Advanced LIGO ramps up, with slight improvements

After a series of upgrades during a 10-month shutdown, Advanced LIGO will begin its second observing campaign with a modest boost in performance over its first run. Early tests show that the interferometer in Livingston, Louisiana, is about 15–25% more sensitive to gravitational waves, whereas the Hanford, Washington, instrument is just about at its previous level. As of 14 November, both facilities had begun their engineering runs, the last step before the science phase.

Although the LIGO collaboration was hoping for a more substantial performance boost for the imminent observing run (O2), the interferometer tandem should nonetheless spot several black hole mergers. In its famed first run (O1), Advanced LIGO detected two pairs of inspiraling black holes and fairly strong evidence for a third. “The bottom line is that [the sensitivity] is better than it was at the beginning of O1,” says Peter Fritschel , LIGO’s chief detector scientist. “We expect to get more detections.”

Over the course of the upgrades, which began in January after the four-month O1 period, the Livingston detector leapfrogged its sister facility in performance. With its sensitivity enhancement, the Livingston interferometer should be able to spot the merger of two neutron stars 90 megaparsecs away. (The LIGO collaboration uses the average distance for a detectable neutron star–neutron star merger as a measure of sensitivity.) That translates to 750 Mpc for detecting a coalescing pair of 30-solar-mass black holes.

Hanford is operating at a sensitivity similar to what it had during O1, with a neutron star merger range of 65–80 Mpc. Overall, Advanced LIGO will be operating at the lower end of the collaboration’s O2 target of 80–120 Mpc. The LIGO team easily met its 40–80 Mpc goal for O1. The volume of space accessible to LIGO, and thus the rate of gravitational-wave detections, grows with the cube of those distances.

The difference in fortunes of the two detectors probably relates to the attention they received during the maintenance period, Fritschel says. At Hanford, scientists worked to supply the interferometer with ever-increasing laser power. In O1 the laser power in each interferometer arm was 100 kW; the ultimate design goal is seven times that. Earlier this year Hanford scientists boosted the power to 200 kW but ran into problems that cost sensitivity. Fritschel and his colleagues have decided to operate Hanford with 120 kW of laser power in the arms during O2, a boost that aids sensitivity at frequencies above 100 Hz but not at the lower frequencies most tuned to neutron star mergers.

Meanwhile, the improvements at Livingston were more basic, including some that scientists had wanted to make even before O1. (In fact, LIGO cofounder Rainer Weiss had almost shut down the Livingston facility to make some fixes just days before the first detected signal.) Researchers installed baffles to minimize scattered light and photodiodes with higher quantum efficiency.

Despite the sensitivity stagnation at Hanford, the overall signal-to-noise ratio for detecting gravitational waves has improved, Fritschel says. He adds that the sensitivities for both interferometers may increase slightly as the researchers make tweaks during operation. And now the LIGO team can pore over the problems they ran into at Hanford, in the hopes of creating a troubleshooting guide for future laser-upgrade sessions.

The O2 run is slated to last about six months. If all goes according to plan, the two LIGO detectors will be joined by Advanced Virgo near Pisa, Italy, for the latter part of the observing window.