Simultaneous Multiband Photometry of the Early Optical Afterglow of GRB 240825A with Mephisto

Gamma-ray bursts (GRBs) are the most luminous catastrophic events in the universe. They are believed to originate from a relativistic jet powered by a newborn black hole or magnetar. Based on the duration, GRBs are generally classified as long and short GRBs (Kouveliotou et al., 1993). The long GRBs have been identified as originating from core collapses of massive stars with fast rotation based on their association with broad-line Type Ic supernovae (e.g., Galama et al., 1998; Woosley & Bloom, 2006), and the short GRBs are believed to originate from the merger of two compact objects including at least one neutron star (i.e., a binary neutron star or a binary consisting of a neutron star and a black hole) (e.g., Eichler et al., 1989; Paczynski, 1991). The afterglow of a GRB usually refers to the temporal phase of emission after the prompt gamma-ray emission has ended. As a consequence of the interaction of relativistic jets with the circumburst medium (e.g., the interstellar medium (ISM) or the stellar wind of the GRB progenitor), the relativistic ejecta from the central engine decelerates and generates a pair of relativistic shocks: a forward shock propagating into the circumburst medium and a reverse shock penetrating the GRB ejecta itself. The forward and reverse shocks accelerate particles in the circumburst medium and the ejecta, respectively, and produce multiwavelength nonthermal emission of the afterglow via the synchrotron radiation (Rees & Meszaros, 1992; Meszaros & Rees, 1993; Paczynski & Rhoads, 1993; Katz, 1994; Mészáros & Rees, 1997; Sari et al., 1998). Observations show that some afterglows of GRBs exhibit complex behaviors (e.g., Akerlof et al., 1999; Harrison et al., 1999; Berger et al., 2003) that demand sophisticated physical conditions of emission, such a medium of non-uniform density (Dai & Lu, 1998b; Chevalier & Li, 2000), continuous energy injection into the blastwave (Dai & Lu, 1998a; Zhang & Mészáros, 2001), joint forward- and reverse-shock emission (Mészáros & Rees, 1997; Kobayashi & Zhang, 2003; Zhang et al., 2003), etc.

The duration of the multiwavelength emission (ranging from radio to gamma-ray bands) of GRB afterglows ranges from seconds to days. Information on the emission in both temporal and spectral domains is required to study the properties of the GRB central engine and of the circumburst medium. Very recently, GRB 240825A was triggered by the Swift Burst Alert Telescope (BAT) at 15:52:59 UT (Gupta et al., 2024) and the Fermi Gamma-ray Burst Monitor (GBM) at 15:53:00 UT (Fermi GBM Team, 2024) on Aug. 25, 2024. The Swift–BAT light curve showed a bright peak of a complex structure of a duration of $\gtrsim$10 s, suggesting a long GRB candidate. The Swift–BAT count rate peaked at $\sim$1 sec after the trigger and reached $\sim 1.6\times 10^{5}$ counts/s in the band of 15-350 keV. The UVOT on board of the Swift took an exposure of 150 s duration with the white filter 92 seconds after the BAT trigger, and measured the optical afterglow at a position of ${\rm RA}=344.57192$ deg and ${\rm Dec}=1.02686$ deg, with a 90%-confidence error radius of about 0.74 arcsec (Gupta et al., 2024). Most multiband or multimessenger facilities all over the world performed follow-up observations of GRB 240825A, including Mephisto (Zhang et al., 2024), Nanshan/HMT (Jiang et al., 2024), Skynet (Dutton et al., 2024), AKO (Odeh & Guessoum, 2024a, b), Global MASTER-Net (Lipunov et al., 2024c, a), GMG (Li et al., 2024; Wang et al., 2024a), LCO (Izzo & Malesani, 2024), Fermi-LAT (Di Lalla et al., 2024), MASTER (Lipunov et al., 2024b), Montarrenti Observatory (Leonini et al., 2024), SVOM/C-GFT (SVOM/C-GFT Team et al., 2024) (37292, 37373), VLT/X-shooter (Martin-Carrillo et al., 2024), REM (Brivio et al., 2024), AstroSat CZTI (Joshi et al., 2024), MISTRAL/T193 OHP (Le Floc’h et al., 2024), Konus-Wind (Frederiks et al., 2024), PRIME (Guiffreda et al., 2024), KAIT (Zheng et al., 2024), NUTTelA-TAO / BSTI (Maksut et al., 2024), TNG (Melandri et al., 2024), SAO RAS (Moskvitin et al., 2024b, a), ALMA (Laskar et al., 2024), GECAM (Wang et al., 2024b), VLA (Peña et al., 2024), IceCube (IceCube Collaboration, 2024), iTelescope (Romanov, 2024), SVOM/VT (SVOM/VT commissioning Team et al., 2024), etc. The redshift of GRB 240825A was measured at $z=0.659$ based on the detection of multiple absorption features of Fe ii, Fe ii^∗, Mn ii, Mg ii, Mg i, Ca ii, and Na i of the host galaxy (Martin-Carrillo et al., 2024).

