Abstract
The LIGO/Virgo detected a gravitational wave (GW) event, named GW200224_222234 (also known as S200224ca) and classified as a binary-black hole coalescence, on 2020 February 24. Given its relatively small localization skymap (71 deg2 for a 90% credible region; revised to 50 deg2 in GWTC-3), we performed target-of-opportunity observations using the Subaru/Hyper Suprime-Cam (HSC) in the r2 and z bands. Observations were conducted on 2020 February 25 and 28 and March 23, with the first epoch beginning 12.3 hr after the GW detection. The survey covered the highest-probability sky area of 56.6 deg2, corresponding to a 91% probability. This was the first deep follow-up (mr ≳ 24, mz ≳ 23) for a binary-black hole merger covering >90% of the localization. By performing image subtraction and candidate screening including light-curve fitting with transient templates and examples, we found 22 off-nucleus transients that were not ruled out as the counterparts of GW200224_222234 with our Subaru/HSC data alone. We also performed GTC/OSIRIS spectroscopy of the probable host galaxies for five candidates; two are likely to be located within the 3D skymap, whereas the others are not. In conclusion, 19 transients remain as possible optical counterparts of GW200224_222234; but we could not identify a unique promising counterpart. If there are no counterparts in the remaining candidates, the upper limits of the optical luminosity are erg s−1 and
erg s−1 in the r2 and z bands, respectively, at ∼12 hr after GW detection. We also discuss improvements in the strategies of optical follow-ups for future GW events.
Export citation and abstract BibTeX RIS
![](https://content.cld.iop.org/journals/0004-637X/947/1/9/revision1/apjacbd42license.gif)
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
1. Introduction
In general relativity, massive objects radiate energy via the distortion of spacetime when their motion is accelerated. This energy radiation predicted by Einstein is called a gravitational wave (GW; Einstein 1916, 1918). Astronomical objects or phenomena are expected to be sources of GW signals with a large amplitude, which may be detected by current instruments. For example, binary systems composed of compact objects such as black holes (BHs) or neutron stars (NSs) emit strong GWs at their coalescence.
In the second observation run of the GW interferometers LIGO and Virgo, they detected a GW signal from a binary-NS (BNS) coalescence using three detectors, and the localization area was constrained to 28 deg2 for a 90% credible region (GW170817; Abbott et al. 2017). The electromagnetic (EM) counterpart of GW events was observed for the first time by multiple observatories across the EM spectrum from radio to γ-rays (Soares-Santos et al. 2017; Cowperthwaite et al. 2017; Nicholl et al. 2017; Chornock et al. 2017; Valenti et al. 2017; Tanvir et al. 2017; Díaz et al. 2017; Arcavi et al. 2017; Lipunov et al. 2017; Pian et al. 2017; Troja et al. 2017; Smartt et al. 2017; Andreoni et al. 2017; Coulter et al. 2017; Kasliwal et al. 2017; Evans et al. 2017; Drout et al. 2017; Utsumi et al. 2017; Tominaga et al. 2018a). It was demonstrated that BNS mergers are accompanied by explosive EM emissions called kilonovae (e.g., Kasen et al. 2013; Perego et al. 2017; Tanaka et al. 2017; Shibata et al. 2017; Rosswog et al. 2018; Kawaguchi et al. 2018; Banerjee et al. 2020).
In contrast to NS mergers, binary-BH (BBH) mergers are not considered to be accompanied by any EM emission. However, the Fermi Gamma-ray Burst Monitor (GBM) reported the presence of a weak γ-ray transient after the detection of GW150914 (Connaughton et al. 2016). Although the physical association between GWs and γ-ray signals is ambiguous, and it is unclear whether the γ-ray signal is truly astronomical because of the low flux, various scenarios have been proposed, such as BBHs surrounded by preexisting material, for example, a circumbinary disk (Martin et al. 2018), an accretion disk surrounding a galactic center BH (Bartos et al. 2017), and remnants of gravitational collapse (Janiuk et al. 2017). Another case is the detection of an optical counterpart candidate of GW190521 by the Zwicky Transient Facility (ZTF19abanrhr). Graham et al. (2020) suggested that the EM flare is consistent with the behavior expected from a kicked BBH merger in an accretion disk of an active galactic nucleus (AGN; McKernan et al. 2019) and ruled out other scenarios (e.g., the intrinsic variability of an AGN, a supernova, a microlens, or tidal disruption). Graham et al. (2023) comprehensively searched for EM counterparts to BBH mergers and identified nine candidates. To test various scenarios for EM radiation from a BBH merger, it is still important to perform follow-ups of the BBH mergers.
The LIGO/Virgo collaboration began their third observation run (O3) in 2019 April and detected a GW event named GW200224_222234 (also known as S200224ca) using three detectors on 2020 February 24 at 22:22 UTC (LIGO Scientific Collaboration & Virgo Collaboration 2020; The LIGO Scientific Collaboration et al. 2021). They released a preliminary localization skymap derived using a software called BAYESTAR (Singer & Price 2016) on 2020 February 24 at 22:32 UTC. In this release, the GW event was classified as a BBH coalescence with a >99% confidence level and a false-alarm rate of 1.6 × 10−11 Hz (approximately one in 1975 yr). The luminosity distance was 1583 ± 331 Mpc, corresponding to a redshift of 0.30 ± 0.05, and the 90% localization sky area was as narrow as 71 deg2.
Upon receiving the alert for GW200224_222234, the Japanese collaboration for Gravitational wave ElectroMagnetic follow-up (J-GEM; Morokuma et al. 2016; Utsumi et al. 2017; Sasada et al. 2021) triggered a target-of-opportunity (ToO) observation to search for its EM counterpart on 2020 February 25 at 10:43 UTC (Ohgami et al. 2020) using the Hyper Suprime-Cam (HSC; Miyazaki et al. 2012; Kawanomoto et al. 2018; Miyazaki et al. 2018; Komiyama et al. 2018; Furusawa et al. 2018), which is a wide-field imager installed on the prime focus of the Subaru Telescope. Its field of view (FoV) of 1.77 deg2 is the largest of the current 8 m class telescopes, which makes the Subaru/HSC the most efficient instrument for the optical survey. The first exposure commenced at approximately 12.3 hr after GW detection, and the observation area reached 56.6 deg2. We also conducted additional ToO observations using the Subaru/HSC on 2020 February 28 and March 23, in the same fields as the first epoch observation. The LIGO and Virgo collaboration published the GWTC-3 catalog (The LIGO Scientific Collaboration et al. 2021) including the observations O1, O2, O3a, and O3b and also released the reanalyzed the localization skymap. The 90% localization sky area was updated from 71 deg2 to 50 deg2. Our observation area corresponds to a cumulative probability of 91% in the updated skymap.
In this paper, we describe the details of the follow-ups of GW200224_222234 using the Subaru/HSC, the candidate selection algorithm, and a list of candidates, including spectroscopic observations of the probable host galaxies of the five candidates to measure their spectroscopic redshifts using the Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy 16 (OSIRIS), which is an imager and spectrograph for the optical wavelength range, installed at the 10.4 m Gran Telescopio CANARIAS (GTC). All magnitudes are given in AB magnitudes.
