Optical follow-up observation for GW event S190510g using Subaru/Hyper Suprime-Cam
Takayuki Ohgami, Nozomu Tominaga, Yousuke Utsumi, Yuu Niino, Masaomi Tanaka, Smaranika Banerjee, Ryo Hamasaki, Michitoshi Yoshida, Tsuyoshi Terai, Yuhei Takagi, Tomoki Morokuma, Mahito Sasada, Hiroshi Akitaya, Naoki Yasuda, Kenshi Yanagisawa, Ryou Ohsawa
aa r X i v : . [ a s t r o - ph . H E ] J a n Publ. Astron. Soc. Japan (2018) 00(0), 1–11doi: 10.1093/pasj/xxx000 Optical follow-up observation for GW eventS190510g using Subaru/Hyper Suprime-Cam
Takayuki O
HGAMI , Nozomu T OMINAGA
1, 2 , Yousuke U
TSUMI , YuuN IINO
4, 5 , Masaomi T
ANAKA , Smaranika B ANERJEE , Ryo H AMASAKI ,Michitoshi Y OSHIDA , Tsuyoshi T ERAI , Yuhei T AKAGI , TomokiM OROKUMA , Mahito S ASADA
8, 9 , Hiroshi A
KITAYA , Naoki Y ASUDA ,Kenshi Y ANAGISAWA
10, 8, 11 , Ryou O
HSAWA and the J-GEM collaboration Department of Physics, Faculty of Science and Engineering, Konan University, 8-9-1Okamoto, Kobe, Hyogo 658-8501, Japan Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University ofTokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), SLAC National AcceleratorLaboratory, Stanford University, 2575 Sand Hill Road, Menlo Park, CA 94025, USA Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa,Mitaka, Tokyo 181-0015, Japan Research Center for the Early Universe, Graduate School of Science, The University ofTokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Astronomical Institute, Tohoku University, Sendai 980-8578, Japan Subaru Telescope, National Astronomical Observatory of Japan, 650 North A‘ohoku Place,Hilo, HI 96720, USA Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama,Higashi-Hiroshima, Hiroshima 739-8526, Japan Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, 2-12 Hoshigaoka,Mizusawa, Oshu, Iwate 023-0861, Japan National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Hongo,Bunkyo-ku, Tokyo 113-0033, Japan ∗ E-mail: [email protected]
Received ; Accepted
Abstract
A gravitational wave event, S190510g, which was classified as a binary-neutron-star coales-cence at the time of preliminary alert, was detected by LIGO/Virgo collaboration on May 10,2019. At 1.7 hours after the issue of its preliminary alert, we started a target-of-opportunityimaging observation in Y -band to search for its optical counterpart using the Hyper Suprime-Cam (HSC) on the Subaru Telescope. The observation covers a 118.8 deg sky area cor-responding to 11.6% confidence in the localization skymap released in the preliminary alertand 1.2% in the updated skymap. We divided the observed area into two fields based on theavailability of HSC reference images. For the fields with the HSC reference images, we appliedan image subtraction technique; for the fields without the HSC reference images, we soughtindividual HSC images by matching a catalog of observed objects with the PS1 catalog. Thesearch depth is 22.28 mag in the former method and the limit of search depth is 21.3 mag in the © 2018. Astronomical Society of Japan. Publications of the Astronomical Society of Japan , (2018), Vol. 00, No. 0 latter method. Subsequently, we performed visual inspection and obtained 83 candidates usingthe former method and 50 candidates using the latter method. Since we have only the 1-dayphotometric data, we evaluated probability to be located inside the 3D skymap by estimatingtheir distances with photometry of associated extended objects. We found three candidates arelikely located inside the 3D skymap and concluded they could be an counterpart of S190510g,while most of 133 candidates were likely to be supernovae because the number density ofcandidates was consistent with the expected number of supernova detections. By comparingour observational depth with a light curve model of such a kilonova reproducing AT2017gfo, weshow that early-deep observations with the Subaru/HSC can capture the rising phase of bluecomponent of kilonova at the estimated distance of S190510g ( ∼
