Early Chandra X-ray Spectroscopy of the Nuclear Transient AT2019pev

ATel #13163; J. M. Miller, A. Zoghbi, M. Reynolds (Univ. of Michigan), J. J. Drake (SAO), S. Gezari, C. Miller, R. Mushotzky (Univ. of Maryland), J. Irwin (Univ. of Alabama), P. Jonker, J. S. Kaastra (SRON), A. Levan (Radboud Univ.), D. Maitra (Wheaton College), F. Paerels (Columbia Univ.), E. Ramirez-Ruiz (Univ. of California, Santa Cruz), R. Saxton (ESA)
on 3 Oct 2019; 16:45 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

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AT2019pev is a nuclear transient that exhibits an array of optical emission lines, associated with a galaxy at a redshift of z = 0.097 (Gezari et al. 2019; ATEL #13127). Initial observations with Swift measured an X-ray flux an order of magnitude higher than archival ROSAT observations; later observations with NICER (Kara et al. 2019; ATEL #13132) measured a flux of 1 E-11 erg/cm2/s, an order of magnitude brighter than initial Swift observations and 100 times brighter than ROSAT observations. This extraordinary flux increase marks AT2019pev as a potential TDE candidate, though an unusual AGN flare or other extreme event cannot be excluded (Bowen lines have also been seen in flaring NLSy1s, though the flaring mechanism is unclear in those cases; Trakhtenbrot et al. 2019). Radio observations did not detect the source (Williams et al. 2019; ATEL #13135), indicating that AT2019pev is likely not a "jetted" TDE, if it is indeed a TDE.

We observed AT2019pev with Chandra, starting on 2019-10-01, at 03:13:20, for a total exposure of 50 ks, and a net exposure of 45.4 ks. The LETG was used to disperse the incident flux on the HRC-S camera. Standard procedures were used to reduce the data, using the CIAO suite. The source is variable at the 25% level, on time scales of 2-4 ks. The time-averaged first-order spectra and responses were combined using the tool "combine_grating_spectra", and then grouped using a variety of tools, and fit using XSPEC and SPEX. Fitting the spectrum in XSPEC in the 5-40 Angstroms range after "optimal" binning, the continuum is consistent with a steep power-law with index Gamma = 3.3(1), or with a disk blackbody function with kT = 0.24(1) keV. In either case, the spectrum is broadly consistent with an observed flux of F ~ 1.35 E-11 erg/cm2/s in the 5-40 Angstroms band (0.30-2.5 keV), or an emitted flux of F ~ 2.3 E-11 erg/cm2/s, equating to L_X ~ 4.8 E+44 erg/s. If this marks the Eddington luminosity of the black hole, the inferred mass is M ~ 3.7 E+6 Msun. Drawing upon archival and more recent optical data, it is possible that AT2019pev is strongly X-ray dominated, with log(X/O) ~ 2.

The spectrum does not contain strong narrow lines, similar to those in ASASSN-14li. There is only weak evidence of potentially photoionized absorption in the spectrum: including an XSTAR absorber tuned for Seyfert-1 AGN measures a column density of N_H ~ 2.8 E+21 cm^-2, log xi ~ 2.95, and v ~ -0.01c (blue-shifted). There is also weak evidence of broadened re-emission from the same gas, peaking in the O VII-VIII range; it is possible that this is instead collisionally ionized gas that is less directly related to the accretion flow. Fitting within SPEX, and binning by factors ranging between 5-10, a multiple-component continuum with both disk blackbody and power-law components is required, though the power-law cannot be tightly constrained. There is again weak evidence for a similar photoionized absorber when the "pion" photoionized plasma model is added to the model. Broadened plasma emission again appears to improve the fit.

If AT2019pev proves to be a TDE, it is important that a high-resolution spectrum has been obtained only days after the putative disruption event. ASASSN-14li, in contrast, was only observed with Chandra months after the likely disruption. The soft, thermal continuum observed from AT2019pev is qualitatively consistent with that observed from ASASSN-14li, and may indicate that a disk has formed within days of a potential disruption event. Alternatively, the thermal continuum could represent an optically-thick, super-Eddington inner accretion zone rather than a true disk spectrum. The putative fast outflow may be consistent with this explanation. Additional observations at all wavelengths are strongly encouraged.

We thank the Chandra Director and planning team for executing this observation very quickly.