2. Observations with the Subaru/HSC and Data Analysis
2.1. Target-of-opportunity Observation
We conducted optical imaging observations using the Subaru/HSC on 2020 February 25 (day 1), 28 (day 4), and March 23 (day 28) in the r2 and z bands. To trigger our ToO follow-up as rapidly as possible, we conducted the first observations in the r2 band, which had already been set in the Subaru/HSC at the time of the GW alert. The first exposure commenced on 2020 February 25 at 10:43 UTC, corresponding to 12.3 hr after GW detection. We selected 60 observation pointings to cover the high-probability area in the BAYSTAR skymap for the HEALPix grid with a resolution of NSIDE = 64, which corresponds to 0.84 deg2 pixel−2, enabling overlap of the FoVs. Figure 1 shows the survey pointing map. Our survey area covered 56.6 deg2, corresponding to a cumulative probability of 91% in the localization skymap refined in the GWTC-3 catalog using the IMRPhenomXPHM model (Pratten et al. 2021), which models higher-order spherical harmonics and spin precession. In this study, we used the localization skymap created using the IMRPhenomXPHM model as the 3D localization of GW200224_222234.
Figure 1. Observation pointings with the Subaru/HSC (filled red circles). The left and right panels are the preliminary localization skymap of GW200224_222234 (BAYESTAR; LIGO Scientific Collaboration & Virgo Collaboration 2020) and a refined version using the IMRPhenomXPHM model (GWTC-3 catalog; The LIGO Scientific Collaboration et al. 2021), respectively. The white contour lines indicate the 90% credible regions.
Download figure:
Standard image High-resolution imageOn 2020 February 25 and 28, we observed these pointings with 30 s of exposure each in pointing ID order and revisited them in each band. The revisits were conducted at least 1 hr apart with a 1' offset in each pointing to fill the CCD gaps. Note that some areas were observed again at a time interval of less than 1 hr because of the overlap between each pointing. On 2020 March 23, we changed the exposure time and the order of the pointings. The pointings at higher elevation were observed earlier. In the z band, we took the shots with a 35 s exposure each and revisited them. In the r2 band, we took the shots with 50 s exposures for the first 32 pointings and 70 s exposures for the remaining 28 pointings. The central coordinates and exposure times are shown in Table 1 (this table is published in its entirety in machine-readable format).
Table 1. Central Coordinates and Exposure Times of the Observation Pointings with the Subaru/HSC
Central Coordinate | Exposure Time | |||||||
---|---|---|---|---|---|---|---|---|
Pointing ID | R.A. (J2000.0) | Decl. (J2000.0) | 2020-02-25 (Day 1) | 2020-02-28 (Day 4) | 2020-03-23 (Day 28) | |||
(HH:MM:SS.ss) | (DD:MM:SS.s) | r2 | z | r2 | z | r2 | z | |
00 | 11:40:18.75 | −14:28:39.0 | 30 s × 2 | 30 s × 2 | 30 s × 2 | 30 s × 2 | 50 s | 35 s × 2 |
01 | 11:34:41.25 | −12:01:28.9 | 30 s × 2 | 30 s × 2 | 30 s × 2 | 30 s × 2 | 70 s | 35 s × 2 |
02 | 11:31:52.50 | −10:11:59.7 | 30 s × 2 | 30 s × 2 | 30 s × 2 | 30 s × 2 | 50 s | 35 s × 2 |
03 | 11:43:07.50 | −15:05:41.2 | 30 s × 2 | 30 s × 2 | 30 s × 2 | 30 s × 2 | 70 s | 35 s × 2 |
04 | 11:37:30.00 | −12:38:08.3 | 30 s × 2 | 30 s × 2 | 30 s × 2 | 30 s × 2 | 70 s | 35 s × 2 |
⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮ |
Only a portion of this table is shown here to demonstrate its form and content. A machine-readable version of the full table is available.
Download table as: DataTypeset image
2.2. Data Reduction and Image Subtraction
We reduced the observational data using hscPipe v4.0.5 (Bosch et al. 2018), which is a standard analysis pipeline for the HSC. This pipeline provides full packages for data analyses, which include image subtraction and source detection. We evaluated the limiting magnitudes of the stacked images in each epoch as follows: We assumed an aperture with a diameter of twice the full width at half maximum (FWHM) of the point-spread function (PSF) and distributed it randomly, avoiding existing sources. Measuring the 5σ deviations of fluxes in this aperture, we obtained a map of the 5σ limiting magnitudes for each stack image. Table 2 lists the mode, 17 maximum, and minimum values of the 5σ limiting magnitudes. The 5σ limiting magnitudes scattered with a range of ∼1 mag, and there were differences of approximately 1 mag between those of the r2 and z bands. Note that the limiting magnitudes in the r2 band on 2020 March 23 had a small positional dependence owing to the different exposure time.
Table 2. Mode, Maximum, and Minimum Values of the 5σ Limiting Magnitudes in the Stacked Images
Date | Filter | Limiting Magnitude (AB) | ||
---|---|---|---|---|
(Days from GW Detection) | Mode | Max | Min | |
2020-02-25 | r2 | 25.29 | 25.71 | 24.51 |
(Day 1) | z | 23.56 | 24.14 | 22.84 |
2020-02-28 | r2 | 24.61 | 25.13 | 23.98 |
(Day 4) | z | 23.21 | 23.76 | 22.54 |
2020-03-23 | r2 | 25.53 | 26.25 | 24.64 |
(Day 28) | z | 23.83 | 24.64 | 22.85 |
Download table as: ASCIITypeset image
We performed image subtraction for the science images obtained in the first (one day from GW detection; day 1) and second (day 4) epochs using the images taken in the third (day 28) epoch as reference images. The subtraction package is included in hscPipe v4.0.5 and is based on an algorithm in which the FWHM of the reference images is fitted to that in the science images via convolution using kernels to make their PSFs equivalent, as proposed in Alard & Lupton (1998) and Alard (1999). Table 3 lists the mode, maximum, and minimum values of the 5σ limiting magnitudes evaluated from the difference images. The positional dependence of the 5σ limiting magnitudes in the r2 band is shown in Figure 2.
Figure 2. Map (top) and histogram (bottom) of the 5σ limiting magnitudes in the difference images within the first (day 1–day 28; left) and second epochs (day 4–day 28; right) in the r2 band. The color indicates higher/lower (in red/blue) values than the mode value of each epoch.
Download figure:
Standard image High-resolution imageTable 3. Mode, Maximum, and Minimum Values of the 5σ Limiting Magnitudes in the Difference Images
Difference Image | Filter | Limiting Magnitude (AB) | ||
---|---|---|---|---|
Mode | Max | Min | ||
Day 1−28 | r2 | 24.99 | 25.56 | 24.29 |
z | 23.28 | 23.89 | 22.54 | |
Day 4−28 | r2 | 24.39 | 25.15 | 23.76 |
z | 22.97 | 23.57 | 22.23 |
Download table as: ASCIITypeset image
2.3. Source Detection and Screening
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. As a result, we obtained 5213 variable point sources. To evaluate the completeness of transient detection with image subtraction and our detection criteria, we randomly injected artificial point sources with various magnitudes into the observed images and detected them in the difference images using the same detection criteria. Figure 3 shows the completeness of transient detection in the difference images within the first (top: day 1–28) and second epochs (bottom: day 4−28). The vertical dotted lines indicate the mode values of the 5σ limiting magnitudes, and these magnitudes were approximately comparable to the magnitude for a completeness of 23% (day 1–28, r), 29% (day 1–28, z), 39% (day 4–28, r), and 32% (day 4–28, z).
Figure 3. Completeness of the transient detection in the difference images (top: day 1–28; bottom: day 4–28). The dashed curves are the detection fractions of artificial point sources as a function of magnitude. The solid curves are the detection fractions of the remaining artificial point sources after applying criteria (i)–(iv). The vertical dotted lines indicate the mode values of the 5σ limiting magnitudes.
