Journal of the Korean Astronomical Society https://doi.org/10.5303/JKAS.2021.54.3.89 54: 89 ∼ 102, 2021 June pISSN: 1225-4614 · eISSN: 2288-890X Published under Creative Commons license CC BY-SA 4.0 http://jkas.kas.org

SOMANGNET:SMALL TELESCOPE NETWORKOF KOREA Myungshin Im1,2, Yonggi Kim3,4, Chung-Uk Lee5, Hee-Won Lee6, Soojong Pak7,8, Hyunjin Shim9, Hyun-Il Sung5, Wonseok Kang10, Taewoo Kim10, Jeong-Eun Heo11, Tobias C. Hinse12, Masateru Ishiguro1,2, Gu Lim1,2, Cuc T. K. Ly´12, Gregory S. H. Paek1,2, Jinguk Seo1,2, Joh-na Yoon4, Jong-Hak Woo1,2, Hojae Ahn7, Hojin Cho1,2, Changsu Choi1,2, Jimin Han7, Sungyong Hwang1,2, Tae-Geun Ji7, Seong-Kook J. Lee1,2, Sumin Lee8, Sunwoo Lee7, Changgon Kim7, Dohoon Kim8, Joonho Kim1,2, Sophia Kim1,2, Mankeun Jeong1,2, Bomi Park1,2, Insu Paek1,2, Dohyeong Kim13, and Changbom Park14 1SNU Astronomy Research Center, Department of Physics and Astronomy, Seoul National University, Gwanak-gu, Seoul 08826, Korea; [email protected] 2Astronomy Program, Department of Physics and Astronomy, Seoul National University, Gwanak-gu, Seoul 08826, Korea 3Dept. of Astronomy & Space Science, Chungbuk National University, Chungdae-ro, Seowon-gu, Cheongju, Cheongbuk 28644, Korea 4Chungbuk National University Observatory, Eunjin-ro 29, Munbaek-Myeon, Jincheon-gun, Cheongbuk 27867, Korea 5Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Korea 6Department of Astronomy and Space Science, Sejong University, 209 Neungdong-ro, Kwangjin-gu, Seoul 05006, Korea 7School of Space Research, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 17104, Korea 8Department of Astronomy and Space Science, Kyung Hee University, 1732 Deogyeong-daero, Gigeung-gu, Yongin-si, Gyeonggi-do 17104, Korea 9Department of Earth Science Education, Kyungpook National University, 80 Daehak-rho, Buk-gu, Daegu, 41566, Korea 10National Youth Space Center, Goheung, Jeollanam-do, 59567, Korea 11Gemini Observatory/NSF’s National Optical-Infrared Astronomy Research Laboratory, Casilla 603, La Serena, Chile 12Department of Astronomy, Space Science and Geology, Chungnam National University, 99 Daehak-ro, 34134 Daejeon, South Korea 13Department of Earth Sciences, Pusan National University, Busan 46241, Korea 14Korea Institute for Advanced Study, 85 Hoegi-ro, Dongdaemun-gu, Seoul 02455, Korea Received January 29, 2021; accepted April 28, 2021

Abstract: Even in an era where 8-meter class telescopes are common, small telescopes are considered very valuable research facilities since they are available for rapid follow-up or long term monitoring observations. To maximize the usefulness of small telescopes in Korea, we established the SomangNet, a network of 0.4–1.0 m class optical telescopes operated by Korean institutions, in 2020. Here, we give an overview of the project, describing the current participating telescopes, its scientific scope and operation mode, and the prospects for future activities. SomangNet currently includes 10 telescopes that are located in Australia, USA, and Chile as well as in Korea. The operation of many of these telescopes currently relies on operators, and we plan to upgrade them for remote or robotic operation. The latest SomangNet science projects include monitoring and follow-up observational studies of , supernovae, active galactic nuclei, symbiotic , solar system objects, neutrino/gravitational-wave sources, and . Key words: telescopes — methods: observational — instrumentation: photometers — instrumentation: spectrographs — surveys

1.I NTRODUCTION future. Indeed, small telescopes played important roles Small telescopes, namely optical telescopes with the in the first identification and the immediate follow-up primary mirror aperture sizes at a range of ∼0.4–1 m, observations of the electromagnetic (EM) counterpart of are still considered valuable research facilities. Despite the gravitational wave (GW) event GW170817 (e.g., Ab- their limited light gathering power in comparison to bott et al. 2017a; Arcavi et al. 2017; Coulter et al. 2017; 4-to-8 meter class telescopes, the small telescopes have Lipunov et al. 2017; Valenti et al. 2017; Im et al. 2017; specific advantages like wide fields of view thanks to Troja et al. 2017). Small telescopes have continued to their short focal lengths, and readily available telescope play important roles in MMA for performing follow-up time that is suitable for intense monitoring observations observations of GW sources (e.g., Agayeva et al. 2020; and follow-up observations of transients. Im et al. 2020) as well as neutrino sources (e.g., Hwang With the advance of time-domain astronomy and et al. 2021; Morokuma et al. 2021). Small telescopes multi-messenger astronomy (MMA), the importance of are also supporting time-critical observations and rapid small telescopes is expected to rise even more in the near transient follow-ups. As a result, there exist a number of networks of small telescopes located worldwide, e.g., Corresponding author: M. Im 89 90 Im et al. (SomangNet Collaboration)

