Host Galaxy Line Diagnostics for the Candidate Tidal Disruption Events XMMSL1 J111527.3+180638 and PTF09axc

TL;DR

This study analyzes host galaxy spectra of two candidate TDEs to determine their nature, revealing one as a Seyfert galaxy likely caused by AGN variability and the other as a star-forming galaxy, supporting its TDE candidacy.

Contribution

It provides a detailed spectral analysis of host galaxies to distinguish between TDEs and AGN activity, highlighting discrepancies between optical diagnostics and infrared colors.

Findings

Host galaxy of XMMSL1 J111527.3+180638 is a Seyfert galaxy.

Host galaxy of PTF09axc is star-forming, supporting TDE candidacy.

Infrared WISE colors do not indicate an active nucleus in either galaxy.

Abstract

We present results of our analysis of spectra of the host galaxies of the candidate Tidal Disruption Events (TDEs) XMMSL1 J111527.3+180638 and PTF09axc to determine the nature of these transients. We subtract the starlight component from the host galaxy spectra to determine the origin of the nuclear emission lines. Using a Baldwin-Phillips-Terlevich (BPT) diagram we conclude that the host galaxy of XMMSL1 J111527.3+180638 is classified as a Seyfert galaxy, suggesting this transient is likely to be caused by (extreme) variability in the active galactic nucleus. We find that the host of PTF09axc falls in the 'star-forming' region of the BPT-diagram, implying that the transient is a strong TDE candidate. For both galaxies we find a WISE-colour difference of $W1-W2<0.8$, which means there is no indication of a dusty torus and therefore an active galactic nucleus, seemingly contradicting our BPT finding for the host of XMMSL1 J111527.3+180638. We discuss possible reasons for the discrepant results obtained through the two methods.