Classification of AT2019azh as an Eddington-limited tidal disruption flare

ATel #12568; S. van Velzen (UMd and NYU), S. Gezari (UMd), T. Hung (UCSC), P. Gatkine (UMd), S. B. Cenko (GSFC), A. Ho (Caltech), S. R. Kulkarni (Caltech), A. Mahabal (Caltech)
on 12 Mar 2019; 12:55 UT
Credential Certification: Suvi Gezari (suvi@astro.umd.edu)

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We report follow-up observations of the bright nuclear transient, AT2019azh, detected by ASAS-SN on UT 2019 Feb 22.02 as ASASSN-19dj (ATel #12526) and by ZTF as ZTF17aaazdba on 2019 Feb 12.40, in the center of a nearby E+A galaxy KUG 0180+227 at z=0.022 (luminosity distance of 96 Mpc). The source was spectroscopically followed up by NUTS on 2019 Feb 22.03 (ATel #12529) and ePESSTO on 2019 Feb 25.16 (ATel #12530). Both remarked on the featureless blue spectrum, and ePESSTO also noted the presence of narrow absorption features at the redshift of the host galaxy.

Here we report ZTF and Neil Gehrels Swift telescope observations that demonstrate that the transient has entered a > 15-day long plateau phase in the optical and UV. Swift observations were obtained on 2019 Mar 2.76 (PI: Arcavi) and 2019 Mar 11.45 (PI: Gezari). We measure a nearly constant flux in the UV and optical bands (within +/- 0.1 mag) of uvw2=14.5 mag, uvm2=14.7, g=15.9 mag, r=16.0 mag starting 2019 Feb 24.25, and continuing until the latest Swift observation on 2019 Mar 11.45. The ZTF photometry and host-subtracted Swift photometry indicates a high temperature of log(T) = 4.5 +/- 0.1. The latest ZTF photometry also confirms the transient is consistent with originating from the center of its host galaxy; we measure a mean offset of 0.07 +/- 0.31 arcsec.

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 Feb 24, and our latest follow-up spectrum from on 2019 Mar 10, obtained with the Discovery Channel Telescope (DCT) DeVeny spectrograph, indicates that the spectrum continues to be blue and featureless, with narrow emission ([O III]5007) and Balmer absorption features associated with the host galaxy.

Swift XRT observations on 2019 Mar 11.45 detect 5 soft photons consistent with a count rate of 0.0095 cts/sec, which assuming a thermal spectrum typical of tidal disruption events of kT=0.06 keV, corresponds to an absorbed flux of 2.3e-13 erg/s/cm^2 and a luminosity of 2.5e41 ergs/sec for a luminosity distance of 96 Mpc.

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. Furthermore, we postulate that the plateau in the light curve is a signature of Eddington limited accretion for the <4e6 Msun central black hole (estimated from the velocity dispersion measured by SDSS DR14 of 70 km/s; smaller than its resolution of 90 km/s, and assuming the black hole mass scaling relation from Gultekin et al. 2009).

We have triggered XMM, VLA, and more Swift monitoring observations, including Swift UVOT grism observations. We strongly encourage further follow-up of this source, in particular spectroscopic monitoring and multi-band imaging.

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 distribution service provided by DIRAC@UW. Alert filtering is being undertaken by the GROWTH marshal system, supported by NSF PIRE grant 1545949.