X-ray state change in 4U 1608-52

The neutron star X-ray transient 4U 1608-52 is often in a state of outburst (e.g. ATel #1113); the Swift/BAT light curve shows three clear outburst peaks so far in 2009. The source usually remains in a hard X-ray state during its outbursts. Here we report an X-ray state transition to a soft state around MJD 54960-75 (2009-05-09 - 24) and accompanying optical monitoring observations before and after the transition. The hard X-ray (15-50 keV) flux as measured by BAT on Swift had risen to 0.025 +- 0.003 counts/cm^2/sec (~ 110 mCrab) on MJD 54960, but then began to drop; by MJD 54973 the source was not detected at the 3 sigma level. Meanwhile, the RXTE ASM 2-12 keV flux continues to rise. On MJD 54960 the flux was 4.4 +- 0.4 counts/sec (~ 330 mCrab) and on MJD 54986 it was 11.2 +- 0.5 counts/sec (~ 840 mCrab). The last clear transition to a soft state was in October 2007, when the source rose to 3.6 Crab (2-12 keV) in RXTE ASM while the hard X-rays (15-50 keV; Swift/BAT) decreased from 320 mCrab to 42 mCrab in four days. This soft state lasted 40-50 days; if the 2009 soft state is of similar length a transition back to the hard state is expected at the end of June/beginning of July. We encourage multi-wavelength observations across the transition where possible.