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

Abstract

On 2019 April 25.346 and 26.640 UT the Laser Interferometer Gravitational-Wave Observatory and the 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 d L = 156 ± 41 Mpc, while S190426c is possibly the first NS-black hole (BH) merger ever detected, at d L = 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 deg 2 are likely essential.

GROWTH on S190425z: Searching Thousands of Square Degrees to Identify an Optical or Infrared Counterpart to a Binary Neutron Star Merger with the Zwicky Transient Facility and Palomar Gattini-IR

“…This transient was imaged 16 times for a period of 3.8 days by a variety of observing groups (Ahumada et al 2019b(Ahumada et al , 2019cPerley et al 2019bPerley et al , 2019c. Nicholl et al (2019) first classified ZTF19aasckkq as a Type IIb SN at z∼0.05, which is consistent with the galaxy redshift (Hosseinzadeh et al 2019). In Figure 5, we highlight the presence of He I, Hα, and Hβ absorption features in the first spectrum we acquired with P200+DBSP, confirming its classification as a SN IIb at a redshift of z=0.0528.…”

“…The forced photometry search did not show activity prior to the merger. This transient was imaged five times during the first 24 hr and it was classified as a SN Ia by Nicholl et al (2019) at a redshift of z=0.145 (Hosseinzadeh et al 2019). The absolute magnitude at discovery was r=−19.2, a few magnitudes brighter than what is expected from a KN.…”

A Galaxy-targeted Search for the Optical Counterpart of the Candidate NS–BH Merger S190814bv with Magellan

“…Upon receipt of the LVC public alert for S190814bv, our automated software generated a list of galaxies from the Galaxy List for the Advanced Detector Era (GLADE) catalog (Dálya et al 2018) covering the 90% confidence volume and ranked by probability within the volume (see Hosseinzadeh et al 2019 for a detailed discussion).…”

Searching for electromagnetic counterparts to gravitational-wave merger events with the prototype Gravitational-Wave Optical Transient Observer (GOTO-4)

“…Numerous wide-field follow-up missions have tiled GW error boxes searching for transients, including the All-Sky Automated Survery for Supernovae (ASASSN; Shappee et al 2014), the Asteroid Terrestrial impact Last Alert System (ATLAS; Tonry et al 2018), the Deca-Degree Optical Transient Imager (DDOTI; Watson et al 2016), the Dark Energy Survey (DES; Dark Energy Survey Collaboration et al 2016), the Global Rapid Advanced Network Devoted to the Multimessenger Addicts (GRANDMA; Antier et al 2020b), KMT-Net (Kim et al 2016), the Mobile Astronomical System of TElescope Robots (MASTER; Lipunov et al 2010), MeerLICHT (Bloemen et al 2016), PanSTARRS (Kaiser et al 2010), Searches After Gravitational waves Using ARizona Observatories (SAGUARO; Lundquist et al 2019), the Télescope à Action Rapide pour les Objets Transitoires (TAROT; Boër 2001), the Visible and Infrared Survey Telescope for Astronomy (VISTA;Sutherland et al 2015), the VLT Survey Telescope (VST; Capaccioli & Schipani 2011) and the Zwicky Transient Facility (ZTF; Bellm et al 2019). No associated transients were identified (Anand et al 2020;Antier et al 2020b,a;Coughlin et al 2020;Sagués Carracedo et al 2020), but constraining limits were placed on a number of milestone events, including S190814bv, the first NSBH merger candidate identified in GW (Dobie et al 2019;Gomez et al 2019;LIGO Scientific Collaboration & Virgo Collaboration 2019;Ackley et al 2020;Andreoni et al 2020;Vieira et al 2020;Watson et al 2020), and several candidate BNS systems (Goldstein et al 2019;Hosseinzadeh et al 2019;Lundquist et al 2019), including the unusually massive GW190425 (Coughlin et al 2019;Hosseinzadeh et al 2019;Lundquist et al 2019;Abbott et al 2020b).…”