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

• Published in: The Astrophysical journal 2020-10, Vol.902 (1), p.47
• Main Authors: Miller, A. A., Yao, Y., Bulla, M., Pankow, C., Bellm, E. C., Cenko, S. B., Dekany, R., Fremling, C., Graham, M. J., Kupfer, T., Laher, R. R., Mahabal, A. A., Masci, F. J., Nugent, P. E., Riddle, R., Rusholme, B., Smith, R. M., Shupe, D. L., Roestel, J. van, Kulkarni, S. R.
• Format: Article
• Language: English
• Subjects: Astronomy; Astrophysics; Companion stars; Explosions; Fireballs; Observational astronomy; Power law; Red shift; Supernovae; Surveys; Thermonuclear explosions; Type Ia supernovae; White dwarf stars
• ISSN: 0004-637X; 1538-4357; 1538-4357
• DOI: 10.3847/1538-4357/abb13b

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 rZTF-band ( ), as expected in the expanding fireball model. There are, however, individual SNe that are clearly inconsistent with . 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 = 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.