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Published November 1, 2021 | Accepted Version + Published
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Faintest of Them All: ZTF 21aaoryiz/SN 2021fcg—Discovery of an Extremely Low Luminosity Type Iax Supernova


We present the discovery of ZTF 21aaoryiz/SN 2021fcg—an extremely low luminosity Type Iax supernova. SN 2021fcg was discovered by the Zwicky Transient Facility in the star-forming galaxy IC0512 at a distance of ≈27 Mpc. It reached a peak absolute magnitude of M_r = −12.66 ± 0.20 mag, making it the least luminous thermonuclear supernova discovered to date. The E(B − V) contribution from the underlying host galaxy is unconstrained. However, even if it were as large as 0.5 mag, the peak absolute magnitude would be M_r = −13.78 ± 0.20 mag—still consistent with being the lowest-luminosity SN. Optical spectra of SN 2021fcg taken at 37 and 65 days post-maximum show strong [Ca II], Ca II, and Na I D emission and several weak [Fe II] emission lines. The [Ca II] emission in the two spectra has extremely low velocities of ≈1300 and 1000 km s⁻¹, respectively. The spectra very closely resemble those of the very low luminosity Type Iax supernovae SN 2008 ha, SN 2010ae, and SN 2019gsc taken at similar phases. The peak bolometric luminosity of SN 2021fcg is ≈ 2.5^(+1.5)_(-0.3) x 10⁴⁰ erg s⁻¹, which is a factor of 3 lower than that for SN 2008 ha. The bolometric lightcurve of SN 2021fcg is consistent with a very low ejected nickel mass (M_(Ni) ≈ 0.8^(+0.4)_(-0.5) x 10⁻³ M⊙). The low luminosity and nickel mass of SN 2021fcg pose a challenge to the picture that low-luminosity SNe Iax originate from deflagrations of near-M_(ch) hybrid carbon–oxygen–neon white dwarfs. Instead, the merger of a carbon–oxygen and oxygen–neon white dwarf is a promising model to explain SN 2021fcg.

Additional Information

© 2021 The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2021 August 13; revised 2021 October 7; accepted 2021 October 7; published 2021 October 27. We thank the anonymous reviewer for comments that helped improve the paper. We thank Harsh Kumar, Varun Bhalerao, and G. C. Anupama for photometric observations with the GROWTH-India telescope (https://sites.google.com/view/growthindia/). This Letter is based on observations obtained with the Samuel Oschin Telescope 48 inch and the 60 inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under grant No. AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, and IN2P3, France. Operations are conducted by COO, IPAC, and UW. SED Machine is based upon work supported by the National Science Foundation under grant No. 1106171. This work is also based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. SED Machine is based upon work supported by the National Science Foundation under grant No. 1106171. The ZTF forced photometry service was funded under the Heising-Simons Foundation grant 12540303 (PI: Graham). K.M. is funded by the EU H2020 ERC grant No. 758638. S.S. and E.C.K. acknowledge support from the G.R.E.A.T research environment, funded by Vetenskapsrådet, the Swedish Research Council, project number 2016-06012. E.C.K. acknowledges support from The Wenner-Gren Foundations. M.R. has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement 759194—USNAC).

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Published - Karambelkar_2021_ApJL_921_L6.pdf

Accepted Version - 2110.04306.pdf


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August 22, 2023
October 23, 2023