230 Mpc).
Key words:
Gravitational waves – Stars: — neutron – nuclear reactions, nucleosynthesis, abundances
A multi-messenger observation with gravitational waves(GW) and electromagnetic (EM) waves is crucial for un-derstanding physical processes of compact star coalescence.Neutron-star (NS) mergers are expected to be accompa-nied by EM emissions called “kilonova” (or “macronova”)powered by radioactive decays of r -process nuclei (Li &Paczy´nski 1998; Kulkarni 2005; Metzger et al. 2010); there-fore, the EM emission from BNS-merger events aids in un-derstanding the origin of heavy elements produced by the r -process (Metzger et al. 2010; Kasen et al. 2013; Barnes& Kasen 2013; Tanaka & Hotokezaka 2013; Tanaka et al.2014; Kasen et al. 2015).The localization area of GW observations can be an or-der of 10 deg for the best case, but can be as large as 1000deg . It has been quite large for locating a galaxy hosting asystem that caused the GW event. Therefore, EM follow-up observations had been expected to play a key role inidentifying the counterpart. The first identification of EMcounterpart to GW was achieved in the event of the firstdetection of GW from a neutron star merger (GW170817).GW170817 was localized with three interferometers in thesecond observing run (O2) of the LIGO/Virgo collabora-tion (Abbott et al. 2017). The identification of the EMcounterpart was made by several observatories on earthincluding space from radio to gamma ray (Arcavi et al.2017; Coulter et al. 2017; D´ıaz et al. 2017; Evans et al.2017; Lipunov et al. 2017; Soares-Santos et al. 2017; Tanviret al. 2017; Tominaga et al. 2018; Valenti et al. 2017).Untargeted wide-field surveys are important for identi-fying the uniqueness of the counterpart. The Japanese col-laboration for Gravitational wave ElectroMagnetic follow-up (J-GEM; Morokuma et al. 2016) conducted coordi-nated observations (Utsumi et al. 2017) and deep blind z -band imaging surveys to identify an EM counterpart us-ing Hyper Suprime-Cam (HSC) on the Subaru Telescope (Miyazaki et al. 2018; Kawanomoto et al. 2018; Komiyamaet al. 2018; Furusawa et al. 2018). They succeededin independently identifying the counterpart (AT2017gfo;Tominaga et al. 2018). HSC is a 1.5 deg φ wide-field op-tical imager, which is the largest among the current exist-ing telescopes with an aperture larger than 8 m. Whilegalaxy-targeted and untargeted wide-field surveys identi-fied AT2017gfo, wide-field survey observations with theSubaru/HSC and Blanco/Dark Energy Camera (DECam)succeeded in identifying the uniqueness of AT2017gfo witha high completeness by ruling out the other candidatesincluding transients which are not associated with galaxy.Kilonova models can broadly reproduce the time evolu-tion of optical and near-infrared emissions of AT2017gfo(Shibata et al. 2017; Tanaka et al. 2017; Kasen et al.2017; Perego et al. 2017; Kawaguchi et al. 2018; Rosswoget al. 2018). However, the observed emissions display bluecomponents in the early-phase spectra, and the origin ofthe emission is unclear. Two models for the early blue com-ponent are proposed: radioactive heating model (a kilo-nova model having higher electron fraction Tanaka et al.2017; Villar et al. 2017; Waxman et al. 2018) and shockheating model (a cocoon emission model Kasliwal et al.2017; Piro & Kollmeier 2018). These models can repro-duce the EM emission after 0.5 days from the explosion,at which the first observation of AT2017gfo was performed.They predict different behaviors that predate 0.5 days fromthe explosion (Arcavi 2018), i.e. a cocoon model showsa higher luminosity than the radioactive kilonova model;therefore, the earlier observations for future events are im-portant to discriminate these models.The LIGO/Virgo collaboration started their third ob-servation run (O3) in April 2019. They detected a BNSevent named GW190425 (Abbott et al. 2020) on April25, 2019 at 08:18:05 UTC (GCN, The LIGO ScientificCollaboration and the Virgo Collaboration 2019a) for thefirst time in O3. On May 10, 2019 at 02:59:39 UTC, they ublications of the Astronomical Society of Japan , (2018), Vol. 00, No. 0 detected the third BNS event in O3, S190510g, using threeinterferometers (GCN, The LIGO Scientific Collaborationand the Virgo Collaboration 2019b). They analyzed theGW signal using BAYESTAR pipeline (Singer & Price2016) and released a preliminary localization skymap onMay 10, 2019 at 04:03:45 UTC. The 50% and 90% confi-dence regions correspond to the areas of 575 deg and 3462deg , respectively. The luminosity distance was 269 ± . × − Hz (about one in 37 years).On receiving this alert, we conducted a target of op-portunity (ToO) imaging observation (GCN, The J-GEMcollaboration 2019), which covered 118.8 deg correspond-ing to the integrated probability of 11.6% in the localiza-tion skymap, using the Subaru/HSC. After our observa-tions, we received an improved localization skymap whichis reanalyzed with the LALInference pipeline (Veitch et al.2015) by the LIGO/Virgo collaboration on May 10, 2019 at10:06:59 UTC (GCN, The LIGO Scientific Collaborationand the Virgo Collaboration 2019c). The 90% localizationarea and the luminosity distance were revised to 1166 deg and 227 ±
92 Mpc, respectively. The integrated probabil-ity in our observation area decreased to 1.2% of the totalprobability owing to the revision. In this alert, the prob-ability of the event being a BNS-merger event decreasedto 42% (the probability of it being a terrestrial event in-creased to 58%) with an FAR of 8 . × − Hz (about onein 3.6 years).In this paper, we describe the details of the observationof GW event S190510g using the Subaru/HSC, the candi-date selection, and a list of candidates. We investigate thenature of the candidates by estimating a contaminationfrom supernovae. Finally, we discuss the future prospectsfor optical-follow-up observations using Subaru/HSC. Inthis paper, all magnitudes are given as AB magnitudes.
