Optical follow-up observation for GW event S190510g using Subaru/Hyper Suprime-Cam

Abstract

A gravitational wave event, S190510g, which was classified as a binary-neutron-star coalescence at the time of preliminary alert, was detected by LIGO/Virgo collaboration on 2019 May 10. At 1.7 hours after the issue of its preliminary alert, we started a target-of-opportunity imaging observation in the Y band to search for its optical counterpart using the Hyper Suprime-Cam (HSC) on the Subaru Telescope. The observation covers a 118.8 deg2 sky area corresponding to $11.6\%$ confidence in the localization skymap released in the preliminary alert and $1.2\%$ in the updated skymap. We divided the observed area into two fields based on the availability of HSC reference images. For the fields with the HSC reference images, we applied an image subtraction technique; for the fields without the HSC reference images, we sought individual HSC images by matching a catalog of observed objects with the PS1 catalog. The search depth is 22.28 mag in the former method and the limit of search depth is 21.3 mag in the latter method. Subsequently, we performed visual inspection and obtained 83 candidates using the former method and 50 candidates using the latter method. Since we only have the one-day photometric data, we evaluated the probability of candidates being located inside the 3D skymap by estimating their distances with photometry of associated extended objects. We found three candidates are likely located inside the 3D skymap and concluded they could be a counterpart of S190510g, while most of the 133 candidates were likely to be supernovae because the number density of candidates was consistent with the expected number of supernova detections. By comparing our observational depth with a light-curve model of such a kilonova reproducing AT2017gfo, we show that early deep observations with the Subaru/HSC can capture the rising phase of the blue component of a kilonova at the estimated distance of S190510g (∼230 Mpc).

Opacity of the Highly Ionized Lanthanides and the Effect on the Early Kilonova

“…For the optical g-, r-, i-, z-filters, the brightness of the kilonovae for the lanthanide-rich ejecta is close to the detection limit of the existing observing facilities like Zwicky Transient Facility (ZTF, the limiting magnitudes for ZTF in g-, r-, i-bands are 21.1 mag, 20.9 mag, 20.2 mag, respectively, for the 30 s exposure time, Dekany et al 2020). The detection of early kilonova in optical bands seems more feasible for the observing facilities with deeper observing limits, such as, Dark Energy Camera (DECam, the limiting magnitudes in i-, z-bands are 22.5 mag, 21.8 mag, respectively, for the 90 s exposure time, Chase et al 2022), Subaru-HSC, which can reach a depth of ∼ 24 mag for 2 × 30 s exposure time (e.g., Ohgami et al 2021Ohgami et al , 2023. More promisingly, such signals in the optical bands can easily be detected by the upcoming wide-field survey such as Vera Rubin Observatory (Chase et al 2022).…”

Follow-up Survey for the Binary Black Hole Merger GW200224_222234 Using Subaru/HSC and GTC/OSIRIS

“…After image subtraction, we applied the following criteria to exclude bogus detections (e.g., caused by bad pixels and failure of image subtraction) and select point sources from the difference images as in Tominaga et al (2018a) and Ohgami et al (2021): (i) A signal-to-noise ratio of the PSF flux (S/N) PSF > 5; (ii) (b/a)/(b/a) PSF > 0.65, where a and b are the lengths of the major and minor axes, respectively, of the shape of a source; (iii) 0.7 < FWHM/(FWHM) PSF < 1.3; and (iv) PSF-subtracted residual with <3σ standard deviation in the difference image. Furthermore, we imposed the following criterion to exclude moving objects such as minor planets: (v) Detection at least twice in the difference images.…”

“…Next, we evaluated our detection criteria and screening process by comparing our results with the expected number of SN detections. We estimated the expected number of SN detections by summing up mock-SN samples weighted with cosmological histories of SN rates using the observation depth, as in Niino et al (2014) and Ohgami et al (2021). We assumed the SN rate of Okumura et al (2014) and Dahlen et al (2012) for the Type Ia SN and CCSN, respectively.…”

A Systematic Exploration of Kilonova Candidates from Neutron Star Mergers During the Third Gravitational Wave Observing Run

“…We consider follow-up reported in the GCNs and literature (submitted or published papers) by numerous EM counterpart follow-up groups (Coughlin et al 2019;Goldstein et al 2019;Gomez et al 2019;Hosseinzadeh et al 2019;Ackley et al 2020;Anand et al 2020;Andreoni et al 2020a;Antier et al 2020a,b;Garcia et al 2020;Gompertz et al 2020;Kasliwal et al 2020;Morgan et al 2020;Thakur et al 2020;Tucker et al 2021;Vieira et al 2020;Watson et al 2020;Becerra et al 2021;Kilpatrick et al 2021;Oates et al 2021;Ohgami et al 2021). We note that the majority of these works were focused on follow-up of GW190814, the most preciselylocalized event potentially involving a NS throughout O3.…”

“…In addition, tens of merger events involving a NS were announced via the Gamma-ray Coordinates Network circulars (GCNs) in O3 that did not pass the thresholds for inclusion in the published samples. Despite these numerous opportunities and subsequent efforts by the community, no credible EM counterpart to a GW event has been identified since AT 2017gfo (as chronicled in the O3 EM follow-up literature; Coughlin et al 2019;Goldstein et al 2019;Gomez et al 2019;Hosseinzadeh et al 2019;Lundquist et al 2019;Anand et al 2020;Andreoni et al 2020a;Antier et al 2020a,b;Ackley et al 2020;Garcia et al 2020;Gompertz et al 2020;Kasliwal et al 2020;Morgan et al 2020;Thakur et al 2020;Vieira et al 2020;Watson et al 2020;Alexander et al 2021;Becerra et al 2021;de Wet et al 2021;Dobie et al 2021;Kilpatrick et al 2021;Oates et al 2021;Ohgami et al 2021;Paterson et al 2021;Tucker et al 2021;de Jaeger et al 2022). A significant challenge for EM follow-up is the need to search large localization areas, which spanned ∼10-10,000 deg 2 for events in O3.…”