Chandra LETG Spectroscopy of the Tidal Disruption Candidate ASASSN-14li

ATel #6800; J. M. Miller (Michigan), B. Cenko (GSFC), S. Gezari (Maryland), K. Gultekin (Michigan), J. A. Irwin (Alabama), J. Kaastra (SRON), P. Maksym (Alabama), R. Mushotzky (Maryland), F. Paerels (Columbia), E. Ramirez-Ruiz (UCSC), M. Reynolds (Michigan)
on 10 Dec 2014; 22:58 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

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We report on a Chandra observation of the candidate tidal disruption flare ASASSN-14li (Jose et al. 2014, ATEL #6777). The high flux from this nearby event offered an excellent opportunity to search for lines that might illuminate the aftermath of the disruption. Owing to the negligible column density to this source and its intrinsically soft spectrum, we requested an 80 ks DDT observation using the LETG+HRC-S. The first 35 ks was obtained starting on Dec 8 2014, and we report on a first analysis of the summed first-order spectra from this segment.

The source appears to be detected out to 170 Angstroms (0.073 keV). There is no evidence of neutral obscuration from gas along the line of sight. Ultraviolet observations are again strongly encouraged.

Above and below the 20-40 Angstroms band, the spectrum diverges from a blackbody; it is not clear if this is astrophysical. Within the 20-40 Angstroms (0.3-0.62 keV) band, the source is well described by a fiducial blackbody, kT = 67 +/- 1 eV, giving a flux of 2.8 E-11 erg/cm/cm/s (extrapolated over the 0.3-1 keV band), corresponding to L ~ 2.8 E+43 erg/s for a host galaxy at 90 Mpc.

Highly ionized charge states of C, N, and O fall within this band, but there is no clear evidence of emission or absorption features in this segment of the spectrum. The strongest feature is a putative absorption line at 31 Angstroms that could correspond to N I; however, the absence of other neutral lines in absorption, and the lack of edges associated with neutral gas along the line of sight, renders this identification unlikely. The Fe L band is similarly free of atomic features.

Ongoing Swift observations show a relatively steady flux over many days. If ASASSN-14li is indeed a tidal disruption event, this would indicate an extended Eddington-limited phase, and the implied luminosity would then signal a log(M/Msun) ~ 6 black hole after bolometric corrections. The cool thermal emission could plausibly represent a newly-formed accretion disk, or an optically-thick photosphere if the source is in an extreme super-Eddington state.

We thank Belinda Wilkes and the Chandra mission for quickly executing observations of a particularly special source.