We commenced a follow-up observation for the GW eventS190510g using Subaru/HSC on May 10, 2019 at 05:46:27UTC, 1 h 43 min after the issue of the preliminary-alertand 2 h 47 min after the GW detection. Our original planwas to perform i - and z -band observations for the GWfollow-up; however, we conducted the observation in Y -band for this event because only the Y -band filter wasavailable that night. We selected 120 healpix grids withhigh probabilities in the BAYESTAR localization skymapwith a HEALPix resolution of NSIDE = 64, which corre-sponds to 0.84 deg /pix , allowing the field of views (FoVs)to overlap each other. The HSC pointings were set as the central coordinates of the each grid (Table 1). We exposedthe 120 pointings with 30 s each and revisited them witha 1-arcmin offset in each pointing at least one hour apart.The exposure time was determined considering the obser-vation time of the half night and an exposure interval ofapproximately 34 s (Utsumi et al. 2012).The survey pointings and 90% contour for theBAYESTAR skymap are shown in the bottom left panelof Fig. 1. The observed area of 118.8 deg correspondsto the integrated probability of 11.6% in the BAYESTARlocalization skymap. The skymap is revised significantlyto the LALInference localization skymap. In this updatedskymap, the integrated probability in the observed area de-creases to 1.2%. The 90% contour in the updated skymapis shown in the right panel of Fig. 1.We reduce the observational data with hscPipe v4.0.5(Bosch et al. 2018). This pipeline is a standard reduc-tion pipeline for HSC and provides complete packages forthe analyses of image data, including bias subtraction, flatfielding, astrometry, flux calibration, mosaicing, warping,stacking, image subtraction, source detection, and sourcemeasurement. We estimate 5 σ limiting magnitudes in thesingle-exposure images by measuring standard deviationsof sky fluxes in randomly distributed apertures with a di-ameter of twice the full width at half maximum (FWHM)of a point spread function (PSF). Subsequently, we foundthat the Y -band limiting magnitudes have a mode value of22.30 mag. The mode value of the seeings in each single-exposure image is approximately 0.6 arcsec. In this section, we describe the methods used to search fortransient objects related to S190510g and the correspond-ing results. We apply the following two methods: One is tosearch for transients with an image-subtraction technique,which is useful for searching transient objects embeddedin host galaxies. However, deep reference images are notavailable for all the survey pointings as shown in Fig. 2.The area with deep Subaru/HSC reference images is 25.9deg while the area without deep reference image is 92.9deg . Therefore, we adopt the second method for the re-maining area. This method searches the single-exposureimages without using the image subtraction by matchingknown objects in the Pan-STARRS1 (PS1; Flewelling et al.2016) catalog. Publications of the Astronomical Society of Japan , (2018), Vol. 00, No. 0
BAYESTAR LALInferenceBAYESTAR LALInference
Fig. 1.
Preliminary ( top left , BAYESTAR; GCN, The LIGO Scientific Collaboration and the Virgo Collaboration 2019b) and updated ( top right , LALInference;GCN, The LIGO Scientific Collaboration and the Virgo Collaboration 2019c) localization skymaps of S190510g. White contour lines and red-filled circles( bottom ) represent the localization area corresponding to the 90% confidence region and our survey pointings, respectively. The observed area of 118.8 deg corresponds to the 11.6% and 1.2% of the total probabilities in the BAYESTAR and LALInference skymaps, respectively. Fig. 2.
Coverage of the deep Subaru/HSC reference images. Blue areashows the footprint of the reference images we used. Red-filled circles rep-resent our observation pointings.
First, we apply image subtraction for the fields with deepreference images. We use images obtained in the HSCSubaru Strategic Program (SSP; Aihara et al. 2018) as thereference images. These reference images were taken from March 25, 2014 to April 8, 2019, and the total exposuretime for each field is 200 s. The limiting magnitude of ref-erence images is 23.3 mag according to the HSC ExposureTime Calculator (ETC), and is substantially deeper thanour observations. Thus, the search depth in the subtractedimages are determined by our images because they domi-nate the noise of the subtracted images. The subtractionpackage in the hscPipe implementation is based on an al-gorithm proposed in Alard & Lupton (1998) and Alard(1999). The seeings in the reference images are blurred tothose in our observation images by being convolved withkernels to make their PSFs equivalent in this algorithm.The mode value of 5 σ limiting magnitudes for the im-ages subtracted with the reference images is 22.28 mag,obtained using the same method as described in Section2. The depth is not degraded even after the subtractionbecause the reference images are sufficiently deep.We then perform a candidate selection in the differentimages to exclude bogus detections (e.g., caused by badpixels in the reference images or failure of the image sub-traction) and moving objects by referring to the criteria https://hscq.naoj.hawaii.edu/cgi-bin/HSC ETC/hsc etc.cgi ublications of the Astronomical Society of Japan , (2018), Vol. 00, No. 0 used in Tominaga et al. (2018). First, we set the follow-ing five criteria; (i) | ( S/N ) PSF | >
5, (ii) ( b/a ) / ( b/a ) PSF > .
65 where a and b are the lengths of major and minoraxes of the shape of an object, respectively (iii) 0 . < FWHM / (FWHM) PSF < .
