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SUPERNOVAE

iPTF16geu: A multiplyimaged, gravitationally lensedtype Ia supernovaA. Goobar,1* R. Amanullah,1 S. R. Kulkarni,2 P. E. Nugent,3,4 J. Johansson,5 C. Steidel,2

D. Law,6 E. Mörtsell,1 R. Quimby,7,8 N. Blagorodnova,2 A. Brandeker,9 Y. Cao,10

A. Cooray,11 R. Ferretti,1 C. Fremling,12 L. Hangard,1 M. Kasliwal,2 T. Kupfer,2

R. Lunnan,2,9 F. Masci,13 A. A. Miller,14,15 H. Nayyeri,11 J. D. Neill,2 E. O. Ofek,5

S. Papadogiannakis,1 T. Petrushevska,1 V. Ravi,2 J. Sollerman,12 M. Sullivan,16

F. Taddia,12 R. Walters,2 D. Wilson,11 L. Yan,2 O. Yaron5

We report the discovery of a multiply imaged, gravitationally lensed type Ia supernova,iPTF16geu (SN 2016geu), at redshift z = 0.409. This phenomenon was identified becausethe light from the stellar explosion was magnified more than 50 times by the curvature ofspace around matter in an intervening galaxy.We used high-spatial-resolution observationsto resolve four images of the lensed supernova, approximately 0.3 arc seconds from thecenter of the foreground galaxy. The observations probe a physical scale of ~1 kiloparsec,smaller than is typical in other studies of extragalactic gravitational lensing. The largemagnification and symmetric image configuration imply close alignment between the linesof sight to the supernova and to the lens. The relative magnifications of the four imagesprovide evidence for substructures in the lensing galaxy.

One of the foundations of Einstein’s theoryof General Relativity is that matter curvesthe surrounding space-time. For the rarecases of nearly perfect alignment of anastronomical source, an intervening mas-

sive object, and the observer, multiple images ofa single source can be detected—a phenomenonknown as strong gravitational lensing.Although many strongly lensed galaxies and

quasars have been detected to date, it has proved

extremely difficult to findmultiply imaged lensedsupernova (SN) explosions. Type Ia supernovae(SNe Ia) are particularly interesting sources be-cause of their “standard-candle” nature. Theseexplosions have nearly identical peak luminosity,which makes them excellent distance indicatorsin cosmology (1). For lensed SNe Ia, the standard-candle property allows the flux magnification tobe estimated directly, independent of any modelrelated to the lensing galaxy (2, 3). This removestwo important degeneracies in gravitational lens-ing measurements: the mass-sheet degeneracy (4)and the source-plane degeneracy (5).PS1-10afx, a lensed SN Ia at redshift z = 1.388

with a large amplification (m ~ 30) wheremultipleimages could have been expected, was reportedseveral years ago (6). A foreground lens was lateridentified at z = 1.117 (7). At the time of the dis-covery, several interpretations were discussed, in-cluding a superluminous supernova (8). Becausethe lensed SN Ia hypothesis did not gain accept-ance until long after the explosion had faded, nohigh-spatial-resolution imaging could be carriedout in that case to verify the strong lensing na-ture of the system. Multiple images of anothersupernova, SN Refsdal (9), were discovered in aHubble Space Telescope (HST) survey of themassive galaxy clusterMACS J1149.6+2223. As thesource was identified as a core-collapse super-nova, it could not be used to measure the lensingmagnification directly.Thanks to the well-known characteristics of

their time-dependent brightness in optical andnear-infrared (NIR) filters (the SN light curves),

multiply imaged SNe Ia are also ideally suitedfor measuring time delays in the arrival of theimages. This provides a direct probe of theHubbleconstant, the cosmological parametermeasuringthe expansion rate of the universe (10), as well asleverage for studies of dark energy (11, 12), thecosmic constituent responsible for the acceleratedexpansion of the universe.The intermediate Palomar Transient Factory

