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Does Light Emerge from a Black Hole Merger?: Subaru+GTC Collaboration to Target Gravitational Wave Events

April 12, 2023
Last updated: July 1, 2023

A research team led by researchers from the National Astronomical Observatory of Japan (NAOJ) and the Instituto de Astrofísica de Canarias (IAC) in Spain performed follow-up observations of a gravitational wave event using the Subaru Telescope and the Gran Telescopio CANARIAS (GTC) to search for electromagnetic emission from a binary black hole coalescence. The collaboration between the wide-field deep imaging capability of the Subaru Telescope and the observing flexibility of GTC played an important role in this study. The cooperative follow-up of a large number of gravitational wave events will elucidate the nature of these enigmatic phenomena.

Does Light Emerge from a Black Hole Merger?: Subaru+GTC Collaboration to Target Gravitational Wave Events Figure1

Figure 1: The combination of the deep and wide field capability of the Subaru Telescope and the flexible spectroscopic follow-up by GTC plays a key role in revealing the nature of electromagnetic radiation related to gravitational wave events. (Credit: Gabriel Pérez, IAC)

Since the first direct detection of gravitational waves in 2015, gravitational wave astronomy has attracted much attention from astronomers all over the world. In 2017, a gravitational wave event from a binary neutron star merger GW170817 was detected; this event was the first and only gravitational wave event for which the electromagnetic counterpart has been firmly detected so far.

Gravitational wave telescopes detect not only gravitational waves from binary neutron star coalescences, but also from black hole-neutron star coalescences and binary black hole coalescences. Indeed, the gravitational wave events from binary black hole coalescences account for more than 90% of all gravitational wave detections so far.

Due to the strong gravity field, not even light can escape from a black hole, and thus it is naively not expected that binary black hole coalescences are accompanied by electromagnetic emission. However, the detection of an electromagnetic emission was claimed for the gravitational wave event GW190521 from a binary black hole coalescence in 2019. Multiple mechanisms for the electromagnetic emission have been theoretically proposed. Therefore, multi-wavelength follow-up observations are important to clarify whether binary black hole coalescences are accompanied by electromagnetic emission, and, if so, how bright the emission is.

Dr. Josefa Becerra, who is responsible for this research at IAC says, "Even though the theoretical framework for the electromagnetic emission of binary black hole coalescences is still under discussion, the observations are crucial to probe the different possibilities."

On February 24, 2020, the gravitational wave telescopes LIGO and Virgo, detected gravitational waves from a binary black hole coalescence named GW200224_222234 (hereafter "GW200224"). The 'eyesight' of gravitational wave detectors is generally poor, and the accuracy of the arrival direction is typically comparable with about 2000 full moons (500 square degrees). However, the arrival direction of GW200224 was well determined, about 50 square degrees, because of its strong gravitational wave radiation. Then the research team led by Dr. Takayuki Ohgami at NAOJ and Dr. Josefa Becerra González at IAC performed follow-up observations of GW200224 with Hyper Suprime-Cam (HSC) on the Subaru Telescope and the Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) on GTC.

Dr. Tomoki Morokuma at Chiba Institute of Technology, a Co-Principal Investigator of the GTC-Subaru Collaboration, says, "The cooperation of Subaru and GTC telescopes provide us with a very unique synergy to perform deep and wide photometric follow-up observations with the Subaru Telescope together with the powerful spectroscopic capabilities provided by GTC. This collaboration offers a key opportunity to reveal the nature of the possible electromagnetic radiation from binary black hole coalescences for the first time."

The team started a deep and wide-field search with HSC for an electromagnetic counterpart, which is expected to be a "transient" object. The team succeeded in observing almost the entire possible arrival direction (91%) of GW200224 just 12 hours after the gravitational wave event detection. This is the first achievement of deep follow-up for a binary black hole coalescence covering more than 90% of the possible arrival direction.

Does Light Emerge from a Black Hole Merger?: Subaru+GTC Collaboration to Target Gravitational Wave Events Figure2

Figure 2: Possible arrival direction of GW200224_22234 (90% probability), represented by a white line, determined by gravitational wave telescopes and the area observed by HSC, represented by red circles. A red circle represents the size of HSC's field of view, equivalent to nine full moons. The yellow circle shows the size of a full moon for comparison. High resolution image is here (2.3MB). (Credit: NAOJ/Tominaga/PanSTARRS)

By scrutinizing the luminosity variations of the transient objects found by the HSC observation, and by performing spectroscopic follow-up observations with OSIRIS to measure the distances to some of them, the team eventually identified 19 candidate objects. However, none of these objects were strongly suggested to be related to GW200224.

Dr. Nozomu Tominaga at NAOJ says, "Using the Subaru Telescope and GTC, we could make an unprecedented wide and deep follow up of a gravitational wave event from a binary black hole coalescence, but we detected no obvious electromagnetic counterpart. We need to continue follow ups of the gravitational waves from binary black hole coalescences to search for firm electromagnetic emission and study their diversity."

Gravitational wave observations will resume in May 2023 with a total of four gravitational wave detectors, LIGO (x2), Virgo, and KAGRA. With their improved performance, they will detect more gravitational wave events from binary neutron star coalescences and binary black hole coalescences. The research team will continue with follow-up observations using the Subaru Telescope and GTC to reveal the diversity of gravitational wave events.


These results appeared as Ohgami et al. "Follow-up survey for the binary black hole merger GW200224_222234 using Subaru/HSC and GTC/OSIRIS" in The Astrophysical Journal on April 12, 2023.

These collaborative observations were possible thanks to the leadership of the National Astronomical Observatory of Japan (NAOJ) and the Instituto de Astrofísica de Canarias (IAC).This study was supported by MEXT KAKENHI (JP17H06363), JSPS KAKENHI (JP19H00694, JP20H00158, JP20H00179, JP21H04997), and financial support from the Spanish Ministry of Science and Innovation (MICINN) through the Spanish State Research Agency, under Severo Ochoa Program 2020-2023 (CEX2019-000920-S) and the projects PID2019-107988GB-C22 and PID2019-105552RB-C43.

About the Subaru Telescope
The Subaru Telescope is a large optical-infrared telescope operated by the National Astronomical Observatory of Japan, National Institutes of Natural Sciences with the support of the MEXT Project to Promote Large Scientific Frontiers. We are honored and grateful for the opportunity of observing the Universe from Maunakea, which has cultural, historical, and natural significance in Hawai`i.

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