (ATel #3148, #3150, #3159, #3167,
#3168, #3179, #3203, #3225, #3229, #3230, #3232, #3266, #3299).
After IGR J17091-3624 underwent the typical hard-to-soft state
transition (e.g. ATel #3179
), it was not detected any more by
(and Rodriguez et al. 2011, A&A, 533, 4). Unfortunately, MAXI cannot
resolve IGR J17091-624 given the nearby bright X-ray source GX 349+2
(T. Mihara, private communication).
This particular outburst turned out to be very interesting because the
source suddenly exhibited strong and highly-structured X-ray
variability which so far had only been observed from the very-bright
black hole X-ray transient GRS 1915+105 (e.g. Altamirano et al. 2011,
ApJL, 742, 17). Swift and RXTE monitored the source during its 2011
outburst until end November - beginning of December 2011, when no more
observations were possible due to visibility constraints.
To determine the current X-ray level of activity of IGR J17091-3624,
we acquired several Swift/XRT observations: on Jan 26th and Jan 31st
2012 IGR J17091-3624 was clearly detected, at a 0.5-10 keV count rate
~15+/-2 cts/sec. This is about a factor of ~2.5 lower than when it was
last seen in 2011.
Spectra of the 2012 observations were extracted following Evans et
al. (2009, MNRAS, 397, 1177). The 0.5-10 keV source spectrum is well
fitted with an absorbed (Nh~1.35) power law of index ~2.6, leading to
an unabsorbed 0.5-10 keV (2.-10 keV) flux of 1.96e-09 ergs/cm2/s
(6.04e-10 ergs/cm2/s), implying a luminosity of 1.5e37 erg/s (4.6e36
erg/s) or 9.4e37 erg/s (2.9e37 erg/s) for 8 kpc and 20 kpc,
Aperiodic timing analysis of the Swift/XRT data does not reveal any
significant feature in the power spectrum. This, together with the
rather soft energy spectra we detect might imply that IGR J17091-3624
is in a black hole ``normal soft state'' and therefore could undergo
the soft-to-hard transition soon. However, given the similarities with
GRS 1915+105, this could also be an additional variability class to
those reported for IGR J17091-3624 so far (e.g. Altamirano et
al. 2011, ApJL, 742, 17).
IGR J17091-3624 will be monitored with Swift/RXTE twice a week during
February. Followup observations at other wavelengths are encourage.
We thank the Swift/Team for scheduling our observations.