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MDM Redshift of the Host of ASASSN-17gs

ATel #10491; R. Chornock (Ohio University) & R. Margutti (Northwestern)
on 14 Jun 2017; 10:48 UT
Credential Certification: Ryan Chornock (chornock@ohio.edu)

Subjects: Optical, X-ray, Gamma Ray, >GeV, Blazar, Transient, Tidal Disruption Event

Referred to by ATel #: 10495, 10642

The optical transient ASASSN-17gs = AT2017egv (http://www.astronomy.ohio-state.edu/~assassin/transients.html, https://wis-tns.weizmann.ac.il/object/2017egv) was associated with the new Fermi LAT source J1544-0649 and a new, bright X-ray source Swift 154419.7-064915 by Ciprini et al. (ATel #10482). The potential host galaxy near this position is identified in NED as 2MASX J15441967-0649156, but with no known redshift. We obtained an optical spectrum (range 4000-9000 Angs) of this object with the OSMOS spectrograph on the MDM 2.4 m Hiltner telescope on the night of 2017 June 14 UT. The object has a blue continuum with several weak absorption features present. Cross-correlation with a library of single stellar population age galaxy templates finds best matches with old stellar populations at a redshift of z=0.171. The absorption features in the object spectrum (including Ca II H+K, Na I D, and Mg I b) are significantly weaker than in the templates, consistent with dilution with by a featureless blue continuum.