Korea

60°N LOAO

WIT 30°N SAO, KHAO

0° CBNUO LSGT KCT, RASA36

30°S DOAO

60°S

30°W 0° 30°E 60°E 90°E 120°E 150°E 180° 150°W 120°W 90°W 60°W 30°W

Figure 1. The locations of the SomangNet telescopes. SomangNet includes telescopes on several sites across the world. the Las Cumbres Observatory Global Telescope Net- ing of knowledge and pave the way for new syner- work, LCOGT (Brown et al. 2013), the Global Rapid getic projects. Running observatories requires hands-on Advanced Network Devoted to the Multi-messenger Ad- know-how to fix problems encountered during the ob- dicts, GRANDMA (Agayeva et al. 2020), the Optical servatory operation, and a common collaborative frame- and Infrared Synergetic Telescopes for Education and work provides a way to exchange knowledge and tech- Research, OISTER (e.g., Itoh et al. 2013; Ishiguro et nical/scientific resources for mutual benefits. The com- al. 2015), the Burst Observer and Optical Transient Ex- bined capabilities of these telescopes will allow more ploring System, BOOTES (Castro-Tirado et al. 1999), science projects to be carried out than what would be Global Relay of Observatories Watching Transients Hap- possible for a single facility. pen, GROWTH (e.g., Kasliwal et al. 2019), the Mobile With these advantages in mind, we started a project Astronomical System of Telescope-Robots, MASTER to systematically organize Korean-owned small tele- (Lipunov et al. 2004; Gorbovskoy et al. 2013), the All-Sky scopes for astronomical research. The project is named Automated Survey for Supernovae, ASAS-SN (Shappee SomangNet, where the word “Somang” has two mean- et al. 2014; Kochanek et al. 2017), and the Catalina ings in Korean: it is an abbreviation of “small telescope”, Real-time Transient Survey, CRTS (Drake et al. 2009). and also means “wish”. The project idea was first dis- In Korea, there are dozens of small optical tele- cussed in 2019 within the Optical/IR/UV Astronomy scopes with apertures ∼0.4 m or larger, but most of division of the Korean Astronomical Society, and after them are being used for educational activities, and reg- a short preparation period, the project started in 2020 ular, full night operations of small telescopes are rare. with support from various organizations. Connecting these telescopes together for a common pur- In this paper, we give an overview of the SomangNet. pose can provide a number of advantages. Section2 summarizes the SomangNet telescopes and their capabilities, Section3 describes the way how So- First, it can mitigate the bad weather conditions in mangNet is being run, and Sections4 and5 describe Korea for a more efficient execution of observational pro- the current and future science projects and educational grams. Since various telescopes are located all over the activities of SomangNet. Future prospects and the sum- country, a single observational program can be carried mary of the paper are given in Section6. out at one of the sites that happens to have good weather even if the other sites suffer from bad weather at that 2.S OMANGNET TELESCOPES time. This is especially advantageous for time-critical Currently, there are 10 telescopes participating in So- observation with a stringent time window. mangNet. These telescopes are all operated by Korean Second, mutual collaboration allows researchers to institutions. Many of these are only capable of imaging access telescopes with different capabilities and instru- observations, but a few of them have wide-field imaging ments. Some telescopes can be suited for wide-field (> 1 deg) or spectroscopy capabilities. Some of the surveys, and there can be other telescopes with spec- telescopes are located in the USA, Chile, and Australia, troscopic capabilities. Sharing of these resources opens allowing SomangNet to access the southern sky and access for a wide range of observational programs. time zones that cannot be covered by facilities in Korea. Third, collaborative activities can lead to the shar- The locations of the SomangNet telescopes are shown SomangNet: Small Telescope Network of Korea 91 in Figure1, and the capabilities of these telescopes are pointing based on (Ji et al. 2021, in prepa- summarized in Table1. We provide short descriptions ration). The script mode of KAOS enables automatic of the SomangNet telescopes below, in order of the ob- observations that follow a preset schedule written in a servatory longitude from the Prime Meridian toward predefined script format. Typical seeing FWHM values east. have been recorded as 200. 0, although sub-arcsec seeing has been reported at rare occasions. The point-source, 2.1. SNU Astronomical Observatory (SAO) 1.0 m 5-σ limiting magnitude is about R ∼ 18 AB mag with Telescope 5 min of single exposure under clear and dark sky with The SAO 1 m telescope saw the first light in March 2018, the typical seeing. and since then has been used regularly for educational activities and research-oriented observations. It is lo- 2.4. DOAO 1.0 m Telescope cated at the Seoul National University (SNU) Gwanak This 1 m telescope is located at the Deokheung Optical campus and housed in the SAO dome. Manufactured Astronomy Observatory (DOAO) of the National Youth by Planewave Instruments, Inc., the telescope has dual Space Center (NYSC) in Goheung, Korea. It is equipped Nasmyth foci with a tertiary mirror that can be easily with a remote observation system in order to provide flipped to change the path of the light from one focus access for students in the frame of educational and re- to the other. The Nasmyth foci are usually equipped search activities. The main camera is SOPHIA-2048B by with an imager with a field size of about 20.00 at one Princeton Instruments, with a 2k × 2k back-illuminated end and a low resolution spectrograph (R ∼ 600) on CCD chip; its field of view is 13.02 × 13.02 at a pixel the other end, allowing both imaging and spectroscopic scale of 000. 387/pixel. The filter system consists of two observations being carried out in one night. The im- wheels with eight positions with UBVRI and narrow ager is currently equipped with UBVRI and Hα filters. band filters such as Hα,Hβ, [O iii], [N ii], and [S ii]. In Typical seeing FWHM values have been recorded as 200. 0, 2019, the median seeing was about 200. 4 (FWHM). The although sub-arcsecond seeing has been reported at rare telescope is also equipped with an Echelle spectrograph, occasions. Despite the severe light pollution by the city eShel by Shelyak, with a spectral resolution R ∼ 10, 000 light of Seoul, the point-source, 5-σ limiting magnitude over the wavelength range 430–780 nm. By moving the is about R ∼ 19.7 AB mag with 10 min of exposure time diagonal mirror, it can be remotely switched from imag- under a seeing of 200. The telescope can be operated ing mode to spectroscopy mode. The telescope can be remotely, although routine remote observations have not automatically operated with a self-designed scheduler yet started. for monitoring transients and variability of stars and active galactic nuclei (AGNs). 2.2. Kyung Hee Astronomical Observatory (KHAO) 0.8 m Telescope 2.5. Chung-Buk National University Observatory The KHAO 0.8 m telescope is the primary telescope of (CBNUO) 0.6 m Telescope Kyung Hee Astronomical Observatory. The aperture The CBNUO wide-field 0.6 m telescope was installed in size of the telescope is 76 cm and the focal ratio is f/7 in 2010 October and saw its first light in November 2010. Ritchey-Chretien configuration. The mount is a single Since then, it has been used regularly for educational ac- fork equatorial type. The science camera uses a 4k × 4k tivities and research-oriented observations. It is located CCD camera that covers a field of view of 23.07 × 23.07 at the Munbaek-myeon Jincheon-gun Chungcheongbuk- at a pixel scale of 000. 35. We are currently testing the do of Korea, and housed in a fully open dome. Manufac- Kyung Hee University Automatic Observing Software tured for the Near Earth Space Survey (NESS) project (KAOS) for semi-automatic observations (Ji et al. 2021). (Yu et al. 2010), the telescope features a primary focus type f/2.92, BVR filters, and three corrector lenses that 2.3. Kyung Hee Astronomical Observatory 0.4 m can cover FoV up to 2◦.0 × 2◦.0. Typical seeing FWHM Telescope values are 200. 5, although 100. 5 seeing has been reported The KHAO 0.4 m telescope is a part of the Maemi at rare occasions. The main observation targets have Dual Field Telescope System (MDFTS) of KHAO which been variable stars, exoplanets, and sometimes artificial consists of the 0.4 m telescope and a 0.1 m wide-field satellites. The telescope can slew fast, with a maximum telescope. The 0.4 m telescope is equipped with an speed of 15◦.0/s; the tracking accuracy is 100. 0/1000 s imager that covers a diagonal field of view (FoV) of 260. without guiding and 100. 0/9 h with autoguiding. The telescopes are located on the Kyung Hee University Global Campus. The 0.4 m telescope has Schmidt- 2.6. Lee Sang Gak Telescope (LSGT) Cassegrain optical design (originally f/10) with a ×0.67 LSGT is a 0.43 m Dall-Kirkham type telescope owned focal reducer (LX200 ACF, Meade Instruments Co.). by SNU. This telescope is located at the Siding Spring The pixel scale is 000. 38/pixel, and the on-chip 2 × 2 pixel Observatory in Australia (Im et al. 2015a). Currently, binning mode is usually used to avoid oversampling the telescope is equipped with the SNUCAM-II camera and to improve the signal-to-noise ratio. Observations which uses a deep depletion CCD chip for red-sensitive are obtained using the standard Johnson & Cousins observation. The camera covers a field of view of 15.07 UBVRI filter system. MDFTS is controlled by KAOS, by 15.07 at a pixel scale of 000. 92 (Choi & Im 2017). which provides extremely accurate sub-arcsecond level SNUCAM-II is equipped with a 20-position filter wheel 92 Im et al. (SomangNet Collaboration)

Table 1 The SomangNet Telescopes

Telescope Aperture/ Instruments Imager 5-σ Deptha Location Longitude/ Altitude Name F-ratio Field of View Med. Seeing Latitude (m) SAO 1.0 m 4k x 4k CCD R = 19.7 Seoul, 126:57:15.7E 21.02 × 21.02 175 1 mb f/6.0 R ∼ 600 Spec. 200 Korea 37:27:25.7N KHAO 0.8 m R = TBD Yongin, 127:04:54.6E 4k × 4k CCD 23.07 × 23.07 141 0.8 m f/7.0 TBD Korea 37:14:19.2N KHAO 0.4 m R = 18.4 Yongin, 127:04:56.1E 3.3k x 2.5k CCD 210 × 160 119 0.4 m f/6.7 200 Korea 37:14:20.7N DOAO 1.0 m 2k x 2k CCD R = 20.6 Goheung, 127:26:48.6E 13.02 × 13.02 81 1 mb f/8.0 Echelle Spec. 200. 4 Korea 34:31:34.5N CBNUO 0.6 m R = 18.6 Jincheon, 127:28:31.2E 4k × 4k CCD 1◦. 19 × 1◦. 19 87 0.6 m f/2.92 200. 5 Korea 36:46:53.5N 0.43 m SNUCAM-II r = 20.1 Siding Spring 149:04:11.2E LSGTc 15.07 × 15.07 1122 f/6.8 1k x 1k CCD 300. 0 Obs., Australia 31:16:24.1S LOAO 1.0 m 4k x 4k R = 20.7 Mt. Lemmon, 110:47:19.2W 28.01 × 28.01 2776 1 m f/8.0 CCD 300. 1 Arizona, US 32:26:32.2N 0.25 m V = 19.3 McDonald Obs., 104:01:22.1W WITc 4k x 4k CCD 2◦. 34 × 2◦. 34 2057 f/3.6 600. 0 Texas, US 30:40:17.5N 0.36 m 4.5k × 3.5k R = 20.0 R´ıoHurtado 70:51:11.8W KCTd 23.07 × 17.09 1710 f/7.2 CMOS 300. 1 Valley, Chile 30:31:34.7S SNU 0.36 m 4k × 4k r = 19.2 R´ıoHurtado, 70:45:47.2W 2◦. 67 × 2◦. 67 1525 RASA36e f/2.2 CMOS 600. 5 Coquimbo, Chile 30:28:20.8S

