O4 Hardware Injections

During the fourth observing run, O4, simulated signals were added to the LIGO detectors for testing and calibration.

The simulated signals are known as hardware injections, because they were created by manipulating mirrors in the arms of the interferometers. Times of hardware injections are marked in LIGO data files by injection data-quality channels. See the injection bit mask definition for details (the link refers to the 16 kHz files, the data-quality categories for the files at 4 kHz are identical).

Further details on hardware injections are in Validating gravitational-wave detections: The Advanced LIGO hardware injection system available from arXiv:1612.07864 [astro-ph.IM] and doi:10.1103/PhysRevD.95.062002.

Segment Lists when Injections are Not Present

Details on the different types of injections will be given in the next sections, information about injection-free times is available in segment lists from the Timeline application.

• The NO_CBC_HW segment lists show times when CBC injections were not present: H1 O4a | L1 O4a
• The NO_BURST_HW segment lists show times when Burst injections were not present: H1 O4a | L1 O4a
• The NO_CW_HW segment lists show times when CW injections were not present: H1 O4a | L1 O4a
• The NO_STOCH_HW segment lists show times when stochastic injections were not present: H1 O4a | L1 O4a
• The NO_DETCHAR_HW segment lists show times when detector characterization injections were not present: H1 O4a | L1 O4a

O4 Transient Hardware Injections

During observing time in O4 (including also the few segments of data released before the start of run) there were no injections of type Compact Binary Coalescence (CBC), Burst, Stochastic (STOCH) and Detector Characterization (DETCHAR). Note that, in the Timeline segment lists, injections times are marked also for periods when data are not released. This could be useful mainly for LVK members.

O4 Continuous Wave Hardware Injections

Continuous Wave (CW) injections (also known as pulsar injections) were added throughout O4. Each CW injection corresponds to a simulation of a gravitational wave signal of the form expected from a rapidly rotating, isolated neutron star (often observed as a radio pulsar).

The LIGO CW injection system was not active at all times during the run. For a list of times when this system was NOT active, see the O4a CW no-Injection Timelines for LIGO.

The parameters are defined as in arxiv:0508096. The parameters f0 and phi0 are both defined at a reference time (epoch) which are chosen to be the beginning of LIGO's 3rd science run (S3, GPS 751680013 == 2003-11-01T00:00:00 UTC) [injections 0-9], the start of Virgo's 2nd run (VSR2, GPS 930582085 == 2009-07-02T15:01:10) [injections 10-15] or New Years' Day 2019 (GPS 1230336018 == 2019-01-10 T00:00:00 UTC) [binary injections 16-17]. The following two parameters are also listed for each simulated pulsar:

• Aplus = 0.5*h0*(1 + cos²(iota))
• Across = h0*cos(iota)

For more information:

• The recovery of similar pulsar hardware injections is described in the following papers from the LIGO Scientific Collaboration: Phys. Rev. D 85, 022001, 2012 (arXiv).
• The recovery of a few simulated pulsars in Virgo data is described in Astrophys.J.737:93,2011 (arxiv).
• A search for pulsars in O1 data is reported in Astrophys. J. 839:12,2017 (arXiv).
• A search for pulsars in S6 data is reported in Astrophys. J. 785:2,2014 (arXiv).
• A search for pulsars in S5 data is reported in Astrophys. J. 713:1, 2010 (arXiv).

• August 26, 2025: First version