(iPTF) searches the sky for new transient phe-nomena at optical wavelengths. It uses imagedifferencing between repeated observations (13)with a large field-of-view camera (7.3 squaredegrees) at the 48-inch telescope (P48) at thePalomar Observatory (14). The first detection ofiPTF16geu, with a statistical significance of fivestandard deviations (5s), is from 5 September2016. The new source was first recognized by ahuman scanner on 11 September (15). iPTF16geu(also known as SN 2016geu) was found near thecenter of the galaxy SDSS J210415.89-062024.7,at right ascension 21h04m15s.86 and declination–06°20′24″.5 (J2000 equinox).Spectroscopic identification was carried out

with the Spectral Energy Distribution (SED) Ma-chine (16) at the Palomar 60-inch telescope (P60)on 2 October 2016, and iPTF16geu was found tobe spectroscopically consistent with a normal SNIa at z ≈ 0.4 (Fig. 1). Further spectroscopicobservations from the Palomar 200-inch tele-scope (P200) and the 2.5-m Nordic Optical Tel-escope (NOT) were used to confirm the SN Iaidentification and to establish the redshift of thehost galaxy, from narrow sodium (Na I D) ab-sorption lines, as z = 0.409. The P200 and NOTspectra also showed absorption features fromthe foreground lensing galaxy at z = 0.216. Toestimate the velocity dispersion of the lensinggalaxy, we fit two Gaussian functions with a com-mon width to the Ha and [N II] emission lines inthe P200 spectrum in Fig. 1D. After taking theinstrumental resolution into account, wemeasures = 3:6þ0:9

−0:6 Å, corresponding to a velocity disper-sion of sv = 163þ41

−27 km s–1.Photometric observations of iPTF16geu col-

lected at P48 andwith the SEDMachine RainbowCamera (RC) at P60 between 5 September and13 October 2016 (Fig. 2) were used to estimatethe peak flux and light curve properties of theSN with the SALT2 light curve fitting tool (17).The best-fit light curve template, also shown inFig. 2, confirms that the observed light curveshapes are consistent with a SN Ia at z = 0.409.These fits also indicate some reddening of thesupernova, which suggests that iPTF16geu suffersfrom moderate extinction by dust. This wouldproduce dimming at optical wavelengths of 20to 40%, with the largest losses in the g-band ob-servations. Thanks to the standard-candle natureof SNe Ia, after correcting the peak magnitudefor light curve properties (18, 19), the flux of theSN was found to be ~30 standard deviationsbrighter than expected for the measured red-shift. This suggested that iPTF16geu was gravita-tionally lensed, and we estimated the lensingamplification to be m ~ 52. Expressed in astro-nomicalmagnitudes,Dm= –4.3 ± 0.2mag,where

RESEARCH

Goobar et al., Science 356, 291–295 (2017) 21 April 2017 1 of 4

1Oskar Klein Centre, Department of Physics, StockholmUniversity, Albanova University Center, SE 106 91 Stockholm,Sweden. 2Cahill Center for Astrophysics, California Instituteof Technology, Pasadena, CA 91125, USA. 3Department ofAstronomy, University of California, Berkeley, CA 94720, USA.4MS 50B-4206, Lawrence Berkeley National Laboratory,Berkeley, CA 94720, USA. 5Department of Particle Physics andAstrophysics, Weizmann Institute of Science, Rehovot 7610001,Israel. 6Space Telescope Science Institute, Baltimore, MD 21218,USA. 7Department of Astronomy, San Diego State University,San Diego, CA 92182, USA. 8Kavli IPMU (WPI), University ofTokyo Institutes for Advanced Study, Kashiwa, Chiba 277-8583,Japan. 9Department of Astronomy, Stockholm University,Albanova, SE 10691 Stockholm, Sweden. 10eScience Instituteand Department of Astronomy, University of Washington, Seattle,WA 98195, USA. 11Department of Physics and Astronomy,University of California, Irvine, CA 92697, USA. 12Oskar KleinCentre, Department of Astronomy, Stockholm University,Albanova University Center, SE 106 91 Stockholm, Sweden.13Infrared Processing and Analysis Center, California Instituteof Technology, Pasadena, CA 91125, USA. 14Center forInterdisciplinary Exploration and Research in Astrophysics andDepartment of Physics and Astronomy, Northwestern University,Evanston, IL 60208, USA. 15Adler Planetarium, Chicago, IL60605, USA. 16Department of Physics and Astronomy,University of Southampton, Southampton SO17 1BJ, UK.*Corresponding author. Email: ariel@fysik.su.se

