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GRO J1655-40 rapidly evolving

ATel #440; Jeroen Homan (MIT)
on 12 Mar 2005; 20:24 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Jeroen Homan (homan@merate.mi.astro.it)

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

Referred to by ATel #: 441, 442, 443

Daily RXTE/PCA observations of GRO J1655-40 show that the source is rapidly evolving. Between March 5.7 (ATel #436) and March 10.7 the 2-25 keV flux gradually increased by a factor of 5.2, to 8.2e-9 erg/s/cm^2, with the spectral power-law index increasing from ~1.5 to ~1.7 and the QPO frequency from 0.31 Hz to 2.3 Hz.

On March 11.7 the flux had suddenly increased by a factor of 3.2 in a single day, to 2.6e-8 erg/s/cm^2. The spectrum was also much softer, with a power-law index of ~2.2 and a prominent thermal disk component (kT~1.1 keV). The March 11.7 power spectrum showed evolution on a time scale of a few thousand seconds. QPOs were detected at ~3.3 Hz and ~6.8 Hz, with indications for features around 0.07 Hz and 260 Hz.

The sudden softening and increase in flux indicate that the source may soon (within a few days) make a transition to the steep power-law state (McClintock & Remillard 2005, astro-ph/0306213). Such transitions often result in major changes in the optical/IR spectrum of black holes, as well as in strong radio flares. Radio, optical and near-IR observations of GRO J1655-40 in the next few days are therefore strongly encouraged. RXTE will keep observing the source on a daily basis.

RXTE/PCA light curves and hardness curves of the source can be found at: http://tahti.mit.edu/opensource/1655

I will try to update the page on a daily basis.