Chandra Fails to Detect X-ray Emission from Type Ia SN 2018fhw/ASASSN-18tb

TL;DR

This study reports non-detection of X-ray emission from the low luminosity Type Ia supernova 2018fhw, setting upper limits and comparing with other Type Ia-CSM supernovae to understand their circumstellar environments.

Contribution

It provides the first X-ray upper limits for SN 2018fhw and compares these with other Type Ia-CSM supernovae to explore their circumstellar medium properties.

Findings

No X-ray emission detected from SN 2018fhw.

Upper limits are below the luminosity of X-ray detected Type Ia-CSM SNe.

Circumstellar medium density likely decreased from 10^8 to below 5×10^6 cm^-3.

Abstract

We report on Chandra X-ray observations of ASASSN-18tb/SN 2018fhw, a low luminosity Type Ia supernova 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 $\times 10^{39}$ erg s$^{-1}$ are calculated, depending on the assumed spectral model, temperature and column density. These are compared to two 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 H$\alpha$ line suggests that the density of the surrounding medium at the time of H$\alpha$ line detection could have been as high as 10$^8$ 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.