GROWTH on S190814bv: Deep Synoptic Limits on the Optical/Near-Infrared Counterpart to a Neutron Star-Black Hole Merger
Igor Andreoni, Daniel A. Goldstein, Mansi M. Kasliwal, Peter E. Nugent, Rongpu Zhou, Jeffrey A. Newman, Mattia Bulla, Francois Foucart, Kenta Hotokezaka, Ehud Nakar, Samaya Nissanke, Geert Raaijmakers, Joshua S. Bloom, Kishalay De, Jacob E. Jencson, Charlotte Ward, Tomás Ahumada, Shreya Anand, David A. H. Buckley, Maria D. Caballero-García, Alberto J. Castro-Tirado, Christopher M. Copperwheat, Michael W. Coughlin, S. Bradley Cenko, Mariusz Gromadzki, Youdong D. Hu, Viraj R. Karambelkar, Daniel A. Perley, Yashvi Sharma, Azamat F. Valeev, David O. Cook, U. Christoffer Fremling, Harsh Kumar, Kirsty Taggart, Ashot Bagdasaryan, Jeff Cooke, Aishwarya Dahiwale, Suhail Dhawan, Dougal Dobie, Pradip Gatkine, V. Zach Golkhou, Ariel Goobar, Andreas Guerra Chaves, Matthew Hankins, David L. Kaplan, Albert K. H. Kong, Erik C. Kool, Siddharth Mohite, Jesper Sollerman, Anastasios Tzanidakis, Sara Webb, Keming Zhang
DDraft version January 1, 2020
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GROWTH on S190814bv: Deep Synoptic Limits on the Optical/Near-Infrared Counterpart to a Neutron Star–BlackHole Merger
Igor Andreoni and Daniel A. Goldstein ∗ these authors contributed equally to this workMansi M. Kasliwal, Peter E. Nugent,
2, 3
Rongpu Zhou, Jeffrey A. Newman, Mattia Bulla,
5, 6
Francois Foucart, Kenta Hotokezaka, Ehud Nakar, Samaya Nissanke,
10, 11
Geert Raaijmakers,
10, 11
Joshua S. Bloom,
3, 2
Kishalay De, Jacob E. Jencson,
1, 12
Charlotte Ward, Tom´as Ahumada, Shreya Anand, David A. H. Buckley, Maria D. Caballero-Garc´ıa, Alberto J. Castro-Tirado,
16, 17
Christopher M. Copperwheat, Michael W. Coughlin, S. Bradley Cenko,
19, 20
Mariusz Gromadzki, Youdong Hu,
16, 22
Viraj R. Karambelkar, Daniel A. Perley, Yashvi Sharma, Azamat F. Valeev, David O. Cook, U. Christoffer Fremling, Harsh Kumar, Kirsty Taggart, Ashot Bagdasaryan, Jeff Cooke,
26, 27
Aishwarya Dahiwale, Suhail Dhawan, Dougal Dobie,
28, 29
Pradip Gatkine, V. Zach Golkhou,
30, 31, † Ariel Goobar, Andreas Guerra Chaves, Matthew Hankins, David L. Kaplan, Albert K. H. Kong, Erik C. Kool, Siddharth Mohite, ‡ Jesper Sollerman, Anastasios Tzanidakis, Sara Webb,
27, 26 and Keming Zhang ‡ California Institute of Technology, 1200 East California Blvd, MC 249-17, Pasadena, CA 91125, USA Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA Department of Physics and Astronomy and PITT PACC, University of Pittsburgh, PA, 15260, USA Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, SE-106 91 Stockholm, Sweden The Oskar Klein Centre, Department of Physics, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden Department of Physics, University of New Hampshire, 9 Library Way, Durham NH 03824, USA Department of Astrophysical Sciences, Princeton University, Peyton Hall, Princeton, NJ 08544, USA Department of Astrophysics, Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv, 69978, Israel GRAPPA, Anton Pannekoek Institute for Astronomy and Institute of High-Energy Physics, University of Amsterdam, Science Park904, 1098 XH Amsterdam, The Netherlands Nikhef, Science Park 105, 1098 XG Amsterdam, The Netherlands University of Arizona, Steward Observatory, 933 N. Cherry Avenue, Tucson, AZ 85721, USA Department of Astronomy, University of Maryland, College Park, MD 20742, USA South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town, South Africa Astronomical Institute, Academy of Sciences of the Czech Republic, Boˇcn´ı II 1401, CZ-141 00 Prague, Czech Republic Instituto de Astrof´ısica de Andaluc´ıa (IAA-CSIC), Glorieta de la Astronom´ıa s/n, E-18008, Granada, Spain Departamento de Ingenier´ıa de Sistemas y Autom´atica, Escuela de Ingenieros Industriales, Universidad de M´alaga, Unidad Asociadaal CSIC, C. Dr. Ortiz Ramos sn, 29071 M´alaga, Spain Astrophysics Research Institute, Liverpool John Moores University,IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK Astrophysics Science Division, NASA Goddard Space Flight Center, MC 661, Greenbelt, MD 20771, USA Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland Universidad de Granada, Facultad de Ciencias Campus Fuentenueva S/N CP 18071 Granada, Spain Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnii Arkhyz, 369167 Russia IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Australian Research Council Centre of Excellence for Gravitational Wave Discovery (OzGrav), Swinburne University of Technology,Hawthorn, VIC, 3122, Australia Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, VIC, 3122, Australia Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia CSIRO Astronomy and Space Science, P.O. Box 76, Epping, New South Wales 1710, Australia DIRAC Institute, Department of Astronomy, University of Washington, 3910 15th Avenue NE, Seattle, WA 98195, USA [email protected] a r X i v : . [ a s t r o - ph . H E ] D ec Andreoni & Goldstein and the GROWTH Collaboration The eScience Institute, University of Washington, Seattle, WA 98195, USA Center for Gravitation, Cosmology and Astrophysics, Department of Physics, University of Wisconsin–Milwaukee, P.O. Box 413,Milwaukee, WI 53201, USA Institute of Astronomy, National Tsing Hua University, Hsinchu 30013, Taiwan The Oskar Klein Centre & Department of Astronomy, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden
ABSTRACTOn 2019 August 14, the Advanced LIGO and Virgo interferometers detected the high-significancegravitational wave (GW) signal S190814bv. The GW data indicated that the event resulted from aneutron star–black hole (NSBH) merger, or potentially a low-mass binary black hole merger. Dueto the low false alarm rate and the precise localization (23 deg at 90%), S190814bv presented thecommunity with the best opportunity yet to directly observe an optical/near-infrared counterpart to aNSBH merger. To search for potential counterparts, the GROWTH collaboration performed real-timeimage subtraction on 6 nights of public Dark Energy Camera (DECam) images acquired in the threeweeks following the merger, covering >
98% of the localization probability. Using a worldwide networkof follow-up facilities, we systematically undertook spectroscopy and imaging of optical counterpartcandidates. Combining these data with a photometric redshift catalog, we ruled out each candidate asthe counterpart to S190814bv and we placed deep, uniform limits on the optical emission associatedwith S190814bv. For the nearest consistent GW distance, radiative transfer simulations of NSBHmergers constrain the ejecta mass of S190814bv to be M ej < . M (cid:12) at polar viewing angles, or M ej < . M (cid:12) if the opacity is κ < g − . Assuming a tidal deformability for the neutron starat the high end of the range compatible with GW170817 results, our limits would constrain the BHspin component aligned with the orbital momentum to be χ < . Q <
6, with weakerconstraints for more compact neutron stars. We publicly release the photometry from this campaignat this http url. INTRODUCTIONMergers of binaries containing neutron stars andstellar-mass black holes (NSBH mergers) have long beentheorized as potential sites of r -process nucleosynthesis(Lattimer & Schramm 1974), that should be detectableby networks of laser interferometers as gravitationalwave (GW) sources (Abadie et al. 2010), potentiallyharboring optical counterparts (Metzger & Berger 2012)that could be used to help constrain the equation-of-state (EOS) of dense nuclear matter (Geesaman 2015;Coughlin et al. 2019b), measure the Hubble constant H (Schutz 1986), and probe radiation hydrodynamics inasymmetric conditions and the limits of nuclear stability(Fern´andez & Metzger 2016). On 2019 August 14, theLIGO and Virgo interferometers detected S190814bv,the first high-confidence GW signal associated withan NSBH merger (The LIGO Scientific Collaborationand the Virgo Collaboration 2019a,b), confirming thatNSBH mergers exist and that they produce gravitationalwaves.Electromagnetic emission from NSBH mergers, whichis critical to achieve many of the science goals described ∗ Hubble Fellow † Moore-Sloan, WRF Innovation in Data Science, and DIRAC Fellow ‡ LSSTC Data Science Fellow in the previous paragraph, is currently the subjectof considerable theoretical uncertainty (e.g., Mingarelliet al. 2015; Hotokezaka & Nakar 2019; Barbieri et al.2019). At this time, it is not clear whether optical/near-infrared (NIR) counterparts to NSBH mergers exist,and, if they do, what their properties might be. Theuncertainty in the nature of electromagnetic counter-parts to NSBH mergers is driven primarily by (1) un-certainties in the optical opacity of r -process elementsin low ionization states, which may be the dominantopacity affecting spectrum synthesis in NSBH opticalcounterparts (“kilonovae,” or “macronovae”), (2) a lackof knowledge regarding the EOS of dense nuclear mat-ter, which directly affects the distribution of the mergerejecta and the post-merger nucleosynthesis, (3) an in-complete theoretical picture of the properties of NSBHmatter outflows for all potential progenitor configura-tions, and (4) the complexity of the multiphysics simu-lations required to predict the observable properties ofNSBH mergers, which at various stages must include so-phisticated treatments of magnetohydrodynamics, Gen-eral Relativity, neutrino transport, radiation transport,and nucleosynthesis.The dynamics of NSBH mergers is profoundly differ-ent from the dynamics of binary neutron star (BNS)mergers (see Nakar, in preparation for a review), buttheir EM counterparts are expected to share some sim- ROWTH on S190814bv (cid:12) tidal tail can be dynami-cally created. These dynamical ejecta have a low elec-tron fraction Y e , favoring heavy element production viar-process. An accretion disk can then form around theBH with mass ∼ (cid:12) . About ∼
