We present photometric and spectroscopic observations of the unusual Type Ia supernova ASASSN-18tb , including a series of SALT spectra obtained over the course of nearly six months and the first observations of a supernova by the Transiting Exoplanet Survey Satellite ( TESS ) .
We confirm a previous observation by ( ) showing that ASASSN-18tb is the first relatively normal Type Ia supernova to exhibit clear broad ( \sim 1000 km s ^ { -1 } ) H \alpha emission in its nebular phase spectra .
We find that this event is best explained as a sub-Chandrasekhar mass explosion producing M _ { Ni } \approx 0.3 M _ { \odot } .
Despite the strong H \alpha signature at late times , we find that the early rise of the supernova shows no evidence for deviations from a single-component power-law and is best fit with a moderately shallow power-law of index 1.69 \pm 0.04 .
We find that the H \alpha luminosity remains approximately constant after its initial detection at phase +37 d , and that the H \alpha velocity evolution does not trace that of the Fe III ~ { } \lambda 4660 emission .
These suggest that the H \alpha emission arises from a circumstellar medium ( CSM ) rather than swept up material from a non-degenerate companion .
However , ASASSN-18tb is strikingly different from other known CSM-interacting Type Ia supernovae in a number of significant ways .
Those objects typically show an H \alpha luminosity two orders of magnitude higher than what is seen in ASASSN-18tb , pushing them away from the empirical light curve relations that define ‘ ‘ normal ’ ’ Type Ia supernovae .
Conversely , ASASSN-18tb exhibits a fairly typical light curve and luminosity for an underluminous or transitional SN Ia , with M _ { R } \approx - 18.1 mag .
Moreover , ASASSN-18tb is the only SN Ia showing H \alpha from CSM interaction to be discovered in an early-type galaxy .