In this work, we present and analyze simultaneous multiband photometry of the optical afterglow of GRB 240825A based on the observations of the Multi-channel Photometric Survey Telescope (Mephisto). We adopt the Swift–BAT trigger time 15:52:59 UT on 2024 August 25 as the phase zero $T_{0}$. All times in the results and figures are relative to $T_{0}$. The paper is organized as follows. We describe our photometric observations and present the measurements of the optical afterglow of GRB 240825A in Section 2. We analyze the physical properties of both the optical afterglow of GRB 240825A and of the host galaxy in Section 3. The temporal and spectral evolutions of the afterglow are discussed in Section 3.1 and the spectral properties of the host galaxy are discussed in Section 3.2. The results are discussed and summarized in Section 4. The convention $Q_{x}=Q/10^{x}$ is adopted in cgs units unless otherwise specified.

The photometric observations of GRB 240825A were conducted with the Mephisto telescope. Mephisto is developed and operated by the South-Western Institute for Astronomy Research, Yunnan University. It is located at the Lijiang Observatory (IAU code: 044) of Yunnan Astronomical Observatories, Chinese Academy of Sciences (Wang et al., 2019), and is the first wide-field multi-channel photometric telescope of its kind worldwide. Currently, Mephisto is in the commissioning phase, equipped with two Andor Technology single-chip (e2v CCD231-C6 $6144\times 6160$ sensor) CCD cameras for the blue ($uv$) and yellow ($gr$) channels, respectively. The red channel ($iz$) is equipped with a single-chip (e2v CCD 290-99 $9216\times 9232$, 10$\mu$m sensor) camera developed at the NAOC. The spatial sampling of the red channel images, $0^{\prime\prime}.286/\text{pix}$, is better than that of the yellow and blue channel images, $0^{\prime\prime}.429/\text{pix}$. The fields of view of all those channels are $\sim$43 arcmin $\times$43 arcmin. Those will be increased to 2 sq. deg. once the mosaic cameras currently under development are implemented, expected to be in the spring of 2025. The three CCD cameras allow simultaneous imaging in the $u$ or $v$ (blue), $g$ or $r$ (yellow), and $i$ or $z$ (red) bands for each pointing. The wavelength ranges of the $u,v,g,r,i$, and $z$ filters are 320–365, 365–405, 480–580, 580–680, 775–900, and 900–1050 nm, respectively, with central wavelengths at 345, 385, 529, 628, 835, and 944 nm, respectively (Yang et al., 2024; Chen et al., 2024). A comparison of the transmission curves of the Mephisto filters and of other systems is shown in Figure 1 of Yang et al. (2024).

