Since September 2011, when the S6e science run was finished, GEO600 has been engaged in the so-called Astrowatch program. While the LIGO and Virgo detectors pursued their respective advanced detector upgrade programs, GEO600 was the most sensitive, and the only interferometric, gravitational wave detector in operation. GEO is also undergoing the upgrade program (GEO-HF). Unlike the other detectors, GEO600 continued to take science quality data for most of the regular work hours during the upgrade.
Although we are insensitive to the frequent weak events that are usually searched for using a multi detector network, we instead have a small chance to observe rare loud events that may occur during this single detector era. To guide our search for gravitational waves, we rely on coincidence detection with electromagnetic or astro-particle signals that indicate a source from which we would also expect gravitational waves. An example candidate from which GEO could observe gravitational waves is the red supergiant star Betelgeuse. This star is quite close on the galactic scale (~600 light years from the earth), and is expected to explode in a type II supernova. If this star were to go supernova while GEO was observing, then this would be an ideal event for GEO to observe gravitational waves.
GEO is very stable and can operate with a high duty cycle, which primarily is limited by upgrade and commissioning work. Over the last two years, we have taken data in Astrowatch mode for about 2/3 of the time. Running in Astrowatch mode requires extra care when making changes to the detector. At the end of each day the detector must be put into a stable state, which can be left unattended. Over weekends and holidays we ensure that at least one operator, post-doc, or graduate student is monitoring the state of the detector. This requires a bit of infrastructure, which allows us to monitoring of the detector status from home, or to be alerted by an automatic text message if something goes wrong with the detector. This works fine for ~80% of weekends, but some times someone has to go to the site to solve the problem. A lot of effort has been put into making the re-locking procedure automatic, and this works fine most of the time. Even so, we loose around 1% of observation time due to situations where the detector looses lock, and is not able to automatically relock.
Since 2010 GEO has been running with injected squeezed vacuum, which is a novel technique to reduce noise at the higher side of the frequency band. We at GEO are testing the long-term stability of squeezing, test its compatibility with the main detector, and try to improve the amount of noise reduction seen from the squeezing. So far the squeezing works well, and is applied to GEO for 90% of the time. But of course - its yet another subsystem of the whole detector that needs to be taken care of.
GEO600 will continue in this Astrowatch mode of operation, while performing upgrades to improve the sensitivity. This will continue while the advanced LIGO and Virgo detectors come online, at which point they will be running with lower duty cycles during periods of intense commissioning. Therefore it will be important that GEO continues to operate with high duty cycle during this period.