Optical spectroscopy of ASASSN-15lh reveal no clear signs of interaction with an H-rich circumstellar environment

ATel #8216; D. Milisavljevic (Smithsonian Astrophysical Observatory), D. J. James (Cerro Tololo InterAmerican Observatory), J. L. Marshall (Texas A&M University), D. Patnaude (Smithsonian Astrophysical Observatory), R. Margutti (New York University), J. Parrent (Harvard University), A. Kamble (Harvard University)
on 26 Oct 2015; 15:34 UT
Credential Certification: D. Milisavljevic (dmilisav@cfa.harvard.edu)

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ASASSN-15lh is thought to be a highly superluminous supernova (ATel #7642,#7774; Dong et al. 2015; arXiv:1507.03010v2). Motivated by the recent report of a rebrightening in its ultraviolet light curve (ATel #8086) and X-ray non-detection (ATel #8089), we obtained a moderate resolution (R ~ 2000) optical spectrum (range 4200-9400 Angstroms) of ASASSN-15lh on October 18.15 UT with the Walter Baade Magellan 6.5m telescope (+ IMACS) at Las Campanas Observatory. These data show a predominantly blue continuum of emission, along with subtle, broad, and unidentifiable absorption features having rest frame minima around 3870 A and 5150 A (z = 0.2326; ATel#7774). Most importantly, the spectrum lacks the broad (FWHM ~ 4000 km/s) Halpha emission feature that accompanied the optical rebrightening of the superluminous supernova iPTF13ehe, associated with interaction between the supernova and an H-rich circumstellar environment (Yan et al. 2015, arXiv:1508.04420). Our observations disfavor the notion that a similar scenario may explain the increase in ultraviolet emission from ASASSN-15lh (ATel #8086). Clearly, the various non-standard emission properties of ASASSN-15lh continue to challenge interpretations of its possible energy sources (56Ni radioactivity, magnetar, and strong interaction with a nearby environment), and ongoing monitoring at all accessible wavelengths is needed to understand its true nature.