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ATEL # 1749; Z. Wang (McGill University)
on 29 Sep 2008; 17:12 UT
Password Certification: Z. Wang (email@example.com)
Subjects: Optical, Binaries, Pulsars, Transients
Referred to by ATEL #: 1752
After the transient X-ray binary SAX J1808.4-3658 was reported to be outbursting in X-rays on 2008 Sept. 21 (ATEL #1728), we have requested Canada-France-Hawaii Telescope (CFHT) TOO observations of the source. On 2008 Sept. 29 (UTC), our first one-hour r' imaging observation was carried out. The detector used is MegaPrime, a wide-field optical imager. In the observation, the source was detected, with r'= 17.41+/-0.04.
Using a color of V-R= 0.3 (Wang et al. 2001, ApJ, 563, L61) obtained during the source's 1998 outburst, we find V= 17.5. Also as V-I= 0.6 in 1998, we estimate I= 16.9 for the source. Comparing to I= 16.93 reported by Maitra et al. (ATEL #1748) six days ago, it is likely that the source has not been fading.
The source has brightened by 3.7 mag in r' compared to in quiescence. In our Gemini observations of the source made in 2008 May, it was found to have an average magnitude of r'=21.1.
We thank CFHT for granting us the TOO observations of the source. We also thank Nadine Manset at CFHT for helping with the observations.