X-ray Absorption Lines in the Candidate Tidal Disruption Event ASASSN-14li

ATel #6825; J. M. Miller (Michigan), J. J. Drake (SAO), J. Kaastra (SRON), F. Paerels (Columbia), R. Mushotzky (Maryland), S. Gezari (Maryland), K. Gultekin (Michigan), M. Reynolds (Michigan), A. Zoghbi (Michigan), E. Ramirez-Ruiz (UCSC), S. B. Cenko (NASA/GSFC), G. Brown (Warwick), A. Levan (Warwick)
on 16 Dec 2014; 01:23 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

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We report on a 94 ks observation of the TDE candidate ASASSN-14li, obtained on 8 December 2014 with XMM-Newton.

The high resolution first-order RGS spectra show a number of significant absorption features in the 20-40 Angstroms band, most notably at 24.6, 25.25, 27.9, 29.9, 31.1, and 32.9 Angstroms. It is worth noting that the features at ~25 and ~31 Angstroms correspond to the strongest line candidates in the Chandra/LETGS-HRC spectrum of ASASSN-14li (Miller et al. 2014, ATEL #6800). To our knowledge, this is the first detection of discrete X-ray spectral features in a TDE or TDE candidate. The results signal that the environment close to disruptions can be probed effectively through X-ray spectroscopy.

The RGS continuum is described adequately with a simple blackbody (kT = 53 eV), modified by interstellar absorption (1.8 E+20 /cm/cm).

Some of these lines may plausibly be attributed to different charge states of N. Overall, the character of the spectrum is similar to atmospheric absorption from white dwarfs in outburst or super-soft phases (such spectra can include blends and require detailed modeling for secure line identifications; see, e.g., Rauch et al. 2010 ApJ 717 363). This may not be unexpected: if ASASSN-14li is indeed in a super-Eddington phase, common physics may cause it to resemble cataclysmic variable in a super-Eddington outburst. The spectrum does not closely resemble a typical Seyfert warm absorber.

The nature of this spectrum raises the possibility that the source is indeed a Galactic white dwarf. However, given the close proximity of this event to the center of a plausible host galaxy, given the lack of a previously known white dwarf or nova at this location, and given the emergence of a broad H-alpha line at the putative host galaxy redshift (Jose et al. 2014; ATEL #6777), a chance alignment is highly improbable. If the absorption line at 25.25 Angstroms is N VII, its redshift would be commensurate with that of the host galaxy and further exclude a Galactic object.

We thanks Norbert Schartel and the XMM-Newton team for executing this TOO program.