Follow-up of the Neutron Star Bearing Gravitational-wave Candidate Events S190425z and S190426c with MMT and SOAR

• Published in: Astrophysical journal. Letters 2019-07, Vol.880 (1), p.L4
• Main Authors: Hosseinzadeh, G., Cowperthwaite, P. S., Gomez, S., Villar, V. A., Nicholl, M., Margutti, R., Berger, E., Chornock, R., Paterson, K., Fong, W., Savchenko, V., Short, P., Alexander, K. D., Blanchard, P. K., Braga, J., Calkins, M. L., Cartier, R., Coppejans, D. L., Eftekhari, T., Laskar, T., Ly, C., Patton, L., Pelisoli, I., Reichart, D. E., Terreran, G., Williams, P. K. G.
• Format: Article
• Language: English
• Subjects: Afterglows; binaries: close; Binary stars; Black holes; Depth indicators; Emission analysis; Gamma emission; Gamma ray bursts; Gamma rays; Gravitation; Gravitational waves; Interferometers; methods: observational; Near infrared radiation; Neutron stars; Neutrons; Observatories; Searching; stars: black holes; stars: neutron
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ab271c

On 2019 April 25.346 and 26.640 UT the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo gravitational-wave (GW) observatory announced the detection of the first candidate events in Observing Run 3 that contained at least one neutron star (NS). S190425z is a likely binary neutron star (BNS) merger at dL = 156 41 Mpc, while S190426c is possibly the first NS-black hole (BH) merger ever detected, at dL = 377 100 Mpc, although with marginal statistical significance. Here we report our optical follow-up observations for both events using the MMT 6.5 m telescope, as well as our spectroscopic follow-up of candidate counterparts (which turned out to be unrelated) with the 4.1 m SOAR telescope. We compare to publicly reported searches, explore the overall areal coverage and depth, and evaluate those in relation to the optical/near-infrared (NIR) kilonova emission from the BNS merger GW170817, to theoretical kilonova models, and to short gamma-ray burst (SGRB) afterglows. We find that for a GW170817-like kilonova, the partial volume covered spans up to about 40% for S190425z and 60% for S190426c. For an on-axis jet typical of SGRBs, the search effective volume is larger, but such a configuration is expected in at most a few percent of mergers. We further find that wide-field γ-ray and X-ray limits rule out luminous on-axis SGRBs, for a large fraction of the localization regions, although these searches are not sufficiently deep in the context of the γ-ray emission from GW170817 or off-axis SGRB afterglows. The results indicate that some optical follow-up searches are sufficiently deep for counterpart identification to about 300 Mpc, but that localizations better than 1000 deg2 are likely essential.