40% of the diskmass is ejected, constituting a secular ejecta component(e.g., Siegel & Metzger 2018; Fern´andez et al. 2019).This component may contain polar winds with veloci-ties 0.1–0.15 c during an efficient accretion phase, fol-lowed by more isotropic, neutron-rich winds with lowervelocities. Simulations suggest that Y e of the secularejecta is higher than that of the dynamical one, espe-cially along the polar regions, where the ejecta may befree of the high opacity lanthanides (Miller et al. 2019;Christie et al. 2019).For comparison, the GW170817 kilonova was found tohave a “red” component with mass M ∼ . M (cid:12) , likelyencompassing both the tidal ejecta and the post-mergerdisk wind, with velocity v ∼ . c (e.g., Kasen et al.2017). The nature of the blue component of GW170817is harder to explain, however polar winds from efficientaccretion onto the BH formed during a NSBH mergermay result in a similarly blue transient at early times.If the NS is disrupted within the ISCO, the mass ofboth the dynamical and the disk ejecta is expected to besmall, < − M (cid:12) , reducing the likelihood of producingany observable EM counterpart.To help characterize the uncertain nature of elec-tromagnetic emission from NSBH mergers, we presentdeep, synoptic, and red-sensitive limits on the opti-cal/NIR emission from the NSBH merger S190814bv.We obtained the limits from public, multi-band observa-tions of the localization region of S190814bv conductedby the Dark Energy Survey GW (DES-GW) collabora-tion (Soares-Santos et al. 2019a), who used the DarkEnergy Camera (DECam, Flaugher et al. 2015) to tile >
98% of the localization probability roughly 10 times ineach of the i and z bands.Section 2 gives an overview of the GW event and Sec-tion 3 describes the DECam follow-up. Our analysismethods are described in Section 4 and the results offollow-up observations of candidates of interest are pre- sented in Section 5, using a Planck Collaboration et al.(2016) cosmology to compute absolute magnitudes. InSection 6, we quantify the completeness of our galaxycatalogs. In Section 7, we use the limits obtained in thepreceding analysis to constrain the ejecta mass, opacity,and viewing angle of S190814bv. The constraints on theejecta mass are used to characterize the spin and themass ratio of the progenitor binary. We summarize ourresults and present concluding remarks in Section 8. S190814bvThe LIGO Scientific Collaboration and the Virgo Col-laboration (2019a) detected the GW event S190814bvon 2019-08-14 21:10:39 UT, using four independentpipelines processing data from three GW interferom-eters (LIGO Hanford, LIGO Livingston, and Virgo)in triple coincidence. The false alarm rate of theevent was 2 × − Hz, or approximately one in 10 years. The GW event was first classified as “MassGap” with >
99% probability. A “mass gap” systemrefers to a binary where the lighter companion has mass3 M (cid:12) < M < M (cid:12) , and no material is expected to beejected. The classification of S190814bv was revisedabout 12 hours later (The LIGO Scientific Collabora-tion and the Virgo Collaboration 2019b) based on newparameter estimation obtained with the LALInferenceoffline analysis pipeline (Veitch et al. 2015; Abbott et al.2016) to an “NSBH” event with >
99% probability. Therefined analysis also indicated that there should be < M ≤ M (cid:12) and the heavier component has M ≥ M (cid:12) . The maxi-mum mass of a neutron star, according the most extremeviable EOS, is M ns , max ≈ . M (cid:12) ( ¨Ozel & Freire 2016).It is thus possible, given the LIGO/Virgo definition of“NSBH,” that GW events classified as “NSBH” may ac-tually be mergers of black holes having M ≥ M (cid:12) withlower-mass black holes having M ns , max ≤ M ≤ M (cid:12) .As the masses of the components of S190814bv are notyet public, we cannot yet comment on this possibility.S190814bv was localized to 23 deg at 90% confidence.For comparison, the BNS merger GW170817 was local-ized to 28 deg (Abbott et al. 2017), then refined to16 deg (Abbott et al. 2019), and the three GW eventcandidates including neutron stars identified during O3before S190814bv were localized to 7461 deg (S190425z;The LIGO Scientific Collaboration & The Virgo Collab- Andreoni & Goldstein and the GROWTH Collaboration oration 2019a), 1131 deg (S190426c; The LIGO Scien-tific Collaboration & The Virgo Collaboration 2019b),and 1166 deg (S190510g; LIGO Scientific Collaboration& Virgo Collaboration 2019), with S190510g having asignificant probability of being non-astrophysical in ori-gin. The precise localization of S190814bv is largely dueto the fact that (1) it was detected with three GW inter-ferometers and (2) it had a favorable location in the skywith respect to the antenna pattern of the detectors.Despite the small localization area, the GW anal-ysis places S190814bv at the fairly large distance of267 ±
52 Mpc (The LIGO Scientific Collaboration andthe Virgo Collaboration 2019b). This corresponds to avolume of 5 . × Mpc for 90% area and 1 σ distance,or a volume of 1 . × Mpc for 90% area and 2 σ dis-tance. The distance probability distribution is broadlyGaussian (although skewed) pixel by pixel, but not overthe whole map, generally. The S190814bv skymap isrelatively small, so the effect is less evident than forlarger skymaps, where a pixel-by-pixel approach is par-ticularly appropriate. We focus the analysis presentedin this paper to the 2 σ volume, corresponding to theredshift range 0 . < z < . DATASETS190814bv was initially classified as a “Mass Gap”event, where both the more massive object and thelighter companion are likely black holes. Therefore,S190814bv was considered a suitable candidate for DE-Cam follow-up under the NOAO program ID 2019B-0372 (PI Soares-Santos), which conducts observationsof binary black hole (BBH) mergers, with the resultingdata becoming immediately public. The program wastriggered within a few hours of the merger, before therefined classification issued by The LIGO Scientific Col-laboration and the Virgo Collaboration (2019b). Thefirst exposure was taken roughly 7 hours after the mergerat UTC 2019–08–15 06:32:43. Data were acquired on sixdistinct Chilean calendar nights (2019–08–14, 2019–08–15, 2019–08–16, 2019–08–17, 2019–08–20, and 2019–08–30), lasting from 1.5 to 4.5 hours each night. The moonand weather conditions steadily improved between thefirst and the last nights of the run, and the exposuretimes were more than twice as long in each filter at theend of the run than the beginning, resulting in a greaterachieved depth. Figure 1 shows the locations of the DE-Cam exposures obtained during the run and processedin this analysis relative to the LALInference skymap ofS190814bv. METHODSWe processed the raw DECam data as they weretaken, using the pipeline described in Goldstein et al. (2019), now running on the Amazon Web Services Elas-tic Compute Cloud (EC2) for increased reliability. Foreach exposure, a c5.18xlarge spot EC2 instance with72 vCPUs and 144GB of RAM was launched to as-trometrically and photometrically calibrate the DE-Cam CCD images in parallel, make references, per-form subtractions, identify candidates, filter them using autoScan (Goldstein et al. 2015), and perform aperturephotometry. Each exposure took roughly 20 minutes toprocess, and the results were stored on the Amazon Sim-ple Storage Service (S3). The median depths achievednightly during the follow-up campaign with DECam arepresented in Table 1.4.1.