3, (iv) PSF-subtracted residual < σ -standard-deviation range (in the difference image),and (v) detected at least twice and one hour apart. Thecriteria (ii) and (iii) are adopted to identify point sources,and (iv) is required to confirm that the objects can be de-scribed by a PSF. The criterion (v) is applied to excludemoving objects, such as minor planets. We find 1000 ob-jects satisfying these criteria. Figure 3 shows a flowchartof the selection process for these objects. Since our obser-vation area corresponds to the footprints of the PS1 sur-vey (Chambers et al. 2016), we match these objects withthe PS1 catalog (Flewelling et al. 2016) to exclude objectsassociated with known stellar-like objects (point source)within 1 arcsec. To classify the point sources, we use aflag of extended in objInfoFlag in the PS1 catalog. Bythis process, 228 objects are excluded.Next, we classify the remaining 772 objects by their an-gular separation θ sep from the nearby extended objects.We use the catalog of extended objects taken from thePS1 catalog. We obtain 369 objects located at the centerof an extended object ( θ sep < ′′ ), 113 objects located atoff-center (1 ′′ < θ sep < ′′ ), and 290 objects that have noclose extended objects ( θ sep > ′′ ). Moreover, we statis-tically evaluate the probability of the associated extendedobjects inside the 3D localization map using the observedmagnitudes and the luminosity function of galaxies. Wecalculate a probability P that an extended object is lo-cated inside a 3 σ range of the LALInference 3D skymapfollowing the method described in Tominaga et al. (2018).The P is defined as follows: P ( λ j , m j ) := R D mean +3 σ D D mean − σ D φ A dD R ∞ φ A dD , (1)where φ = φ ( λ, M ) is the luminosity function of galaxies ata rest wavelength λ derived from the rest-frame UBV RI -luminosity functions (Ilbert et al. 2005) and the
Planck cosmology (Planck Collaboration et al. 2014), A = A ( D )is an observed surface area at a distance of D , D mean and σ D are mean value and standard deviation of a probabilitydistribution of the distance, respectively, M = M ( D ; m j )is an absolute magnitude of a galaxy with observer-frame j -band apparent magnitude m j at a distance of D , and λ = λ ( D ; λ j ) is the rest wavelength redshifted from aobserved wavelength λ j at a distance of D . We cor-rect the Galactic extinction (Schlafly & Finkbeiner 2011) when we convert the apparent magnitudes to the absolute http://irsa.ipac.caltech.edu/applications/DUST/ Match 228Matched point source in PS1 catalog (< 1”) No match 772 Outside (<50%) 100Sources 1000Visual inspectionAssociated with extended object in PS1 catalog Off-center (1” < θ sep < 15’’) 113Location in 3D skymap ( P ) No close objects ( θ sep > 15”) 290
73 6 3
Inside (>50%) 12 Outside (<50%) 356 Inside (>50%) 1 θ sep < 1”) 369 Fig. 3.
Flowchart of the candidate screening and the classification processfor the objects in the difference images. Numbers in each box represent thenumber of remaining objects at each step. Thick boxes indicate the candi-dates after the visual inspection. magnitudes. We use r - and/or i -band PSF-magnitudes( rMeanPSFMag , iMeanPSFMag in PS1 catalog) of the ex-tended objects for conversion to M and classify the ob-jects according to whether P is higher than 50% or not(“Inside” or “Outside” 3D skymap, respectively). The ob-jects classified as “Outside” are likely to be unrelated tothe GW event. If both rMeanPSFMag and iMeanPSFMag areset to − P are not evaluated, and these objectsare classified as “No Info.”Finally, we perform a visual inspection because bogusdetections remain in these candidates. After the visualexclusion of bogus detections, we finally obtain 83 can-didates. Figure 4 shows some example images of thesecandidates. A detailed information of these candidates isshown in Table 2 (“Off-center” and “No close objects”) andTable 3 (“Center of extended object”). Since only Cand-A10 has a high P , we conclude this source as a finalcandidate of an electromagnetic counterpart of S190510g.However, this source may result from a variability of an ac-tive galactic nucleus because it is located at center of theextended object. We cannot evaluate P of three candi-dates (Cand-A07, Cand-A08 and Cand-A09) because thesehave no close extended object and thus cannot rule outtheir possibility to be the counterpart of S190510g. Next, we examine the fields without the HSC-SSP refer-ence images. We construct the candidate catalog from thesingle exposure images, rather than using the method inSection 3.1. Here, we also focus on point source-like tran-sients in the single-exposure images before stacking. Weperform a forced photometry to the single-exposure im-ages with the hscPipe and select objects by the following
Publications of the Astronomical Society of Japan , (2018), Vol. 00, No. 0
Fig. 4.
Examples of the candidates obtained in the fields with HSC-SSP reference images with the image subtraction; HSC-SSP reference images (Ref), ourobservation images (New) and difference images (Diff). criteria: (i) extendedness equals to 0 . < σ depth in y -band of thePS1 3 π steradian survey. Therefore, the search depth islimited to 21.3 mag by the depth of the PS1 catalog. Forfinding sources on a bright region of an extended object,the effective search depth can be shallower than isolatedsources: this is because the source on extended objects isdetected only when the source is bright enough to make asignificant local minimum of brightness between the sourceand the peak of extended source (Magnier et al. 2020). Wefind 664477 objects satisfying these criteria.Since these objects include stars, we perform an ob-ject screening with criteria similar to the one in Section3.1. We show a flowchart for this screening process in Fig.5. First, we discard 647557 objects positionally coincidingwith point-like objects in the PS1 catalog within 1 arcsec,and obtain the remaining 16920 objects. Next, we clas-sify them based on whether they have PS1 objects whichis extended within 15 arcsecs or not. We obtain 6644 ob-jects without close objects and 10276 objects associatedwith the extended PS1 objects. If the objects located atthe center of the extended object include non-transient ob-jects, it is difficult to visually classify them without imagesubtraction. We discard 6843 objects associated with theextended PS1 objects within 1 arcsec, and then obtain 3433“Off-center” objects. Considering P of the extended ob-jects associated with the “Off-center” objects, 159 objectsare classified as “Inside” and 2842 objects are classified The extendedness has the Double type; however, it has only two values1.0 and 0.0 in hscPipe v4.0.5. Cited from PanSTARRS1 Quick Facts in https://panstarrs.stsci.edu/ as “Outside.” Both rMeanPSFMag and iMeanPSFMag of thePS1 objects associated with the remaining 432 objects areset to −
