LIGO and Virgo’s third observing run revealed the first neutron star–black hole (NSBH) merger candidates in gravitational waves. These events are predicted to synthesize r-process elements1,2 creating optical/near-infrared ‘kilonova’ emission. The joint gravitational wave and electromagnetic detection of an NSBH merger could be used to constrain the equation of state of dense nuclear matter3, and independently measure the local expansion rate of the Universe4. Here, we present the optical follow-up and analysis of two of the only three high-significance NSBH merger candidates detected to date, S200105ae and S200115j, with the Zwicky Transient Facility5. The Zwicky Transient Facility observed ~48% of S200105ae and ~22% of S200115j’s localization probabilities, with observations sensitive to kilonovae brighter than −17.5 mag fading at 0.5 mag d−1 in the g- and r-bands; extensive searches and systematic follow-up of candidates did not yield a viable counterpart. We present state-of-the-art kilonova models tailored to NSBH systems that place constraints on the ejecta properties of these NSBH mergers. We show that with observed depths of apparent magnitude ~22 mag, attainable in metre-class, wide-field-of-view survey instruments, strong constraints on ejecta mass are possible, with the potential to rule out low mass ratios, high black hole spins and large neutron star radii.
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This work was supported by the GROWTH (Global Relay of Observatories Watching Transients Happen) project funded by the National Science Foundation under PIRE grant no. 1545949. GROWTH is a collaborative project among California Institute of Technology (USA), University of Maryland College Park (USA), University of Wisconsin Milwaukee (USA), Texas Tech University (USA), San Diego State University (USA), University of Washington (USA), Los Alamos National Laboratory (USA), Tokyo Institute of Technology (Japan), National Central University (Taiwan), Indian Institute of Astrophysics (India), Indian Institute of Technology Bombay (India), Weizmann Institute of Science (Israel), The Oskar Klein Centre at Stockholm University (Sweden), Humboldt University (Germany), Liverpool John Moores University (UK) and University of Sydney (Australia). This work was based on observations obtained with the 48-inch Samuel Oschin Telescope and the 60-inch Telescope at the Palomar Observatory as part of the ZTF project. ZTF is supported by the National Science Foundation under grant no. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington (UW), Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee and Lawrence Berkeley National Laboratories. Operations are conducted by Caltech Optical Observatories, IPAC, and UW. The work is partly based on the observations made with the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias, in the island of La Palma. The KPED team (M.W.C., R.G.D., D.A.D., M.F., S.R.K., E.S. and R.R.) thanks the National Science Foundation and the National Optical Astronomical Observatory for making the Kitt Peak 2.1-m telescope available. We thank the observatory staff at Kitt Peak for their efforts to assist Robo-AO KP operations. The KPED team thanks the National Science Foundation, the National Optical Astronomical Observatory, the Caltech Space Innovation Council and the Murty family for support in the building and operation of KPED. In addition, they thank the CHIMERA project for use of the Electron Multiplying CCD (EMCCD). SED Machine is based upon work supported by the National Science Foundation under grant no. 1106171 The ZTF forced-photometry service was funded under the Heising-Simons Foundation grant #12540303 (PI: Graham). M.W.C. acknowledges support from the National Science Foundation with grant no. PHY-2010970. S.A. gratefully acknowledges support from a GROWTH PIRE grant (1545949). Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. E.C.K. acknowledges support from the G.R.E.A.T. research environment and the Wenner-Gren Foundations. F.F. gratefully acknowledges support from NASA through grant 80NSSC18K0565, from the NSF through grant PHY-1806278, and from the DOE through CAREER grant DE-SC0020435.
The authors declare no competing interests.
Peer review information Nature Astronomy thanks Aaron Zimmerman and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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5-σ limiting magnitudes as a function of time for a, S200105ae (ZTF), b, S200115j (ZTF), and c, S190814bv (DECam) with the left, middle, and right panels corresponding to observations on the first, second, and third nights for S200105ae and S190814bv and first, second, and fourth nights for S200115j. The red and green triangles correspond to the r- and g-band limits for ZTF, while the yellow and black triangles correspond to the i- and z-band limits for DECam; the open triangles correspond to serendipitous observations and closed ToO observations. The large differences in limiting magnitude from observation to observation are due to poor weather.