Download figure:
Standard image High-resolution imageFigure 4 shows a flowchart of the candidate screening and classification process for the detected sources. We first checked whether the sources were associated with known objects (e.g., variable stars and AGNs) by matching these sources with the Pan-STARRS1 (PS1) catalog (Flewelling et al. 2020) within 1''. Furthermore, to classify the PS1 objects, we used a flag, objInfoFlag, that indicates whether an object is extended. As a result, we found that 2743 and 1883 sources were associated with star-like objects and extended objects, respectively. The former are likely to have originated from stellar variabilities, and thus we excluded them as candidates of an EM counterpart of GW200224_222234. The latter could be variabilities of AGNs, an EM signal from a BBH coalescence, or something else. The rate of BBH coalescence may be enhanced in galactic nuclei (e.g., McKernan et al. 2019; Tagawa et al. 2020a, 2020b). The light curves of AGNs are stochastic (e.g., Vanden Berk et al. 2004), and thus a clear difference between AGN variability and a catastrophic phenomenon such as BBH coalescence is the long-term variability. However, it was not possible to distinguish them from our three-night observation over one month alone. Therefore, we also excluded the 1883 sources associated with extended objects in this study. Further examination is left to future studies. After this selection, 587 sources remained in the sample.
Figure 4. Flowchart of the candidate screening and classification process for the detected sources after image subtraction. The number in each box represents the number of remaining sources after each process.
Download figure:
Standard image High-resolution imageAs the next step, we performed a visual inspection to exclude bogus detections and obtained 223 plausible candidates. Then, we classified these candidates according to whether the PS1 catalog included extended objects within an angular separation, θsep, from them. Here, we classified them into two groups: "off-center" candidates (1'' ≤ θsep ≤ 15'') and "hostless" candidates (θsep > 15''). We conservatively adopted a threshold (θsep = 15'') corresponding to a separation of ∼60 kpc at the distance of GW200224_222234 (z ∼ 0.3). This separation is larger than the typical size of galaxies (e.g., Kauffmann et al. 2003; Shen et al. 2003). Then, we investigated off-center candidates with large separation individually. As a result, we obtained 118 off-center candidates and 105 hostless candidates.
Furthermore, we calculated the probability P3D for 118 off-center candidates. P3D represents how likely the extended objects were to be located inside the 3D skymap of GW200224_222234. The definition and calculation method are described in Tominaga et al. (2018a). We used the r- and/or i-band Kron magnitudes (rMeanKronMag, iMeanKronMag in the PS1 catalog) of the extended objects to derive the absolute magnitudes of probable host galaxy in the observer frame. We classified them with a threshold of P3D = 0.5 and obtained 93 likely candidates within the 3σ error of the arrival distance of GW200224_222234 (P3D ≥ 0.5) and 16 candidates that were likely outside (P3D < 0.5). The remaining 9 candidates had no information of the Kron magnitudes in the r and i bands in the PS1 catalog (rMeanKronMag = iMeanKronMag = −999) and were classified as "No Info." In the following, we investigate the nature of 93 candidates inside the 3D skymap, 16 candidates outside the 3D skymap, 9 candidates without host galaxy information, and 105 hostless candidates.
3. Light-curve Fitting
3.1. Method
We performed light-curve fitting using transient templates and examples to exclude common transients such as supernovae (SNe) from the 223 candidates. We adopted a template set including the transient templates of Type Ia SNe (Hsiao et al. 2007) and core-collapse SNe (CCSNe; Type Ibc, IIP, IIL, and IIn; Nugent et al. 2002), and examples of rapid transients (RTs; Drout et al. 2014) as in Tominaga et al. (2018b). RTs were included in the template set despite their low rate (Drout et al. 2014) because their nature is still under debate, and their timescale might be similar to that of a possible EM counterpart of BBH coalescence (McKernan et al. 2019). The template set did not include other types of transients, such as superluminous SNe (Inserra et al. 2021) and SNe amplified by a gravitational lens (Oguri & Marshall 2010), because there is no indication that they are associated with BBH coalescence, and their rates are sufficiently low that no detection was expected in our observation (e.g., Quimby et al. 2014; Moriya et al. 2019). Note that the probability that the nature of detected candidates is that of these transients cannot be excluded.
We derived the light curves of 223 candidates via forced PSF photometry of the difference images at their location. The difference fluxes of the candidates were measured using the difference images with the local background subtracted. Note that the difference fluxes could not be the genuine fluxes of candidates because the time interval between the science and reference images was shorter than the typical timescales of SNe. Thus, forced PSF photometry was also performed for the stacked images without the local background subtracted. The measured fluxes included the fluxes of their host galaxies as well as the genuine fluxes of candidates and thus were adopted as the upper limits of the genuine fluxes of candidates. Then, we performed template fitting not only on the difference fluxes, but also on the upper limits.
We considered a variation in the explosion date and an intrinsic variation in templates and examples. The light-curve templates were derived with an explosion date , redshift z, and variations as conducted in Tominaga et al. (2018b). To account for the variations in supernova properties, we derived the peak absolute B-band magnitude MB
using Equation (4) in Barbary et al. (2012) by considering the stretch s, color c, and intrinsic variation I for the template of a Type Ia SN. For CCSNe and RTs, we parameterized MB
and the color excess of their host galaxy EB−V
characterizing the extinction of the host galaxy. We assumed that the extinction curve of the host galaxy was the same as that in our Galaxy (Pei 1992).
To evaluate the difference between the observed light curves and the template, we defined the following ξ value:
![Equation (1)](https://content.cld.iop.org/journals/0004-637X/947/1/9/revision1/apjacbd42eqn1.gif)
where the subscripts d and s indicate quantities derived from the difference images (d) and stacked images (s), respectively, Nobs is the number of data points of the observation, fobs and σobs are the observed flux and its error, respectively, and ftemp(p) is the template flux calculated using a template parameter set p. When , we set
to infinity because the flux in the stacked image is an upper limit of the genuine flux of the candidate.
The 5σ limiting magnitudes in the difference images correspond to 0.37 μJy (day 1−28) and 0.64 μJy (day 4−28) in the r2-band flux, and 1.77 μJy (day 1−28) and 2.36 μJy (day 4−28) in the z-band flux. Most of these candidates were detected in the r2 band, but not in the z band. If the difference flux was lower than the flux corresponding to the 5σ limiting magnitudes, we required the template flux to be fainter than the flux corresponding to the 5σ limiting magnitudes.
We applied the Metropolis–Hastings (MH) algorithm, which is a type of Markov chain Monte Carlo (MCMC) method, for the template fitting. This method can derive best-fit template parameter sets without being trapped by the local minima of ξ by probabilistically moving to a location where ξ is smaller in a parameter space. We applied the top-hat function as a prior probability distribution with the same parameter ranges as adopted in Tominaga et al. (2018b). The ranges were obtained from Barbary et al. (2012) for the Type Ia SN, from Dahlen et al. (2012) for CCSNe, and an assumption of ±1 mag variation for RTs, as shown in Table 4. We derived a Markov chain using 50,000 samples with each redshift from 0.00 to 0.50 every 0.05 using each template model (Type Ia SN, six CCSNe, and nine RTs). Each sample of the Markov chain was proposed from a Gaussian distribution with the standard deviation shown in Table 4, using the current value as the median. The initial values of each parameter were set to the central value of each range. We adopted a large number of burn-in steps to ensure a negligible dependence on the initial values. To narrow down possible templates, we selected samples with a loose threshold of ξ < 25 from all MCMC samples.
