Classification of AT2019ehz as a tidal disruption flare
ATel #12789; S. Gezari (UMd), S. van Velzen (UMd and NYU), D. A. Perley (LJMU), T. Hung (UCSC), S. B. Cenko (GSFC), S. Frederick (UMd), M. Graham (Caltech), S. R. Kulkarni (Caltech)
on 21 May 2019; 17:34 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Suvi Gezari (suvi@astro.umd.edu)
Subjects: Optical, Ultra-Violet, X-ray, Request for Observations, Transient, Tidal Disruption Event
Referred to by ATel #: 12813
We report follow-up observations of the nuclear transient, AT2019ehz, in the center of galaxy SDSS J140941.88+552928.0, discovered by Gaia on UT 2019 April 29 as Gaia19bpt with G=18.49 mag and reported first to TNS. The first detection of AT2019ehz by the ZTF MSIP Survey was made on 2019 April 23 and reported to the TNS as ZTF19aarioci with g=19.81 mag and r=20.03 mag. Due to the persistent blue color (g-r ~ -0.3 mag) of this nuclear transient and its relatively slow rise to peak, we flagged the transient as a photometric tidal disruption event (TDE) candidate for further follow-up observations.
Our Spectral Energy Distribution Machine (SEDM) (range 350-950nm, spectral resolution R~100) mounted on the Palomar 60-in (P60) telescope (Blagorodnova et al. 2018, PASP, 130, 5003) spectrum on 2019 May 02, and our latest follow-up spectrum from the Liverpool Telescope SPRAT low-resolution spectrograph (R~350) on 2019 May 19, indicate a spectrum that is blue and mostly featureless, with a tentative detection of Ca II H,K absorption features associated with the host galaxy at z~0.12, consistent with the SDSS photo-z=0.12+/-0.06.
Here we also report Neil Gehrels Swift telescope observations on 2019 May 17 (PI: Gezari) that demonstrate that the nuclear transient is UV bright (uvw2=17.62 +/- 0.06 mag), and is detected in the soft X-rays (< 1.5 keV) with 0.005 +/- 0.002 cts/sec, corresponding to an X-ray flux of 1e-13 erg/cm^2/s, or an X-ray luminosity of ~4e42 ergs/s for a luminosity distance of 565 Mpc. The ZTF photometry and host-subtracted Swift photometry indicates a temperature of the UV/optical component of T = 2e4 K and a total UV/optical blackbody luminosity of 3e44 ergs/s. The latest ZTF photometry also confirms the transient is consistent within 2 sigma with originating from the center of its host galaxy; we measure a mean and rms offset of 0.18 +/- 0.11 arcsec. Finally the estimated host galaxy mass from broadband photometry of log(M/Msun) = 9.7 suggests a ~10^6 Msun black hole, and a peak flare luminosity close to the Eddington luminosity.
We classify this source as a tidal disruption event (TDE) based on the location in the center of the host galaxy, the persistent blue colors and spectrum, high blackbody temperature, and lack of spectroscopic features associated with a supernova or AGN. We have triggered XMM, VLA, and more Swift monitoring observations. Higher spectral resolution optical spectra and multi-band imaging are encouraged.
ZTF is a project led by PI S. R. Kulkarni at Caltech (see ATel #11266),
and includes IPAC; WIS, Israel; OKC, Sweden; JSI/UMd, USA; UW,USA; DESY,
Germany; NRC, Taiwan; UW Milwaukee, USA and LANL USA. ZTF acknowledges
the generous support of the NSF under AST MSIP Grant No 1440341. Alert
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by the GROWTH marshal system, supported by NSF PIRE grant 1545949.