The simultaneous multiband photometric observations with Mephisto began 128 seconds after the Swift Burst Alert Telescope (BAT) triggered and located GRB 240825A. For dense sampling and better signal-to-noise ratios (SNRs), the exposure times of the three channels were adjusted to compensate the different bandwidths and efficiencies of the three channels. From 15:55:07 to 16:55:05 UTC on 2024-08-25, initial exposure times were 180, 60, and 120 seconds in the blue ($uv$), yellow ($gr$), and red ($iz$) channels, respectively. During the night, 3, 10, and 5 frames were first collected in the $vrz$ bands, followed by the same number of exposures in the $ugi$ bands. The sequence was then repeated with additional 3, 10, and 5 frames in the $ugi$ bands, and finally, the same exposures in the $vrz$ bands. In total, 6, 6, 20, 20, 10, and 10 frames were obtained in the $u,v,g,r,i$, and $z$ bands, respectively, during the first night. In the following three nights, the exposure times were increased to 300 seconds per band, and 14, 9, 14, 9, 9, and 9 frames in total were collected in the $u,v,g,r,i$, and $z$ bands, respectively.

The raw frames were preprocessed using a dedicated pipeline developed for Mephisto (Fang et al. 2024, in preparation), including bias and dark subtraction, flat-fielding, and removal of satellite trails and cosmic rays. Astrometric calibration was performed using stars from the Gaia DR3 catalog (Gaia Collaboration et al., 2023) as references. Aperture photometry was then performed using SExtractor (Bertin & Arnouts, 1996). The photometric calibration was carried using recalibrated Gaia BP/RP (XP) spectra with the Synthetic Photometry Method (Huang et al., 2024; Xiao et al., 2023). Synthetic magnitudes in the AB system were calculated by convolving the XP spectra with the transmission curves of the Mephisto in different bands. The XP spectra cover wavelengths from 336 to 1020 nm, which do not fully encompass wavelength ranges of the Mephisto $u$ and $z$ bands. Therefore, we applied linear extrapolation to the XP spectra to cover missing wavelengths. The typical uncertainty in synthetic $u$ and $z$ magnitudes resulting from this extrapolation was estimates to be 0.016 mag and 0.0007 mag. We then determined the difference $\Delta m$ between the instrumental and synthetic magnitudes for good-quality non-variable stars in each frame. To mitigate the gain measurement errors, we calculated the weighted mean value of $\Delta m$ in each CCD output and applied it for the photometric calibration of targets located in that output. The weights were calculated from errors incorporating considerations from both the synthetic and instrumental magnitude uncertainties. The uncertainties in the photometric calibration were estimated to be better than 0.03, 0.01, and 0.005 mag in the $u$, $v$, and $griz$ bands, respectively. Note that the transmission efficiency curves of Mephisto in the individual bands were calculated based on the measured efficiency curves of the individual elements in the optical path from the primary mirror to the detector.

For observations obtained in the second to fourth nights, we co-added multiple exposures in each band to enhance the SNR for better target detection. The image with the smallest median FWHM was selected as the reference for flux alignment. Flux levels of the remaining images were then scaled to that of the reference image using stars with SNRs $>$20. The same procedures as described above were applied to obtain aperture photometry in the co-added images. Photometric calibration was carried out using the zero-point values estimated from the reference image. The 3-sigma limiting magnitudes were estimated from the co-added images if the source was not clearly detected.

