Chapter V: Results of the PyCBC Search across LIGO–Virgo’s First Three
5.1 Gravitational-wave Transient Catalogs
GWTC-1 [220] is the first catalog detailing the GW signals detected during O1 and O2 by the LIGO Scientific and Virgo collaboration. Two matched-filter pipelines, PyCBC andGstLAL, and one minimally modeled cWB pipeline were employed for GWTC-1. PyCBC and cWB did not include Virgo data for analysis while GstLAL
analyzed Virgo data for August 2017. Note the PyCBC ranking statistic of GW candidates for the LIGO two-detector network was simpler at the time of GWTC-1 than described in Chapter4. During O1, three BBHs were detected [223] while O2 saw the first detection of a BNS [4, 5, 227] and an additional seven detections of BBHs [224–226]. In addition, a list of 14 marginal event candidates with a FAR less than 1 per 30 days is provided in GWTC-1. Sensitivity of a GW interferometer is conventionally measured in terms of the inspiral range of a 1.4𝑀+1.4𝑀 BNS, which is the distance such a system can be detected with a SNR of 8. During O1 and O2, the BNS inspiral ranges span from 60 Mpc to 80 Mpc for LIGO Hanford and from 60 Mpc to 100 Mpc for LIGO Livingston. During the one-month of Virgo’s up-time, its BNS inspiral range was roughly at 25 Mpc.
GWTC-2
Thanks to several upgrades during the commissioning period prior to the start of O3a, the three detectors significantly improved their sensitivities, with median BNS inspiral ranges achieving 108 Mpc for LIGO Hanford, 135 Mpc for LIGO Livingston and 45 Mpc for Virgo [228]. The sensitivity increase drastically boosted the number of detections to 39 over O3a, compared to 11 across O1 and O2 [220]. Of the 39 new GW candidates reported by GWTC-2 over O3a, 26 were previously identified by low-latency searches and announced in near real-time through GCN Notices and Circulars while 13 were found by more in-depth offline reanalyses. GWTC-2 imposes a FAR threshold of 2 per year in each of the three offline search pipelines (cWB1, GstLAL, PyCBC), with an expected noise contamination percentage of less than 10%. At this FAR threshold, the hyper volume surveyed byPyCBCis 0.296 Gpc3 yr [228]. In GWTC-2,PyCBCdid not scan the Virgo data because the three-detector searches were not integrated into the pipeline. Among the 39 reported candidates, 27 were detected byPyCBC, fewer than the number of 36 forGstLAL primarily due to the non-inclusion of Virgo data. When accounting for the difference in analyzed data, bothGstLALandPyCBChave detected a comparable amount of GW signals.
GWTC-2.1
In GWTC-2.1, we use the final version of the O3a strain data with improved cali- bration [23] and non-linear noise subtraction around 60 Hz [244,245]. In addition toPyCBC andGstLAL, MBTAin its offline configuration was employed for the first
1cWBdid not detect any events that were not also seen by the two CBC pipelines; no unmodeled burst events have been confidently detected in O1 through O3.
Figure 5.1: Masses of BHs and NSs in the stellar graveyard detected through GWs and EM waves. This plot contains all GW events detected through GWTC-3 with 𝑝astro > 0.5 [247]. Credit: LVK / Aaron Geller / Northwestern University.
time in GWTC-2.1. In this analysis, PyCBC was extended to search data from the three-detector LIGO–Virgo network, along with updates to the event ranking statistic [174] and the 𝑝
astro [246] calculation and a new method to estimate source class probability [213]. GWTC-2.1 imposes a looser FAR threshold of 2 per day, resulting in a deeper list of 1201 candidate events [229]. Astronomical investi- gations of subthreshold GW candidates may lead to multi-messenger discoveries, enhancing our understanding of such systems. Added with the requirement of hav- ing a probability of astrophysical origin greater than 0.5 (𝑝
astro > 0.5), a subset of 44 high-significance candidates were identified, of which 36 have been reported in GWTC-2.
GWTC-3
GWTC-3 is the most comprehensive catalog of GW events up to date, recording all 90 signals with 𝑝
astro > 0.5 found by the LVK collaboration across LIGO–Virgo’s first three observing runs [6]. Out of the 35 new significant events discovered during O3b, 18 were previously broadcast in low-latency through GCN Notices and Circulars and 17 were presented for the first time. The noise contamination rate is expected to be∼10–15%. Additionally, there are 1048 subthreshold candidates with a FAR less than 2 per day but that do not surpass the 𝑝
astro threshold. Significant candidates from O3b are comprised of BBHs and NSBHs, with none from BNSs.
Figure 5.2: The number of confidently identified CBC candidates with a probability of astrophysical origin𝑝
astro >0.5 versus the detector network’s effective surveyed BNS time–volume [228]. The colored bands mark the different observing runs. The solid black line indicates the cumulative number of probable candidates. The blue line, dark blue band and light blue band are the median, 50% confidence interval and 90% confidence interval for a Poisson distribution fit to the number of candidates at the end of O3b [6].
Masses of BHs and NSs detected through GWs and EM waves are plotted in Fig.5.1, containing all GW events in GWTC-3 with 𝑝
astro > 0.5. Compared to O3a, the median BNS inspiral ranges are 115 Mpc for LIGO Hanford, 133 Mpc for LIGO Livingston and 51 Mpc for Virgo. Fig. 5.2 shows the increase in the number of candidates across observing runs. ThePyCBCpipeline employed for GWTC-3 has been detailed in Chapter4, with a new introduction of the use of graphics processing unit (GPU) cores or distributed computing through the Open Science Grid (OSG) [248, 249] for faster computation. Among the 35 events, 29 were found by one or both of thePyCBC-broadandPyCBC-BBHanalyses. 21 events were found by two or more offline search pipelines (PyCBC,GstLAL,MBTA,cWB).