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the uncertainty is dominated by the brightnessdispersion of normal SNe Ia. Because the mag-nification is derived from comparing the ob-served brightness of iPTF16geu to other SNe Ia(20) within a narrow redshift range around z =0.409, the measurement of the lensing magni-fication is independent of any assumptions oncosmology (e.g., the value of the Hubble constantor other cosmological parameters). The lensingmagnification is also independent of a lensmodel, which is the only way to determine themagnification for almost all other strong lensingsystems.The optical observations from Palomar, with a

typical angular resolution (atmospheric seeing)of 2″, were insufficient to spatially resolve anymultiple images that could result from the stronglensing nature of the system (Fig. 3A). We there-fore obtained Ks-band (2.2 mm) observations fromtheEuropeanSouthernObservatory (ESO)with theNasmyth Adaptive Optics System Near-InfraredImager and Spectrograph (NaCo) at the Very LargeTelescope (VLT). An angular resolution of ~0.3″[full width at half maximum (FWHM)] was ob-tained at the location of the target. Adaptive opticscorrections of the seeing were performed usinga natural bright star, ~30″ southeast of the SNlocation, indicated in Fig. 3 along with the SDSSpre-explosion image of the field (21).

The near-IR image from VLT indicated thatthe structurewas as expected in a strongly lensedsystem, with higher flux in the northeastern andsouthwestern regions of the system than in thecenter (Fig. 3B). Multiple images of the systemwere first resolved with observations from theKeck observatory atNIRwavelengths, using LaserGuide Star Adaptive Optics (LGSAO) with theOH-Suppressing Infrared Imaging Spectrograph(OSIRIS) instrument, which yielded an imagequality of 0.07″ FWHM in the H-band centeredat 1.6 mm (Fig. 3C).Shown in Fig. 4 are LGSAO observations of

iPTF16geu using the Near-Infrared Camera 2(NIRC2) at the Keck telescope on 22 Octoberand 5November 2016, in the Ks-band and J-band(1.1 mm), respectively, and optical images obtainedwithHSTon25October2016.TheHSTobservationswere carried out through the F475W, F625W, andF814W filters, where the names correspond to theapproximate location of the central wavelength innanometers.The observations exhibit four images of

iPTF16geu, 0.26″ to 0.31″ from the center of thelensing galaxy, with nearly 90° azimuthal sep-arations. The extended host galaxy, warped bythe lens to form a partial Einstein ring, is brighterin the NIR spectrum relative to the observationsthrough optical filters. Thus, the fainter individ-

ual SN images are poorly resolved for the obser-vations with the longest wavelengths in Fig. 4.Furthermore, the SN Ia spectral energy distri-bution (redshifted to z = 0.4) peaks within theF625W and F814W filters [see, e.g., (22)]. Dim-ming by interstellar dust in the line of sight isroughly inversely proportional to wavelengthin the optical and NIR spectra (23). The biggestimpact from extinction by dust is therefore ex-pected for the shortest wavelength, in F475Wfilter observations, where the two faintest SN im-ages cannot be detected above the backgroundlight. The low-spatial-resolution light curves inFig. 2 are dominated by the two brightest SNimages, labeled 1 and 2 in Fig. 4D. The F625W-F814W magnitude difference (color) of theresolved images measured with HST indicatessmall differences in relative extinction amongthe SN images, except for image 4, which appearsto have about two magnitudes of additionaldimming in F814W.Unaccounted dimming of light by scattering

on dust grains in the line of sight would lead toan underestimation of the lensing amplification.If corrections for differential extinction in theintervening lensing galaxy among the SN imagesare included, the result is a wider range for thelensing magnification of iPTF16geu, between –4.1and –4.8 mag (24).