Observatories are ordered toward E in longitude from the Prime Meridian. The accuracy of the longitudes, latitudes, and the altitudes are a few tenths of seconds, and a few meters, respectively. More accurate coordinates of some of the telescopes will be presented elsewhere (Sung et al. 2021, in preparation). a 5-σ depths are given for an exposure time of 10 min under median seeing. b Telescopes with spectroscopic capabilities (see text for details). c Telescopes with low resolution imaging spectroscopic capabilities. d Managed and maintained by DeepSkyChile. e Managed and maintained by ObsTech. that houses the broad-band u, g, r, i, z, and Y filters as UBVRI filters as well as z and Y filters. The telescope well as 13 medium-band filters with a band-width of has been used for monitoring observations of variable ∼ 50 nm each. With this suite of medium-band (MB) fil- stars, nearby galaxies, supernovae (SNe), and AGNs, ters, very low resolution imaging spectroscopy (R ∼ 10 as well as Target of Opportunity (ToO) observations of to 20) is possible from 425 to 1025 nm. The typical transients (Urata et al. 2009; Lee et al. 2010; Jang et al. seeing FWHM for the LSGT is about 200. 5. 5-σ image 2011; Urata et al. 2014). depths are g = 19.9, r = 19.5, and i = 18.9 mag for point-source detections with 180 sec exposure time un- der a typical seeing of 300. 0; the corresponding numbers for the medium-band filters are about 0.6 mag shallower 2.8. Wide-field Integral-field-unit Telescope (WIT) (Choi & Im 2017). The telescope is routinely used in robotic mode, but can also be operated remotely in real This is a 0.25 m telescope owned by SNU which is time. Detailed descriptions of LSGT and SNUCAM-II remotely operated from Korea by SNU and KHU. The can be found in Im et al.(2015a) and Choi & Im(2017). telescope is located at the McDonald observatory, Texas, USA; operations started in 2016. WIT is a Takahashi 2.7. LOAO 1.0 m Telescope CCA-250 telescope with a focal reducer that brings down The Mt. Lemmon Optical Astronomy Observatory the focal ratio to f/3.6 from the original f/5. It is now (LOAO) 1 m telescope is a telescope owned by the Korea equipped with a 4k × 4k CCD camera that covers a wide Astronomy and Space Science Institute (KASI). It was field of view of 2.34 × 2.34 deg2, and an 8 position filter established in 2003 and is located on top of Mt. Lemmon wheel that includes 5 MB filters covering a wavelength near Tucson, Arizona, USA (Han et al. 2005). Observa- range of 550 to 800 nm at 50 nm band width each for low tions are carried out remotely by operators residing at resolution imaging spectroscopy (like LSGT). Detailed KASI. An imager with a 4k × 4k back-illuminated CCD descriptions of WIT can be found in Hwang et al. (2021) chip is the main instrument of the telescope, which has and Ji et al.(2021). SomangNet: Small Telescope Network of Korea 93

Figure 2. SomangNet opera- tional scheme. Observation plan- ning and scheduling are handled by the SomangNet headquarter. The telescopes that are indepen- dently operated by each owner institution are placed in the red circle, and those that are being operated by SOSC are circled in green including possible fu- ture additions as “Telescope 1” and “Telescope 2”. The facilities in the overlapping area are the telescopes that are both indepen- dently operated and can be re- motely operated through SOSC.

2.9. KIAS Chamnun Telescope (KCT) The SomangNet headquarter is responsible for ob- servation planning, including the handling of the logis- The Chamnun telescope is a Dall-Kirkham type 0.36 tics of observation proposals from potential users (both m optical telescope manufactured by Planewave Instru- those in participating institutes and general users who ments, Inc. It is owned by the Korea Institute for are not associated with the participating institutes) and Advanced Study (KIAS) and was installed at the Deep- the proposal review, and assignment and scheduling of SkyChile site at the Rio Hurtado valley in Chile in observation time. The headquarter also oversees the March 2020. The name “Chamnun” stands for “true SomangNet activities, provides help to the SomangNet eye” in Korean. The telescope is equipped currently users regarding issues related to operations, such as with a fast-reading 4656 × 3520 pixel CMOS camera questions regarding proposals and data reduction. The covering a field of view of 23.07 × 17.09 at a pixel scale of 00 headquarter will manage a data archive system where 0. 306, and is equipped with UBVRI filters for imaging the SomangNet data will be stored or be linked to ex- observations. The median telescope seeing FWHM is ternal sites. 300. 14 so far, with a point source detection limit (5-σ) of 20.0 mag in R-band for an exposure time of 10 minutes. SOSC is responsible for supporting observations Currently, it is manually operated remotely from Korea, of several remotely operated telescopes. Upon requests and works are ongoing to implement robotic operation from the headquarter, it will arrange necessary activities capability. Details on this telescope will be presented in to obtain data from remotely accessible telescopes and Lim et al. (2021, in preparation). archive the data obtained from such telescopes. SOSC also takes the responsibility for activities related to au- 2.10. SNU RASA36 tomation of telescopes for remote and robotic operations, and user support related to remote observations. One ex- The SNU RASA36 telescope is a wide-field Schmidt ample is the addition of other telescopes to SomangNet telescope by Celestron with an aperture size of 0.36 and helping with hardware and the works needed to m. It is owned by SNU and installed at the El Sauce get those telescopes to work smoothly for remote and Observatory in Chile. With a 4k × 4k CMOS camera robotic operations. Through this kind of activity, we installed at its prime focus, the SNU RASA36 telescope hope to add several telescopes to SomangNet in the ◦ ◦ offers a wide field of view of 2.67 × 2.67 with a pixel size future. of 200. 35. Typical seeing achieved by this telescope is 600. 47, Besides the telescopes managed by SOSC (those in and the 5-σ detection limit (point source) is r = 18.6 the green circle in Figure2), there are telescopes that mag for 60 sec exposure time. Currently, only an r-band are managed by the owner institutions (telescopes in filter is installed, but we plan to install a filter changer the red circle in Figure2). The telescopes that can be unit that can house up to 4 filters. Currently, it is operated both independently by the owner institution manually operated remotely from Korea, and the work and remotely through SOSC are placed in the overlap is ongoing to implement robotic operation capability. area of the red and green circles. For the telescopes being More information on this telescope will be presented in independently operated, each institution is responsible Jeong et al. (2021, in preparation). for the data acquisition activity of its telescope, for both science and calibration data. Each user of the data 3.S OMANGNET OPERATION is currently responsible for the reduction of the data Figure2 shows the operational scheme of SomangNet. taken from different telescopes. We are preparing a There are two major organizational components: the data reduction pipeline that can handle the reduction of SomangNet headquarter at Seoul National University, the data taken by SomangNet telescopes, and the data and the SomangNet observation support center (SOSC) reduction and the archiving can be handled at a central at Chungbuk National University Observatory. location. 94 Im et al. (SomangNet Collaboration)

