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INTEGRAL observes fading of GRO J1655-40 above 20 keV

ATel #442; S. Shaw (Southampton, UK & ISDC, Geneva), E. Kuulkers (ESA/ESAC, Spain), A. Paizis (ISDC, Geneva & IASF, Milano), N. Mowlavi (ISDC, Geneva), T. Courvoisier (ISDC, Geneva), K. Ebisawa (NASA/GSFC, USA), P. Kretschmar (ESA/ESAC, Spain), C. Markwardt (NASA/GSFC, USA),T. Oosterbroek (ESA/ESTEC, Netherlands), A. Orr (ESA/ESTEC, Netherlands) & R. Wijnands (UvA, Netherlands)
on 16 Mar 2005; 17:12 UT
Credential Certification: Nami Mowlavi (Nami.Mowlavi@obs.unige.ch)

Subjects: X-ray, Gamma Ray, Binary, Black Hole

We have observed a decrease in the the hard X-ray flux (above 20 keV) from the black hole binary GRO J1655-40, of about a factor of two (in four days), using INTEGRAL. This object has been observed for ~12 ksec approximately every 3 days since 17 Feb 2005 during the INTEGRAL Galactic Bulge Monitoring observations ( ATel #438 ).

In the latest observation, on MJD 53444.2 (15 March 2005), the measured flux in the ISGRI instrument was ~18 and ~5 cps in the 20--60 and 60--150 keV bands respectively. In the same energy bands the Crab gives about 180 and 50 cps, so both measurements correspond to ~100 mCrab.

In the previous observation, on MJD 53440.0 (11 March 2005) at the peak of the recent activity, the average flux was more than 2 times higher at ~48 cps in 20--60 keV (~270 mCrab) and ~11 cps (~220 mCrab) in 60--150 keV.

The fading of the hard X-ray flux from this source accompanies the decrease in the radio reported in ATel #441 and the apparent softening of the 2--25 keV RXTE/PCA spectrum given in ATel #440.

Note that the source was observed at a large off-axis angle (~ 15 degrees) and so is in the partially coded field of view of ISGRI and is not visible with JEM-X.

Light curves and images can be found at the following web page and will be updated as soon as possible after following INTEGRAL Galactic Bulge Monitoring observations, which are conducted every 3 days:

http://isdc.unige.ch/Science/BULGE