999 in the PS1 catalog. Therefore, those P arenot evaluated, and are classified as ‘No Information’.Finally, we conduct a visual inspection to remove the re-maining bogus objects from 10077 objects (6644 “No closeobjects,” 432 “No information,” 159 “Inside,” and 2842“Outside”). Here, we compare our images with the PS1stacked images in g -, r -, i -, z -, and y -band. We discard theobjects when a counterpart below the detection thresholdcan be recognized in the PS1 catalogs. These objects inthe PS1 images are significantly faint and slightly visible.We then obtain 50 candidates after the visual inspectionas summarized in Table 4 (“Associated”) and Table 5 (“Noclose objects”). Figure 6 shows some examples of the can-didate. Since two sources (Cand-B01 and Cand-B02) havehigh P , we also include these sources to final candidatesof an electromagnetic counterpart of S190510g. We cannotexclude the possibility to be the counterpart of S190510gfor 40 objects tagged “No close object” and a “No infor-mation” because we cannot evaluate P of them. The candidates include objects unrelated to the GW event,such as supernova (SN). However, it is difficult to deter-mine their nature because we have only 1-day photomet-ric observations. Thus, we compare our results with theexpected number of SN detections and consider the con-tamination from them. As shown below, this comparisondemonstrates that our sample is dominated by SNe. Here,we adopt a similar method to that introduced in Niinoet al. (2014). The expected number is estimated by sum- ublications of the Astronomical Society of Japan , (2018), Vol. 00, No. 0 Match 647557 No match 16920Associated 10276No close objects 6644Center 6843Location in 3D skymap ( P ) Sources 664477Visual inspection Off-center 3433Center of extended object ( θ sep < 1”)Matched point source in PS1 catalog (< 1”)Associated with extended object in PS1 catalog ( θ sep < 15”) No information 432 Inside (>50%) 159 Outside (<50%) 2842
Fig. 5.
Flowchart of the candidate screening and classification process forthe selection from the single-exposure images in the fields without HSC-SSPreference images.
Fig. 6.
Examples of the candidates in the fields without HSC-SSP referenceimages; reference images in PS1- y band (Ref) and our observation imageswith HSC (New). ming mock-SN samples brighter than magnitudes corre-sponding to the limiting magnitude at redshift weightedwith cosmological histories of SN rates. We assume Type-Ia SN rate of Okumura et al. (2014) and core-collapse SN(Ib, Ic, IIL, IIP and IIn) rates of Dahlen et al. (2012). TheSN light curves are generated from SN-spectrum evolutionsprovided by Hsiao et al. (2007) for Type-Ia SN. For core-collapse SN, we generate the light curves from the tem-plates provided by Nugent et al. (2002) . The luminositydistributions of SNe are taken from Barbary et al. (2012)and Dahlen et al. (2012). We sample SNe whose bright-ness is rising in y -band assuming the reference images are The templates including the core-collapse SNe are publicly available at thewebsite of P. Nugent (https://c3.lbl.gov/nugent/nugent templates.html).
Fig. 7.
Expected number density of SN detections in Y -band. The verticalaxis indicates the area number density of SNe brighter than the limiting mag-nitudes in the horizontal axis. The cyan region is the ± error originatingfrom the uncertainty of the core-collapse-SN rate density. The dots witherror bars are obtained by the number of candidates obtained in the fieldswith/without the HSC-SSP reference images ( filled/open circle ). taken 500 days before the detection. Although the Y -bandbandpasses are different between HSC and PS1, we assumesame filters for two observations in this estimation. The ef-fect of the difference is negligible compared to other effectsof model uncertainties.Under the above conditions, we derive the expectednumber density as a function of the limiting magnitude(Fig. 7). The cyan region represents the ±
50% error orig-inating from the uncertainty of the core-collapse SN ratedensity. The dots with error bars are given by
N/S field ,where S field is the area corresponding to the fields withor without the HSC-SSP reference images ( filled or opencircle ), and N is the number of candidates found in eachfield. The horizontal error bar of filled circle represents the1 σ -standard-deviation range of the limiting magnitudes inthe difference images. The vertical error bars represent theuncertainties defined as √ N/S field by assuming that thenumber of candidates follows a Poisson distribution withthe expected value of N . Although there is a probabil-ity that we overlooked some SNe because we neglected theobjects located at the center of extended objects in thefields without the HSC-SSP reference images, these dotsare within the error region and consistent with the simu-lation. Therefore, most of our candidates are likely to beSNe. The Dark Energy Survey also performed a follow-upobservation covering 84 deg (65% of the total probabilityregion) using a DECam with the depths of 20.58 mag ( z -band), 21.72 mag ( r -band), and 21.67 mag ( g -band) andreported that the result of their all candidates are consis-tent with supernovae (DES Collaboration et al. 2020). Publications of the Astronomical Society of Japan , (2018), Vol. 00, No. 0
In this section, we compare our search depths with a kilo-nova model, and discuss future prospects for the follow-upobservations with Subaru/HSC. When we convert the ab-solute magnitudes to the apparent magnitudes, we correctthe Galactic extinction by assuming E ( B − V ) = 0 .