Photometric redshifts
At the distance to S190814bv ( ∼
250 Mpc), spectro-scopic redshift catalogs are largely incomplete (D´alyaet al. 2018, Cook et al, in preparation). We there-fore relied primarily on photometric redshifts of tran-sient host galaxies to assess whether transient candi-dates had distances consistent with the GW distance ofS190814bv. We carried out an offline analysis of theDESI Legacy Imaging Surveys (Dey et al. 2019) DataRelease 8 (DR8), which includes model-based photom-etry from the DECam and from the
Wide-field InfraredSurvey Explorer ( WISE , Wright et al. 2010), to estimatephotometric redshifts for the galaxies in the S190814bvlocalization region. By applying a Random Forest al-gorithim to the DR8 data (Zhou et al. 2019, in prepa-ration), we generated a photometric redshift catalog forthe entire DR8 footprint.Due to its inclusion of data from Dark Energy Survey(DES, Dark Energy Survey Collaboration et al. 2016)observations, the catalog fully covered the S190814bvlocalization region. Catalogued sources with m z > z = 21 has a negligible impact on ourcompleteness, with an expected loss in luminosity frac-tion of < > × the average photometric redshift uncertainty of all thesources of a similar magnitude within a 1 deg radius( ± . Gaia parallaxes that are not compatiblewith 0 ± .
081 mas obtained from the analysis of paral-laxes measured for quasars (Luri et al. 2018). Spectro-
ROWTH on S190814bv h m h m m m h m -20°-25°-30°-35° Right Ascension D e c li n a t i o n i-band h m h m m m h m -20°-25°-30°-35° Right Ascension D e c li n a t i o n z-band Figure 1.
Top row –
Locations of DECam exposures processed in this analysis (black circles) relative to the S190814bvLALInference skymap (The LIGO Scientific Collaboration and the Virgo Collaboration 2019b), with color linearly proportionalto localization probability density.
Bottom row –
Bounding box of the top two plots (black square) relative to a global projectionof the LALInference skymap.
Andreoni & Goldstein and the GROWTH Collaboration scopic redshifts (primarily from the 2dF Galaxy RedshiftSurvey, Colless et al. 2001) were considered instead ofphotometric redshifts when available.4.2.
Candidate selection
We used the GROWTH Marshal (Kasliwal et al. 2019)to display, filter, and assess candidates detected with ourimage-subtraction pipeline. During the scanning pro-cess, 519 candidates were saved that were located insidethe 95% probability area of the skymap.The candidates were cross-matched to known solarsystem objects from the IAU Minor Planet Center usingthe astcheck utility. The cross-match radius betweencandidates and known solar system objects was 100 (cid:48)(cid:48) .In addition to excluding known asteroids from our tran-sient list, we identified elongated candidates (likely tobe fast-moving uncatalogued solar system objects) byvisual inspection and removed them.The selection criteria for candidates to be reported inthis work were defined as follows:1. No match with moving objects reported in the IAUMinor Planet Center.2. At least 2 detections in any filter with a time base-line of ≥
30 minutes to further reject fast movingobjects.3. Location within the 95% probability contour of theLALInference skymap.4. The distance to a possible host must be consistentwith the distance range expected for S190814bv(accounting for 2 × the standard deviation of thedistance probability distribution, which translatesinto a redshift range of 0 . < z < . m z < × the uncertainty on the photometricredshifts.5. At least 3 detections with an autoScan classifica-tion score > . Most coordinates on the skymap had at least 20visits (see Figure 1).Candidates discovered in real time were reported tothe Transient Name Server (TNS). New candidates(Andreoni et al. 2019; Goldstein et al. 2019b,a) andtransient follow-up were reported via Gamma-ray Coor-dinates Network (GCN) circulars during the follow-upcampaign. We used a radius of 20 (cid:48)(cid:48) to cross-match ourcandidates with the photometric redshift catalog (seeSection 4.1), which corresponds to a physical distanceof 16 kpc at z = 0 .
037 and of 36 kpc at z = 0 . Candidate follow-up methods
The spectroscopic results presented in this paper in-clude data obtained using Near Infrared Echellete Spec-trometer (NIRES) and the Low Resolution ImagingSpectrometer (LRIS, Oke et al. 1995) at W. M. KeckObservatory. The NIRES data were reduced usingthe
Spextool code (Cushing et al. 2004) adapted forNIRES. The LRIS data were processed using lpipe ,the fully automated reduction pipeline for longslit spec-troscopy described in Perley (2019). We observed threepotential candidates with the 10.4m Gran Telescopio deCanarias (GTC, PI A. Castro-Tirado), located at theobservatory of Roque de los Muchachos in La Palma(Canary Islands, Spain), equipped with the OpticalSystem for Imaging and low-intermediate-ResolutionIntegrated Spectroscopy (OSIRIS, Cepa et al. 2000).GTC/OSIRIS spectra for the three targets were ob-tained either with the R1000B or with the R1000Rgrisms and a 1 arcsec slit covering the 3,700˚A–7,500˚Aor 5,100˚A–10,000˚A range. The slit was placed in orderto cover the candidate location and the host galaxy cen-tre. Data were reduced and calibrated using standardroutines. Optical images in the r -band filter were alsotaken for the candidates with GTC. Photometric zeropoints and astrometric calibration were computed usingthe Pan-STARRS catalogue (Chambers et al. 2016). Wethen performed point spread function (PSF) matchingphotometry of the targets. Spectroscopy of one candi-date of interest was also obtained with 10m SouthernAfrican Large Telescope (SALT, Buckley et al. 2006,PI Buckley) equipped with the Robert Stobie Spectro-graph (RSS, Burgh et al. 2003; Kobulnicky et al. 2003).The primary data reduction of the SALT/RSS spec-trum was done using the PySALT package (Crawfordet al. 2010), which accounts for basic CCD character- https://wis-tns.weizmann.ac.il ROWTH on S190814bv Average Date ∆ t m lim,i m lim,z m lim,i m lim,z P enc P enc P enc (UT) (days) 5 σ -phot 5 σ -phot detection limit detection limit ( i ) ( z ) ( i + z )2019-08-15 08:18 0.46 21.1 20.9 20.4 20.3 92% 94% 94%2019-08-16 07:57 1.45 21.8 22.0 21.0 21.1 97% 97% 98%2019-08-17 06:59 2.41 22.3 22.3 21.3 21.4 97% 97% 98%2019-08-18 07:32 3.43 22.9 22.9 22.1 22.3 97% 97% 98%2019-08-21 06:21 6.38 23.4 23.2 22.8 22.6 93% 93% 94%2019-08-31 06:11 16.37 24.2 · · · · · · · · · Table 1.
Median depth achieved during the follow-up of S190814bv. The dates correspond to the central time between thefirst and the last epoch acquired on each observing night, and ∆ t indicates the time lag from the merger time (The LIGOScientific Collaboration and the Virgo Collaboration 2019a). The photometric depth corresponds to 5 σ photometric magnitudelimits (column 3 and 4) and detection depth indicates the detection limit of the image-subtraction pipeline. All magnitudes arecalibrated to the AB system. The last three columns present the integrated probability of the S190814bv LALInference skymapobserved on each observing night, with the last column considering the observations in either i or z filters. istics (e.g., cross-talk, bias and gain correction) and cos-mic ray removal. Standard IRAF/Pyraf routines werethen used to undertake wavelength and relative flux cali-brations. Due to the design of SALT, which has a chang-ing field-dependent entrance pupil, spectrophotometricstandard observations can only provide relative fluxes(Buckley et al. 2018).The photometric evolution of the most promising can-didates was monitored using the optical imaging compo-nent of the Infrared-Optical suite of instruments (IO:O)on the 2m Liverpool Telescope (LT, Steele et al. 2004)at Observatorio del Roque de los Muchachos. All im-ages were processed with the LT IO:O pipeline and im-age subtraction was performed automatically using Pan-STARRS (Chambers et al. 2016) imaging as a reference,using the methods described in Fremling et al. (2016).Optical photometric follow-up data were also acquiredusing the Las Cumbres Observatory (LCO) telescopenetwork under proposal ID 2019B-0244 (PI Coughlin).The LCO photometry was measured after subtractingreference images from the Legacy Surveys archive usingthe
HOTPANTS package (Becker 2015). At infrared wave-lengths we obtained photometry using the Wide-field In-frared Camera (WIRC, Wilson et al. 2003) on the Palo-mar 200-inch Hale telescope (P200). The P200/WIRCdata were reduced using a reduction pipeline developedby members of our team (De et al., in preparation). RESULTSIn this section, we describe the follow-up observa-tions that were conducted to characterize each of the21 objects that we selected as candidate counterparts toS190814bv using the methods described in Section 4. Inaddition, we discuss a selection of candidates that didnot pass our selection criteria, but that were reportedand extensively followed up in the first three weeks after S190814bv. Most of the objects presented individuallywere spectroscopically classified.
DG19qabkc/AT2019nqc — The candidate was first re-ported in Andreoni et al. (2019) and appeared to be ∼ (cid:48)(cid:48) offset from its host galaxy. Although no spectro-scopic redshift was available, the photometric redshiftplaced the host in the correct distance range (Goldsteinet al. 2019b). The candidate was photometrically con-firmed in the optical (Herner et al. 2019b; Dichiara et al.2019b,c) and we detected the transient in the near in-frared at magnitude J ∼ . ± . F < . × − erg cm − s − (Evans et al. 2019)was placed using data acquired with the X–ray Tele-scope (XRT, Burrows et al. 2005) on the space-based Neil Gehrels Swift Observatory , hereafter referred toas
Swift . We observed DG19qabkc/AT2019nqc withSALT/RSS starting on 2019-08-23 22:46:10 and twoconsecutive 1200 s exposures were obtained using thePG300 transmission grating, which covered the spec-tral region 3300–9800˚A. The seeing was ∼ . (cid:48)(cid:48) and a1 . (cid:48)(cid:48) slit was used, giving an average resolving powerof ∼ ∼ α line with a P-Cygni profile dominates the spectrum,with a weak H β line in absorption, consistent with aredshift of z = 0 . z = 0 . ± .
