1 Draft version May 21, 2020 2 Typeset using LATEX twocolumn style in AASTeX63 3 GW190814: Gravitational Waves from the Coalescence of a 23 M Black Hole 4 with a 2.6 M Compact Object 5 LIGO Scientific Collaboration and Virgo Collaboration 6 7 (Dated: May 21, 2020) 8 ABSTRACT 9 We report the observation of a compact binary coalescence involving a 22.2 { 24.3 M black hole and 10 a compact object with a mass of 2.50 { 2.67 M (all measurements quoted at the 90% credible level). 11 The gravitational-wave signal, GW190814, was observed during LIGO's and Virgo's third observing 12 run on August 14, 2019 at 21:10:39 UTC and has a signal-to-noise ratio of 25 in the three-detector 2 +41 13 network. The source was localized to 18.5 deg at a distance of 241−45 Mpc; no electromagnetic 14 counterpart has been confirmed to date. The source has the most unequal mass ratio yet measured +0:008 15 with gravitational waves, 0:112−0:009, and its secondary component is either the lightest black hole 16 or the heaviest neutron star ever discovered in a double compact-object system. The dimensionless 17 spin of the primary black hole is tightly constrained to 0:07. Tests of general relativity reveal no ≤ 18 measurable deviations from the theory, and its prediction of higher-multipole emission is confirmed at −3 −1 19 high confidence. We estimate a merger rate density of 1{23 Gpc yr for the new class of binary 20 coalescence sources that GW190814 represents. Astrophysical models predict that binaries with mass 21 ratios similar to this event can form through several channels, but are unlikely to have formed in 22 globular clusters. However, the combination of mass ratio, component masses, and the inferred merger 23 rate for this event challenges all current models for the formation and mass distribution of compact- 24 object binaries. 25 1. INTRODUCTION 48 The first six months of the third observing run (O3) 49 were completed between April 1 and September 30, 26 The first two observing runs (O1 and O2) with 50 2019. The LVC recently reported on the discovery of 27 Advanced LIGO (Aasi et al. 2015) and Advanced 51 GW190425, the coalescence signal of what is most likely 28 Virgo (Acernese et al. 2015) opened up the field of 52 a BNS with unusually large chirp mass and total mass 29 gravitational-wave astrophysics with the detection of 53 compared to the Galactic BNSs known from radio pul- 30 the first binary black hole (BBH) coalescence signal, 54 sar observations (Abbott et al. 2020a). Another dis- 31 GW150914 (Abbott et al. 2016a). Another nine such 55 covery from O3 is that of GW190412, the first BBH 32 events (Abbott et al. 2016b, 2019a) were discovered by 56 33 coalescence with an unequivocally unequal mass ratio the LIGO Scientific and Virgo Collaborations (LVC) +0:12 57 q = m2=m1 of 0:28 (all measurements are reported 34 during this period, and additional events were reported −0:06 58 as symmetric 90% credible intervals around the median 35 by independent groups (Zackay et al. 2019a,b; Venumad- 59 of the marginalized posterior distribution, unless other- 36 hav et al. 2019; Nitz et al. 2020). The first binary neu- 60 wise specified). It is also the first event for which higher- 37 tron star (BNS) coalescence signal, GW170817, was dis- 61 multipole gravitational radiation was detected with high 38 covered during the second of these observing campaigns 62 significance (Abbott et al. 2020d). 39 (Abbott et al. 2017a, 2019b). It proved to be a multi- 63 Here we report on another O3 detection, GW190814, 40 messenger source with emission across the electromag- 64 the signal of a compact binary coalescence with the 41 netic spectrum (Abbott et al. 2017b), with implications 65 42 most unequal mass ratio yet measured with gravitational for the origin of short gamma-ray bursts (Abbott et al. +0:008 66 waves: q = 0:112 . The signal was first identified 43 2017c), the formation of heavy elements (Chornock et al. −0:009 67 in data from two detectors, LIGO Livingston and Virgo, 44 2017; Tanvir et al. 2017; Abbott et al. 2017d; Rosswog 68 on 2019 August 14, 21:11:00 UTC. Subsequent analysis 45 et al. 2018; Kasliwal et al. 2019; Watson et al. 2019), 69 of data from the full three-detector network revealed a 46 cosmology (Abbott et al. 2017e, 2019c) and fundamen- 70 merger signal with signal-to-noise ratio (SNR) of 25. 