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ZTF Early Observations of Type Ia Supernovae. II. First Light, the Initial Rise, and Time to Reach Maximum Brightness

  • Author(s): Miller, AA
  • Yao, Y
  • Bulla, M
  • Pankow, C
  • Bellm, EC
  • Cenko, SB
  • Dekany, R
  • Fremling, C
  • Graham, MJ
  • Kupfer, T
  • Laher, RR
  • Mahabal, AA
  • Masci, FJ
  • Nugent, PE
  • Riddle, R
  • Rusholme, B
  • Smith, RM
  • Shupe, DL
  • Roestel, JV
  • Kulkarni, SR
  • et al.

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While it is clear that Type Ia supernovae (SNe) are the result of thermonuclear explosions in C/O white dwarfs (WDs), a great deal remains uncertain about the binary companion that facilitates the explosive disruption of the WD. Here, we present a comprehensive analysis of a large, unique data set of 127 SNe Ia with exquisite coverage by the Zwicky Transient Facility (ZTF). High-cadence (six observations per night) ZTF observations allow us to measure the SN rise time and examine its initial evolution. We develop a Bayesian framework to model the early rise as a power law in time, which enables the inclusion of priors in our model. For a volume-limited subset of normal SNe Ia, we find that the mean power-law index is consistent with 2 in the r ZTF-band (α r=2.01± 0.02), as expected in the expanding fireball model. There are, however, individual SNe that are clearly inconsistent with α r=2. We estimate a mean rise time of 18.9 days (with a range extending from ∼15 to 22 days), though this is subject to the adopted prior. We identify an important, previously unknown, bias whereby the rise times for higher-redshift SNe within a flux-limited survey are systematically underestimated. This effect can be partially alleviated if the power-law index is fixed to α = 2, in which case we estimate a mean rise time of 21.7 days (with a range from ∼18 to 23 days). The sample includes a handful of rare and peculiar SNe Ia. Finally, we conclude with a discussion of lessons learned from the ZTF sample that can eventually be applied to observations from the Vera C. Rubin Observatory.

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