XMM-Newton Observations of the TDE AT 2021ehb

ATel #15205; J. M. Miller, M. T. Reynolds, A. Zoghbi (Univ. of Michigan); J. Drake (SAO); C. Miller, R. Mushotzky (Univ. of Maryland); L. Dai (Univ. of Hong Kong); P. Jonker (SRON); E. Ramirez-Ruiz (UCSC); B. Cenko (NASA/GSFC)
on 6 Feb 2022; 14:46 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

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Based on the recent evolution of AT 2021ehb, we triggered an approved XMM-Newton ToO program to obtain spectra of this tidal disruption event. The first two in a sequence of observations were obtained on 27 January 2022 and 29 January 2022. Owing to limited visibility within each orbit, the EPIC-pn exposure time was 13 ks in each case.

The continuum in both EPIC-pn spectra can be fit with a steep power-law (Gamma_27 = 2.9, F_0.3-10 = 4.5 E-12 erg/cm2/s; Gamma_29 = 3.4, F_0.3-10 = 4.3 E-12 erg/cm2/s). The addition of a simple blackbody function (kT = 0.11 keV) improves each fit, but the addition of an outflow with v = -0.15c is a greater improvement (ATEL #15179). The best fits are found when both additional components are included. It is interesting to note that a multicolor disk blackbody does not improve the fits. The short exposures have only permitted limited sensitivity in the RGS, but the summed exposure from the full sequence of observations will enable tests of these models.

Both exposures show interesting features in the Fe K band. The observation obtained on 27 January shows evidence of weak emission lines at 6.1 keV and 6.55 keV (in the host frame). This separation is too large for the pair to be red-shifted Fe XXV and Fe XXVI lines. It is possible to account for the putative line pair using a neutral Fe K disk line emitted from a narrow ring between 50-60 GM/c^2, seen at a low inclination (18-20 deg). The observation on 29 January shows evidence of an emission line at 5.6 keV that can be fit with a simple Gaussian.

In both spectra, the features in the Fe K band are only significant at the 3-4 sigma level (single-trial significance), but it is interesting to speculate on potential origins, especially given the unusual late-time X-ray rise of AT 2021ehb (ATEL #15179). Red-shifted lines can be observed momentarily as a flare of hard X-rays sweeps over the inner accretion disk. A corona consisting of discrete flares that are asymmetrically distributed in azimuth might also be able to generate such line profiles. Contributions from a variable background source are also possible. Red-shifted lines are also expected if gravitational light bending is important (e.g., Wilkins et al. 2021) or in a lensing scenario (e.g., Krawczynski et al. 2019). The increase in X-ray emission long after the disruption event in AT 2021ehb is qualitatively similar to the evolution of OGLE16aaa, which has been discussed in terms of a disruption event in a black hole binary system (Shu et al. 2020), though reduced obscuration or disk slimming at late times may also be viable (e.g., Wen et al. 2020).

We thank Norbert Schartel and the XMM-Newton planning team for executing this program.

Krawczynski, H., et al., 2019, ApJ, 870, 125
Shu, X., et al., 2020, NatCO, 11, 5876
Wen, S., et al., 2020, ApJ, 897, 80
Wilkins, D. R., et al., 2021, Nature, 595, 675