We retrieved images of the host galaxy of GRB 240825A from the Panoramic Survey Telescope and Rapid Response System (PanSTARRS) (Kaiser et al., 2002) archive¹¹1http://ps1images.stsci.edu/cgi-bin/ps1cutouts and conducted aperture photometry on the stacked images in the PanSTARRS-1 $rizy$ bands. Since the source was not detected in the stacked PanSTARRS-1 $g$-band image, we estimated the 5-$\sigma$ limiting magnitude for this band. Photometric calibration was performed using the zero-point values stored in the image headers. The limiting magnitude obtained for the $g$-band is 23.70 mag, while the measured magnitudes in the $rizy$ bands are 22.61$\pm$0.19, 22.09$\pm$0.13, 22.03$\pm$0.26, and 21.71$\pm$0.21 mag, respectively, before corrected for the foreground extinction of the Milky Way. We also retrieved the target images from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP, Aihara et al., 2018), a wide-field multi-band imaging survey conducted with the Subaru 8.2 m telescope. The survey reaches a 5-$\sigma$ depth of 26.5, 26.5, 26.2, 25.2, and 24.4 mag in the $grizy$ bands, respectively (Aihara et al., 2022). We performed aperture photometry on the images of those bands. The images were calibrated against PanSTARRS and have a constant zero-point of 27.0 mag. The measured magnitudes in the $grizy$ bands are 23.56$\pm$0.13, 22.72$\pm$0.09, 21.93$\pm$0.06, and 21.79$\pm$0.06, and 21.54$\pm$0.06 mag, respectively, before the Galactic extinction correction. Comparing the above values with our own measurements where we positively detected the target in the $gri$ bands on the second night and in the $griz$ bands in the third night, it appears that the GRB afterglow still has some contribution to the detected signals in the second and third nights. We also queried the Sloan Digital Sky Survey (SDSS) DR17 archive²²2https://skyserver.sdss.org/dr17. The query yields host galaxy magnitudes before the Galactic extinction correction of 25.42$\pm$1.15, 23.12$\pm$0.31, 22.16$\pm$0.19, 21.82$\pm$0.23, and 21.69$\pm$0.70 mag in the SDSS $ugriz$ bands. Comparison with our data further indicates that contributions from the GRB afterglow persist in our observations on the second and third nights. However, we note that the queried SDSS magnitudes have almostly reached or exceeded the detection depths limits of the SDSS photometric survey ³³3https://www.sdss4.org/dr17/imaging/other_info/#DepthsoftheSDSSphotometricsurvey. Thus, caution is warranted when interpreting these values.

The foreground extinction from the Milky Way in the line of sight of GRB 240825A is $E_{B-V}=0.0534$ mag (Schlegel et al., 1998; Schlafly & Finkbeiner, 2011). Using the extinction law of Fitzpatrick (1999) assuming a total to selective extinction ratio of $R_{V}=3.1$, the extinction values in the $uvgriz$ bands are 0.264, 0.241, 0.172, 0.135, 0.086, 0.070 mag, respectively. In Figure 1, we present the observed light curves of the optical afterglow of GRB 240825A obtained with Mephisto in the $uvgriz$ bands, after correcting for the Galactic extinction. The data presented in Figure 1 are listed in Table LABEL:tab of the Appendix along with the mid times of the measurements. Our measurements show that the early optical afterglow reaches 16.49, 14.32, 13.54 mag in the $vrz$ bands, respectively, at $\sim$130 s, and 18.89, 17.31 16.33 mag in the $ugi$ bands, respectively, at $\sim$1000 seconds after the GRB trigger. The optical afterglow in multi-bands showed a significant power-law decay. On the fourth day after the GRB trigger, our measured magnitudes approach those of the host galaxy as given by the SDSS, PanSTARRS, and HSC-SSP observation obtained before the burst.

GRB 240825A was recently triggered by Swift–BAT at 15:52:59 UT on Aug. 25, 2024. In this work, we present the simultaneous multiband photometry of its early optical afterglow obtained with the Mephisto telescope which is capable of simultaneously imaging the same patch of sky in three bands. Mephisto observations began 128 seconds after the Swift–BAT trigger, and the light curves in the $uvgriz$ bands follow a single power-law decay. This early set of observations provides us with a strong constraint on the onset of the afterglow at the blastwave deceleration radius as well as on the initial Lorentz factor $\Gamma_{0}$ of the GRB ejecta. For two scenarios of the circumburst medium being composed of the general ISM and of the stellar winds, we find $\Gamma_{0}\gtrsim 139~{}{\rm and}~{}52$, respectively.

