X-ray state change in 4U 1608-52

ATel #2072; D. M. Russell (Univ. of Amsterdam), F. Lewis (Las Cumbres Observatory Global Telescope (LCOGT)/Open Univ.), P. Roche (LCOGT/Cardiff Univ.)
on 9 Jun 2009; 15:40 UT
Credential Certification: David M. Russell (D.M.Russell@uva.nl)

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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.

We have been monitoring the optical counterpart of 4U 1608-52 with the Faulkes Telescope South at Siding Spring in Australia since August 2007 in R and i' filters. The source brightened from R ~ 20.4; i' ~ 19.4 on MJD 54966.6 near the start of the state transition to R ~ 19.7; i' ~ 19.1 on MJD 54979.5 in the soft state (magnitude errors ~ 0.1 mag; variability of amplitude ~0.2 mag is present in i'-band on MJD 54966.6). The R-i' colour is bluer after the state transition, which could indicate a hotter (it is also brighter) outer accretion disc (e.g. Maitra & Bailyn 2008) or a soft state suppression of the non-thermal synchrotron emission from a jet (e.g. Homan et al. 2005). We also report a short (< 6 minute) ~0.5 mag i'-band flare on MJD 54966.556. The origin of this flare is uncertain but could be the optical reprocessing of a type I X-ray burst (we do not have simultaneous X-ray coverage).

The Swift/BAT transient monitor results are provided by the Swift/BAT team. Quick-look ASM/RXTE results are provided by the ASM/RXTE team. The Faulkes Telescope observations are part of an optical monitoring project of low-mass X-ray binaries (Lewis et al. 2008). The Faulkes Telescope Project is an educational and research arm of the Las Cumbres Observatory Global Telescope (LCOGT). FL acknowledges support from the Dill Faulkes Educational Trust.

4U 1608-52 light curves