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.