Table 4. Template Parameters Applied in the MCMC Method
Parameter | Range† | SD of Sampling‡ |
---|---|---|
Common | ||
Explosion Time ![]() | [−∞ , ∞] | 0.1 |
Type Ia SN | ||
Color c | [−0.2, 0.8] | 0.01 |
Stretch s | [0.6, 1.2] | 0.01 |
Intrinsic Variation I | [−0.3, 0.3] | 0.01 |
Type IIL normal | ||
Peak Magnitude MB | [−16.09, −18.05] | 0.01 |
Color Excess EB−V | [0, 1] | 0.01 |
Type IIL bright | ||
Peak Magnitude MB | [−17.92, −19.96] | 0.01 |
Color Excess EB−V | [0, 1] | 0.01 |
Type IIP | ||
Peak Magnitude MB | [−14.43, −18.91] | 0.01 |
Color Excess EB−V | [0, 1] | 0.01 |
Type IIn | ||
Peak Magnitude MB | [−16.98, −20.66] | 0.01 |
Color Excess EB−V | [0, 1] | 0.01 |
Type Ibc normal | ||
Peak Magnitude MB | [−16.09, −18.05] | 0.01 |
Color Excess EB−V | [0, 1] | 0.01 |
Type Ibc bright | ||
Peak Magnitude MB | [−18.46, −20.30] | 0.01 |
Color Excess EB−V | [0, 1] | 0.01 |
Rapid Transient (PS1-10ah) | ||
Peak Magnitude MB | [−16.63, −18.63] | 0.01 |
Color Excess EB−V | [0, 1] | 0.01 |
Rapid Transient (PS1-10bjp) | ||
Peak Magnitude MB | [−17.20, −19.20] | 0.01 |
Color Excess EB−V | [0, 1] | 0.01 |
Rapid Transient (PS1-11bbq) | ||
Peak Magnitude MB | [−18.48, −20.48] | 0.01 |
Color Excess EB−V | [0, 1] | 0.01 |
Rapid Transient (PS1-11qr) | ||
Peak Magnitude MB | [−18.03, −20.03] | 0.01 |
Color Excess EB−V | [0, 1] | 0.01 |
Rapid Transient (PS1-12bb) | ||
Peak Magnitude MB | [−15.29, −17.29] | 0.01 |
Color Excess EB−V | [0, 1] | 0.01 |
Rapid Transient (PS1-12bv) | ||
Peak Magnitude MB | [−18.44, −20.44] | 0.01 |
Color Excess EB−V | [0, 1] | 0.01 |
Rapid Transient (PS1-12brf) | ||
Peak Magnitude MB | [−17.39, −19.39] | 0.01 |
Color Excess EB−V | [0, 1] | 0.01 |
Rapid Transient (PS1-13ess) | ||
Peak Magnitude MB | [−17.49, −19.49] | 0.01 |
Color Excess EB−V | [0, 1] | 0.01 |
Rapid Transient (PS1-13duy) | ||
Peak Magnitude MB | [−17.77, −19.77] | 0.01 |
Color Excess EB−V | [0, 1] | 0.01 |
Note. [†] Range of top-hat distributions for prior distribution. [‡] Standard deviation of the proposed distribution for sampling.
Additionally, for the candidates associated with the PS1 extended object (i.e., off-center candidates), we confirmed the consistency between the redshifts of transient templates and the distance information of the host galaxies by applying the following criteria:
- Criterion 1–1: We eliminated templates whose redshifts were outside the redshift error range (2σ) of the PS1 extended object.
- Criterion 1-2: If the PS1 objects exhibited a large angular separation, θsep > 10'', we did not apply criterion 1–1 because the associated PS1 objects were potentially not true host galaxies.
- Criterion 2: When ξ > 25 for θsep < 10'' with criterion 1–1, we allowed the templates at a redshift higher than that of the PS1 extended objects because a true faint host galaxy may exist behind the PS1 extended object even if θsep < 10''.
Here, we used the photometric redshifts (photo-z) from the Sloan Digital Sky Survey (SDSS; York et al. 2000) catalog as the redshift of the PS1 extended object when available. If not, we estimated a single-band redshift zsingle and its standard deviation σz using the following formulae:
![Equation (2)](https://content.cld.iop.org/journals/0004-637X/947/1/9/revision1/apjacbd42eqn2.gif)
![Equation (3)](https://content.cld.iop.org/journals/0004-637X/947/1/9/revision1/apjacbd42eqn3.gif)
where ϕ = ϕ(λ, M) is the luminosity function of galaxies at a rest wavelength λ derived from the luminosity functions in the U, B, V, R, and I bands provided in Ilbert et al. (2005) and Planck cosmology (Planck Collaboration et al. 2014), A = A(D) is the surface area observed at a distance D, M = M(D; mj ) is the absolute magnitude derived with the jth band (r or i) apparent magnitude mj at D in the observer frame, and λ = λ(D; λj ) is the rest wavelength redshifted from an observed wavelength λj at D. We also estimated zsingle and σz for candidates with an SDSS photometric redshift (Table 5). Their comparison illustrates their approximate consistency (Figure 5). Here, we corrected for the Galactic extinction (Schlafly & Finkbeiner 2011) 18 when we derived absolute magnitudes from apparent magnitudes.
Figure 5. Comparison between the SDSS photometric redshifts and the redshift estimated with Equations (2) and (3).
Download figure:
Standard image High-resolution imageTable 5. Information of the Candidates that Are Consistent with the Transient Templates or Examples
Name | Coordinate (J2000.0) |
![]() | zsingle | σz | zSDSS | σSDSS |
![]() | Probable Transient Template Sets | |
---|---|---|---|---|---|---|---|---|---|
R.A. (HH:MM:SS.ss) | Decl. (DD:MM:SS.s) | ('') | |||||||
JGEM20abe | 11:43:12.36 | −15:39:25.6 | 8.4 | 0.37 | 0.13 | ⋯ | ⋯ | 77 | CCSNe (Ibc and IIL) |
JGEM20abf | 11:43:11.52 | −15:28:16.3 | ⋯ | ⋯ | ⋯ | ⋯ | ⋯ | ⋯ | CCSNe (Ibc, IIn and IIL) |
JGEM20acf | 11:41:48.24 | −14:55:29.3 | ⋯ | ⋯ | ⋯ | ⋯ | ⋯ | ⋯ | CCSNe (Ibc, IIn and IIL) |
JGEM20adp | 11:50:40.82 | −15:14:02.8 | ⋯ | ⋯ | ⋯ | ⋯ | ⋯ | ⋯ | Type Ia, CCSNe (Ibc, IIn and IIL) |
JGEM20adq | 11:50:42.55 | −15:13:57.4 | ⋯ | ⋯ | ⋯ | ⋯ | ⋯ | ⋯ | CCSNe (Ibc, IIn and IIL) |
JGEM20ads | 11:50:52.06 | −15:10:58.4 | 2.4 | 0.15 | 0.06 | ⋯ | ⋯ | 97 | Type Ia, CCSNe (Ibc, IIn, IIP and IIL) |
JGEM20aej | 11:49:46.03 | −15:01:31.8 | ⋯ | ⋯ | ⋯ | ⋯ | ⋯ | ⋯ | Type Ia, CCSNe (Ibc, IIn, IIP and IIL), RTs (13duy) |
JGEM20afe | 11:48:22.75 | −15:44:46.3 | 12.9 | 0.33 | 0.12 | ⋯ | ⋯ | 93 | CCSNe (Ibc and IIL) |
JGEM20cvb | 11:40:58.56 | −11:26:57.5 | 2.0 | ⋯ | ⋯ | ⋯ | ⋯ | ⋯ | Type Ia |
JGEM20ewa | 11:37:13.22 | −8:21:04.7 | 3.6 | 0.15 | 0.06 | ⋯ | ⋯ | 82 | Type Ia |
⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮ | ⋮ |
Note. [†] We indicate with a ellipsis when a candidate is not associated with PS1 extended objects.
