Radio detection of the tidal disruption event AT2019azh

ATel #12870; M. Perez-Torres (IAA-CSIC, Spain), J. Moldon (JBCA, Manchester), S. Mattila (Univ. of Turku, Finland), A. Alberdi (IAA-CSIC, Spain), R. Beswick (JBCA, Manchester), S. Ryder (Macquarie Univ., Australia), E. Varenius (JBCA, Manchester), M. Fraser (Univ. College, Dublin), E. Kankare (Univ. of Turku, Finland), E. Kool (Stockholm Univ., Sweden).
on 16 Jun 2019; 15:46 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Miguel A. Perez-Torres (torres@iaa.es)

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We report the first detection at radio wavelengths of the nuclear transient AT2019azh, which was discovered by ASAS-SN on UT 2019 Feb 22.02 (ATel #12526) in the center of the E+A galaxy KUG 0180+227 at z=0.022 (D=96 Mpc). Based on the location in the center of the host galaxy, the persistent blue colors and spectrum (ATel #12529, #12530), high blackbody temperature and lack of spectroscopic features associated with a supernova or AGN, AT2019azh has been classified as a tidal disruption event (ATel #12568).

Here we report electronic Multi-Element Remotely Linked Interferometer Network (e-MERLIN) observations of AT2019azh at a frequency of 5.075 GHz on 2019 May 21 and 2019 June 11, using a total bandwidth of 512 MHz. The array included five antennas (Mk2, Pi, Da, De, and Cm), but not the Lovell telescope, resulting in synthesized Gaussian beams of 80x30 (70x30) milliarcseconds at position angle 29 deg (30 deg) on the May and June epochs, respectively.

We centered our observations at the RA(J2000.0)= 08:13:17.054 and DEC(J2000.0)=+22:38:54.49, and imaged a 4x4 sq. arcsecond region around that position. We clearly detected AT2019azh at 5 GHz in both epochs, with peak flux densities of 354 +/- 17 (581 +/- 40) microJy/b on 2019 May 21 and 2019 June 11, respectively, and at the same position: RA(J2000.0)=08:13:16.9509; DEC(J2000.0)=22:38:53.896. The positional uncertainty is of 12 and 4 mas in RA and DEC, respectively. The Gaia DR2 (Brown et al. 2018, A&A, 616, 1) coordinates of the host galaxy are RA(J2000.0)=08:13:16.951562 +/- 0.000037; DEC(J2000.0)=22:38:53.90033 +/- 0.00033, so the separation between the e-MERLIN and the Gaia DR2 position is of 9.2 and 4.3 milliarcsec in RA and DEC, respectively, i.e., within 1-sigma of the quoted radio uncertainty.

The above radio brightness peaks correspond to monochromatic 5.07 GHz luminosities of (3.90 +/- 0.19)*1e27 erg/s/Hz and (6.40 +/- 0.44)*1e27 erg/s/Hz at an age of the TDE of about 88 and 109 days, respectively. These values are similar to those displayed by Arp299B-AT1 (Mattila et al. 2018, Science, 6401, 482) and ASASSN-14li (Alexander et al. 2016, ApJ, 819, L25) at similar ages, suggesting that AT2019azh may evolve in a similar fashion. The difference in flux density between the first and the second epochs suggests that AT2019azh is still in its optically thick phase at 5.0 GHz, and has not yet reached its maximum at this frequency.

Further approved e-MERLIN and VLBI observations are underway to trace the radio evolution of AT2019azh, and we encourage the monitoring of the object at other wavelengths. We thank the e-MERLIN staff for supporting our observing program.