Goobar et al., Science 356, 291–295 (2017) 21 April 2017 2 of 4

Fig. 1. Spectroscopic identification of iPTF16geu as a type Ia supernovaand measurements of the redshifts of the SN host galaxy and the inter-vening lensing galaxy. Measurements of the SN spectral energy distributionFlobtainedwith the P60, P200, andNOT telescopes are best fitted by a normalSN Ia spectral template. (A) Comparison with a nearby SN Ia, SN 2011fe, red-shifted to z=0.409 (green line) (22) at a similar rest-frame phase, expressed inunits of days with respect to the time of the optical light curve maximum.The

spectra also reveal narrow absorption and emission lines, marked by thedashed vertical lines, fromwhich the redshifts of the lens (z = 0.216, blue lines)and SN host galaxy (z = 0.409, red lines) were determined. (B toD) Zoomed-inview in rest-frame wavelengths of the Ca II H&K absorption features (B), theNa I D absorption features (C), and theHa and [N II] emission lines (D).TheHaand [N II] emission lines at z = 0.216 were used to fit the velocity dispersion ofmatter in the lensing galaxy, sv = 163þ41

−27 km s–1.

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The SNmultiple-image positions in Fig. 3wereused to construct a lensing model, with an iso-thermal ellipsoid galaxy lens (25, 26) with ellip-

ticity ee = 0.15 ± 0.07 andmassM = 1.70 (±0.06) ×1010 solar masses inside an ellipse with majoraxis of 1.13 kpc and minor axis of 0.97 kpc. De-

tails of the lensing model are presented in (24).The lens model can be independently verifiedthroughcomparisonsbetween themodel-predictedand observed velocity dispersion of the lensinggalaxy. From the model we derive an estimate,smodv = 156 ± 4 km s–1, in good agreement with

the measured value of the velocity dispersion(Fig. 1D).However, the adopted smooth isothermal el-

lipsoid lensmodel predicts brightness differencesbetween the multiple SN images that are in dis-agreement with the observations. Including cor-rections for extinction in the resolved SN imagesin the F814W filter, we find large discrepanciesbetween the model and measured magnitude dif-ferences for the multiple images of iPTF16geu:Dmobs

1 j −Dmmod1 j = (–0.3, –1.6, –1.5) mag for j = 2,

3, and 4, where the indices follow the numberingscheme adopted in Fig. 4. The observed discrep-ancy between the smooth model predictions forthe SN images 1 and 2 compared to 3 and 4(brighter by factors of 4 and 3, respectively)cannot be accounted for by time delays betweenthe images, as they are predicted to be <35 hours(24). Graininess of the stellar distribution anddark matter sub-halos in the lens galaxy, in ad-dition to the smooth mass profile, can cause var-iations to magnification without altering imagelocations. These milli- and microlensing effects(27, 28), small enough not to cause additional

Goobar et al., Science 356, 291–295 (2017) 21 April 2017 3 of 4

Fig. 2. Multicolor light curveof iPTF16geu showing that thesupernova is 4.3 magnitudes(30 standard deviations)brighter than expected.Themagnitudes are measured withrespect to time of maximumlight (modified Julian date57653.10) in the R-band at P48and in the g-, r-, and i-bandswith the SED Machine RCat P60. The filter transmissionfunctions are shown in (24).The solid lines show the best-fitted SALT2 (17) model to thedata. The dashed lines indicatethe expected light curves at z =0.409 (without lensing); thebands represent the standarddeviation of the brightnessdistribution for SNe Ia. To fit theobserved light curves, a bright-ness boost of 4.3 magnitudesis required.

Fig. 3. Image of the field of iPTF16geu show-ing the spatial resolution of the ground-basedinstruments used in this work. Left: Pre-explosionmulticolor image from SDSS indicating the brightnatural guide star. (A) Position of the SN detection inthe R-band at P48 (zoomed-in view near the galaxySDSS J210415.89-062024.7). (B and C) Improvedspatial resolution with the use of NGSAO (B) andLGSAO (C).