Science programs are divided into regular programs that was found near an IMSNG target, SN 2020scc, is and ToO programs. The regular programs include long- about 25 kpc away from NGC 2782, and possibly a SN term monitoring and time-critical observations, and will that occurred in the outskirt of NGC 2782 (Moran et al. be scheduled on a semester-by-semester base. ToO pro- 2020). Stacked IMSNG images are also used for finding grams are usually programs aimed at transient observa- faint merging features within (Hong et al. 2015) and low tions or other urgent time-critical observations. These surface brightness satellite galaxies around the target programs are now invoked by the headquarter or the galaxies (Im et al. 2019). end-user, but we are also planning to implement a broker system that gathers the numerous transient alerts that 4.2. Solar System Science are expected in the LSST era. SomangNet is a useful for solar system research. Particu- Currently participating institutions are SNU, larly, it has great potential for fast follow-up observations CBNU and CBNUO, KHU and KHAO, KASI, NYSC, of sudden brightness changes of asteroids and comets. KIAS, Kyungpook National University, Sejong Univer- Various mechanisms have been proposed to explain these sity, and Pusan National University, where the latter outbursts, such as impact of small asteroids (Kim et three provide logistics support to SomangNet. al. 2017), breakup of fast-rotating bodies via YORP spinup (Jewitt et al. 2017), and sudden sublimation of 4.S OMANGNET SCIENCE PROJECTSAND internal super-volatile ices (Skorov et al. 2017). Because HIGHLIGHTS many of these events are observable on time scales of With many small telescope at different locations around one week to several months at optical magnitudes 10–15, the world, SomangNet has the potential to carry out monitoring observations with SomangNet will provide a number of scientific projects. We describe current valuable information on the outburst mechanisms. SomangNet science programs and selected research high- One successful example is a sudden brightening lights. of the comet 252P/LINEAR, which was detected by LSGT and the InfraRed Survey Facility, IRSF (one of 4.1. Intensive Monitoring Survey of Nearby Galaxies, the OISTER telescopes). From the data, Kwon et al. IMSNG (2019) conjectured a sudden ejection of large compact IMSNG is a project that performs daily monitoring of dust particles. 60 galaxies located at < 50 Mpc to catch the early light Figure4 shows the asteroid 6478 Gault with a curves of SNe (Im et al. 2019). These 60 galaxies are dust tail, observed by the SAO 1-m telescope. Based chosen to be bright in near-ultraviolet, and have high on the rotation period of 2 hours, Kleyna et al.(2019) probability of hosting SNe events. The survey limit argued that the dust particles were ejected via landslides of 50 Mpc corresponds to the distance where IMSNG or re-configurations due to rapid rotation after YORP telescopes can detect the peak shock-heated cooling spinup. Since there are only 14 known cases for which emission of a SN with a progenitor with R = 1 R possible outburst mechanisms have been studied (Jewitt (Im et al. 2019) which will be as bright as R ∼ 19.5 mag. et al. 2019), further observations of other events will Using the SomangNet telescopes around the world, high be crucial for understanding the impact frequency and cadence (twice per day) observations are possible. High internal structure of comets. cadence observations of early light curve are used for Additional possible observation programs using So- constraining the progenitor star sizes of SNe: since the mangNet include: light curve and spectroscopy studies shock-heated emission lines arise immediately after the of asteroids and comets (including extrasolar objects), SNe explosion, they decay quickly within hours to days and long-term spectroscopic observations of the surface with an initial strength that is proportional to the size compositions of satellites to examine seasonal variations. of the progenitor star system (e.g.,Kasen 2010; Nakar & Sari 2010; Rabinak & Waxman 2011; Piro & Nakar 2013, 4.3. Active Galactic Nuclei 2014; Piro & Morozova 2016; Noebauer et al. 2017). Active galactic nuclei (AGNs) are powered by supermas- One good example for the SN early light curve sive black holes (SMBHs) or intermediate mass black study is SN 2015F whose early light curve was caught holes (IMBHs) at centers. Typically, AGNs ex- by LSGT, one of the SomangNet telescopes (e.g., Im hibit variability over days to , but some show even et al. 2015b). With the LSGT data that captured the hour-scale variability (e.g., see Kim et al. 2018 and daily cadence light curve of SN 2015F at both before references therein). Various aspects of AGN variabil- and after the explosion, we were able to rule out a type ity can be studied with SomangNet, but here, we de- Ia SN progenitor system that is made of a scribe the reverberation mapping projects. Reverbera- and a red (Im et al. 2015b), demonstrating tion mapping is a way of measuring the distance between that a small telescope can be a powerful tool to probe the SMBH/IMBH and AGN broad emission line region SNe early light curves and their progenitor stars. The (BLR) by measuring the time lag between the contin- high cadence observations also allow constructing dense uum (the light emitted by the central accretion disk) light curves of various SNe. In 2020, 6 SNe occurred and the broad emission line light curves (Blanford & among the IMSNG galaxies, and additional 6 SNe near McKee 1982; Peterson et al. 2004). Through a virial rela- IMSNG targets. Figure3 shows images of SNe that tion that connects this distance and the width (velocity were caught by IMSNG during 2020. One of the SNe dispersion) of the broad emission lines, one can derive SomangNet: Small Telescope Network of Korea 95

SN2020oi @NGC4321 SN2020bio @NGC5371 SN2020dpw @NGC6951 SN2020jfo @NGC4303

15°51' 40°29'30" 4°27'

50' 00" 28' 66°07'

28'30" 49' 29'

00"

48' 08' 30' 27'30" 12h23m00s 22m56s 52s 48s 13h55m42s 39s 36s 33s 20h37m10s 00s 12h22m00s 21m55s 50s 45s SN2020uxz @NGC0514 SN2020abqw @NGC3646 AT2020akl @NGC5668 field SN2020enm @IC1222

12°54'30" 46°12'40"

20°10'00" 4°32'30" 45"

55'00" 33'00" 20" 50"

30" 55" 30" 40"

34'00" 13'00"

11'00" 56'00" 05" 30"

1h24m10s 08s 06s 04s 11h21m44s 42s 40s 38s 14h33m03s 00s 32m57s 54s 16h35m08s 07s 06s SN2020scc @NGC2782 field SN2020unm @NGC2856 SN2020zyy @NGC2207 field SN2020acjg @NGC3294 field

37°27'30" 49°14'45" 40°04'30"

-21°30'30" 28'00"

15'00" 05'00" 30"

31'00" 30" 15" 29'00"

9h14m09s 06s 9h24m19s 18s 17s 16s 15s 14s 6h16m36s 10h36m45s 42s 39s 36s

Figure 3. SNe and possible SNe caught by SomangNet telescopes during 2020. The first six SNe, in the red images, occurred in the main IMSNG galaxies; the next six, in the blue images, are SNe that occurred in the auxiliary IMSNG galaxies or in the field of view of the main IMSNG galaxies. Among them, SN 2020scc could be a SN that occurred in the outskirts of a main IMSNG galaxy, NGC 2782. Each postage stamp image size corresponds to 20 kpc × 20 kpc. When converting the angular sizes of the images to 20 kpc, we assumed the galaxy distances in Bai et al.(2015). Circles with a 5 00 radius are drawn to indicate the location of SNe. the SMBH/IMBH mass reliably (e.g., Peterson et al. in 2015, by targeting ∼50 luminous AGNs out to z = 0.5. 2004). With readily available telescope time, this kind SomangNet telescopes, including LOAO and DOAO, as of long-term monitoring observation can be efficiently well as the MDM (Michigan-Dartmouth-MIT) observa- carried out with the SomangNet telescopes. tory1 telescopes are the main facilities for photometric monitoring, which traces the variation of continuum flux 4.3.1. Medium-band AGN Reverberation Mapping of target AGNs. For spectroscopic observations, the Lick With the MB capabilities of LSGT and WIT, we are 3-m telescope has been used as the main facility. The undertaking a reverberation mapping project where we initial results of Hβ line reverberation mapping for a few use medium-band filters to sample the time lag between targets were reported by Rakshit et al.(2019), and for variations of the AGN continuum light and Hα or Hβ more than a dozen targets reliable lag measurements are emission lines. The MB-based reverberation mapping expected by completing the 5 monitoring campaign has been successful for AGNs with small time lags (on in 2021. This project demonstrates that SomangNet time scales of weeks; Kim et al. 2019), and we are facilities can play an important role by providing essen- now extending the study to a sample that contains tial or complementary photometry data for challenging longer time lags (months to ∼year; Kim et al. 2021, in reverberation mapping projects. preparation) 4.3.3. Intermediate Mass Black Hole in NGC 4395 4.3.2. SNU AGN Monitoring Project IMBHs are one of the core subjects in studies of the SomangNet facilities have been used for the SNU AGN origin of SMBHs and their co-evolution with their host Monitoring Project (SAMP), which aims at determining 1 emission line time lag and black hole mass for relatively The MDM observatory is owned and operated by a consor- tium of five educational institutions: Dartmouth College, The high-luminosity AGNs as outlined by Woo et al.(2019). Ohio State University, Columbia University, The University of The campaign started as a five year monitoring project Michigan, and Ohio University. 96 Im et al. (SomangNet Collaboration)

