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Swift-UVOT Ultra-Violet/Optical Observations of GRO J1655-40

ATel #496; Albert K. H. Kong, Jeroen Homan, and Walter H. G. Lewin (MIT)
on 19 May 2005; 18:00 UT
Credential Certification: Albert Kong (akong@cfa.harvard.edu)

Subjects: Optical, Ultra-Violet, X-ray, Binary, Black Hole, Transient

We report Swift-UVOT observations of GRO J1655-40 during the recent outburst. GRO J1655-40 has been observed with Swift for 9 times since April and the Ultra-Violet/Optical telescope (UVOT) obtained images through the U, B, V, UVW1, and UVW2 filters during 3 of the observations. GRO J1655-40 was clearly detected and we derived the following magnitudes:

2005 Apr 10: U=16.51+/-0.16, B=16.60+/-0.09, V=15.12+/-0.09, UVW1=18.12+/-0.24, UVW2=19.15+/-0.28
2005 May 14: U=17.38+/-0.18, B=17.47+/-0.11, V=15.86+/-0.10, UVW1=18.90+/-0.27, UVW2=20.18+/-0.33
2005 May 16: U=18.08+/-0.18, B=17.91+/-0.10, V=16.32+/-0.10, UVW1=19.52+/-0.28, UVW2=20.27+/-0.32

The bandpass of the UVW1 and UVW2 filters is centered at 251nm (width=70nm) and 188nm (width=76nm), respectively. We note that the B and V magnitudes are consistent with ground-based observations (ATEL #485).

Simultaneous X-ray observations taken with the XRT show an increasing trend in luminosity; the mean 0.3-10 keV count rate is 816 c/s (Apr 10), 1235 c/s (May 14), and 1697 c/s (May 16). In particular, the last two observations show significant variability, consistent with the RXTE observations (ATEL #487). The anti-correlation between UV/optical and X-ray fluxes is probably related to the state change that occurred on May 10 (ATEL #485, #486, and #487).

The Swift-UVOT magnitudes are based on a preliminary zero-point, measured in orbit, and will require refinement with further calibration.