10 mag(Schlafly & Finkbeiner 2011).Here, we adopt a kilonova based on the radiative trans-fer simulations by Banerjee et al. (2020) for comparisonwith the early phase of brightness evolution. We assumethat a kilonova with the same propertieis with GW170817is located at the distance of 227 Mpc reported in S190510g.Figure 8 shows multi-color ( i -, z - and Y -band) light curveof a kilonova model with an ejecta mass M ej = 0 . M ⊙ andan electron fraction Y e = 0 . − .
40 (no lanthanide). Thisparameter set can explain the observed early multi-colorlight curve of AT2017gfo. Properties of the light curvesare affected by choice of the ejecta mass and electron frac-tion. The peak bolometric luminosity roughly scales withthe Mej to the power of 0.35 (e.g. Fern´andez & Metzger2016; Tanaka 2016; Metzger 2019). Also, the electron frac-tion Ye influences the light curves through the opacity: forexample, if the Y e is low ( Y e < . Y e = 0 . − .
40) in the early time.For observational limiets, we show the limiting mag-nitude in the difference images ( green solid line ), the 5 σ depth of the PS1- y band catalog ( gray dots line ) and thelimiting magnitudes in HSC- z or HSC- i orange dotslines ), respectively, by horizontal lines. The values in HSC- z and HSC- i × ∼
60 deg in both z - and i - bands withthis exposure time during a half night of the telescope time.We note that the limiting magnitudes could be shallower( ∼ Y -band observation.This comparison demonstrates that observations usingSubaru/HSC can detect the kilonova emission in i -, z -,and Y -bands during peak times even at 227 Mpc. ForS190510g, our observations covered approximately 0.1-0.3days (2.8-7.0 hours), as shown in the gray shaded area inFig. 8. If the emission is purely powered by radioactivedecays, the emission still rises in those phases, as shownin Fig. 8. However, if ejecta are further heated by co-coon produced by the interaction between the relativisticjets and the ejecta, the emission might be brighter thanthis kilonova model (Arcavi 2018). Therefore, the early observations will be important to provide constraints onthe emission models.Observations with the Subaru/HSC can detect kilonovafainter than GW170817/AT2017gfo even at the distance ofS190510g (227 Mpc). For example, if the M ej is 0.01 M ⊙ with all the other parameters fixed, the peak bolometricluminosity is decreased from our fiducial model by 40%.Then, the peak of the light curves gets ∼ Y e < . i - and r -bands.Here, we also emphasize the importance of the deep ref-erence image. The horizontal solid line shows our limitingmagnitude for the fields with HSC-SSP reference image,while the dashed line shows the limit of search depth inthe fields without the reference. In the latter case, obser-vations are limited by the depth of the PS1 images, andthus, we cannot detect transient sources fainter than 21.3mag. For this particular kilonova model at 227 Mpc dis-tance, if the deep reference image is available, the firstdetection in Y -band would be 0.4 days compared to 1.5days without the deep reference image.Finally, we consider future prospects in the fourth andfifth observing runs (O4 and O5) of the GW interferom-eters. Colored squares in Fig. 9 correspond to a BNSrange and an observation period in the each observing runshown in the document of observing run plans . Dots witherror bars are distances (left vertical axis) of GW eventsreported in O1, O2, and O3. The right vertical axis refersto a peak magnitude of the light curve in z -band in thesame kilonova model as Fig. 8 on an assumption that it islocated at a distance shown in the left vertical axis. Thelimiting magnitude evaluated with the exposure time of60 s in HSC- z band is 23.45 mag ( black dashed line ). Weexpect that observations with HSC will sufficiently attainthe BNS ranges expected in O4 or O5.Here, we estimate how many survey observations forGW events classified as BNS can be performed withSubaru/HSC. Assuming the Subaru telescope can view thesky above 20 ◦ of elevation, the total area of visible sky isapproximately 90% of the whole sky in a day. Since thenature seeing becomes poor in the elevations lower than20 ◦ , they are undesirable for the good imaging quality.Furthermore, typical hours of “night”, 8 hours, reducesthis ratio to approximately 30%. In addition, considering https://dcc.ligo.org/public/0161/P1900218/002/SummaryForObservers.pdf ublications of the Astronomical Society of Japan , (2018), Vol. 00, No. 0 Difference images ( 30sec, HSC- Y )5 σ depth of PS1 catalog ( PS1- y ) i z ) Fig. 8.
Multi-color ( i -, z - and Y -band) light curve of the kilonova model withan ejecta mass M ej = 0 . M ⊙ and an electron fraction Y e = 0 . − . (The i - and z -band cases are shown in Banerjee et al. (2020)), assumingthat it is located at the distance of 227 Mpc reported in S190510g. Horizontallines indicate the limiting magnitude in the difference images ( green solidline ), the 5 σ depth of PS1 y -band catalog ( gray dots line ) and the limitingmagnitudes in HSC- z or HSC- i band calculated by HSC Exposure TimeCalculator under the assumption of 30 s × orange dottedlines ), respectively. A gray-filled square shows a range of time and depthwe observed. A bottom panel is the enlarged figure of the dashed-frame partin the upper panel. nights available for HSC ToO observations, which is esti-mated as 30% (Utsumi et al. 2018), the resulting chancewe can conduct the follow-up observation will be approx-imately 9% of BNSs, of which the most probable point islocated in the observable sky for HSC. Assuming the BNSrate of 110-3840 Gpc − yr − shown in Abbott et al. (2019)and the mean ranges of LIGO during each observing run,the numbers of BNS-merger events expected to be detectedby LIGO are 0 . ∼ . . ∼ . . ∼ . . ∼ . . ∼ . . ∼ . The GW detection S190510g, that may include NSs, hadbeen reported by the LIGO/Virgo Collaboration on May10, 2019. For this event, we performed a ToO observationwith Subaru/HSC in Y -band for the optical-counterpart Fig. 9.