001 (Lopez-Cruz et al. 2019a).We extensively monitored the transient with LCO andLT imaging. The photometry that we obtained (databehind Figure 3) confirms a slow evolution compatiblewith supernova behavior.
Andreoni & Goldstein and the GROWTH Collaboration
DG19wxnjc/AT2019npv — When the candidate was dis-covered (Goldstein et al. 2019a) it appeared to be off-set from its host galaxy, the photometric redshift ofwhich ( z = 0 . ± . z = 0 . Swift /XRT (
F < . × − erg cm − s − ,Evans et al. 2019) and no radio counterpart was detectedwith the Australian Square Kilometre Array Pathfinder(ASKAP) ( S < µ Jy; Dobie et al. 2019) andKarl G. Jansky Very Large Array (VLA) ( S < µ Jy; Mooley et al. 2019). Palmese et al. (2019) re-ported a possible archival detection in DES data, ques-tioning the transient nature of DG19wxnjc/AT2019npv.Annis et al. (2019) produced precise photometry ob-tained with nightly stacks of DECam data, indicatingthe transient to be reddening at a rate ∆( i − z ) ∼ .
05 mag day − . The transient was also monitoredphotometrically with LCO and LT (see Coughlin et al.2019a, and data behind Figure 3), which produced de-tections in the r, i and z bands and a marginal detection g (cid:38) . J band, and did not detect the source to a5 σ limit of 21.4 AB mag, although we caution that thephotometry is contaminated by host galaxy light.We obtained one NIR spectrum of DG19wxnjc withNIRES on the Keck II telescope on 2019 August 24(De et al. 2019c,b). We acquired two sets of ditheredABBA exposures on the transient location for a total ex-posure time of 40 mins. The telluric standard HIP 7202was used for flux calibration. The reduced and stackedspectra showed a largely featureless continuum between1.0 and 2.5 µ m (Figure 2) along with a prominent P-Cygni profile near 1.08 µ m with an absorption velocityof ≈ − . This feature is consistent with He Iat the redshift of the host galaxy, in addition to a weakhint for another He I feature at 2.05 µ m, confirming theclassification of this source as a Type Ib/c supernovaand unrelated to S190814bv. Gomez et al. (2019) con-firmed the SN Ib classification using the IMACS opticalspectrograph on the Magellan telescope. desgw-190814j/AT2019nxe — The candidate was an-nounced by Soares-Santos et al. (2019c) and wasindependently detected with our image-subtractionpipeline on multiple z -band epochs with internal nameDG19zcrpc. The photometric redshift of the host is z = 0 . ± . g -band evolution be-tween 2019-08-22 and 2019-08-25 and color r − i (cid:39) z = 0 . ± . DG19rzhoc/AT2019num — We identified this candidatein DECam data (Goldstein et al. 2019a) and it wasindependently confirmed in the same dataset (Herneret al. 2019a), in images taken with the Reionizationand Transients Infrared Camera (RATIR ) on the 1.5mHarold Johnson Telescope at the Observatorio Astro-nomico Nacional on Sierra San Pedro Martir (Dichiaraet al. 2019a), and in VLT Survey Telescope (VST) im-ages (Yang et al. 2019). We performed photometricfollow-up with LCO and LT (data behind Figure 3)which revealed the transient to be slowly evolving onday timescales. A Swift /XRT upper limit was placedat
F < . × − erg cm − s − (Evans et al. 2019).We obtained one epoch of P200/WIRC imaging of thesource in J band and did not detect the source to a 5 σ limit of 21.4 AB mag, although we caution that the tran-sient location is contaminated heavily with host galaxylight. DG19rzhoc/AT2019num was spectroscopicallyclassified as a Type II SN at redshift z = 0 .
113 usingthe Goodman High Throughput Spectrograph (GHTS)on the 4.1m Southern Astrophysical Research (SOAR)telescope (Tucker et al. 2019b).
PS19epf/AT2019noq — The candidate was identified withthe Pan-STARRS1 telescope and reported on 2019-08-15 (Huber et al. 2019). We independently detectedPS19epf/AT2019noq in DECam data starting on 2019-08-15 06:44:29 with internal name DG19lsugc. The tran-sient was classified as SN II at redshift z = 0 .
07 usingSOAR/GHTS (Rodr´ıguez et al. 2019). A pre-detectionof the transient in ZTF data 2 weeks before the GWevent further excluded its association with S190814bv.
DG19wgmjc/AT2019npw — This candidate was discov-ered in DECam data (Andreoni et al. 2019) and flaggedas a high priority target because of the photometric red-shift of the putative host z = 0 . ± .
054 being com-patible with the distance of S190814bv (Goldstein et al.2019b). The transient was confirmed with optical ob-servations with other telescopes such as the DiscoveryChannel Telescope (DCT; Dichiara et al. 2019b,c), VST(Yang et al. 2019), and with our P200/WIRC imaging ROWTH on S190814bv
Swift /XRT upper limitwas placed at
F < . × − erg cm − s − (Evanset al. 2019). DG19wgmjc/AT2019npw was eventuallyclassified as a Type IIb SN at redshift z = 0 .
163 usingSOAR/GHTS (Tucker et al. 2019b).
DG19sbzkc/AT2019ntr — We initially identified this can-didate in DECam data (Goldstein et al. 2019a) and thedetection was confirmed using RATIR (Dichiara et al.2019a) and VST (Yang et al. 2019). We note thatthis transient did not pass the stricter selection criteriaadopted in this work (see Section 4.2) because its loca-tion was visited 5 times, less than the 10-visit thresholdthat we imposed. DG19sbzkc/AT2019ntr was spectro-scopically classified as a SN II at redshift z = 0 . desgw-190814q/AT2019obc — The candidate was foundand announced by the DESGW team (Soares-Santoset al. 2019d) and we independently detected it withour automatic pipeline from 2019-08-16 05:56:59 withinternal name DG19lkunc. Our DECam photometryusing Pan-STARRS1 templates was consistent with aflat evolution until 2019-08-21 (Fremling et al. 2019).We acquired P200/WIRC near-infrared imaging in Ksband on MJD 58 717.492 and the transient was not de-tected down to a 5 σ limit of Ks > .
72 AB magnitude.desgw-190814q/AT2019obc was classified as a SN Ia fewdays past its peak at redshift z = 0 . ± .
005 usingGTC/OSIRIS (Castro-Tirado et al. 2019).
ZTF19abkfmjp/SN2019mbq — The transient was discov-ered with ZTF on 2019-07-30 (Nordin et al. 2019), beforeS190814bv, and it was classified as a SN II at redshift z = 0 . ± .
013 with the SED Machine (Blagorodnovaet al. 2018) on the 60-inch telescope at Palomar Obser-vatory. We automatically found the transient (dubbedDG19fcmgc) in DECam data and it was also reported bytwo other groups via GCN (Soares-Santos et al. 2019a;Yang et al. 2019). Given the pre-detection with ZTFand the SN classification, ZTF19abkfmjp/SN2019mbqcannot be associated with S190814bv.
DG19gxuqc/AT2019paa — We obtained a spectrum ofthis nuclear candidate with Keck/LRIS. The spectrumwas host-dominated, with common emission lines fromthe galaxy that allowed us to place the host at redshift z = 0 . − . This limit was adopted based on the photometric evo-lution of GW170817, the best-studied kilonova to date.GW170817 faded faster (almost 2 mag in g in 24 hours)and reddened faster (from g − z = − . . z = 0 . ± .
