2023MNRAS.520.1362D

We report on Chandra X-ray observations of ASASSN-18tb/SN 2018fhw, a low luminosity Type Ia supernova (SN) that showed a H line in its optical spectrum. No X-ray emission was detected at the location of the SN. Upper limits to the luminosity of up to 3 × 10³⁹ erg s^–1 are calculated, depending on the assumed spectral model, temperature, and column density. These are compared to Type Ia-CSM SNe, SN 2005gj, and SN 2002ic that have been observed with Chandra in the past. The upper limits are lower than the X-ray luminosity found for the Type Ia-CSM SN 2012ca, the only Type Ia SN to have been detected in X-rays. Consideration of various scenarios for the Hα line suggests that the density of the surrounding medium at the time of Hα line detection could have been as high as 10⁸ cm^–3, but must have decreased below 5 $\times \, 10^6$ cm^–3 at the time of X-ray observation. Continual X-ray observations of SNe which show a H line in their spectrum are necessary in order to establish Type Ia SNe as an X-ray emitting class.