Only a portion of this table is shown here to demonstrate its form and content. A machine-readable version of the full table is available.
Download table as: DataTypeset image
3.2. Results of the Fitting Analysis
Of the 223 candidates for which we performed light-curve fitting, 200 candidates were consistent with the templates of SNe, three candidates were consistent only with the template of RTs, and the remaining 20 candidates were not consistent with any templates or examples. Table 5 shows the coordinates, θsep, P3D, and probable transient templates of the 200 candidates consistent with the templates. A fitting result of JGEM20ewa with ξ = 0.6 is shown as an example of the candidates that are consistent with the transient template (Figure 6). We concluded that the most probable origin of this candidate is a Type Ia SN for a redshift of z = 0.25. The best-fit light curves are shown as solid lines in Figure 6. The top and bottom panels show the PSF flux measured in the stacked and difference images, respectively.
Figure 6. Example of the fitting results. We show the light curve of JGEM20ewa (points with error bars), and the best-fit light curve of the Type Ia SN template (solid curves); z = 0.25, days, c = −0.15, s = 0.60, and I = −0.02. The vertical axes of the top and bottom panels indicate the upper limits measured in the stacked images and the difference fluxes measured in the difference images, respectively.
Download figure:
Standard image High-resolution imageOf the 200 candidates consistent with the templates of SNe, 105 candidates were associated with the PS1 extended objects (i.e., off-center candidates), and the remaining 95 candidates had no PS1 extended objects within <15''.
Criterion 1–1 was applied to 102 out of the 105 off-center candidates, and these candidates were excluded from the final candidates. Two of the remaining candidates (JGEM20fvn and JGEM20gqu) were associated with the PS1 extended objects of z = 0.56 ± 0.05 and 0.34 ± 0.04, respectively. The positions of the candidates and PS1 objects are shown in Figure 7 in the left (JGEM20fvn) and middle panels (JGEM20gqu). Because JGEM20fvn and JGEM20gqu exhibited θsep = 13''8 and 11''0 respectively, criterion 1–2 was applied to them. We found that the templates of Type Ibc CCSNe with z = 0.2 and 0.15 reproduced the light curves of JGEM20fvn and JGEM20gqu, respectively. These results indicated that the PS1 objects associated with JGEM20fvn and JGEM20gqu were unlikely to be true host galaxies and were likely to be Type Ibc CCSNe with z = 0.2 and 0.15. Thus, we ruled out these two objects from the final candidates. Criterion 2 was applied to the one remaining candidate (JGEM20hgo). With criterion 2, we found that a template of Type Ibc CCSN with z = 0.1 can reproduce the light curve of JGEM20hgo. The redshift of the template was higher than the redshift of the PS1 object associated with JGEM20hgo (z = 0.03 ± 0.01). Therefore, we also ruled out JGEM20hgo from the final candidates because it could be a Type Ibc CCSN behind the PS1 object. The stacked and difference images of JGEM20hgo and the associated PS1 object are shown in right panels of Figure 7. The parameters θsep and P3D of candidates consistent with the transient templates are shown in Table 5.
Figure 7. Observed (top) and difference (bottom) r2-band images of JGEM20fvn, JGEM20gqu, and JGEM20hgo on 2020 February 25. Each image displays the 20'' × 20'' region. Although the PS1 extended objects (red pluses) are associated with the candidates (reverse L markers), these are unlikely to be the true host galaxies of the candidates.
Download figure:
Standard image High-resolution imageOf the 20 candidates that were inconsistent with the templates and examples, one candidate (JGEM20cvb) exhibited a high ξ value of 2175. However, it apparently matched the template light curve, as shown in Figure 8. This high ξ could be attributed to the underestimation of photometric errors or the uncertainties on the template fluxes. For instance, ξ was improved to ∼2.4 by assuming a template difference flux error of 0.1 mag. Thus, we concluded that the origin of JGEM20cvb is a Type Ia SN and excluded it from the final candidates.
Figure 8. Light curve of JGEM20cvb (points with error bars), and the best-fit light curve of the Type Ia SN template (solid curves); z = 0.05, d, color = −0.20, s = 1.07, and I = 0.05.
Download figure:
Standard image High-resolution imageWe also performed template fitting without the single-band photometric redshifts for the candidates associated with extended objects without SDSS photometric redshifts. We found that the redshifts of the best-fit templates were consistent for most of the candidates and confirmed that the consistent templates did not change for candidates other than the candidates discussed above, that is, JGEM20fvn, JGEM20gqu, and JGEM20hgo.
As a result, 22 objects, that is, 3 objects consistent only with RTs and 19 objects inconsistent with all templates and examples, remained as the final candidates of the optical counterpart of GW200224_222234. Their coordinates, θsep, P3D, and fitting results are shown in Table 6 and Figure 9.
Figure 9. Light curves of 19 candidates inconsistent with the templates of SNe and 3 candidates consistent with RTs alone. The best-fit (but poor-fit for the former 19 candidates) templates or examples are also shown for comparison. The vertical axis of each panel indicates the upper limits measured in the stacked images (top) and the difference fluxes measured in the difference images (bottom).
Download figure:
Standard image High-resolution imageTable 6. Information on Candidates that Are Inconsistent with the Templates of SNe and Their Associated PS1 Extended Objects
Name | Coordinate (J2000.0) |
![]() |
![]() | Best-fit Transient Templates | |
---|---|---|---|---|---|
R.A. (HH:MM:SS.ss) | Decl. (DD:MM:SS.s) | ('') | |||
JGEM20acc | 11:41:52.70 | −15:08:22.9 | ⋯ | ⋯ | CCSN Type Ibc (ξ ∼ 33) |
JGEM20aeh | 11:50:20.04 | −15:12:54.0 | ⋯ | ⋯ | CCSN Type Ibc (ξ ∼ 55) |
JGEM20boq | 11:39:19.68 | −12:44:10.0 | 8.9 | 75 | CCSN Type Ibc (ξ ∼ 334) |
JGEM20bsr | 11:37:26.83 | −12:05:34.4 | ⋯ | ⋯ | RT 10bjp (ξ ∼ 16) |
JGEM20cpd | 11:35:05.23 | −11:06:33.8 | 14.9 | 94 | CCSN Type Ibc (ξ ∼ 281) |
JGEM20dig | 11:41:48.72 | −11:45:02.9 | 11.4 | 78 | CCSN Type Ibc (ξ ∼ 26) |
JGEM20dte | 11:27:56.54 | −9:05:45.6 | ⋯ | ⋯ | CCSN Type Ibc (ξ ∼ 159) |
JGEM20ejv | 11:47:24.98 | −9:40:32.5 | 6.3 | 75 | CCSN Type Ibc (ξ ∼ 62) |
JGEM20ekz | 11:45:32.81 | −9:42:04.7 | ⋯ | ⋯ | CCSN Type Ibc (ξ ∼ 84) |
JGEM20env | 11:33:13.03 | −8:10:35.0 | 14.3 | ⋯ | CCSN Type IIL (ξ ∼ 331) |
JGEM20enw | 11:32:50.98 | −7:56:35.2 | 14.9 | 74 | RT 13duy (ξ ∼ 1) |
JGEM20eso | 11:28:42.14 | −7:48:41.4 | ⋯ | ⋯ | RT 13duy (ξ ∼ 10) |
JGEM20fci | 11:44:18.89 | −8:08:34.4 | ⋯ | ⋯ | Type Ia SN (ξ ∼ 1137) |
JGEM20fud | 11:31:23.86 | −5:55:16.0 | 7.0 | 89 | Type Ia SN (ξ ∼ 1562) |
JGEM20fxo | 11:40:19.39 | −6:36:34.9 | ⋯ | ⋯ | CCSN Type Ibc (ξ ∼ 77) |
JGEM20fyv | 11:40:00.36 | −6:01:27.5 | 5.2 | 99 | CCSN Type Ibc (ξ ∼ 422) |
JGEM20gdm | 11:45:52.56 | −6:13:49.4 | 1.6 | 91 | CCSN Type IIL (ξ ∼ 27) |
JGEM20hdq | 11:32:35.04 | −3:56:15.7 | 3.0 | 60 | RT 10bjp (ξ ∼ 187) |
JGEM20hea | 11:32:20.45 | −3:40:50.9 | 3.7 | 60 | CCSN Type Ibc (ξ ∼ 394) |
JGEM20hen | 11:32:21.91 | −3:09:21.6 | 7.3 | 98 | Type Ia SN (ξ ∼ 348) |
JGEM20hfc | 11:31:30.55 | −3:45:54.7 | 2.7 | 94 | CCSN Type Ibc (ξ ∼ 87) |
JGEM20hkw | 11:35:25.49 | −4:11:36.6 | ⋯ | ⋯ | CCSN Type Ibc (ξ ∼ 37) |
Note. [†] We indicate with an ellipsis when a candidate is not associated with PS1 extended objects.
