NICER X-ray observations of the Tidal Disruption Event (TDE) candidate AT2020ocn

ATel #13864; Dheeraj Pasham (MIT), Keith Gendreau (NASA/GSFC), Ron Remillard (MIT), Michael Loewenstein (NASA/GSFC/CRESST), Zaven Arzoumanian (NASA/GSFC), Erin Kara (MIT)
on 13 Jul 2020; 20:53 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Dheeraj Pasham (drreddy@mit.edu)

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The transient AT2020ocn was discovered on 29 April 2020 by the Zwicky Transient Facility (ZTF). Its position coincides with a galaxy with a measured redshift of 0.0705. Optical spectrum acquired by the Palomar 200in Double Spectrograph on 17 June 2020 reveals a blue excess compared to the archival SDSS spectrum, along with a broad He II 4686 line. These features are consistent with properties of previously known TDE candidates (ATELs: #13863, #13859).

Following the Swift/XRT detection of X-ray brightening of this TDE, NICER made several exposures between 11-12 July 2020. The mean 0.2-2 keV count rate was 2.5 cps. Using the total exposure of 7.6 ks we fit the 0.4-2.0 keV X-ray spectrum with various spectral models. First, we tried a single-temperature blackbody at a redshift of 0.0705 modified by Galactic absorption of 1.28e20 cm^-2, i.e., tbabs*zashift(bbodyrad) in XSPEC. This resulted in a poor fit with systematic residuals. Replacing the blackbody with a disk improves the fit but systematic residuals were still evident. Adding a power-law component, i.e., tbabs*zashift(bbodyrad+pow), gives an acceptable fit (reduced chi-squared of 1.17 with 103 degrees of freedom [dof]) with no obvious systematic residuals. However, the implied index is steep, 4.3(+0.5,-0.3), which is much steeper than the power-law component typically seen in AGN. The best-fit blackbody temperature was 0.14+-0.01 keV. The implied unabsorbed 0.4-2.0 keV and 0.1-10 keV fluxes (luminosities) are 1e-12 erg/s/cm^2 (1.6e43 erg/s) and 1.8e-11 erg/s/cm^2 (2.2e44 erg/s), respectively.

We also tried a model consisting of two blackbodies: tbabs*zashift(bbodyrad+bbodyrad). That again gave an acceptable fit (reduced chi-squared of 1.28 with 107 dof) with cooler and hotter thermal temperatures of 0.055+-0.01 and 0.143+-0.007 keV, respectively. There was however a systematic residual above roughly 1.2 keV. The unabsorbed 0.4-2.0 keV and 0.1-10 keV fluxes (luminosities) implied by this model are 1.1e-12 erg/s/cm^2 (1.5e43 erg/s) and 5.8e-12 erg/s/cm^2 (7e43 erg/s), respectively. The relative contributions of the cool and the warm blackbody components to the total flux are roughly 70 and 30%, respectively. A composite spectrum consisting of blackbody components with significantly different temperatures is not typical of TDEs, but the early results for AT2020ocn may be a consequence of changes in temperature over the multiple NICER exposures that were combined to improve modeling statistics. Multiple exposures per day are planned with NICER to investigate possible rapid blackbody evolution. The most recent NICER snapshots on 13 July indicate the source flux is on a downward trend.

NICER carries out prompt follow-up observations of transients and is planning to systematically follow up alerts from LIGO and other X-ray-bright extragalactic transients in the future.

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.