We present observations of SN 2022joj, a peculiar Type Ia supernova (SN Ia)
discovered by the Zwicky Transient Facility (ZTF). SN 2022joj exhibits an
unusually red gZTF−rZTF color at early times and a rapid
blueward evolution afterwards. Around maximum brightness, SN 2022joj shows a
high luminosity (MgZTF,max≃−19.7 mag), a blue
broadband color (gZTF−rZTF≃−0.2 mag), and shallow Si
II absorption lines, consistent with those of overluminous, SN 1991T-like
events. The maximum-light spectrum also shows prominent absorption around 4200
\r{A}, which resembles the Ti II features in subluminous, SN 1991bg-like
events. Despite the blue optical-band colors, SN 2022joj exhibits extremely red
ultraviolet − optical colors at maximum luminosity (u−v≃1.6 mag and
uvw1−v≃4.0 mag), suggesting a suppression of flux between
∼2500--4000 \r{A}. Strong C II lines are also detected at peak. We show
that these unusual spectroscopic properties are broadly consistent with the
helium-shell double detonation of a sub-Chandrasekhar mass
(M≃1M⊙) carbon/oxygen (C/O) white dwarf (WD) from a
relatively massive helium shell (Ms≃0.04--0.1M⊙), if
observed along a line of sight roughly opposite to where the shell initially
detonates. None of the existing models could quantitatively explain all the
peculiarities observed in SN 2022joj. The low flux ratio of [Ni II]
λ7378 to [Fe II] λ7155 emission in the late-time nebular
spectra indicates a low yield of stable Ni isotopes, favoring a
sub-Chandrasekhar mass progenitor. The significant blueshift measured in the
[Fe II] λ7155 line is also consistent with an asymmetric chemical
distribution in the ejecta, as is predicted in double-detonation models.Comment: 24 pages, 11 figures, 6 tables. Submitted to Ap