Figure 4. Composite image of asteroid 6478 Gault, taken by Figure 5. Color composite image of the blazar TXS 0506+056, the SAO 1 m telescope in R−band. The image size is 50 × 50, taken with WIT on 2017 October 17 (UT). The large image the total integration time is 153 min. shows the whole field of view of WIT, the inset image shows an 8.08 × 8.08 area centered on TXS 0506+056. The location of TXS 0506+056 is marked in the inset image, the 90% galaxies. While AGNs powered by IMBHs are likely IceCube neutrino containment region is indicated by the red present in the universe, it is difficult to detect or confirm contour in the large image. The color composite is composed their existence. Reverberation mapping is one of the of images taken with the m575, m675, and m775 filters, methods to determine the mass of IMBH candidates, where m stands for “medium-band”, and the number stands likely providing important constraints for IMBH pop- for the central wavelength of each filter in units of nm. ulations. SomangNet facilities have been used for a reverberation mapping study of NGC 4395, which is a dwarf galaxy hosting a low-luminosity AGN (Woo et al. 2019; Cho et al. 2020). Woo et al.(2019) used have been following up neutrino events with SomangNet a number of SomangNet facilities along with various telescopes to search for any abrupt changes in the optical international facilities for monitoring variation of the properties of blazars or other sources in the neutrino localization areas of so-called “track” events that are AGN in NGC 4395 in 2017 and 2018. The photome- 2 try was performed in the V-band and narrow Hα-band, typically localized to within about 1 deg for neutrino respectively, providing continuum and emission line vari- energies with > 1 TeV (Aarsten et al. 2017a,b; Reimann ability, with a 5 minute cadence over several consecutive 2019). Track events are neutrino events that produce nights. Based on the cross correlation analysis between high energy muons near the IceCube detector with the V-band and Hα-band light curves, Woo et al.(2019) muons traveling a few km emitting Cherenkov radiation reported a time lag of 83±14 min of the Hα emission (Aarsten et al. 2017b). There are also other events, line, which translates to a black hole mass of ∼10,000 cascade events, that have a larger positional error with a median value of 16 deg2 (Aarsten et al. 2017c; Reimann M . These results indicate that NGC 4395 hosts an actively mass-accreting IMBH (see also Cho et al. 2020). 2019), for which we have not yet started optical follow-up observations due to their large positional errors. 4.4. MMA with Neutrino and EM Signals So far, there are several events with possible EM SomangNet is being used for MMA studies of IceCube counterparts (e.g., Kadler et al. 2016; Stein et al. 2021), neutrino events. Present-day neutrino observatories but only one event has been widely accepted as be- routinely detect high energy neutrino events (energies ing identified with an EM counterpart. This event is at > 100 TeV; e.g., IceCube Collaboration 2013). These IceCube-170922A which has been linked to the blazar high energy neutrinos are theorized to originate from TXS 0506+056(IceCube Collaboration et al. 2018). Our many different astrophysical phenomena. Therefore, WIT study of the temporal variation of spectral en- identifying the physical origin and mechanism of high ergy distribution of TXS 0506+056 revealed that TXS energy cosmic neutrinos has been a key topic of MMA. 0506+056 is similar to a Flat Spectrum Radio Quasar One of the candidate sites for the production of (FSRQ), a type of blazar with high energy output high energy neutrinos is blazar jets (Mannheim 1995; (Hwang et al. 2021). Figure5 shows the WIT images of Halzen 2017; Meszaros 2017; Ansoldi et al. 2018). We the field around TXS 0506+056. SomangNet: Small Telescope Network of Korea 97

4.6. Symbiotic Stars

Symbiotic stars are wide binary systems containing a mass-losing giant and a hot white dwarf (e.g., Kenyon 1986). The coexistence of a nebular region photoionized by the hot source and a dense neutral region in the stellar wind from the giant provides an ideal environment for operation of Raman scattering by atomic hydrogen H i (e.g., Schmid 1989). Raman-scattered O vi bands at 6830 A˚ and 7088 A˚ are spectral features exclusive to symbiotic stars, detected in about half of known galactic symbiotic systems (e.g., Allen 1979; Akras et al. 2019). Taking advantage of the large width (>30 A)˚ and high flux of the line, the RAMSES II (RAMan Search for Extragalactic Symbiotic Stars) team initiated an optical survey using a narrow band filter centered at O vi 6830 A˚ with the Gemini 8.1 m telescopes to search for new extragalactic symbiotic stars (Angeloni et al. 2019). Motivated by the RAMSES II survey, two 50 A˚ wide narrow-band filters have been installed on the SAO Figure 6. Color composite image (gri) of the optical coun- 1-m telescope: one on-band, centered on O vi 6830 A,˚ terpart of GW170817 and its host galaxy NGC 4993, taken and one off-band, centered at 6780 A˚ in the adjacent con- by LSGT. The positions of the GW event (and, equivalently, tinuum. The Raman O vi narrow-band imaging with So- the optical counterpart) and the host galaxy are marked mangNet telescopes will mainly focus on monitoring Ra- with arrows. The image size shown here is 10 × 10. man flux variations in Galactic symbiotic stars. Schmid et al.(1998) suggested that the flux of Raman O vi fea- ture varies depending on the orbital phase because Ra- 4.5. MMA with GW and EM Signals man scattering for O vi is a dipole-type scattering that yields an anisotropic radiation distribution enhanced Since the discovery of the binary neutron star GW event along the forward and backward directions. From their GW170817 (Abbott et al. 2017a,b), MMA with GW and time-series spectroscopic observations of the symbiotic EM signals has been flourishing around the world. A star CD-43◦14304, such phase-dependent flux variations key to the success of MMA studies are timely wide-field of Raman O vi 6830 were indeed detected. The Raman optical observations for catching EM counterparts as O vi feature had a maximum intensity in both conjunc- soon as possible over a very wide GW localization area tions and decreased in the quadrature-phase. Long-term (tens to hundreds of deg2). The SomagNet wide-field photometric observations of symbiotic stars covering and narrow-field telescopes scattered around the world their orbital periods, ranging from a few hundred days are ideal for such following up transients such as GW to a few tens of years, will allow us to derive their orbital sources that could appear anytime, anywhere. We are parameters. now utilizing some of the SomangNet facilities for the Gravitational-wave EM Counterpart Korean Observa- Considering that the Raman O vi feature is ob- tory (GECKO; Im et al. 2020; Kim et al. 2021; Paek et al. served almost exclusively in symbiotic binaries, Raman 2021, in preparation) which is a collaboration for study- narrow-band imaging is one of the most efficient ways ing GW EM counterparts and includes telescopes with to search for new symbiotic stars, as suggested by the larger apertures such as the Korean Microlensing Tele- RAMSES II project. SomangNet telescopes with Ra- scope Network telescopes (Kim et al. 2016). GECKO man filters are ideal for mapping objects brighter and observed 12 GW events in 2019 through 2020 till the closer than those covered by RAMSES II/Gemini with end of the advanced LIGO/Virgo O3 run (Abbott et a larger FoV. We will systematically search for galactic al. 2020). Like other EM follow-up facilities, GECKO Raman emitters within the catalogs of known planetary found no promising EM counterparts during the O3 run. nebulae, B[e] stars, and similar objects. Combined with However, earlier in 2017, LSGT, one of the SomangNet Hα imaging, contamination by other objects will be sig- telescopes, succeeded in catching the optical counterpart nificantly reduced. We will produce extensive catalogs AT2017gfo of GW170817 (e.g., Coulter et al. 2017; Ab- of symbiotic systems and their spatial distribution in bott et al. 2017a) within one day of the GW detection the Milky Way. The SomangNet survey can be extended alert (Figure6). LSGT was able to follow the event for to searches of possible symbiotics in relatively sparse a few days until the EM counterpart became too faint. globular clusters, as proposed recently by Belloni et al. This demonstrates that SomangNet can play an impor- (2020). Furthermore, our survey might lead to discov- tant role in GW EM counterpart observations even for eries of non- objects showing Raman the events that cannot be observed from Korea. O vi features. 98 Im et al. (SomangNet Collaboration)