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resolved image separations, offer a plausibleexplanation for the deviation from the smoothlens model.Available forecasts for wide-field surveys (29)

suggest that about one strongly lensed SN Iacould be expected in our survey, irrespective ofredshift andmagnification, with approximately a30% chance of being in a quad configuration. Foran average ellipticity of the lenses e=0.3 (29), onlyabout 1% of the lensed SNe are expected to havem > ~50 (30). We have performed an indepen-dent rate estimate, with a somewhat simplifiedlensing simulation but including survey-specificparameters, and confirmed that the probabilityof detecting and classifying a highly magnifiedSN Ia like iPTF16geu does not exceed the few-percent level (24).iPTF16geu appears to be a rather unlikely event,

unless the actual rate of very magnified SNe ishigher than anticipated—for example, if the con-tribution from lensing by any kind of substructuresin galaxies is underestimated, or if we are other-wise lacking an adequate description of gravita-tional lensing at the ~1-kpc scale. The physicalscale probed by the resolved images of iPTF16geuis comparable to the smallest of the 299 mul-tiply imaged lensed systems in the Master LensDatabase, http://admin.masterlens.org (31). Usingthe standard-candle nature of SNe Ia, we canmore easily detect strongly lensed systems withsub–arc second angular separations, allowing ex-ploration of the bending of light at scales lessthan ~1 kpc—an otherwise challengingly smalldistance in studies of gravitational lensing (32).As demonstrated with iPTF16geu, discoveredwhile still brightening with a modest-sized tele-scope and suboptimal atmospheric conditions, thelocations of these rare systems can be identified in

advance of extensive follow-up imaging at highspatial resolution.

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4. E. E. Falco, M. V. Gorenstein, I. I. Shapiro,Astrophys. J.289, L1 (1985).5. P. Schneider, D. Sluse, Astron. Astrophys. 559, A37 (2013).6. R. M. Quimby et al., Astrophys. J. 768, L20 (2013).7. R. M. Quimby et al., Science 344, 396–399 (2014).8. R. Chornock et al., Astrophys. J. 767, 162 (2013).9. P. L. Kelly et al., Science 347, 1123–1126 (2015).10. S. Refsdal, Mon. Not. R. Astron. Soc. 128, 307–310 (1964).11. A. Goobar, E. Mörtsell, R. Amanullah, P. Nugent,

Astron. Astrophys. 393, 25–32 (2002).12. S. H. Suyu et al., Astrophys. J. 766, 70 (2013).13. Y. Cao, P. E. Nugent, M. M. Kasliwal, Publ. Astron. Soc. Pac.

128, 114502 (2016).14. N. M. Law et al., Publ. Astron. Soc. Pac. 121, 1395–1408 (2009).15. A. Goobar, Astron. Telegr. 9603, 1 (2016);

www.astronomerstelegram.org/?read=9603.16. A. Ritter, C. C. Ngeow, N. Konidaris, R. Quimby,

S. Ben-Ami, Contrib. Astron. Observ. Skalnate Pleso 43,209–215 (2014); www.astro.sk/caosp/Eedition/FullTexts/vol43no3/pp209-215.pdf.

17. J. Guy et al., Astron. Astrophys. 466, 11–21 (2007).18. M. M. Phillips, Astrophys. J. 413, L105 (1993).19. R. Tripp, Astron. Astrophys. 331, 815–820 (1998).20. M. Betoule et al., Astron. Astrophys. 568, A22 (2014).21. S. Alam et al., Astrophys. J. Suppl. Ser. 219, 12 (2015).22. P. A. Mazzali et al., Mon. Not. R. Astron. Soc. 439, 1959–1979

(2014).23. J. A. Cardelli, G. C. Clayton, J. S. Mathis,Astrophys. J. 345, 245 (1989).24. See supplementary materials.25. A. Kassiola, I. Kovner, in Liege International Astrophysical

Colloquia, vol. 31, J. Surdej, D. Fraipont-Caro, E. Gosset,S. Refsdal, M. Remy, Eds. (1993), p. 571.

26. R. Kormann, P. Schneider, M. Bartelmann, Astron. Astrophys.284, 285 (1994).

27. R. Kayser, S. Refsdal, Nature 338, 745–746 (1989).28. H. J. Witt, S. Mao, Astrophys. J. 429, 66 (1994).29. M. Oguri, P. J. Marshall, Mon. Not. R. Astron. Soc. 405,

2579–2593 (2010).