07 passed ourselection because we considered twice the uncertaintieson photometric redshifts, however its large value sug-gests that the host galaxy is well beyond the distancerange of interest.Three DECam candidates DG19zoonc/AT2019nyy,DG19gyvx/AT2019thm, and DG19ggesc/AT2019pawlie within a 20 (cid:48)(cid:48) radius from galaxies with photometricredshift compatible with S190814bv, however underly-ing galaxies at larger redshifts are most likely their host.DG19ggesc/AT2019paw is also coincident with a redstellar source detected with VISTA (Greggio et al. 2014).Therefore we exclude that DG19zoonc/AT2019nyy,DG19gyvx/AT2019thm, or DG19ggesc/AT2019paw areassociated with S190814bv.Two objects labelled desgw-190814a/AT2019nmd anddesgw-190814b/AT2019nme were reported as transientspossibly associated with S190814bv (Soares-Santos et al.2019a). These candidates were followed up with severaltelescopes whose observations resulted in non-detections(McBrien et al. 2019; Huber et al. 2019; Belkin et al.2019; Evans et al. 2019; Corre et al. 2019). Querying0
Andreoni & Goldstein and the GROWTH Collaboration the IAU Minor Planet Center, we found that desgw-190814a/AT2019nmd is consistent with the known as-teroid (297025) 2010 GA33 (De et al. 2019a). Inspec-tion of DECam images allowed us to show that desgw-190814b/AT2019nme is a Solar System fast movingobject (Goldstein et al. 2019) absent from the MinorPlanet Center database. In conclusion, both desgw-190814a/AT2019nmd and desgw-190814b/AT2019nmewere moving objects unrelated to S190814bv.The transient labelled desgw-190814d/AT2019nqr wasfirst reported by (Herner et al. 2019b). The candidatewas detected twice with our automated pipeline (inter-nal name DG19pihic), but the two detections occurredonly 2.2 minutes apart on 2019-08-16, too close in timeto pass our selection criteria of >
30 minutes between thefirst and last detection. desgw-190814d/AT2019nqr waslater classified as a SN IIb using SOAR/GHTS (Tuckeret al. 2019a).The candidate desgw-190814c/AT2019nqq (Herneret al. 2019b; Tucker et al. 2019a; Goldstein et al. 2019b;De et al. 2019d; Herner et al. 2019a; D’Avanzo et al.2019; Dichiara et al. 2019a) was automatically de-tected with our pipeline (dubbed DG19kxqic), but itwas not included in Table 2 because it lies outsidethe LALInference 95% probability area of S190814bv.desgw-190814c/AT2019nqq was classified as SN II at z = 0 . ± .
001 using GTC/OSIRIS (Lopez-Cruzet al. 2019b).Follow-up observations were performed also for thecandidates named desgw-190814f/AT2019nte (Herneret al. 2019a) and desgw-190814r/AT2019odc (Soares- Santos et al. 2019b). The redshifts of their putativehost galaxies were fixed to z = 0 . ± .
001 for desgw-190814r/AT2019odc (as part of our GTC/OSIRIS ob-servations, Hu et al. 2019) and z = 0 . z = 0 . z = 0 . ± . z = 0 . Table 2 . Subset of candidates discovered or independently detected by the DECam-GROWTH team during the follow-up of S190814bvthat were spectroscopically classified. None of them is a viable optical counterpart to S190814bv. The reported candidates passed theselection criteria described in Section 4.2. Specifically, they lie within the 95% probability region of the LALInference skymap and arewithin 20 (cid:48)(cid:48) from galaxies whose redshifts (2 σ uncertainty) are compatible with the LIGO/Virgo distance (2 σ ). All the transients reportedin this table were detected using the image-subtraction pipeline described in Section 4. [ ∗ ] We note that DG19sbzkc was observed with <
10 visits and was added to this table for completeness.
Name IAU Name RA [deg] Decl. [deg] Offset [ (cid:48)(cid:48) ] spec- z Classification ReferencesDG19qabkc AT2019nqc 22 . − . ± Table 2 continued
ROWTH on S190814bv Table 2 (continued)
Name IAU Name RA [deg] Decl. [deg] Offset [ (cid:48)(cid:48) ] spec- z Classification ReferencesDG19wxnjc AT2019npv 13 . − . . − . . − . . − . − . − . − . ± − . . ± .
013 SN II Soares-Santos et al. (2019a); Yanget al. (2019)
Table 3 . Additional candidates discovered during the follow-up of S190814bv whose host galaxy redshift is compatible with theLIGO/Virgo distance (2 σ ). These candidates are ruled out based on photometric evolution. DG19tedsc was detected for the firsttime on 2019-08-21 in i band, which suggests a slow evolution. The reported candidates passed the selection criteria described inSection 4.2. Name IAU Name RA [deg] Decl. [deg] Offset [ (cid:48)(cid:48) ] z phot σ z (cid:104) m i − m z (cid:105) (cid:104) ˙ m i (cid:105) [mag/day] (cid:104) ˙ m z (cid:105) [mag/day]DG19aferc AT2019tig 14 . − . .
08 0 .
074 0 . · · · − . · · · DG19gxuqc AT2019paa 13 . − . .
40 0 .
116 0 .
06 0 . − .
09 0 . Table 3 continued Andreoni & Goldstein and the GROWTH Collaboration
Table 3 (continued)
Name IAU Name RA [deg] Decl. [deg] Offset [ (cid:48)(cid:48) ] z phot σ z (cid:104) m i − m z (cid:105) (cid:104) ˙ m i (cid:105) [mag/day] (cid:104) ˙ m z (cid:105) [mag/day]PS19ekf a AT2019nbp 11 . − . .
42 0 .
102 0 . − .
18 0 .
01 0 . . − . .
25 0 .
074 0 .
12 0 . − .
01 0 . . − . .
41 0 .
217 0 . · · · . · · · DG19kpykc AT2019nul 13 . − . .
44 0 .
095 0 . − . − . − . . − . .
21 0 .
055 0 . · · · · · · · · · DG19wynuc AT2019tij 12 . − . .
32 0 .
157 0 .
11 0 . − . − . . − . .
70 0 .
218 0 . · · · . · · · DG19bown AT2019tix 12 . − . .
27 0 .
190 0 . − .
14 0 .
02 0 . . − . .
47 0 .
285 0 .
13 0 .
23 0 .
00 0 . . − . .
52 0 .
212 0 . · · · · · · − . . − . .
73 0 .
233 0 . · · · · · · a ] DG19hcsgc, with Pan-STARRS1 pre-discovery on 2019-08-09 GALAXY CATALOG COMPLETENESSThe completeness of a synoptic follow-up campaignsuch as the one conducted with DECam for S190814bv ismainly limited by the area covered and the efficiency ofthe transient detection pipeline. Once these two quanti-ties are set, the ability to detect an EM counterpart be-comes flux limited. Given a mean detection limit of 21.7mag (Table 1), we were able to find transients with abso-lute magnitude M ≤ − . D = 163 Mpc, M ≤ − . D = 267 Mpc, and M ≤ − . D = 371 Mpc.Several publications (for example, Nissanke et al.2013; Singer et al. 2016; Gehrels et al. 2016) advocatethat galaxy-targeted follow-up of GW triggers can bevery effective when the event occurs within tens of mega-parsecs. The discovery of AT2017gfo (the optical coun-terpart to GW170817) using a galaxy-targeted strategyis an example of success of this approach at a distanceof 41 Mpc (Coulter et al. 2017). However, at distancesbeyond ∼
200 Mpc, galaxy-targeted searches becomemore challenging. Gomez et al. (2019) used the Mag-ellan telescope to observe galaxies possibly hosting theS190814bv merger. In their work, Gomez et al. (2019)imaged 96 galaxies at 3 σ magnitude limit i < .
2, cor-responding to M i = − . ≥ . L ∗ .The analysis presented in this paper took advantageof photometric redshifts calculated from Legacy Sur-veys and WISE photometry mainly to exclude from oursample those candidates likely associated with galaxiessignificantly outside the distance range of S190814bv.Astrophysical transients with no clear association to ahost galaxy were not excluded a priori , but their pho-tometric evolution was not rapid enough for them to be considered likely counterparts to S190814bv. Nev-ertheless, we estimate the completeness that we couldreach considering only a sample of transients found inthe proximity of galaxies present in the photometric red-shift catalog. Assuming a conservative limit of B = 21,we obtain a completeness >
97% based on the luminosityfraction given a Schechter luminosity function (Gehrelset al. 2016) in the 2 σ distance range of S190814bv (Fig-ure 4). Although it is likely that the z <
21 excludesa large number of small, faint galaxies with z >
21, weare still nearly complete in luminosity. We note that z -band luminosity is a much better proxy for stellar massthan luminosity in bluer bands such as B , such that thespread in z band mass-to-light ratio is smaller than therange of B / z flux ratios amongst galaxies. As a result,the stellar mass completeness of the z <
21 subset ofDECaLS would be expected to be at least as high as theconservative B luminosity completeness estimated here. DISCUSSIONThe results presented in Section 5 show that no viablecounterpart to S190814bv was discovered. In this sec-tion we discuss the constraints that this non-detectionplaces on the astrophysical properties of the merger ifthe candidate was originally a neutron star–black hole.7.1.
Kilonova models
We used the upper limits obtained with DECam andkilonova simulations to constrain the parameter spaceof the possible EM counterpart to S190814bv. Specif-ically, we consider the kilonova models developed byBulla (2019) and Hotokezaka & Nakar (2019).We first compare DECam limits to 2D kilonova mod-els computed with the Monte Carlo radiative trans-fer code possis (Bulla 2019). These models assumea two-component ejecta geometry, with a lanthanide-rich component distributed around the equatorial plane
ROWTH on S190814bv .
50 0 .
55 0 .
60 0 .
65 0 .
70 0 . Rest wavelength ( µ m) S c a l e d F λ DG 19qabkc / GTC – SN II DG 19zcrpc / GTC – SN IaDG 19lkunc / GTC – SN IaDG 19qabkc / SALT – SN II . . . . . . . Rest wavelength ( µ m) S c a l e d F λ DG 19wxnjc / Keck II – SN Ib/c
Figure 2.