Based on the measured multiband light curves in $uvgriz$ bands, we find that the decay of the flux density of the early optical afterglow satisfies $F_{\nu,{\rm obs}}\propto t^{-\alpha_{\rm obs}}\nu^{-\beta_{\rm obs}}$ with a temporal decay index of $\alpha_{\rm obs}=1.340\pm 0.002$ and a spectral index of $\beta_{\rm obs}=2.477\pm 0.006$. It is noteworthy that the observed SED of GRB 240825A is much softer than those of most other GRBs (e.g., Kann et al., 2010, 2011). In particular, in the golden sample comprising 48 GRBs of Kann et al. (2010), there are only two sources (i.e., GRB 070802 and GRB 080310) with spectral indexes larger than $\beta_{\rm obs}\sim 2$ and most GRBs have $\beta_{\rm obs}\sim 1$ with uncertainties of $0.01-0.1$. This interesting discovery of the very soft SED of GRB 240825A may be attributed to the simultaneous photometry in the wide $uvgriz$ bands of Mephisto, especially for the contribution from the blue bands. And a relatively low uncertainty of the spectral index is obtained due to the same reason.

Assuming that the measured temporal decay index is intrinsic, $\alpha=\alpha_{\rm obs}=1.340$, there are only three main possible scenarios for the standard afterglow model to explain the feature of the optical afterglow:

1) Case I: ISM slow cooling, where $\nu_{a}<\min(\nu_{m},\nu_{c})$ with $\nu_{m}<\nu<\nu_{c}$, or $\nu_{m}<\nu_{a}<\nu_{c}$ with $\nu_{a}<\nu<\nu_{c}$. In this case, an electron spectral index of $p=2.79$ is required and the intrinsic spectral index directly inferred from $\alpha_{\rm obs}$ is $\beta(\alpha_{\rm obs})=0.89$.

2) Case II: wind slow cooling, where $\nu_{a}<\min(\nu_{m},\nu_{c})$ with $\nu_{m}<\nu<\nu_{c}$, or $\nu_{m}<\nu_{a}<\nu_{c}$ with $\nu_{a}<\nu<\nu_{c}$. In this case one has $p=2.12$ and $\beta(\alpha_{\rm obs})=0.56$.

3) Case III: ISM/wind slow/fast cooling, where $\nu>\max(\nu_{c},\nu_{m})$. In this case one has $p=2.45$ and $\beta(\alpha_{\rm obs})=1.23$.

The above results show that our measured spectral index of $\beta_{\rm obs}=2.477$ is much larger than those directly inferred from $\alpha_{\rm obs}$ in the standard afterglow model. The result implies that the blue parts of the optical afterglow of GRB 240825A might be partially absorbed by the circumburst medium, leading to an extra extinction. We then further check whether the extinction evolves with the afterglow decay. Using the afterglow temporal power-law decay to fit independently the light curves in different bands, we infer the magnitudes at different epochs via interpolation. Benefiting from the advantage of the simultaneous imaging in three channels of Mephisto, we find that the SED of the optical afterglow hardens as the afterglow decays, with the inferred color excess $E_{B-V}$ decreasing by $\sim 0.26$ mag from 100 seconds 3000 seconds. This result suggests that the dust in the cirbumburst medium of GRB 240825A evolves as the afterglow decays.