30. K.-H. Chae, Mon. Not. R. Astron. Soc. 346, 746–772(2003).

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ACKNOWLEDGMENTS

Supported by the Swedish National Science Council and theSwedish National Space Board (A.G., R.A., and E.M.); U.S.Department of Energy (DOE) grant DE-AC02-05CH11231 (P.E.N.);European Research Council grant 615929 (M.S.); NASA (S.R.K.);and NSF grant 1545949. The iPTF Swedish collaboration is fundedthrough a grant from the Knut and Alice Wallenberg foundation. Thisresearch used resources of the National Energy Research ScientificComputing Center, supported by DOE contract DE-AC02-05CH11231.Some of the data presented here were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andNASA grant HST-GO-14862.002. The Observatory was madepossible by the generous financial support of the W. M. KeckFoundation. Some of the data presented here were obtained withALFOSC, which is provided by the Instituto de Astrofisica deAndalucia (IAA) under a joint agreement with the University ofCopenhagen and NOTSA. The Space Telescope Science DataAnalysis System (STSDAS) and the command language PyRAFare products of the Space Telescope Science Institute, whichis operated by the Association of Universities for Research inAstronomy (AURA) for NASA. Part of the processing was carried outoff-line using the commercial software package MATLAB andStatistics Toolbox Release 2013a, The MathWorks Inc., Natick, MA.Photometric data used in this paper are available in tables S1, S2,and S5, spectroscopic data are available at public repositoryWISeREP (33) (http://wiserep.weizmann.ac.il) under the ID “SN2016geu”; the positions of the SN images used in the lensing modelare provided in table S4.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/356/6335/291/suppl/DC1Materials and MethodsFigs. S1 to S3Tables S1 to S5References (34–56)

25 October 2016; accepted 24 March 201710.1126/science.aal2729

Goobar et al., Science 356, 291–295 (2017) 21 April 2017 4 of 4

Fig. 4. High-spatial-resolution images from theHubble Space Telescope and the Keck Observatoryused to resolve the positions of the SN images,the partial Einstein ring of the host galaxy, andthe intervening lensing galaxy. (A to C) HST/WFC3observations of iPTF16geu obtained on 25 October2016 in the F475W, F625W, and F814W bands, re-spectively. The images reveal four point sources, ex-cept for F475W where SN images 3 and 4 are toofaint. (D to F) NIR images obtained using adaptiveoptics–aided Keck observations in the J-, H-, andKs-bands, respectively. All four SN images are clearlyseen in the J-band (D). For the H- and Ks-bandimages, both the lensing galaxy at the center of thesystem and the lensed partial Einstein ring of thehost galaxy are visible.

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iPTF16geu: A multiply imaged, gravitationally lensed type Ia supernova

Sullivan, F. Taddia, R. Walters, D. Wilson, L. Yan and O. YaronMasci, A. A. Miller, H. Nayyeri, J. D. Neill, E. O. Ofek, S. Papadogiannakis, T. Petrushevska, V. Ravi, J. Sollerman, M.Blagorodnova, A. Brandeker, Y. Cao, A. Cooray, R. Ferretti, C. Fremling, L. Hangard, M. Kasliwal, T. Kupfer, R. Lunnan, F. A. Goobar, R. Amanullah, S. R. Kulkarni, P. E. Nugent, J. Johansson, C. Steidel, D. Law, E. Mörtsell, R. Quimby, N.

DOI: 10.1126/science.aal2729 (6335), 291-295.356Science 

, this issue p. 291Sciencewill enable cosmological measurements and can be used to probe the distribution of mass in the foreground galaxy.a well-studied variety with reliable properties that can be used to constrain models of the lensing. This distinctive objectcausing it to be highly magnified and splitting the light into four separate images. What is more, it is a type Ia supernova,

have identified a supernova that is strongly lensed by a foreground galaxy,et al.known as gravitational lensing. Goobar General relativity indicates that any sufficiently massive object bends the path of light passing by it. This effect is

Multiple images of a type Ia supernova

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