GRO J1655-40 returning to the hard state
ATel #607; Jeroen Homan (MIT), Jon M. Miller (U. Mich.), Rudy Wijnands (U. Amsterdam), Walter H. G. Lewin (MIT)
on 15 Sep 2005; 15:25 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)
Subjects: Radio, Infra-Red, Optical, X-ray, Binary, Black Hole, Transient
We have been monitoring the ongoing outburst of the black-hole X-ray
transient GRO J1655-40 with more than 400 pointed RXTE observations
since its start in mid February 2005 (ATel #414). During the last
month the source brightness has been steadily declining while
remaining relatively soft. However, in the last few days the spectrum
of GRO 1655-40 has rapidly become harder. Data taken early September 15
show the reappearance of a clear QPO around 13 Hz in the power
spectrum, accompanied by broad band-limited noise. The strength of
the variability in the 0.015-100 Hz range was about 9.5% rms. The QPO
and noise appeared within a time span of 6 hours, although the
variability had already started to increase on September 14 from (~3.5% to
~7% rms). The spectral and variability changes indicate that GRO
J1655-40 is undergoing a transition back to the hard state.
During the 1996/1997 outburst of the source this transition occurred
at a similar flux/luminosity (4.4e-9 erg/s/cm^2 [3-100 keV],
corresponding to 5.3e36 erg/s for a distance of 3.2 kpc). In August
1997 the full transition from the soft state to the hard state took
place in less than two weeks. Based on this, and given that the
transition is already well underway, we expect GRO J1655-40 to reach
the hard state and become active in the radio again within 5 to 10
days. Brightening in the optical and/or IR might also be observed
around that time.
We will continue to monitor the source with RXTE on a daily basis.
Latest results can be found at: http://tahti.mit.edu/opensource/1655/
Observers with programs on other X-ray satellites who wish to observe
the decay/hard state of GRO J1655-40, or search for X-ray jets from
the source should be aware that the visibility windows for the source
end within a few weeks, due to sun angle constraints:
Chandra : October 27
Integral : October 6
Suzaku : October 6
Swift : October 27
RXTE : November 14
XMM-Newton : September 28
