The Neil Gehrels Swift Observatory followed up 18 gravitational wave (GW)triggers from the LIGO/Virgo collaboration during the O3 observing run in2019/2020, performing approximately 6500 pointings in total. Of these events,four were finally classified (if real) as binary black hole (BH) triggers, sixas binary neutron star (NS) events, two each of NSBH and Mass Gap triggers, onean unmodelled (Burst) trigger, and the remaining three were subsequentlyretracted. Thus far, four of these O3 triggers have been formally confirmed asreal gravitational wave events. While no likely electromagnetic counterparts toany of these GW events have been identified in the X-ray data (to an averageupper limit of 3.60 x 10-12 erg cm-2 s-1 over 0.3-10 keV), or at otherwavelengths, we present a summary of all the Swift-XRT observations performedduring O3, together with typical upper limits for each trigger observed. Themajority of X-ray sources detected during O3 were previously uncatalogued;while some of these will be new (transient) sources, others are simply toofaint to have been detected by earlier survey missions such as ROSAT. Theall-sky survey currently being performed by eROSITA will be a very usefulcomparison for future observing runs, reducing the number of apparent candidateX-ray counterparts by up to 95 per cent.
- Gravitational wave
- X-rays: general