Download table as: ASCIITypeset image
4. Spectroscopic Observations with the GTC/OSIRIS
Optical spectroscopic observations were performed using the OSIRIS instrument mounted on the 10.4 m GTC telescope located at the Roque de los Muchachos Observatory (Canary Islands, Spain). Five targets were selected for spectroscopic follow-ups to determine the spectroscopic redshift (spec-z) of the probable host galaxies of the candidates. The selected targets were the PS1 extended objects associated with JGEM20fud, JGEM20fyv, JGEM20gdm, JGEM20hdq, and JGEM20hen, which were sufficiently bright in the z band for short exposures. Owing to observation time constraints, we targeted only these five objects.
The observations were performed on 2021 February 10. The instrumental setup was the spectroscopic long-slit mode, with a R1000R grism and a slit width of 1''. Each target observation was divided into three exposures of 400 s (1200 s in total for JGEM20fud, JGEM20fyv, and JGEM20hen) and 600 s (1800 s in total for JGEM20gdm and JGEM20hdq). In addition to the targets, the spectrophotometric standard star G191-B2B was also observed using the same instrumental setup and observation conditions. Standard calibration images for bias, flat field, and the calibration lamp (HgAr+Xe+Ne) were also taken during the same night. The data were reduced using standard procedures for bias subtraction, flat-field correction, flux calibration, and atmospheric extinction correction.
The flux-calibrated spectra are shown in Figures 10 and 11. The spec-z was derived based on the identification of spectral lines. In addition to the redshifted spectral features from the galaxies, we identified a possible intervening system in the line of sight of JGEM20fud and JGEM20hen, which is consistent with an Mg absorption line located at z = 0.33. We show the coordinate, the z-band magnitude, estimated redshift, and spec-z of the target galaxies in Table 7. If an SDSS photometric redshift is not available, we show zsingle and σz calculated with Equations (2) and (3). However, note that these have large uncertainties because they are calculated from only a single band magnitude.
Figure 10. Optical spectra of candidates JGEM20fud, JGEM20fyv, JGEM20gdm, JGEM20hdq, and JGEM20hen observed with the GTC. The identified spectral features are marked by vertical dotted lines. The Na interstellar absorption feature is denoted by the solid vertical line. The emission lines identified as intervening systems are marked by dashed lines.
Download figure:
Standard image High-resolution imageFigure 11. Continuation of Figure 10.
Download figure:
Standard image High-resolution imageTable 7. Information of the Target Galaxies
Name | Coordinate of the Target Galaxy (J2000.0) |
![]() | Estimated | spec-z | Passoc | |
---|---|---|---|---|---|---|
R.A. (HH:MM:SS.ss) | Decl. (DD:MM:SS.s) | (mag) | Redshift | |||
JGEM20fud | 11:31:24.32 | −05:55:17.8 | 18.7 | 0.22 ± 0.03‡ | 0.357 | 0.76 |
JGEM20fyv | 11:40:00.36 | −06:01:22.1 | 19.0 | 0.26 ± 0.07* | 0.391 | 0.07 |
JGEM20gdm | 11:45:52.63 | −06:13:48.4 | 19.9 | 0.29 ± 0.09* | 0.398 | 1 × 10−3 |
JGEM20hdq | 11:32:35.03 | −03:56:18.6 | 20.1 | 0.56 ± 0.03‡ | 0.632 | 1 × 10−8 |
JGEM20hen | 11:32:22.41 | −03:09:22.9 | 18.4 | 0.36 ± 0.03‡ | 0.438 | 4 × 10−3 |
Note. [†] z-band magnitude of the target galaxy from the PS1 catalog. [‡] Photometric redshift zSDSS from the SDSS catalog. [*] zsingle and σz calculated with Equations (2) and (3).
Download table as: ASCIITypeset image
Figure 12 shows the 2D location and distance of the five candidates. We evaluated their associations with GW200224_222234 by comparing the 3D location of their probable host galaxies with the 3D skymap of GW200224_222234. Here, we define the probability of the association as follows:
![Equation (4)](https://content.cld.iop.org/journals/0004-637X/947/1/9/revision1/apjacbd42eqn4.gif)
where fx
is the probability distribution function (PDF) at the 3D coordinates x, and xgal is the location of the host galaxy. The integration in Equation (4) represents the cumulative probability integrated over the region with . The PDF was obtained as fx
= ρ(R. A., decl.)N(D; μ, σ2) using the 2D probability distribution ρ = ρ(R. A., decl. ) and normal distribution N(D; μ, σ2) of the distance with the mean μ = μ(R. A., decl. ) and standard deviation σ = σ(R. A., decl.) provided for each direction. To perform the integration of Equation (4) numerically, we discretized N(D) into 5000 bins from 0 to 50,000 Mpc. The discretization of ρ was performed through pixelization using HEALPix. Passoc is small when the host galaxy is located outside the highly probable region. We set the threshold to Passoc = 0.01. Table 7 also shows Passoc obtained for the five targets. Although JGEM20gdm, JGEM20hdq, and JGEM20hen were located outside the highly probable region (Passoc < 0.01) and are unlikely to be related to GW200224_222234, JGEM20fud and JGEM20fyv were close to the highly probable region (Passoc = 0.76 and 0.07, respectively) and thus are possible counterparts of GW200224_222234.
Figure 12. (Left) 2D location of the five candidates observed with the GTC/OSIRIS and the localization skymap (IMRPhenomXPHM model) released in the GWTC-3 catalog. The dots are the locations of the candidates. The gray contour line indicates the 90% credible region. (Right) Distance of each candidate and probability distribution at their location as a function of distance. The dots are the distances of the candidates. The horizontal solid and dashed lines indicate the mean and standard deviation of the probability distribution, respectively.
Download figure:
Standard image High-resolution image5. Discussion
5.1. Contamination from Supernovae
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. For the Type Ia SN, the SN light curves were generated from the evolution of the SN spectrum provided by Hsiao et al. (2007). For the CCSN, the light curves were generated from the templates provided by Nugent et al. (2002). 19 We refer to the luminosity distributions of SNe in Barbary et al. (2012) and Dahlen et al. (2012).
