O4 Hardware Injections

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

Some of them simulate GW events by moving mirrors in the arms of the interferometers. These are colloquially known as hardware injections. Times of hardware injections are marked in the 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 | H1 O4b | L1 O4b | V1 O4b
The NO_BURST_HW segment lists show times when Burst injections were not present: H1 O4a | L1 O4a | H1 O4b | L1 O4b | V1 O4b
The NO_CW_HW segment lists show times when CW injections were not present: H1 O4a | L1 O4a | H1 O4b | L1 O4b | V1 O4b
The NO_STOCH_HW segment lists show times when stochastic injections were not present: H1 O4a | L1 O4a | H1 O4b | L1 O4b | V1 O4b
The NO_DETCHAR_HW segment lists show times when detector characterization injections were not present: H1 O4a | L1 O4a | H1 O4b | L1 O4b | V1 O4b

O4 Transient Hardware Injections

During observing time in O4a and O4b (including also the few segments of data released before the start of runs) 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 only for the LIGO detectors. 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 and O4b CW no-Injection Timelines for LIGO.

List of injected pulsar parameters in O4: O4 pulsar injection parameters

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).

Revision History

August 26, 2025: First version
May 26, 2026: Addition of O4b info