Another possibility of the SED hardening shown in the top panel of Figure 2 is due to the passage of a synchrotron characteristic frequency (e.g., $\nu_{m}$ or $\nu_{c}$) within the optical wavelength range. However, such a possibility is disfavored due to the following reasons. The observed very soft SED implies that it should be near the last segment of the spectrum of synchrotron radiation (e.g., Sari et al., 1998). According to the standard afterglow model, the difference between the spectral indexes of the last two segments has two cases: $\Delta\beta=1/2$ or $\Delta\beta=(p-1)/2$ (e.g., Sari et al., 1998; Gao et al., 2013; Zhang, 2018). However, the maximum difference of the observed spectral indexes in the first two days reaches $\Delta\beta_{\rm obs}\gtrsim 1$, which is significantly larger than the predicted $\Delta\beta$ without the extinction. Therefore, the observed SED hardening seems not likely attributed to the passage of a synchrotron characteristic frequency. On the other hand, since the very soft SED directly reveals the existence of the extinction, it is more natural to attribute the SED hardening to the extinction evolution based on the principle of Occam’s razor.

Since long GRBs have been proposed to occur in star-forming dusty regions of a galaxy (Paczyński, 1998), the effects of dust extinction are important for understanding the environments and progenitors of long GRBs. In fact, only about 60% of Swift GRBs are detected in optical wavelengths, and the remaining 40% (so-called “dark GRBs”) is undetectable in the optical bands, as a result of the attenuation of optical photons by the intervening dust (e.g., Draine & Hao, 2002). Some evidence shows that the extinction curve toward the individual GRBs could be described by an SMC-like curve with a steep far-UV rise, implying the predominance of small grains in the local environments near the GRB sources (e.g., Heintz et al., 2017; Zafar et al., 2018). Besides, some early optical afterglows of GRBs, e.g, GRB 111209A (Stratta et al., 2013) and GRB 120119A (Morgan et al., 2014), show significant red-to-blue color change within a few hundred seconds after the prompt emission phase. Such a red-to-blue change also appears in our observations of the early afterglow of GRB 240825A, see the discussion in Section 3.1 shown. Thus, GRBs are good probes of their immediate environment, and the blue optical bands carry information about extinction, probing the existence of dust in the GRB emission region. In physics, Hoang et al. (2020) studied rotational disruption and alignment of dust grains by radiative torques induced by afterglows and found that optical-near-infrared extinction decreases and ultraviolet extinction increases due to the enhancement of small grains. This mechanism might explain the feature of the afterglow of GRB 240825A, see Section 3.3 of Hoang et al. (2020) for a detailed discussion.

Finally, we also analyze the properties of the host galaxy of GRB 240825A, based on the data from the SDSS, PanSTARRS and HSC-SSP surveys. Using the SED models of galaxies (Abdurro’uf et al., 2021), the star formation rate, the stellar mass, the stellar metallicity and the stellar evolving age of the stellar population of the host GRB galaxy are estimated to be $\log({\rm SFR}/M_{\odot}{\rm yr}^{-1})=0.6^{+0.8}_{-3.3}$, $\log(M_{*}/M_{\odot})=10.0^{+0.3}_{-0.3}$, $\log(Z/Z_{\odot})=-1.3^{+0.6}_{-0.5}$, $\log({\rm age_{sys}}/{\rm Gyr})=-0.5^{+0.5}_{-0.3}$, respectively, pointing to a gas-rich, star-forming, medium-size galaxy. Savaglio et al. (2009) presented the properties of the host galaxies of the sample including 46 GRBs. The average stellar mass is $10^{9.3}M_{\odot}$ with 1$\sigma$ dispersion of 0.8 dex. The average metallicity for a subsample is about $1/6$ of the solar’s. The star formation rate is $(0.01-36)M_{\odot}{\rm yr^{-1}}$. It is worth noting that GRB hosts are smaller in size and they have higher stellar mass and star formation rate surface densities than field galaxies (Schneider et al., 2022), and almost $\sim 20\%$ of GRBs are heavily dust-obscured (Perley et al., 2016a, b). For the host galaxy of GRB 240825A, its properties are relatively moderate in the GRB sample but with relatively low metallicity. Although its optical afterglow shows a significant extinction, both the evolving properties of the afterglow and the host location in the color-magnitude diagram suggest that its extinction is more likely from the circumburst medium.