Because our observation depth in the z band was shallower than that in the r2 band, most of the candidates were detected only in the r2 band. Thus, we sampled mock SNe whose brightness was decaying in the r band, assuming the reference images were taken 27 days after detection because the variation in the SN magnitude during 3 days between days 1 and 4 (<0.1 mag) was negligible compared to the standard deviation of the 5σ limiting magnitude of our observation. The number density as a function of limiting magnitude is shown in Figure 13 as a dashed curve. The colored area represents the ±50% error from the 1σ error of the CCSN rate density. In the actual observation, the detection completeness for fainter sources was lower (Figure 3). Therefore, we considered two detections based on the completeness of day 1–28 and day 4–28. The solid curve in Figure 13 is the number density derived by considering the completeness. The dot with error bars was estimated with the number of transients consistent with SNe, , and our observed area of S = 56.6 deg2. The vertical error bar was defined as
by assuming that the number followed a Poisson distribution with the expected value of
. The horizontal error bar is the 1σ error of the 5σ limiting magnitude in the difference images on day 4 in the r2 band.
Figure 13. Expected number density of SN detection in the r band as a function of limiting magnitude. The colored area is the ±50% error originating from the 1σ error of the SN rate density. The dashed curve was obtained with 100% completeness, and the solid curve was obtained with the actual completeness of our survey, shown in Figure 3, under the assumption of two detections. The dot with error bars was obtained using the number density of the transients consistent with the templates of SNe. The vertical error bar is defined as by assuming that the number follows a Poisson distribution with the expected value of
. The horizontal error bar is the 1σ error of the 5σ limiting magnitude in the difference images on day 4 in the r2 band.
Download figure:
Standard image High-resolution imageOur result is consistent with the expected number density of the SN detection within a 1σ error. This verified that our detection criteria and screening process can be used to identify SNe with high completeness. Note that our result was located at the 1σ lower end of the expected value. This might be because we ignored nuclear transients in this search.
5.2. Implication in BBH Merger Models Accompanied by EM Emissions
We finally found 19 candidates of the optical counterpart of GW200224_222234 by performing light-curve fitting and spectroscopic observations. Owing to the limitation of observations, we must exclude sources located at the center of galaxies, and thus these final candidates cannot be considered variabilities of galactic nuclei, such as a kicked BBH merger in an accretion disk shown by McKernan et al. (2019). Yamazaki et al. (2016) reported a BBH merger potentially accompanied by γ-ray emissions caused by a relativistic outflow. They showed that optical afterglow peaks appear ∼4.5 × 104 s after GW detection (almost the same as the start of our first epoch of observations) if the ambient matter density is 0.01 cm−3; but the peak flux is expected to be lower than our observation limits (0.37 μJy in r band) at a distance of 1710 Mpc. Therefore, an afterglow like this could not be detected in our search.
Of the 19 candidates, 16 candidates inconsistent with all templates and examples exhibited an increasing difference flux in one band and a decreasing difference flux in the other band (e.g., JGEM20ejv), or a more rapid decline in the z band than in the r2 band (e.g., JGEM20boq). If we perform further observations to obtain deeper images than the current reference images after the candidates sufficiently decay, we can measure the genuine fluxes of these candidates instead of the difference fluxes and hence identify their natures.
The final 19 candidates are potentially unrelated to GW200224_222234. If there are no counterparts of GW200224_222234, the upper limits of optical luminosity are erg s−1 (
erg s−1) and
erg s−1 (
erg s−1) on day 1 (day 4) in the r2 band and z band, respectively. Note that these upper limits are comparable with the luminosity of a possible EM counterpart of the BBH event GW190521 (1042 erg s−1, McKernan et al. 2019).
5.3. Comparison with Sources Detected by Other Observations
For GW200224_222234, the Neil Gehrels Swift Observatory performed near-UV/X-ray observations covering an area corresponding to 79.2%/62.4% of the GW probability region and reported their detection of eight X-ray sources and three near-UV sources (Klingler et al. 2021; Oates et al. 2021), whereas no significant signal was detected in very high-energy γ-ray (Abdalla et al. 2021). We searched our sources to find out whether these sources were detected. The source JGEM20aoz was located at R. A. = 11h41m4181,
(J2000.0), 4
8 off-center of the error circle of X-ray source 7. JGEM20aoz was detected four times (two epochs and two bands) and exhibited a magnitude of 21.75 mag (21.59 mag) and 21.43 mag (21.25 mag) in the r2 and z bands, respectively, in the first-epoch (second-epoch) difference images. However, we classified it as a star-like object and ruled it out because the coordinates matched that of a point source found in the PS1 catalog. In the SIMBAD archival database, a BL Lac-type object (2FGL J1141.7-1404) is located at a position consistent with JGEM20aoz. Therefore, we conclude that JGEM20aoz likely is an AGN.
Our observation area also covered part of the area observed by the GW program in the Dark Energy Survey (DESGW). They conducted observations of GW200224_222234 using the Dark Energy Camera (DECam) in the i band on February 24, 25, 27, and 2020 March 5, with 10σ limiting magnitudes of 23.17, 23.19, 23.49, and 23.03, respectively, and reported eight transients (Morgan et al. 2020). Comparing the transients found by the DESGW with our candidates, JGEM20gfn matched the coordinates of AT2020ehw within 1''. JGEM20gfn was classified as an SN via light-curve fitting. We could not detect AT2020eho and AT2020eht because they were located at the CCD gap of our difference images. Sources were detected at the location of AT2020ehp and AT2020ehq in our difference images, but they were rejected because of the large elongation, (b/a)/(b/a)PSF = 0.6 and 0.5, respectively. Although no source was found at the position of AT2020ehy in either the stacked or the difference images, AT2020ehv and AT2020ehr were observed in the stacked images, but could not be detected in the difference images with our criterion (S/N)PSF > 5.
5.4. Comparison with a Kilonova Light Curve
Although GW200224_222234 was classified as a BBH merger, we compared our observation result with a kilonova model based on a radiative transfer simulation with an ejecta mass Mej = 0.05 M⊙ and an electron fraction Ye = 0.30–0.40 (Banerjee et al. 2020). This kilonova model can explain the observed early multicolor light curve of GW170817/AT2017gfo. The peak of the bolometric luminosity approximately scales with Mej to the power of 0.35 (see Tanaka 2016; Fernández & Metzger 2016; Metzger 2019). Ye influences the light curves through the opacity. In the low-Ye case (Ye < 0.25), the kilonova ejecta becomes Lanthanide rich and the opacity becomes higher, and thus the light curves in the early phase can be fainter by 2–3 mag than that in the adopted model of Ye = 0.30–0.40.
Figure 14 shows a comparison between the kilonova light curve and the magnitudes of two candidates whose spectroscopic redshifts of their host galaxies are consistent with the 3D skymap of GW200224_222234 (JGEM20fud and JGEM20fyv). All observed magnitudes were brighter and decayed more slowly than the kilonova at the distance of the host galaxies. They did not exhibit rapid decline in the short-wavelength components, which is a major feature of the kilonova model; thus, they are unlikely to be kilonovae.
Figure 14. Comparison of light curves between the candidates whose spectroscopic redshifts of the host galaxies are consistent with the 3D skymap of GW200224_222234 and a kilonova model with an ejecta mass Mej = 0.05 M⊙ and an electron fraction Ye = 0.30–0.40 (Banerjee et al. 2020). We assume that the kilonovae are located at the distance of each candidate.
Download figure:
Standard image High-resolution image5.5. Future Prospects
Here, we discuss the future prospects of follow-ups for kilonova events with the Subaru/HSC in the era of next-generation GW interferometers, such as an optimal upgrade of the LIGO facilities, known as "Voyager." If a BNS merger occurs at the distance of GW200224_222234 (∼1710 Mpc), can we detect it?