BNS range during each observing runshown in the document of observing run plans(https://dcc.ligo.org/public/0161/P1900218/002/SummaryForObservers.pdf).Dots with error bar are the distance (left vertical axis) of GW events reportedin O1, O2 and O3. The right vertical axis refers to a peak magnitude of thelight curve in z -band in the same kilonova model as Fig. 8 on an assumptionthat it is located at a distance shown in the left vertical axis. Black dashedline indicates the expected limiting magnitude in HSC-z band. survey as early as 1.7 hours after the issue of its prelimi-nary alert. Our observation area, which was selected fromthe preliminary localization skymap, covers 118.8 deg . Itcorresponds to 11.6% of the total probability in the lo-calization skymap released in the preliminary alert, and1.2% in the updated skymap. We searched for an optical-counterpart by dividing the observed area into two fields,depending on whether a previous reference HSC image isavailable.For the fields with HSC-SSP reference images, wesearched for optical counterpart by using the image sub-traction. We obtained 83 candidates through screeningsources in the difference images. For the fields withoutHSC-SSP reference images, we searched our individual ob-servation images by matching the observed sources withPS1 catalog, and found 50 candidates except the sources lo-cated at the center of extended object. We, then, estimatetheir distance with photometry of associated extended ob-jects. Finally, we concluded three sources (Cand-A10,Cand-B01 and Cand-B02) as final candidates of the elec-tromagnetic counterpart of S190510g because these candi-date are likely located inside the 3D skymap. We could notrule out the possibility that 44 candidates are related tothe GW event because their distance cannot be estimated.Unfortunately, no spectroscopic observations for them areperformed. The search depth for the second method isshallower than that for the first method because we choseonly brighter source than 21.3 mag to match the sourceswith the PS1 catalog.We estimated the expected number of SN detectionsby performing mock observations. We confirmed that thenumber density of our candidates was consistent with the Publications of the Astronomical Society of Japan , (2018), Vol. 00, No. 0 expected number within the 50% uncertainty. Therefore,it may imply that most of 133 candidates could be SNe.By comparing with a radioactive kilonova model repro-ducing AT2017gfo, which is based on the radiative trasfersimulations with realistic opacity (Banerjee et al. 2020),we found that our observations were sufficiently deep todetect the kilonova emission well before the peak at ∼ . ∼ . . ∼ . . ∼ . within afew hours after the GW event. For future GW observingruns, most of the GW events will be discovered at > Acknowledgments
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Table 1.
Central coordinates of the survey pointings and observation log.
Pointing R.A. decl. taiObs(ID) (J2000) (J2000) (UTC)000 13 h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . ublications of the Astronomical Society of Japan , (2018), Vol. 00, No. 0 Table 1. (Continued)
Pointing R.A. decl. taiObs(ID) (J2000) (J2000) (UTC)047 13 h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
03 +01 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
52 +01 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
02 +00 ◦ ′ ′′ . h m s .
77 +02 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
52 +02 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
53 +01 ◦ ′ ′′ . h m s .
02 +01 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
53 +03 ◦ ′ ′′ . h m s .
52 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
53 +02 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
28 +02 ◦ ′ ′′ . h m s .
53 +04 ◦ ′ ′′ . h m s .
04 +01 ◦ ′ ′′ . h m s .
79 +01 ◦ ′ ′′ . h m s .
28 +04 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
02 +03 ◦ ′ ′′ . h m s .
77 +05 ◦ ′ ′′ . h m s .
04 +02 ◦ ′ ′′ . h m s .
52 +05 ◦ ′ ′′ . h m s .
79 +02 ◦ ′ ′′ . h m s .
54 +01 ◦ ′ ′′ . h m s .
27 +04 ◦ ′ ′′ . h m s .
77 +04 ◦ ′ ′′ . h m s .
30 +06 ◦ ′ ′′ . h m s .
52 +03 ◦ ′ ′′ . h m s .
26 +05 ◦ ′ ′′ . h m s .
54 +02 ◦ ′ ′′ . Publications of the Astronomical Society of Japan , (2018), Vol. 00, No. 0
Table 1. (Continued)
Pointing R.A. decl. taiObs(ID) (J2000) (J2000) (UTC)094 13 h m s .
79 +05 ◦ ′ ′′ . h m s .
53 +04 ◦ ′ ′′ . h m s .
27 +01 ◦ ′ ′′ . h m s .
51 +02 ◦ ′ ′′ . h m s .
77 +02 ◦ ′ ′′ . h m s .
53 +02 ◦ ′ ′′ . h m s .
78 +03 ◦ ′ ′′ . h m s .
03 +04 ◦ ′ ′′ . h m s .
02 +03 ◦ ′ ′′ . h m s .
27 +04 ◦ ′ ′′ . h m s .
78 +05 ◦ ′ ′′ . h m s .
53 +04 ◦ ′ ′′ . h m s .
28 +04 ◦ ′ ′′ . h m s .
77 +05 ◦ ′ ′′ . h m s .
28 +05 ◦ ′ ′′ . h m s .