4.7. Extrasolar Planets Small- to medium-sized telescopes are well-suited for photometric follow-up observations of transiting extra- solar planets orbiting a bright host star. While the ma- jority of planet discoveries are made with space-based telescopes like Kepler (Borucki et al. 2010) and currently TESS (Ricker et al. 2015), these instruments either have a limited life time and/or the original mission-design is limiting the observing frequency of specific targets. As a consequence of observational uncertainties, newly discov- ered transiting planets need to be monitored densely in order to derive the transit ephemerides. The timing er- ror in transit mid-times scales quadratically with orbital . Therefore, frequent follow-up observations allow future transit events to be predicted with sufficient ac- curacy. This is an important aspect to remember when future detailed follow-up observations (with potentially ground-breaking discoveries in the field of exoplanets) are planned involving cost-intensive ground- as well as space-based observing platforms such as the James Webb Space Telescope (Seager et al. 2009, JWST), the Vera C. Rubin Observatory 2, and the Giant Magellan Telescope3. For example, the JWST4 is capable of probing the atmospheric molecular composition of a transiting planet using multi wavelength transmission spectroscopy (Charbonneau et al. 2002; Deming et al. 2005). For such observations, knowledge of the planet transit times is crucial. Furthermore, cost-intensive spectroscopic follow-up observations would greatly benefit from accu- Figure 7. Examples of two point-spread-functions (PSF) of the same star using the CBNUO 0.6 m telescope. Top panel: rate ephemeral information for a transiting planet. The Defocus PSF from a 195 seconds exposure. Bottom panel: spin– angle (angle between the axis In-focus PSF from a 12-second exposure. The brightness and the planet’s orbital rotation axis) can be measured distribution for defocused observations depends on the align- with good accuracy using medium to large aperture tele- ment of the optical axis and mirror components in a telescope, scopes via the Rossiter–McLaughlin (RM) effect (Gaudi potentially causing multiple focal points (astigmatism). The et al. 2007) which causes a characteristic modulation of figure is reproduced from Hinse et al.(2015). the observed stellar during the transit of the planet. Compiling spin–orbit geometries for a extrasolar planets. For a small-aperture telescope, the large number of systems would help to constrain current dominant source of noise is scintillation noise. Secondary planet formation theories and the subsequent dynamical noise sources are CCD read-out and flat-fielding noise. evolution of planetary (Mu˜nozet al. 2018). Scintillation noise can be mitigated and photometric Frequent transit monitoring might lead to detec- precision can be increased by increasing the signal in- tions of additional massive bodies (Holman & Murray stead of decreasing the noise. This can be achieved by 2005). In the presence of a perturbing body, the mid- defocusing, which works well for less crowded star fields. transit time of the transiting planet will experience The CBNUO 0.6 m telescope was operated with the tele- changes in the ephemeris known as transit-timing vari- scope being heavily defocused (Hinse et al. 2015) during ations (TTVs). The nature of the change depends on several transit events. A highlight is the achievement the orbital properties as well as the mass of the per- of a photometric precision of < 1 milli-mag (root-mean- turbing body. In recent years several systems have been square scatter) for a V = 9.8 mag star from a 195 second identified as TTV candidates (e.g., WASP-12; Maciejew- long exposure. By defocusing the telescope, the amount ski et al. 2012), benefiting from continuous long-term of time to collect photons can be maximized since the photometric monitoring by international collaborations. illumination of the CCD remains within the linear (non- The CBNUO 0.6 m telescope (Lee et al. 2011, 2012; saturation) regime for a longer time. This allows for a Kim et al. 2014; Hinse et al. 2015) as well as the NYSC substantial increase in the photometric signal. In Fig- DOAO 1.0 m telescope (Kang et al. 2016) were previ- ure7 we show in-focus and defocus PSFs of the same ously used for photometric observations of transiting target. The CCD read-out noise is low for long-exposure 2https://www.lsst.org time-series observations. The exposure times are limited 3https://www.gmto.org by the need to sample the transit light curve sufficiently, 4https://www.jwst.nasa.gov especially during the ingress and egress phases. There- SomangNet: Small Telescope Network of Korea 99

Figure 8. Transit light curves of TrES-3b (V = 12.4 mag; O’Donovan et al. 2007) observed using the 1.0 m telescope at DOAO. The root-mean-square scatter of the data is about 1.6 milli-magnitudes. Observations were carried out with the telescope in focus mode. fore, the choice of exposure times, and hence the amount of techniques and methods from various areas within of defocusing, depends on the CCD read-out time and mathematics, computer science, engineering, physics as the transit duration. In Figure8 we show two transit well as biology and chemistry (astrobiology), provid- events of the transiting planet TrES-3b (O’Donovan et ing a unique platform to inspire, motivate and educate al. 2007) demonstrating a good photometric precision the next generation of engineers and scientists. With even with the telescope in focus mode. SomangNet, one could use educational resources of a participating observatory for training which likewise 5.E DUCATIONAL AND TRAINING ASPECTS benefits the other telescopes. We plan to explore vari- Astronomy and related sciences often grasp the attention ous educational and public outreach potentials of the of the general public and young students. Currently, SomangNet in future. some of the SomangNet telescopes have their own edu- 6.F UTURE PROSPECTSAND CONCLUSION cational and public outreach activities. Bringing these In this paper, we presented an overview of SomangNet, telescopes together provides an opportunity for enhanc- the Korean small telescope network which currently ing the educational and public outreach activities in a comprises 10 telescopes in Korea, Australia, Chile, and complementary way. For example, observation of special USA. During its first year of operation in 2020, as well astronomical events such as comets and planet transits as through the studies performed by several SomangNet can be coordinated such that one can use a telescope at telescopes in previous years, SomangNet has demon- a site with clear weather for a public outreach activity strated its potential of being a powerful tool for studying of a special event, even if the other sites cannot observe dynamic aspects of the universe by its ability to carry the event due to bad weather. A live feed of the event out long-term monitoring observations, time-critical ob- through the internet can reach out to a much larger servations, and rapid follow-up observations. number of viewers than activities confined to on-site We envision several future activities to improve viewers at a single observatory. the SomangNet capability. First, we plan to imple- With present-day technology, a small to medium- ment robotic/remote operational capabilities to the So- sized telescope can be remodeled/upgraded to allow for mangNet telescopes. With robotic operation, the time remote operation. Frequently, the strongest constraints spent for observations by personnel can be saved, and placed on optical astronomy observations is the need to we can reallocate the saved resource for other activi- carry out observations at night. This problem could be ties. Robotic operation is also advantageous for rapid overcome by engaging in and initiate international col- follow-up observations of transients. Second, we plan laborations with countries in favorable time zones. One to implement scheduling and broker systems for effi- obvious candidate would be Chile which has seen a recent cient observations. The broker system will be useful surge in astronomical and space science research and especially when there are multiple transient alerts for educational activities. Chile is geographically located which immediate observations are required. With the around 12 hrs trailing Korea, thus providing favorable advancement of time-domain astronomy, we expect that conditions for remote observations from Chile during the SomangNet fulfills our wish of making it a major Korean night time and vice versa. Establishing mu- astronomy observational resource in Korea – as the word tual agreements between various universities in South “somang” (“wish”) implies. Korea and Chile would make education in the field of astronomy more accessible for Korean as well as Chilean ACKNOWLEDGMENTS students. We thank the anonymous referee for careful reading of The field of astronomy comprises the application the manuscript and useful suggestions. This research 100 Im et al. (SomangNet Collaboration) was supported by the Korea Astronomy and Space Sci- Arcavi, I., Hosseinzadeh, G., Howell, D. A., et al. 2017, Op- ence Institute under the R&D program (Project No. tical Emission from a Kilonova Following a Gravitational- 2020-1-600-05) supervised by the Ministry of Science wave-detected Neutron-star Merger, Nature, 551, 64 and ICT (MSIT), and the National Research Founda- Bai, Y., Liu, J., & Wang, S. 2015, An Updated Ultraviolet tion of Korea (NRF) Grant No. 2020R1A2C3011091, Catalog of GALEX Nearby Galaxies, ApJS, 220, 6 funded by MSIT. DK acknowledges support by the Belloni, D., Mikolajewska, J. Ilkiewicz,K., et al. 2020, On the Absence of Symbiotic Stars in Globular Clusters, MNRAS, National Research Foundation of Korea (NRF) grant 496, 3436 (No. 2021R1C1C1013580) funded by the Korean gov- Blandford, R. D., & McKee, C. F. 1982, Reverberation ernment (MSIT). SL acknowledges the support from Mapping of the Emission Line Regions of Seyfert Galaxies the National Research Foundation of Korea (NRF) and Quasars, ApJ, 255, 419 grant (No. 2020R1I1A1A01060310) funded by the Ko- Borucki, W. J., Koch, D., Basri, G., et al. 2010, Kepler rean government (MSIT). TCH acknowledges financial Planet-Detection Mission: Introduction and First Results, support from the National Research Foundation (No. Science, 327, 977 2019R1I1A1A01059609). We thank the staff at the iTe- Brown, T. M., Baliber, N., Bianco, F. B., et al. 2013, Las lescope.Net, DeepSkyChile, ObsTech, SAO, CBNUO, Cumbres Observatory Global Telescope Network, PASP, DOAO, LOAO, SOAO, and McDonald observatories for 125, 1031 their help with the observations and maintenance of the Castro-Tirado, A. J., Soldan, J., Bernas, M., et al. 1999, The facilities. This research made use of the data taken with Burst Observer and Optical Transient Exploring System (BOOTES), A&AS, 138, 583 LOAO and SOAO operated by the Korea Astronomy and Charbonneau, D., Brown, T. M., Noyes, R. W., 2002, De- Space Science Institute (KASI), CBNUO of Chungbuk tection of an Extrasolar Planet Atmosphere, ApJ, 568, National University, DOAO of National Youth Space 377 Center (NYSC), the McDonald Observatory, and the Cho, H., Woo, J.-H., Hodges-Kluck, E., et al. 2020, Variabil- Siding Spring Observatory. ity and the Size-Luminosity Relation of the Intermediate- mass AGN in NGC 4395, ApJ, 892, 93 REFERENCES Choi, C., & Im, M. 2017, Seoul National University Camera II (SNUCAM-II): The New SED Camera for the Lee Sang Aartsen, M. G., Ackermann, M., Adams, J., et al. 2017a, Gak Telescope (LSGT), JKAS, 50, 71 The IceCube Realtime Alert System, Astropart. Phy., 92, Coulter, D. A., Foley, R. J., Kilpatrick, C. D., et al. 2017, 30 Swope Supernova Survey 2017a (SSS17a), the Optical Aartsen, M. G., Ackermann, M., Adams, J., et al. 2017b, Counterpart to a Gravitational Wave Source, Science, 358, Multiwavelength Follow-up of a Rare IceCube Neutrino 1556 Multiplet, A&A, 607, 115 Deming, D., Seager, S., & Richardson, L. J., 2005, Infrared Aartsen, M. G., Ackermann, M., Adams, J., et al. 2017c, Radiation from an Extrasolar Planet, Nature, 434, 740 Search for Astrophysical Sources of Neutrinos Using Cas- Drake, A. J., Djorgovski, S. G., Mahabal, A., et al. 2009, First cade Events in IceCube, ApJ, 846, 136 Results from the Catalina Real-Time Transient Survey, Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2017a, Multi- ApJ, 696, 870 messenger Observations of a Binary Neutron Star Merger, Gaudi, B. S., & Winn, J. N., 2007, Prospects for the Char- ApJL, 848, 12 acterization and Confirmation of Transiting Exoplanets Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2017b, via the Rossiter-McLaughlin Effect, ApJ, 655, 550 GW170817: Observation of Gravitational Waves from a Gorbovskoy, E. S., Lipunov, V. M., Kornilov, V. G., et Binary Neutron Star Inspiral, PhRvL, 119, 1101 al. 2013, The MASTER-II Network of Robotic Optical Abbott, R., Abbott, T. D., Abraham, F., et al. 2020, GWTC- Telescopes. First results, Astron. Rep., 57, 233 2: Compact Binary Coalescences Observed by LIGO and Halzen, F. 2017, High-energy Neutrino Astrophysics, NatPh, Virgo During the First Half of the Third Observing Run, 13, 232 arXiv:2010.14527 Han, W., Mack, P., Lee, C.-U., et al. 2005, Development of Agayva, S., Alishov, S., Antier, S., et al. 2020, Grandma: A a 1-m Robotic Telescope System, PASJ, 57, 821 Network to Coordinate Them All, arXiv:2008.03962 Hinse, T. C., Han, W., Yoon, J.-N., et al. 2015, Photometric Ahn, H., Shim, H., Pak, S., & Kang, W. 2021 Flux Calibra- Defocus Observations of Transiting Extrasolar Planets, tion Based on Narrow Band Imaging Observations, AJ, in JASS, 32, 21. preparation Holman, M. J., & Murray, N. W., 2005, The Use of Transit Akras, S., Guzman-Ramirez, L., Leal-Ferreira, M. L., & Timing to Detect Terrestrial-Mass Extrasolar Planets, Ramos-Larios, G. 2019, A Census of Symbiotic Stars in Science, 307, 1288 the 2MASS, WISE, and Gaia Surveys, ApJS, 240, 21 Hong, J., Im, M., Kim, M., & Ho, L. C. 2015, Correlation Allen, D. A. 1979, Symbiotic Stars at Optical Infrared and between Galaxy Mergers and Luminous Active Galactic Radio Wavelengths, Proc. IAU Coll., 46, 125 Nuclei, ApJ, 804, 34 Angeloni, R., Goncalves, D. R., Akras, S., et al. 2019, RAM- Hwang, S., Im, M., Taak, Y. C. 2021, Medium-band Obser- SES II: RAMan Search for Extragalactic Symbiotic Stars – vation of the Neutrino Emitting Blazar, TXS 0506+056, Project Concept, Commissioning, and Early Results from ApJ, 908, 113 the Science Verification Phase, AJ, 157, 156 IceCube Collaboration 2013, Evidence for High-Energy Ex- Ansoldi, S., Antonelli, L. A., Arcaro, C., et al. 2018, The traterrestrial Neutrinos at the IceCube Detector, Science, Blazar TXS 0506+056 Associated with a High-energy 342, 1 Neutrino: Insights into Extragalactic Jets and Cosmic- IceCube Collaboration, Aartsen, M. G., Ackermann, M., et Ray Acceleration, ApJL, 863, L10 al. 2018, Multimessenger Observations of a Flaring Blazar SomangNet: Small Telescope Network of Korea 101