Top panel –
Optical spectroscopic follow-up of candidates found in the localization region. The black lines correspondto binned versions of the unbinned reduced spectra shown in gray. GTC / OSIRIS and SALT spectra of DG 19qabkc show astrong P-Cygni H α line suggesting a Type II supernova at z = 0 .
08. GTC / OSIRIS spectra of DG19lkunc and DG19zcrpc areconsistent with SNe Ia at z = 0 .
21 and z = 0 .
08 respectively.
Lower panel –
Near-infrared spectrum of DG19wxnjc obtainedwith Keck II / NIRES. The spectrum shows a prominent P-Cygni feature at ≈ . µ m, consistent with He I with an absorptionvelocity of 7000 km s − , classifying this source as a Type Ib/c supernova. These classifications rule out associations of thesesources to S190814bv. Andreoni & Goldstein and the GROWTH Collaboration m [ A B ] DG19qabkcAT2019nqc z = 0.077SN II 20.521.021.522.022.523.0 m [ A B ] DG19wxnjcAT2019npv z = 0.056SN Ibc20212223 m [ A B ] desgw-190814jAT2019nxe z = 0.0777SN Ia 20.521.021.522.022.523.0 m [ A B ] DG19rzhocAT2019num z = 0.113SN II0 5 10 15Time since Merger [days]19.819.920.020.120.220.320.4 m [ A B ] PS19epfAT2019noq z = 0.07SN II 0 5 10 15Time since Merger [days] 20212223 m [ A B ] DG19wgmjcAT2019npw z = 0.163SN IIbLCO/gLT/IO:O/gDECam/i LCO/iLT/IO:O/iLCO/r LT/IO:O/rDECam/zLT/IO:O/z Figure 3.
Light curves of the first six candidates presented in Table 2. The LCO and LT photometry upper limits are quotedto 3 σ , while DECam upper limits are quoted to 5 σ . P200/WIRC photometry was not plotted because it is host contaminated.Photometry for all candidates in Tables 2 and 3 is available in machine-readable form online (data behind the figure). We notethat absolute magnitudes were not K corrected and must be considered to be indicative values. and characterized by an half-opening angle φ and alanthanide-poor component at higher latitudes (see Fig-ure 1 in Bulla et al. 2019). Radiative transfer calcula-tions are then performed to predict spectral time seriesfor 11 different viewing angles, from which broad-bandlight-curves can be easily extracted. For our analysis, wechoose φ = 15 ◦ and φ = 30 ◦ guided by numerical sim-ulations (Kawaguchi et al. 2016; Fern´andez et al. 2017) and calculate light curves for ejecta masses M ej between0.01 and 0 . M (cid:12) (step size 0.01 M (cid:12) ).The top panels of Figure 5 show which modelledlight curves are ruled out by DECam i − band (left) and z − band (right) limits for different distance assumptions(215, 267 and 319 Mpc from light to dark blue). Asexpected, more models are brighter than the limits andthus ruled out at closer compared to farther distances. ROWTH on S190814bv F r a c t i o n o f l u m i n o s i t y i n c l u d e d B = 15.5 limitB = 21 limitS190814bv0 50 100 150 200 250 300 350 400Luminosity Distance (Mpc)242220181614 M B B = 15.5 limitB = 21 limitS190814bv
Figure 4.
Luminosity fraction ( upper panel ) and B -bandabsolute magnitude limit ( lower panel ) as a function of lu-minosity distance. These quantities were estimated for theLegacy Surveys DR8 photometric redshifts (orange line) aswell as for the Galaxy List for the Advanced Detector Era(GLADE, D´alya et al. 2018, blue line) assuming the catalogto be complete for B <
21 and
B < . σ distance intervalfor S190814bv (The LIGO Scientific Collaboration and theVirgo Collaboration 2019b) is delimited by the cyan-coloreddashed lines. Interestingly, we find that the most constraining limit isthe z − band point at 3.4 days ( z =22.3 mag), with all theother limits bringing no improvement in terms of rulingout models. We note that comparable deep limits at ear-lier epochs, when the kilonova is intrinsically brighter,would have been extremely important to constrain theparameter space more strongly.The bottom panels of Figure 5 show what region ofthe M ej - viewing angle parameter space is ruled out for φ = 15 ◦ (left) and φ = 30 ◦ (right). The brightest kilo-novae in the modelled grid are predicted at high M ej and for polar viewing angles (system viewed face-on, θ obs = 0 and cos θ obs = 1). These models are there-fore the first to be ruled out by DECam limits (upper- right corner in the M ej - viewing angle parameter space).Stronger constraints are found for closer distances (seeabove) and smaller φ angles as the larger contribution ofthe lanthanide-poor compared to lanthanide-rich com-ponent leads to an intrinsically brighter kilonova. Wenote that the best-fit model to GW170817 in this grid( M ej = 0 . M (cid:12) , φ = 30 ◦ and cos θ obs = 0 .
9, Dhawanet al. 2019) would be slightly fainter and thus hiddenbelow DECam limits at 267 Mpc. To summarise, ejectamasses are constrained to M ej < . M (cid:12) in the mostoptimistic case assuming the nearest consistent distanceof 215 Mpc, φ = 15 ◦ and cos θ obs = 1 (face-on). Amore conservative constraint ( M ej (cid:46) . M (cid:12) ) is in-stead found for farther distances, viewing angles closerto the equatorial plane and larger φ values.Figure 6 presents upper limits on the ejecta mass ob-tained using a different approach. We assume a spher-ical ejecta with a power-law density profile ρ ∝ v − n for v min < v < v max and calculate the emission usingthe heating rate formalism and light curve modeling de-scribed in Hotokezaka & Nakar (2019). The outflow pa-rameters are v min = 0 . v max = 0 .
4c and n = 4 .
5. Thecomposition that we consider is of r-process elementswith atomic mass 85 ≤ A ≤
209 and a solar abundancepattern. The heating-rate calculation includes only β -decay. We assume further that the entire ejecta can becharacterised by a single grey opacity parameter κ andvary the value of κ . The shaded regions in the M ej - κ space in Figure 6 are where the light curve is brighterthan the upper limits we have for this event. The conclu-sion from this figure is that the ejecta cannot have morethan ∼ . M (cid:12) of ejecta that is not lanthanide rich ata distance of 267 Mpc, or ∼ . M (cid:12) at an optimisticdistance of 215 Mpc. This conclusion is in agreementwith the results obtained with the Bulla (2019) kilonovamodels under favorable ( θ (cid:46) ◦ ) viewing angles.7.2. Constraints on the merging binary
At present, constraints on the amount of mass ejectedby the merger can be translated into approximate con-straints on the initial parameters of the possible merg-ing neutron star and black hole (see Coughlin et al.2019c for a summary of other events during O3a). Asa proof of principle, given the mass ratio of the binary Q = M BH /M NS , the dimensionless component of theinitial black hole spin aligned with the orbital angularmomentum ( χ aligned ), and the compactness of the neu-tron star C NS = GM NS / ( R NS c ), we can conservativelyassume that: M ej (cid:38) M dyn + 0 . M out − M dyn ) , where M out represents the mass that remains outsideof the black hole after merger (Foucart 2012; Foucart6 Andreoni & Goldstein and the GROWTH Collaboration . . . . . . . . . . . cos θ obs . . . . . . . . . . M e j ( M (cid:12) ) A ll o w e d r e g i o n φ = 15 ◦
215 Mpc267 Mpc319 Mpc . . . . . . . . . . . cos θ obs . . . . . . . . . . M e j ( M (cid:12) ) A ll o w e d r e g i o n φ = 30 ◦
215 Mpc267 Mpc319 Mpc θ obs relative to face-on (deg) θ obs relative to face-on (deg) Time since merger (days) i ( m ag ) Ruled out models φ = 15 ◦ , ◦
215 Mpc267 Mpc319 Mpc
Time since merger (days) z ( m ag ) Ruled out models φ = 15 ◦ , ◦
215 Mpc267 Mpc319 Mpc
Figure 5.
The limits obtained with DECam observations excluded regions of the parameter space for given kilonova models. Inthis figure we consider models obtained with the Monte Carlo radiative transfer code possis (Bulla 2019) whose key parametersare the viewing angle θ obs , the half-opening angle of an equatorial lanthanide-rich component φ , and the ejecta mass M ej . Top – i (left) and z (right) band light curves of kilonovae ruled out using the multi-band DECam upper limits (Table 1), here markedwith triangles. Bottom –
Using the multi-band DECam upper limits, regions of the ejecta mass and viewing angle parameterspace can be ruled out using φ = 15 ◦ (left) and φ = 30 ◦ (right). The best-fit model to GW170817 in this grid ( M ej = 0 . M (cid:12) , φ = 30 ◦ and cos θ obs = 0 .
