New black hole mergers in the LIGO-Virgo O3 data from a gravitational wave search including higher-order harmonics

Nearly all of the previous gravitational wave (GW) searches in the LIGO-Virgo data included GW waveforms with only the dominant quadrupole harmonic, i.e., omitting higher-order harmonics which are predicted by general relativity. We improved the IAS pipeline by efficiently introducing higher harmonics in the GW templates using the techniques in Wadekar et al. [1, 2]. Using the IAS-HM pipeline on the public LIGO-Virgo data from the O3 run, we find 11 new candidate BBH mergers with $0.52\le p_{\mathrm{a}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{o}}\le 0.88$ (we use the detection threshold as the astrophysical probability, $p_{\mathrm{a}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{o}}$, being over 0.5, following the approach of other pipelines). We broadly recover the high-significance events from earlier catalogs, except a few which were vetoed. We also find that including higher harmonics in our search raises the significance of a few previously reported marginal events (e.g., GW190711_030756). A few notable properties of our new candidate events are as follows. At $>95$% credibility, 4 candidates have primary masses in the intermediate-mass black hole (IMBH) range (i.e., above $\sim$100 $M_{\odot }$). 5 candidates have median mass ratio $q\le 0.5$. 5 candidates have median redshift $z\ge 0.8$. 3 candidates have non-zero ${\chi }_{\mathrm{e}\mathrm{f}\mathrm{f}}$ at $>95\mathrm{\%}$ credibility. While our new candidate events have modest false alarm rates ($\gtrsim 1.5$/yr), a population inference study including these can better inform the parameter space of BHs corresponding to the pair instability mass gap, high redshifts and asymmetric mass ratios.