47 tal physics (Abbott et al. 2017c, 2019d). ' 2 71 The primary component of GW190814 is conclusively +1:1 Normalized energy 72 a black hole (BH) with mass m1 = 23:2 M . Its di- −1:0 0.0 2.5 5.0 7.5 10.0 12.5 15.0 73 mensionless spin magnitude is constrained to χ1 0:07. +0:08 ≤ 74 The nature of the 2:59−0:09 M secondary component is 75 unclear. The lack of measurable tidal deformations and 76 the absence of an electromagnetic counterpart are con- LIGO Hanford 77 sistent with either a neutron star (NS) or a BH given the +41 100 78 event's asymmetric masses and distance of 241−45 Mpc. 79 However, we show here that comparisons with the maxi- 80 mum NS mass predicted by studies of GW170817's rem- 81 nant, by current knowledge of the NS equation of state, 82 and by electromagnetic observations of NSs in binary 10 83 systems indicate that the secondary is likely too heavy 84 to be a NS. Either way, this is an unprecedented source LIGO Livingston 85 because the secondary's well-constrained mass of 2.50{ 100 86 2.67 M makes it either the lightest BH or the heaviest 87 NS ever observed in a double compact-object system. 88 As in the case of GW190412, we are able to measure Frequency (Hz) 89 the presence of higher multipoles in the gravitational ra- 90 diation, and a set of tests of general relativity with the 10 91 signal reveal no deviations from the theory. Treating 92 this event as a new class of compact binary coalescences, Virgo −3 −1 93 we estimate a merger rate density of 1{23 Gpc yr for 100 94 GW190814-like events. Forming coalescing compact bi- 95 naries with this unusual combination of masses at such a 96 rate challenges our current understanding of astrophys- 97 ical models. 98 We report on the status of the detector network and 10 10 8 6 4 2 0 99 the specifics of the detection in Sections 2 and 3. In − − − − − 100 Section 4, we estimate physical source properties with Time (seconds) from 2019-08-14 21:10:39 UTC 101 a set of waveform models, and we assess statistical and 102 systematic uncertainties. Tests of general relativity are Figure 1. Time{frequency representations (Chatterji et al. 103 described in Section 5. In Section 6, we calculate the 2004) of data containing GW190814, observed by LIGO Han- 104 merger rate density and discuss implications for the na- ford (top), LIGO Livingston (middle), and Virgo (bottom). Times are shown relative to 2019 August 14, 21:10:39 UTC. 105 ture of the secondary component, compact binary for- Each detector's data are whitened by their respective noise 106 mation and cosmology. Section 7 summarizes our find- amplitude spectral density and a Q-transform is calculated. 107 ings. The colorbar displays the normalized energy reported by the Q-transform at each frequency. These plots are not used in 108 2. DETECTOR NETWORK our detection procedure and are for visualization purposes only. 109 At the time of GW190814, LIGO Hanford, LIGO Liv- 110 ingston and Virgo were operating with typical O3 sen- 122 of analyzed frequencies. A time{frequency representa- 111 sitivities (Abbott et al. 2020a). Although LIGO Han- 123 tion (Chatterji et al. 2004) of the data from all three 112 ford was in a stable operating configuration at the time 124 detectors around the time of the signal is shown in Fig- 113 of GW190814, the detector was not in observing mode 125 ure1. 114 due to a routine procedure to minimize angular noise 126 We used validation procedures similar to those used 115 coupling to the strain measurement (Kasprzack & Yu 127 to vet previous gravitational-wave events (Abbott et al. 116 2017). This same procedure took place at LIGO Han- 128 2016c, 2019a). Overall we found no evidence that instru- 117 ford around the time of GW170608; we refer the reader 129 mental or environmental disturbances (Effler et al. 2015) 118 to Abbott et al.(2017f) for details of this procedure. 130 could account for GW190814. However, we did identify 119 Within a 5 min window around GW190814, this pro- 131 low-frequency transient noise due to scattered light at 120 cedure was not taking place, therefore LIGO Hanford 132 LIGO Livingston, a common source of noise in all three 121 data for GW190814 are usable in the nominal range 3 133 interferometers (Nuttall 2018). Scattered light features 184 ization computed using the rapid Bayesian algorithm 134 in the strain data are produced when a small fraction 185 BAYESTAR (Singer & Price 2016) that is shown in 135 of the main laser beam reflects off a moving surface and 186 Figure2. The event was initially classified as \Mass- 136 is phase modulated before recombining with the main 187 Gap" (Kapadia et al. 2020; LIGO Scientific Collabora- 137 beam.