Predicting the Appearances of the Death of a Star

Gravitational lensing is the phenomenon of bending of the light in the presence of a high gravitational potential. Einstein, in his general theory of relativity, describes how concentration of matter in space distorts the space around it. Gravitational lensing occurs when the gravitational potential created by a massive body distorts the light coming from a distant source behind it. This leads to phenomena like the Einstein Rings, apparent distortion, multiplication or/and magnification of the original sources. A supernova is a transient astronomical event that occurs during the last evolutionary stages of a massive star or when a white dwarf is triggered into runaway nuclear fusion. The later forms the subtype of supernova called Type Ia supernova. Each supernova is an extremely bright and very powerful explosion of a dying star. Type Ia supernovae serve as Standard Candles, to accurately measure astronomical distances, and are known to be very significant cosmological probes. Sjur Refsdal, in 1964, published two papers discussing the phenomenon of gravitational lensing in great details[1] and subsequently discussing possibility of determining the Hubble’s constant or the rate of expansion of the universe using the time delay measurements between multiple images of the supernovae gravitationally lensed by galaxy clusters[2] . 

It took almost five decades of searches after that to actually observe a multiply imaged gravitationally lensed supernova system. The first confirmed case of strongly lensed, multiply-imaged supernova observation was reported by Kelly et al.[3] in March 2015. SN Refsdal, aptly named after Sjur Refsdal, was reported to be found in the archival data from Nov 10, 2014, of the Hubble Space Telescope, of the galaxy cluster MACS J1149.6+2223. SN Refsdal showed a peculiar case of gravitational lensing of both the supernova and the galaxy in which the supernova resides. The supernova is located in the arm of a face-on spiral galaxy at the redshift of 1.4888. This arm of the spiral galaxy is lensed by the galaxy cluster MACS J1149.6+2223 (redshift 0.54) and the SN Refsdal, residing in this spiral galaxy arm, is lensed by a foreground elliptical galaxy in the same galaxy cluster.

The lens modeling of this system shows that it is a quadruply lensed system as observed in the box 1.1 in Fig 1. Now, because the galaxy hosting the supernova is itself getting multiple-imaged due to lensing, we expect to see the lensing of the supernova too multiple times in each of the three galaxy images. The lens modeling by Kelly et al. showed that an image of the supernova should have appeared in the past in image 1.3 of the galaxy and predicted that one image will appear in the near future (within an year) in central image of the spiral host galaxy, image 1.2, at an approximate position of α = 11h49m36.01s , δ = +22◦23’48.13”(in J2000.0).

Figure 1: Color-composite image of the galaxy cluster MACSJ1149.6+2223: In the Left Panel, the labels 1.1, 1.2 and 1.3 denote the three gravitationally lensed images of the host galaxy formed by the cluster. The image 1.1 clearly shows the four images (the red point sources) reported in 2014. The white curves depict something called the Critical Curves, derived from the lensing theory, that help in predicting the multiplicity in lensed system. Each of the Right Panels shows these three images in enlarged form. The four current images of SN Refsdal, labeled S1 to S4, appear as red point sources. The model reported in Kelly et al. indicates that an image of the SN appeared in the past in image 1.3 and that one will appear in the near future in image 1.2. [Source: Kelly et al. The image combines infrared and optical HST imaging data from the Frontier Fields and GLASS programs, along with images from the CLASH and the Frontier SN programs.] 

Since the end of October 2015, Hubble was made to periodically peer at MACS J1149.5+2223, hoping to observe the predicted rerun of SN Refsdal and prove the models correct. This story of SN Refsdal again came into limelight in Nov 2015, when the group led by Kelly and Rodney announced the observation of a newly emerged source at about the same coordinates as the predicted by lens models for SN Refsdal in the HST observations of Dec 11, 2015[4]. This predicted, yet nonetheless showstopping, reappearance of the SN Refsdal in the image 1.2 of the host galaxy was confirmed by various groups and was shown to match the predictions about the parameters of lensing made from lens modeling of previous SN Refsdal observations. The timing and brightness of the new image were found to be consistent with the blind predictions of a fraction of the models.

Figure 2: Reappearance of the SN Refsdal in 2015: Left Panel: A part of the deep field observation of the galaxy cluster MACS J1149.5+2223 from the Frontier Fields program. The circle indicates the predicted position of the newest appearance of SN Refsdal. To the lower right part of this image, the four images from late 2014 are visible. Right Panel: The image on the top shows observations by Hubble from October 2015, taken at the beginning of the observation program to detect the newest appearance of the supernova. The image on the bottom shows the discovery of the Refsdal Supernova on 11 December 2015, as predicted by several different models. [Source: ESA Hubble Images].

This marked the first time that the appearance of a supernova image at a particular time and location in the sky was successfully predicted in advance. This enabled us for the first time to test lens model predictions of both magnifications and time delays for a lensed supernova, illustrating the discriminatory power of this blind test and its utility to uncover sources of systematic uncertainty in the lens models. With already very few detections of the strongly-lensed, multiply-imaged supernova systems, this new-found power is expected to increase our chances of tracking these explosions of the dying stars and use their observational data to constrain the value of the Hubble’s constant independently. The similar case of lensing of both the supernova and its host galaxy was also reported very recently, in 2021, by Rodney et al.[5]. SN Requiem, observed by HST, hosted by a galaxy at the redshift of 1.95 shows multiple images of host galaxy and the supernova. The lens models for SN Requiem predict the reappearance of the fourth image approximately in the year 2037. With the advancements in our observational capacities, we expect to discover more and more of such multiply images lensed systems and provide independent constraints on the rate of acceleration of the universe.

References: 

1] Refsdal, S., and Bondi, H. The Gravitational Lens Effect. MNRAS. 128(4) (1964). [DOI: 10.1093/mnras/128.4.295] 

2] Refsdal, S. On the Possibility of Determining Hubble’s Parameter and the Masses of Galaxies from the Gravitational Lens Effect. MNRAS (1964). [DOI: 10.1093/mnras/128.4.307]. 

3] P. L. Kelly et al., Multiple images of a highly magnified supernova formed by an early-type cluster galaxy lens. Science. 347, 1123–1126 (2014)[DOI: 10.1126/science.aaa3350]. 

4] P. L. Kelly et al., Deja Vu All Over Again: The Reappearance of Supernova Refsdal. The Astrophysical Journal Letters. 819(1) (2015). [DOI: 10.3847/2041-8205/819/1/L8]. 

5] Steven A. Rodney, A gravitationally lensed supernova with an observable two-decade time delay. Nature Astronomy. 5(11):1-8 (2021). [DOI: 10.1038/s41550-021-01450-9] 

- Prajakta Mane (MS19054)