9, Dhawan et al. 2019) is marked with a yellow star in the right panel. Both the top and bottom plotsshow that constraints on the models are more stringent if lower distances to S190814bv are considered. Here we used distancesof 319 Mpc (dark blue patches), 267 Mpc (light blue patches), and 215 Mpc (cyan patches). (cm /g) M e j ( M ) A l l o w e d r e g i o n Figure 6.
Constraints on the ejecta mass ( M ej ) and opac-ity ( κ ) phase space obtained using multi-band DECam up-per limits (Table 1) and the kilonova models described inHotokezaka & Nakar (2019). Similarly to Figure 5, the con-straints become more significant assuming lower distances tothe merger. et al. 2018), and M dyn denotes the mass ejected dur-ing disruption (Kawaguchi et al. 2016). Both M out and M dyn are predictions from semi-analytical fits to the re-sults of merger simulations. In the above, we have alsoconservatively assumed that (cid:38)
10% of the matter ini-tially bound in an accretion disk around the remnantblack hole will be ejected in magnetically-driven and/orneutrino-driven winds, and during viscous expansion ofthe disk (Fernandez & Metzger 2013; Fern´andez et al.2015; Siegel & Metzger 2017; Christie et al. 2019). In Figure 7, we show constraints on the parame-ter space of NSBH binaries assuming M ej = 0 . M (cid:12) . For low mass black holes leaving remnants comparable to theinitial conditions of existing 3D simulations, (cid:38)
25% of the disk ismost likely ejected, but more compact disks around massive blackholes eject a smaller fraction of their disk. We do not show the conservative case of M ej < . M (cid:12) as itdoes not provide meaningful constraints on the parameter space of ROWTH on S190814bv Figure 7.
Constraints on the parameter space of black hole-neutron star binaries assuming M ej < .
03 M (cid:12) . We showthe highest possible value of the component of the blackhole spin aligned with the orbital momentum as a func-tion of mass ratio Q = M BH /M NS and tidal deformabilityΛ NS of the neutron star. The figure obtained assumes that M NS = 1 . M (cid:12) , but as the ejected mass is approximatelyproportional to M NS (at fixed Λ NS ), any choice in the range M NS ∼ (1 . − . M (cid:12) would give qualitatively similar con-straints. Practically, an upper bound on M ej can be interpretedas a maximum possible value of χ aligned for each choice ofmass ratio and dimensionless neutron star deformability( Q, Λ NS ), or an upper bound on Λ NS at fixed χ aligned .Λ NS is related to the neutron star EOS, and is givenby Λ NS = λc G M , where λ = 2 k R / (3 G ) and k isthe Love number (see e.g., Flanagan & Hinderer 2008;Hinderer et al. 2010).Assuming Λ ∼
800 (the largest value allowed byGW170817 for a 1.4 M (cid:12)
NS, Abbott et al. 2017) and M ej < .
03 M (cid:12) , the data constrain the BH spin compo-nent aligned with the orbital momentum to be χ < . Q <
6. This constraint becomes looserfor more compact stars (lower Λ), and tighter for lesscompact stars (larger Λ). CONCLUSIONIn this paper, we have presented deep synoptic lim-its on the optical counterpart to the NSBH mergerS190814bv by analyzing publicly available data froma DECam imaging campaign. We identified dozens ofcounterpart candidates, and systematically ruled each the binary because the fitting formulae are not properly calibratedabove χ aligned = 0 . of them out using the results of a global follow-up cam-paign undertaken by our group and the community.Real-time data analysis and prompt follow-up allowedthe candidates to be classified at timescales from hoursto days. Based on our lack of identification of an opti-cal counterpart, we used our detection limits and kilo-nova models to constrain the allowable parameter spacefor S190814bv. We found that the ejecta mass can bepoorly constrained at the far end of the distance prob-ability distribution, however limits on the ejecta massof M ej (cid:46) .
05 M (cid:12) can be placed at a luminosity dis-tance of 267 Mpc at polar viewing angles or assumingan opacity κ < g − . A more stringent limit of M ej (cid:46) .
03 M (cid:12) can be placed assuming a distance of215 Mpc. Using the constrains that we obtained forthe ejecta mass, we showed how the phase space of theNSBH binary system can also be constrained. In par-ticular, reliable constraints on the highest possible valueof the BH spin component aligned with the orbital mo-mentum as a function of Q and Λ NS can be placed for M ej < .
03 M (cid:12) . For example, assuming a tidal deforma-bility at the high end of the range allowed by gravita-tional wave observations of GW170817, we can constrainthe spin component to be χ < . Q <
Andreoni & Goldstein and the GROWTH Collaboration
Stockholm University (Sweden), Humboldt University(Germany), Liverpool John Moores University (UK)and University of Sydney (Australia).Daniel A. Goldstein acknowledges support from Hub-ble Fellowship grant HST-HF2-51408.001-A. Supportfor Program number HST-HF2-51408.001-A is pro-vided by NASA through a grant from the SpaceTelescope Science Institute, which is operated bythe Association of Universities for Research in As-tronomy, Incorporated, under NASA contract NAS5-26555. We gratefully acknowledge Amazon Web Ser-vices, Inc. for a generous grant (
PS IK FY2019 Q3Caltech Gravitational Wave ) that funded our useof the Amazon Web Services cloud computing infras-tructure to process the DECam data. P. E. Nugentacknowledges support from the DOE through DE-FOA-0001088, Analytical Modeling for Extreme-Scale Com-puting Environments. D. A. Perley and D. A. Goldsteinperformed the work associated with this project at theAspen Center for Physics which is supported by Na-tional Science Foundation grant PHY-1607611. Thiswork was partially supported by a grant from the Si-mons Foundation. AJCT thanks I. Agudo, J. Cepa, V.Dhillon, J. A. Font, A. Martin-Carrillo, S. R. Oates,S. B. Pandey, E. Pian, R. Sanchez-Ramirez, A. M.Sintes, V. Sokolov and B.-B. Zhang for fruitful conversa-tions. F. Foucart gratefully acknowledges support fromNASA through grant 80NSSC18K0565 and from theNSF through grant PHY1806278. M. Bulla, A. Goobar,E. Kool, S. Dhawan, and J. Sollerman acknowledge sup-port from the G.R.E.A.T research environment fundedby the Swedish National Science Foundation. J. Soller-man acknowledges support from the Knut and AliceWallenberg Foundation. J. S. Bloom and K. Zhang arepartially supported by a Gordon and Betty Moore Foun-dation Data-Driven Discovery grant. D. A. H. Buckleyacknowledges research support from the National Re-search Foundation of South Africa. M. W. Coughlinis supported by the David and Ellen Lee PostdoctoralFellowship at the California Institute of Technology.S. Nissanke and G. Raaijmakers are grateful for supportfrom VIDI, Projectruimte and TOP Grants of the Inno-vational Research Incentives Scheme (Vernieuwingsim-puls) financed by the Netherlands Organization for Sci-entific Research (NWO). H. Kumar and K. Zhang thankthe LSSTC Data Science Fellowship Program, which isfunded by LSSTC, NSF Cybertraining Grant ). The original description ofthe VizieR service was published in A&AS 143, 23.This project used data obtained with the Dark En-ergy Camera (DECam), which was constructed bythe Dark Energy Survey (DES) collaborating insti-tutions: Argonne National Lab, University of Cali-fornia Santa Cruz, University of Cambridge, Centrode Investigaciones Energeticas, Medioambientales yTecnologicas-Madrid, University of Chicago, Univer-sity College London, DES-Brazil consortium, Univer-sity of Edinburgh, ETH-Zurich, University of Illinoisat Urbana-Champaign, Institut de Ciencies de l’Espai,Institut de Fisica d’Altes Energies, Lawrence BerkeleyNational Lab, Ludwig-Maximilians Universitat, Univer-sity of Michigan, National Optical Astronomy Observa-tory, University of Nottingham, Ohio State University,University of Pennsylvania, University of Portsmouth,SLAC National Lab, Stanford University, Universityof Sussex, and Texas A&M University. Funding forDES, including DECam, has been provided by the U.S.Department of Energy, National Science Foundation,Ministry of Education and Science (Spain), Scienceand Technology Facilities Council (UK), Higher Edu-cation Funding Council (England), National Center forSupercomputing Applications, Kavli Institute for Cos-mological Physics, Financiadora de Estudos e Projetos,Funda¸c˜ao Carlos Chagas Filho de Amparo a Pesquisa,