Coincident with High-energy Neutrino IceCube-170922A, Kim, J., Im, M., Paek, G. S. H., et al. 2021, GECKO Science, 361, 1378 Optical Follow-up Observation of Three Binary Black Im, M., Choi, C., & Kim, K. 2015a, Lee Sang Gak Telescope Hole Merger Events, GW190408 181802, GW190412, and (LSGT): A Remotely Operated Robotic Telescope for Ed- GW190503 185404, ApJ, submitted ucation and Research at Seoul National University, JKAS, Kim, S.-L., Lee, C.-U., Park, B.-G., et al. 2016, KMTNET: A 48, 207 Network of 1.6 m Wide-Field Optical Telescopes Installed Im, M., Choi, C., Yoon, S.-C., et al. 2015b, The Very Early at Three Southern Observatories, JKAS, 49, 36 Light Curve of SN 2015F in NGC 2442: A Possible Detec- Kim, Y., Ishiguro, M., & Lee, M. G. 2017, New Observational tion of Shock-heated Cooling Emission and Constraints Evidence of Active Asteroid P/2010 A2: Slow Rotation of on SN Ia Progenitor System, ApJS, 221, 22 the Largest Fragment, ApJL, 842, L23 Im, M., Ko, J., Cho, Y., et al. 2010, Seoul National University Kleyna, J. T., Hainaut, O. R., Meech, K. J., et al. 2019, 4K×4K Camera (SNUCAM) for Maidanak Observatory, The Sporadic Activity of (6478) Gault: A YORP-driven JKAS, 43, 75 Event?, ApJL, 874, L20 Im, M., Yoon, Y., Lee, S.-K., et al. 2017b, Distance and Prop- Kochanek, C. S., Shappee, B. J., Stanek, K. Z., et al. 2017, erties of NGC 4993 as the Host Galaxy of the Gravitational- The All-Sky Automated Survey for Supernovae (ASAS-SN) wave Source GW170817, ApJL, 849, 16 Light Curve Server v1.0, PASP, 129, 104502 Im, M., Choi, C., Hwang, S., et al. 2019, Intensive Monitoring Kwon, Y. G., Ishiguro, M., Kwon, J., et al. 2019, Survey of Nearby Galaxies (IMSNG), JKAS, 52, 11 Near-infrared Polarimetric Study of Near-Earth Object Im, M., Kim, J., & Paek, S. H. 2020, GECKO: Gravitational- 252P/LINEAR: An Implication of Scattered Light from wave EM Counterpart Korean Observatory, Proceedings the Evolved Dust Particles, A&A, 629, A121. of the Yamada Conference LXXI: Gamma-ray Bursts in Lee, I., Im, M., & Urata, Y. 2010, First Korean Observations the Gravitational Wave Era 2019, Yokohama of Gamma-Ray Burst Afterglows at Mt. Lemmon Optical Ishiguro, M., Kurdao, D., Hanayama, H., et al. 2015, Astronomy Observatory (LOAO), JKAS, 43, 95 Dust from Comet 209P/LINEAR during its 2014 Re- Lee, J. W., Youn, J.-H., Kim, S.-L., et al. 2012, The Sub- turn: Parent Body of a New Meteor Shower, the May Saturn Mass Transiting Planet HAT-P-12b, AJ, 143, 95. Camelopardalids, ApJ, 798, 34 Lee, J. W., Youn, J.-H., Kim, S.-L., et al. 2011, Physical Itoh, R., Fukazawa, Y., Tanaka, Y., et al. 2013, Dense Op- Properties of the Transiting Planetary System TrES-3, tical and Near-infrared Monitoring of CTA 102 during PASJ, 63, 301 High State in 2012 with OISTER: Detection of Intra-night Leedj¨arve, L., G´alis, R., Hric, L., Merc, J., & Burmeister, M. ”Orphan Polarized Flux Flare”, ApJ, 768, 24 2016, Spectroscopic View on the Outburst Activity of the Jang, M., Im, M., Lee, I., et al. 2011, Dust Properties in the Symbiotic Binary AG Draconis, MNRAS, 456, 2558 Afterglow of GRB 071025 at z ∼ 5, ApJL, 2011, 741 L20 Lipunov, V. M., Gorbovskoy, E., Kornilov, V. G., et al. 2017, Jewitt, D., Kim, Y., Luu, J., et al. 2019, Episodically Active MASTER Optical Detection of the First LIGO/Virgo Asteroid 6478 Gault, ApJL, 876, L19 Neutron Star Binary Merger GW170817, ApJL, 850, L1 Jewitt, D., Agarwal, J., Li, J., et al. 2017, Anatomy of an Lipunov, V. M., Krylov, A. G., Kornilov, V. G., et al. 2004, Asteroid Breakup: The Case of P/2013 R3, AJ, 153, 223. MASTER: The Mobile Astronomical System of Telescope- Ji, T.-G., Byeon, S., Lee, H.-I., et al. 2021, Development Robots, Astron. Nachr. 325, 580 of Kyung Hee University Automatic Observing Software Maciejewski, G., Dimitrov, D., Seeliger, M., et al. 2012, (KAOS) for Small-Sized Observing System, PASP, in Multi-site Campaign for Transit Timing Variations of preparation WASP-12 b: Possible Detection of a Long-period Signal Kadler, M., Krauß, F., Mannheim, K., et al. 2016, Coinci- of Planetary Origin, A&A, 551, 108. dence of a High-fluence Blazar Outburst with a PeV-energy Mannheim, K. 1995, High-energy Neutrinos from Extragalac- Neutrino Event, Nature Physics, 12, 807 tic Jets, Astropart. Phys., 3, 295 Kang, W., Kim, T., Kwon, S.-G., et al. 2016, Preliminary Meszaros, P. 2017, Astrophysical Sources of High-Energy Result of Transit Observation by NYSC 1 m Neutrinos in the IceCube Era, Ann. Rev. Nucl. Part. Sci., Telescope, 2016, KAS Spring Meeting, poster presentation, 67, 45 Seoul. Moran, S., Fraser, M., Kankare, E., et al. 2020, Spectroscopic Kasen, D. 2010, Seeing the Collision of a Supernova with Its Observation of SN 2020scc by NUTS2 (NOT Un-biased Companion Star, ApJ, 708, 1025 Transient Survey 2), ATel, 14002 Kasliwal, M. M., Cannella, C., Bagdasaryan, A., et al. 2019, Morokuma, T., Utsumi, Y., Ohta, K., et al. 2021 Follow-up The GROWTH Marshal: A Dynamic Science Portal for Observations for IceCube-170922A: Detection of Rapid Time-domain Astronomy, PASP, 131, 38003 Near-Infrared Variability and Intensive Monitoring of TXS Kenyon, S. J. 1986, The Symbiotic Stars (New York: Cam- 0506+056, PASJ, 73, 25 bridge Univ. Press) Mu˜noz,D. J., & Perets, H. B., 2018, Statistical Trends in Kim, C.-H., Song, M.-H., Yoon, J.-N., et al., 2014, BD the Obliquity Distribution of Exoplanet Systems, AJ, 156, Andromedae: A New Short-period RS CVn Eclipsing 253 with a Distant Tertiary Body in a Highly Nakar, E., & Sari, R. 2010, Early Supernovae Light Curves Eccentric Orbit, ApJ, 788, 134 Following the Shock Breakout, ApJ, 725, 904 Kim, J., Karouzos, M., Im, M., et al. 2018, Intra-Night Noebauer, U. M., Kromer, M., Taubenberger, S., et al. 2017, Optical Variability of Active Galactic Nuclei in the Cosmos Early Light Curves for Type Ia Supernova Explosion Mod- Field with the KMTNet, JKAS, 51, 89 els, MNRAS, 472, 2787 Kim, J., Im, M., Choi, C., & Hwang, S. 2019, Medium-band O’Donovan, F. T., Charbonneau, D., Bakos, G. A.,´ et al. Photometry Reverberation Mapping of Nearby Active 2007, TrES-3: A Nearby Massive Transiting Hot Jupiter Galactic Nuclei, ApJ, 884, 103 in a 31 Hour Orbit, ApJ, 663, 37 102 Im et al. (SomangNet Collaboration)