Here we show results for each step of the ZTF filtering scheme for three representative nights covering the events discussed in this paper. Each cell shows the number of candidates that successfully pass a particular filter. The number shown is the result of running a filtering step on the alerts that met previous requirements. We define as ‘Real’ any alert with a real-bogus score greater than 0.25 and ‘not moving’ the candidates that have more than two detections separated by at least 15 minutes. The highlighted numbers represent the amount of candidates that required further vetting, as described in Section 3.
Top row: Coverage of S200105ae, showing the tiles on the 90% probability region of the initial BAYESTAR a, and final LALInference b, skymaps. The color intensity is proportional to the 2-D probability. The mapping of candidates to numbers is 1: ZTF20aaervoa, 2: ZTF20aaertpj, 3: ZTF20aaervyn, 4: ZTF20aaerqbx, 5: ZTF20aaerxsd, 6: ZTF20aafduvt, 7: ZTF20aaevbzl, 8: ZTF20aaflndh, 9: ZTF20aaexpwt, 10: ZTF20aafaoki, 11: ZTF20aafukgx, 12: ZTF20aagijez, 13: ZTF20aafanxk, 14: ZTF20aafujqk, 15: ZTF20aagiiik, 16: ZTF20aafdxkf, 17: ZTF20aagiipi, 18: ZTF20aagjemb, 19: ZTF20aafksha, 20: ZTF20aaertil, 21: ZTF20aafexle and 22: ZTF20aafefxe. Bottom row: Same for S200115j, with the BAYESTAR coverage shown in c, and LALInference coverage shown in d. The mapping of candidates to numbers is 1: ZTF20aagjqxg, 2: ZTF20aafqvyc, 3: ZTF20aahenrt, 4: ZTF20aafqpum, 5: ZTF20aafqulk, and 6: ZTF20aahakkp. We note that we include candidates up to and including within the 95% probability region, and therefore some are outside of the fields we plot here.
Extended Data Fig. 4 Potential constraints on kilonova model parameters based on the deepest limiting magnitudes.
We display constraints on a, S200105ae (ZTF), b, S200115j (ZTF) and c, S190814bv (DECam) for the models in the NSBH grid used here. Top panels: same as Figure 4 but using the deepest (filled triangles) rather than the median limits for each set of S200105ae and S200115j observations. The panel for S190814bv is the same as in Figure 4, with all limits corresponding to the median magnitudes. Bottom panels: regions of the Mej,dyn − Mej,pm parameter space that are ruled out at different distances and for different viewing angle ranges (moving from pole to equator from top to bottom panel).
Extended Data Fig. 5 Potential constraints on the parameters of a NSBH binary associated with S200105ae.
Here we assume that Mej,dyn ≤ 0.02M⊙ and Mej,pm ≤ 0.04M⊙, appropriate for the deepest observations of S200105ae in a face-on orientation. We show the maximum value of the aligned component of the BH spin as a function of the neutron star radius RNS and the binary mass ratio Q = MBH/MNS. The two panels show results assuming that low a, and high b, fractions of the post-merger accretion disk are ejected (see text). Both plots assume MNS = 1.35. Results for different neutron star masses can be estimated from this plot simply by considering a binary with the same Q, χ and compaction MNS/RNS.
Extended Data Fig. 6 Minimum aligned component of the BH spin above which we cannot rule out the presence of a kilonova.
We cannot exclude this region of parameter space because either the resulting kilonova evolves too slowly, or the ejected mass is outside of the grid of models used in this study. In this plot, we consider the worse-case scenario of frem = 0.5.
Extended Data Fig. 7 Potential constraints on the parameters of a NSBH binary associated with S190814bv.
Here we assume that Mej,dyn ≤ 0.01M⊙ and Mej,pm ≤ 0.01M⊙, as appropriate for S190814bv in a face-on orientation in a similar fashion to Extended Data Figure 5 with low a, and high b, fractions of disk ejecta.
Light curves predicted with POSSIS (Ref. 20) for a NSBH model with Mdyn = 0.05M⊙ and Mpm = 0.05M⊙ as seen from a polar a, and equatorial b, viewing angle.
Extended Data Fig. 9 Comparison of peak magnitudes between optical and near-IR bands for NSBH models.
We plot the difference in peak magnitudes between the a, g-band and the near-IR i- and z-bands for the models in the NSBH grid used here. Similarly, in b, we show the difference between r-band and the same near-IR bands.
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Anand, S., Coughlin, M.W., Kasliwal, M.M. et al. Optical follow-up of the neutron star–black hole mergers S200105ae and S200115j. Nat Astron 5, 46–53 (2021). https://doi.org/10.1038/s41550-020-1183-3
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