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Abadie, J., Abbott, B. P., Abbott, R., et al. 2010, Classicaland Quantum Gravity, 27, 173001Abbott, B., et al. 2016, Physical Review Letters, 116,061102—. 2017, Phys. Rev. Lett., 119, 161101Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2019,Physical Review X, 9, 031040Andreoni, I., Goldstein, D. A., Ahumada, T., et al. 2019,GCN, 25362Annis, J., Herner, K., & Soares-Santos, M. 2019, GCN,25458Barbieri, C., Salafia, O. S., Perego, A., Colpi, M., &Ghirlanda, G. 2019, A&A, 625, A152Becker, A. 2015, HOTPANTS: High Order Transform ofPSF ANd Template Subtraction, ascl:1504.004Belkin, S., Pozanenko, A., Inasaridze, R. Y., et al. 2019,GCN, 25392Blagorodnova, N., Neill, J. D., Walters, R., et al. 2018,PASP, 130, 035003Blondin, S., & Tonry, J. L. 2007, ApJ, 666, 1024Bruun, S. H., Sagues Carracedo, A., Chen, T. W., et al.2019, GCN, 25384Buckley, D., Ciroi, S., Gromadzski, M., et al. 2019, GCN,25481Buckley, D. A. H., Swart, G. P., & Meiring, J. G. 2006, inProc. SPIE, Vol. 6267, 62670ZBuckley, D. A. H., Andreoni, I., Barway, S., et al. 2018,MNRAS, 474, L71Bulla, M. 2019, MNRAS, 489, 5037Bulla, M., Covino, S., Kyutoku, K., et al. 2019, NatureAstronomy, 3, 99Burgh, E. B., Nordsieck, K. H., Kobulnicky, H. A., et al.2003, in Proc. SPIE, Vol. 4841, Instrument Design andPerformance for Optical/Infrared Ground-basedTelescopes, ed. M. Iye & A. F. M. Moorwood, 1463–1471 Burrows, D. N., Hill, J. E., Nousek, J. A., et al. 2005,SSRv, 120, 165Castro-Tirado, A. J., Valeev, A. F., Hu, Y. D., et al. 2019,GCN, 25543Cepa, J., Aguiar, M., Escalera, V. G., et al. 2000, in Societyof Photo-Optical Instrumentation Engineers (SPIE)Conference Series, Vol. 4008, Proc. SPIE, ed. M. Iye &A. F. Moorwood, 623–631Chambers, K. C., Magnier, E. A., Metcalfe, N., et al. 2016,arXiv e-prints, arXiv:1612.05560Chen, T. W., Schweyer, T., Nicuesa Guelbenzu, A., et al.2019, GCN, 25457Christie, I. M., Lalakos, A., Tchekhovskoy, A., et al. 2019,MNRAS, 490, 4811Colless, M., Dalton, G., Maddox, S., et al. 2001, MNRAS,328, 1039Corre, D., Blazek, M., Klotz, A., et al. 2019, GCN, 25599Corwin, Jr., H. G., Buta, R. J., & de Vaucouleurs, G. 1994,AJ, 108, 2128Coughlin, M., Ahumada, T., & Anand, S. 2019a, GCN,25477Coughlin, M. W., Dietrich, T., Margalit, B., & Metzger,B. D. 2019b, Monthly Notices of the Royal AstronomicalSociety: Letters, 489, L91Coughlin, M. W., Dietrich, T., Antier, S., et al. 2019c,arXiv:1910.11246Coulter, D. A., Foley, R. J., Kilpatrick, C. D., et al. 2017,Science, 358, 1556Cowperthwaite, P. S., Berger, E., Villar, V. A., et al. 2017,ApJ, 848, L17Crawford, S. M., Still, M., Schellart, P., et al. 2010, inProc. SPIE, Vol. 7737, Observatory Operations:Strategies, Processes, and Systems III, 773725Cushing, M. C., Vacca, W. D., & Rayner, J. T. 2004,PASP, 116, 362 Andreoni & Goldstein and the GROWTH Collaboration
D´alya, G., Galg´oczi, G., Dobos, L., et al. 2018, MNRAS,479, 2374Dark Energy Survey Collaboration, Abbott, T., Abdalla,F. B., et al. 2016, MNRAS, 460, 1270D’Avanzo, P., Giunta, A., Izzo, L., et al. 2019, GCN, 25401De, K., Goldstein, D., Andreoni, I., et al. 2019a, GCN,25348De, K., Jencson, J., Kasliwal, M. M., Goldstein, D., &Andreoni, I. 2019b, GCN, 25478De, K., Kasliwal, M. M., Andreoni, I., Karambelkar, V., &Sharma, Y. 2019c, GCN, 25461De, K., Tinyanont, S., Kamraj, N., et al. 2019d, GCN,25396de Vaucouleurs, G., de Vaucouleurs, A., Corwin, Jr., H. G.,et al. 1991, Third Reference Catalogue of BrightGalaxies. Volume I: Explanations and references. VolumeII: Data for galaxies between 0 h and 12 h . Volume III:Data for galaxies between 12 h and 24 h .Dey, A., Schlegel, D. J., Lang, D., et al. 2019, AJ, 157, 168Dhawan, S., Bulla, M., Goobar, A., Sagu´es Carracedo, A.,& Setzer, C. N. 2019, arXiv e-prints, arXiv:1909.13810Dichiara, S., Troja, E., Watson, A. M., et al. 2019a, GCN,25416Dichiara, S., Troja, E., Cenko, S. B., et al. 2019b, GCN,25374—. 2019c, GCN, 25397Dobie, D., Murphy, T., Lenc, E., et al. 2019, GCN, 25472Drout, M. R., Piro, A. L., Shappee, B. J., et al. 2017,Science, 358, 1570Evans, P. A., Kennea, J. A., Tohuvavohu, A., et al. 2019,GCN, 25400Fern´andez, R., Foucart, F., Kasen, D., et al. 2017, Classicaland Quantum Gravity, 34, 154001Fernandez, R., & Metzger, B. D. 2013, Mon. Not. Roy.Astron. Soc., 435, 502Fern´andez, R., & Metzger, B. D. 2016, Annual Review ofNuclear and Particle Science, 66, 23Fern´andez, R., Tchekhovskoy, A., Quataert, E., Foucart,F., & Kasen, D. 2019, MNRAS, 482, 3373Fern´andez, R., Kasen, D., Metzger, B. D., & Quataert, E.2015, Mon. Not. Roy. Astron. Soc., 446, 750Flanagan, ´E. ´E., & Hinderer, T. 2008, prd, 77, 021502Flaugher, B., Diehl, H. T., Honscheid, K., et al. 2015, AJ,150, 150Foucart, F. 2012, Phys. Rev., D86, 124007Foucart, F., Hinderer, T., & Nissanke, S. 2018, Phys. Rev.,D98, 081501Fremling, C., Goldstein, D., Andreoni, I., & Kasliwal,M. M. 2019, GCN, 25460 Fremling, C., Sollerman, J., Taddia, F., et al. 2016, A&A,593, A68Geesaman, D. 2015, in APS Meeting Abstracts, AA1.001Gehrels, N., Cannizzo, J. K., Kanner, J., et al. 2016, ApJ,820, 136Goldstein, D. A., & Anand, S. 2019, GCN, 25393Goldstein, D. A., Perley, D., Andreoni, I., & Kasliwal,M. M. 2019, GCN, 25355Goldstein, D. A., D’Andrea, C. B., Fischer, J. A., et al.2015, AJ, 150, 82Goldstein, D. A., Andreoni, I., Nugent, P. E., et al. 2019,ApJ, 881, L7Goldstein, D. A., Andreoni, I., Hankins, M., et al. 2019a,GCN, 25393Goldstein, D. A., Andreoni, I., Zhou, R., et al. 2019b,GCN, 25391Gomez, S., Hosseinzadeh, G., Cowperthwaite, P. S., et al.2019, ApJ, 884, L55Gomez, S., Hosseinzadeh, G., Berger, E., et al. 2019, GCN,25483Greggio, L., Rejkuba, M., Gonzalez, O. A., et al. 2014,A&A, 562, A73Herner, K., Palmese, A., Soares-Santos, M., et al. 2019a,GCN, 25398—. 2019b, GCN, 25373Hinderer, T., Lackey, B. D., Lang, R. N., & Read, J. S.2010, prd, 81, 123016Hotokezaka, K., & Nakar, E. 2019, arXiv e-prints,arXiv:1909.02581Hu, Y. D., Castro-Tirado, A. J., Valeev, A. F., et al. 2019,GCN, 25588Huber, M., Smith, K. W., Chambers, K., et al. 2019, GCN,25356Jonker, P., Maguire, K., Fraser, M., et al. 2019, GCN, 25454Kasen, D., Metzger, B., Barnes, J., Quataert, E., &Ramirez-Ruiz, E. 2017, Nature, 551, 80Kasliwal, M. M., Nakar, E., Singer, L. P., et al. 2017,Science, 358, 1559Kasliwal, M. M., Cannella, C., Bagdasaryan, A., et al.2019, PASP, 131, 038003Kawaguchi, K., Kyutoku, K., Nakano, H., et al. 2015,Phys. Rev. D, 92, 024014Kawaguchi, K., Kyutoku, K., Shibata, M., & Tanaka, M.2016, Astrophys. J., 825, 52Kilpatrick, C. D., Foley, R. J., Kasen, D., et al. 2017,Science, 358, 1583Kobulnicky, H. A., Nordsieck, K. H., Burgh, E. B., et al.2003, in Society of Photo-Optical InstrumentationEngineers (SPIE) Conference Series, Vol. 4841,Proc. SPIE, ed. M. Iye & A. F. M. Moorwood, 1634–1644 ROWTH on S190814bv21