Figure 15 shows the r-, i-, and z-band light curves under the assumption that the kilonova is located at the estimated distance of GW200224_222234. The colored area is the uncertainty caused by the 1σ credible range of the distance. The horizontal lines with the upward arrows are the 5σ limiting magnitudes measured in the difference images in each epoch in each band (r2 and z). In the r2 band, although the observation depth in the first epoch was sufficiently deep to detect the rising phase of the light curve, the model light curve decreased rapidly and was significantly fainter than the limiting magnitude in the second epoch. The observation depths in the z band did not reach the brightness in either epoch. Thus, if a BNS merger occurs at the distance of GW200224_222234, we might be able to detect the light curve in the early phase in the r band using the power of a wide-field survey with an 8 m telescope.
Figure 15. Comparison between a kilonova model located at the estimated distance of the GW200224_222234 and the limiting magnitudes of our observation. The solid curves are the light curves of the kilonova model, the same as in Figure 14. The colored area represents the uncertainty caused by the 1σ credible range of the distance.
Download figure:
Standard image High-resolution imageBased on the follow-up of GW200224_222234, the number of sources detected only on day 1 and only in the r2 band (excluded using detection criterion (v)) reached 20137. 20 It is difficult to determine their origin using only single-epoch and single-band data.
Next, we consider what will improve the detection efficiency of future surveys. (1) If we adopt a high-cadence (one day) and continuous (over three days) observation, the kilonova at this distance will be detected on days 1 and 2 and not detected on day 3 in the r band. This will illustrate the rapidly evolving nature of the kilonova. (2) If we adopt the i band instead of the z band, the i-band observation with Subaru/HSC will reach ∼24 mag with 1 min of exposure and may detect the kilonova on day 2. This will constrain the color of the kilonova. These considerations illustrate the power of a dedicated wide-field search with an 8m class telescope, such as Subaru/HSC and the Rubin/Legacy Survey of Space and Time (LSST).
6. Summary and Conclusion
The BBH coalescence GW200224_222234 was detected by the LIGO/Virgo collaboration with their three detectors on 2020 February 24. We performed ToO observations using the Subaru/HSC in the r2 and z bands during three epochs: 2020 February 24 and 28, and March 23. We selected the observation area from the high-probability region in the preliminary localization skymap covering 56.6 deg2. The integrated probability reaches 91% in the localization skymap released in the GWTC-3 catalog.
We searched for the optical counterpart using the image subtraction technique. We adopted the images taken on the third epoch as the reference images and obtained the difference images of the first and second epochs. After screening for the sources detected in the difference images via matching with the PS1 catalog and visual inspection, we found 223 candidates. We could not include sources located at the galactic center owing to the limitations of observation. Subsequently, we classified these candidates using their angular separation from the nearby extended object and distance estimated from the photometric data. Additionally, we investigated their nature using light-curve fitting with the transient template set. We adopted the templates of the Type Ia SN and CCSN and examples of RTs and then found 201 candidates consistent with the SN templates and likely not related to GW200224_222234.
To measure the spectroscopic redshifts of the probable host galaxies of the final candidates, we also performed spectroscopic observations using the GTC/OSIRIS for the extended PS1 objects associated with the five candidates. We found that two targets (JGEM20fud and JGEM20fyv) are likely to be located inside the highly probable region. The other targets are outside the highly probable region and not related to the GW event.
As a result, we found 19 candidates as possible candidates of the optical counterpart of GW200224_222234. The light curves of 3 candidates were consistent only with those of RTs, and the light curves of the other 16 candidates were inconsistent with all transients and examples. These 19 candidates might be unrelated to GW200224_222234, but we could not establish their nature because of the lack of spectroscopic observations for the candidates. If there is no counterpart of GW200224_222234 in the 19 final candidates, the upper limits of the optical luminosity are evaluated as erg s−1 (
erg s−1) and
erg s−1 (
erg s−1) on day 1 (day 4) in the r2 and z band, respectively, from the 5σ limiting magnitudes of our observation. These upper limits are comparable with the brightness of a possible EM counterpart of the BBH event GW190521 (1042 erg s−1; McKernan et al. 2019).
We evaluated our detection criteria and screening process by comparing our result with the expected number of SN detections, which was estimated from a cosmic SN rate. The number density of the candidates consistent with the SN templates was consistent with the expected number density within the 1σ error. This indicated that our method could identify SNe with high completeness.
We also compared our sources with those found by the Swift and DESGW and found that some of our sources were associated with their sources. However, these identical sources are unlikely to be the optical counterpart of the GW.
Additionally, we discuss the implications of our result in several models of a BBH merger accompanied by EM emissions. The kicked BBH merger in an accretion disk reported by McKernan et al. (2019) cannot be the possible nature of the 16 candidates that are inconsistent with all transients because we excluded nucleus transients. The relativistic outflow model reported in Yamazaki et al. (2016) cannot be the possible nature of the candidates either because the expected flux is fainter than the 5σ limiting magnitude of our observations. It is important to perform further observations that are deeper than the observations on day 28 to identify their nature.
We also compared the light curve of the two candidates with the kilonova light curve by adopting a radiative transfer simulation that could explain the multicolor light curve of GW170817/AT2017gfo. These candidates likely did not originate from the same kilonova as AT2017gfo because these were brighter and decayed more slowly than the kilonova. We also discuss future prospects by comparing our observation depths with the kilonova light curve. If we perform a high-cadence (one day) and continuous (over three days) observation, we can reveal the rapidly evolving nature of transients. Moreover, our observation can detect a kilonova or a possible EM counterpart of the BBH event GW190521 (McKernan et al. 2019) at the distance of GW200224_222234. This demonstrates the power of 8m class telescopes, such as Subaru/HSC and Rubin/LSST.
These collaborative observations were possible thanks to the leadership of the National Astronomical Observatory of Japan (NAOJ) and the Instituto de Astrofísica de Canarias (IAC). This research was based in part on data collected at the Subaru Telescope, which is operated by the NAOJ, and in part on observations made using the GTC telescope in the Spanish Observatorio del Roque de los Muchachos of IAC under Director's Discretionary Time. The spectroscopic data were obtained using the instrument OSIRIS, built by a Consortium led by the Instituto de Astrofísica de Canarias in collaboration with the Instituto de Astronomía of the Universidad Autónoma de México. OSIRIS was funded by GRANTECAN and the National Plan of Astronomy and Astrophysics of the Spanish Government. We are grateful to the staff of the Subaru Telescope for their help with the observations of this study. We are honored and grateful for the opportunity to observe the Universe from Maunakea, which has cultural, historical, and natural significance in Hawaii. This study was supported by MEXT KAKENHI (JP17H06363) and JSPS KAKENHI (JP19H00694, JP20H00158, JP20H00179, JP21H04997). J.B.G., F.P., J.A.A.P., I.P.F., and T.M.D. acknowledge financial support from the Spanish Ministry of Science and Innovation (MICINN) through the Spanish State Research Agency, under Severo Ochoa Program 2020–2023 (CEX2019-000920-S) and the projects PID2019-107988GB-C22 and PID2019-105552RB-C43. We would like to thank Editage (www.editage.com) for the English language editing.
Footnotes
- 16
- 17
The mode values are the peak values of the histograms of the 5σ limiting magnitudes. Here, we distributed the 5σ limiting magnitudes into 10 bins ranging between the maximum and minimum values.
- 18
- 19
The light-curve templates of Type Ia SNe and CCSNe are available at the website https://c3.lbl.gov/nugent/nugent_templates.html.
- 20
This number includes bogus detections because we did not perform visual inspection.