Mephisto is developed at and operated by the South-Western Institute for Astronomy Research of Yunnan University (SWIFAR-YNU), funded by the “Yunnan University Development Plan for World-Class University” and “Yunnan University Development Plan for World-Class Astronomy Discipline”. The authors acknowledge supports from the “Science & Technology Champion Project” (202005AB160002) and from two “Team Projects” – the “Innovation Team” (202105AE160021) and the “Top Team” (202305AT350002), all funded by the “Yunnan Revitalization Talent Support Program”. Y.P.Y. is supported by the National Key Research and Development Program of China (2024YFA1611603), the National Natural Science Foundation of China grant No.12473047 and the National SKA Program of China (2022SKA0130100). J.Z. acknowledges financial support from NSFC grant No. 12103063. X. H. and P. Z. are supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDB0550101. L. X. is supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDB0550401. C. W. is supported by the SVOM project, a mission in the Strategic Priority Program on Space Science of CAS. We thank the anonymous referee for providing helpful comments and suggestions. We also acknowledge the helpful discussions with Bing Zhang, Yi-Zhong Fan, Ye Li, and Yun Wang.

The Pan-STARRS1 Surveys (PS1) and the PS1 public science archive have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen’s University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation Grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), the Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation.

The Hyper Suprime-Cam (HSC) collaboration includes the astronomical communities of Japan and Taiwan, and Princeton University. The HSC instrumentation and software were developed by the National Astronomical Observatory of Japan (NAOJ), the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), the University of Tokyo, the High Energy Accelerator Research Organization (KEK), the Academia Sinica Institute for Astronomy and Astrophysics in Taiwan (ASIAA), and Princeton University. Funding was contributed by the FIRST program from the Japanese Cabinet Office, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), the Japan Society for the Promotion of Science (JSPS), Japan Science and Technology Agency (JST), the Toray Science Foundation, NAOJ, Kavli IPMU, KEK, ASIAA, and Princeton University.

This paper makes use of software developed for Vera C. Rubin Observatory. We thank the Rubin Observatory for making their code available as free software at http://pipelines.lsst.io/.

This paper is based on data collected at the Subaru Telescope and retrieved from the HSC data archive system, which is operated by the Subaru Telescope and Astronomy Data Center (ADC) at NAOJ. Data analysis was in part carried out with the cooperation of Center for Computational Astrophysics (CfCA), NAOJ. We are honored and grateful for the opportunity of observing the Universe from Maunakea, which has the cultural, historical and natural significance in Hawaii.

Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the Participating Institutions. SDSS-IV acknowledges support and resources from the Center for High Performance Computing at the University of Utah. The SDSS website is www.sdss.org.

SDSS-IV is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS Collaboration including the Brazilian Participation Group, the Carnegie Institution for Science, Carnegie Mellon University, Center for Astrophysics — Harvard & Smithsonian, the Chilean Participation Group, the French Participation Group, Instituto de Astrofísica de Canarias, The Johns Hopkins University, Kavli Institute for the Physics and Mathematics of the Universe (IPMU) / University of Tokyo, the Korean Participation Group, Lawrence Berkeley National Laboratory, Leibniz Institut für Astrophysik Potsdam (AIP), Max-Planck-Institut für Astronomie (MPIA Heidelberg), Max-Planck-Institut für Astrophysik (MPA Garching), Max-Planck-Institut für Extraterrestrische Physik (MPE), National Astronomical Observatories of China, New Mexico State University, New York University, University of Notre Dame, Observatário Nacional / MCTI, The Ohio State University, Pennsylvania State University, Shanghai Astronomical Observatory, United Kingdom Participation Group, Universidad Nacional Autónoma de México, University of Arizona, University of Colorado Boulder, University of Oxford, University of Portsmouth, University of Utah, University of Virginia, University of Washington, University of Wisconsin, Vanderbilt University, and Yale University.