Peterson, B. M., Ferrarese, L., Gilbert, K. M., et al. 2004, Soares-Santos, M., Holz, D. E., Annis, J., et al. 2017, The Central Masses and Broad-Line Region Sizes of Active Electromagnetic Counterpart of the Binary Neutron Star Galactic Nuclei. II. A Homogeneous Analysis of a Large Merger LIGO/Virgo GW170817. I. Discovery of the Opti- Reverberation-Mapping Database, ApJ, 613, 682 cal Counterpart Using the Dark Energy Camera, ApJL, Piro, A. L., & Morozova, V. S. 2016, Exploring the Potential 848, L16 Diversity of Early Type Ia Supernova Light Curves, ApJ, Stein, R., van Velzen, S., Kowalski, M., et al. 2021, A Tidal 826, 96 Disruption Event Coincident with a High-energy Neutrino, Piro, A. L., & Nakar, E. 2013, What Can We Learn from 2021, Nat. Astron., 5, 510 the Rising Light Curves of Radioactively Powered Super- novae?, ApJ, 769, 67 Tanvir, N. R., Levan, A. J., Gonz´alez-Fern´andez,C., et Piro, A. L., & Nakar, E. 2014, Constraints on Shallow 56Ni al. 2017, The Emergence of a Lanthanide-rich Kilonova from the Early Light Curves of Type Ia Supernovae, ApJ, Following the Merger of Two Neutron Stars, ApJL, 848, 784, 85 L27 Rabinak, I., & Waxman, E. 2011, The Early UV/Optical Troja, E., Piro, L., van Eerten, H., et al. 2017, The X-ray Emission from Core-collapse Supernovae, ApJ, 728, 63 Counterpart to the Gravitational-wave Event GW170817, Rakshit, S., Woo, J.-H., Gallo, E., et al. 2019, The Seoul Nature, 551, 71 National University AGN Monitoring Project. II. BLR Size and Black Hole Mass of Two AGNs, ApJ, 886, 93 Urata, Y., Huang, K., Im, M., et al. 2009, Swift GRB src src Reimann, R., Monitoring and Multi-Messenger Astronomy GRB071010B: Outlier of the Epeak–Eγ and Eiso–Epeak– src with IceCube, Galaxies, 7, 40 tjet Correlations, ApJL, 706, L183 Ricker, G. R., Winn, J. N., Vanderspek, R., et al. 2015, Urata, Y., Huang, K., Takahashi, S., et al. 2014, Synchrotron Transiting Exoplanet Survey Satellite (TESS), JATIS, Self-inverse Compton Radiation from Reverse Shock on 1a4003 GRB 120326A, ApJ, 789, 146 Seager, S., Deming, D., Valenti, J. A., 2009, Transiting Exoplanets with JWST, arXiv:0808.1913 Valenti, S., Sand, D. J., Yang, S., et al. 2017, The Discovery of Schmid, H. M. 1989, Identification of the Emission Bands at the Electromagnetic Counterpart of GW170817: Kilonova λλ 6830, 7088, A&A, 211, L31 AT 2017gfo/DLT17ck, ApJL, 848, L24 Schmid, H. M., Dumm, T., M¨urset,U., et al. 1998, High Woo, J.-H., Cho, H., Gallo, E., et al. 2019, A 10,000-solar- resolution Spectroscopy of Symbiotic Stars: III. Radial mass Black Hole in the Nucleus of a Bulgeless Dwarf Velocity Curve for CD–43◦14304, A&A, 329, 986 Galaxy, Nat. Astron., 3, 755 Shappee B. J., Prieto J. L., Grupe D., et al. 2014, The Man Woo, J.-H., Son, D., Gallo, E., et al. 2019, Seoul National behind the Curtain: X-Rays Drive the UV through NIR University AGN Monitoring Project. I. Strategy And Sam- Variability in the 2013 Active Galactic Nucleus Outburst ple, JKAS, 52, 109 in NGC 2617, ApJ, 788, 48 Skorov, Y. V., Rezac, L., Hartogh, P., et al. 2017, Is Near- Yu, S.-Y., Yi, H.-S., Lee, J. H., et al. 2010, Performance surface Ice the Driver of Dust Activity on 67P/Churyumov- Improvement of Near Earth Space Survey (NESS) Wide- Gerasimenko, A&A, 600, A142. Field Telescope (NESS-2) Optics, JASS, 27, 153