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GRO J1655-40 enters a highly-variable, high-luminosity state

ATel #487; Jeroen Homan (MIT), Jon M. Miller (CfA), Rudy Wijnands (U. of Amsterdam), and Walter H. G. Lewin (MIT)
on 11 May 2005; 15:53 UT
Credential Certification: Jeroen Homan (homan@merate.mi.astro.it)

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

Referred to by ATel #: 489, 496, 614

GRO J1655-40 enters a highly-variable, high-luminosity state Jeroen Homan (MIT), Jon M. Miller (CfA), Rudy Wijnands (U. of Amsterdam), and Walter H.G. Lewin (MIT) Recent RXTE observations of the black hole X-ray transient GRO J1655-40, which is currently in outburst (e.g. ATEL #414,#417,#418,#419,#432,#438), show that the source has entered a highly variable, high-luminosity state. During the past two months the source has been steadily increasing in luminosity, while remaining spectrally soft. On May 10 the unabsorbed 3-200 keV flux increased from 3.9e-8 (erg/s/cm^2) to 6.8e-8 (erg/s/cm^2), compared to 2.9e-8 (erg/s/cm^2) on May 9. This increase was accompanied by a strong hardening of the X-ray spectrum and an increase in the strength of the 0.01-100 Hz variability (from ~5% to ~11% rms). In addition to red noise, which has dominated the power spectrum for the past two months, peaked noise and a weak QPO were observed in the 2-20 Hz range in the latest observation (taken on May 10 18:26 UTC).

Spectral fits were made to three 3-200 keV spectra obtained on May 10, with phenomenological (absorbed) multicolor disk blackbody plus power-law continuum models. These continuum models give an increasing apparent disk color temperature (kT = 1.3-1.5 keV) and varying power-law index (Gamma = 2.7 --> 2.5 --> 2.7), compared to kT = 1.27 and Gamma = 3.3 on May 9. These parameters are typical of the "very high" aka "steep power-law" state in Galactic black holes. Deconvolving the spectra in this way, the implied luminosity of GRO J1655-40 on May 10 rose from 1.2e38 erg/s to 2.7e38 erg/s in the 0.5-200 keV band (for d = 3.2 kpc), or approximately one-third of the Eddington limit for a 6 Msun black hole. The simple models used did not provide formally acceptable fits; the spectra also show indications for ionized disk reflection.

Observations with the RXTE/ASM on May 10 and 11 show that the source varies between ~2 and ~4.5 Crab on a time scale of a few hours (see also ATEL #486). This sudden turn-on of such strong variability is similar to that seen during the first part of the 1996/1997 outburst of GRO J1655-40.

As the X-ray spectral hardness of the source is close to the value during the period in which the source was radio bright (ATEL #419,#425,#434,#437,#443), GRO J1655-40 may show renewed radio activity. The sudden state change has already led to a decrease in the optical/IR flux of the source (ATEL #485). Further observations of GRO J1655-40 in the radio and optical/IR are encouraged in this relatively rare state. Daily RXTE/PCA observations will continue and the most recent results from these observations can be found at: http://tahti.mit.edu/opensource/1655/

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