28 +04 ◦ ′ ′′ . h m s .
03 +04 ◦ ′ ′′ . h m s .
52 +05 ◦ ′ ′′ . h m s .
77 +05 ◦ ′ ′′ . h m s .
02 +05 ◦ ′ ′′ . h m s .
04 +05 ◦ ′ ′′ . h m s .
29 +06 ◦ ′ ′′ . h m s .
52 +05 ◦ ′ ′′ . h m s .
77 +05 ◦ ′ ′′ . h m s .
79 +06 ◦ ′ ′′ . h m s .
52 +06 ◦ ′ ′′ . Table 2.
Candidates obtained in the fields with the HSC-SSP referenceimages with the image subtraction (Off center of the extended objects or Noclose objects).
Name R.A. decl. Mag. † θ sep ‡ P (J2000) (J2000) (AB) [arcsec] [%]Off-center (Outside ∗ )Cand-A01 13 h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
42 +00 ◦ ′ ′′ . h m s .
92 +01 ◦ ′ ′′ . h m s .
39 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
60 +00 ◦ ′ ′′ . h m s .
04 +04 ◦ ′ ′′ . h m s .
63 +04 ◦ ′ ′′ . † Magnitudes in the difference image before the Galactic extinction correction. ‡ Angular separations from the extended object in the PS1 catalog. ∗ Outside 3 σ region of 3D localization map ( P < ublications of the Astronomical Society of Japan , (2018), Vol. 00, No. 0 Table 3.
Candidates obtained in the fields with the HSC-SSP referenceimages using the image subtraction (Center of extended object).
Name R.A. decl. Mag. P Name R.A. decl. Mag. P (J2000) (J2000) (AB) [%] (J2000) (J2000) (AB) [%]Inside ( P ≥ h m s . − ◦ ′ ′′ . P ≤ h m s . − ◦ ′ ′′ . h m s .
70 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
54 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
81 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
65 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
65 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
24 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
40 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
76 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
29 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
35 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
12 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
66 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
98 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
12 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
63 +00 ◦ ′ ′′ . h m s .
39 +00 ◦ ′ ′′ . h m s .
63 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
86 +00 ◦ ′ ′′ . h m s .
53 +00 ◦ ′ ′′ . h m s .
19 +00 ◦ ′ ′′ . h m s .
61 +01 ◦ ′ ′′ . h m s .
24 +00 ◦ ′ ′′ . h m s .
14 +00 ◦ ′ ′′ . h m s .
45 +00 ◦ ′ ′′ . h m s .
35 +01 ◦ ′ ′′ . h m s .
55 +00 ◦ ′ ′′ . h m s .
02 +01 ◦ ′ ′′ . h m s .
41 +00 ◦ ′ ′′ . h m s .
47 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
56 +00 ◦ ′ ′′ . h m s .
38 +00 ◦ ′ ′′ . h m s .
71 +00 ◦ ′ ′′ . h m s .
20 +00 ◦ ′ ′′ . h m s .
39 +00 ◦ ′ ′′ . h m s .
60 +00 ◦ ′ ′′ . h m s .
93 +01 ◦ ′ ′′ . h m s .
26 +00 ◦ ′ ′′ . h m s .
98 +01 ◦ ′ ′′ . h m s .
36 +00 ◦ ′ ′′ . h m s .
20 +01 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
90 +02 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
72 +01 ◦ ′ ′′ . h m s .
87 +00 ◦ ′ ′′ . h m s .
71 +01 ◦ ′ ′′ . h m s .
90 +00 ◦ ′ ′′ . Publications of the Astronomical Society of Japan , (2018), Vol. 00, No. 0
Table 4.
Candidates obtained in the fields without HSC-SSP referenceimages (Off-center of extended object).
Name R.A. decl. Mag. θ sep P ‡ (J2000) (J2000) (AB) [ ′′ ] [%]Inside ( P ≥ h m s .
14 +03 ◦ ′ ′′ . h m s .
38 +03 ◦ ′ ′′ . P ≤ h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
25 +04 ◦ ′ ′′ . h m s .
39 +04 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
22 +03 ◦ ′ ′′ . h m s .
25 +04 ◦ ′ ′′ . h m s .
75 +03 ◦ ′ ′′ . ‡ Candidates classified as “No Information” are not evaluated P , becauseboth rMeanPSFMag and iMeanPSFMag are set to − Table 5.
Candidates obtained in the fields without HSC-SSP reference images (No close objects).
Name R.A. decl. Mag. Name R.A. decl. Mag.(J2000) (J2000) (AB) (J2000) (J2000) (AB)Cand-B11 13 h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
87 +00 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
82 +05 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
11 +05 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
27 +02 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
73 +02 ◦ ′ ′′ . h m s .
30 +05 ◦ ′ ′′ . h m s .
26 +02 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
99 +04 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
90 +03 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
34 +00 ◦ ′ ′′ . h m s .
69 +02 ◦ ′ ′′ . h m s .
90 +02 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
53 +04 ◦ ′ ′′ . h m s .
49 +03 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
23 +03 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
90 +03 ◦ ′ ′′ . h m s .
70 +04 ◦ ′ ′′ . h m s .
64 +03 ◦ ′ ′′ . h m s .
55 +00 ◦ ′ ′′ . h m s .
02 +03 ◦ ′ ′′ . h m s . − ◦ ′ ′′ . h m s .
39 +03 ◦ ′ ′′ ..