AstroNote 2021-286

AT2021adlw was discovered by the Zwicky Transient Facility (ZTF, Graham et al. 2019, Bellm et al. 2019) on MJD 59523.5 in the nearby star-forming galaxy NGC 3813 (z=0.0049, 30 Mpc). The transient was discovered at m_r = 18.70 +/- 0.14 mag. Using the distance to NGC 3813 of ~30 Mpc from NED, the absolute magnitude is M_r = -13.6 +/- 0.2 mag. 

We obtained an optical spectrum of AT2021adlw with the medium resolution (R~3000) Double Beam Spectrograph (DBSP, Oke et al. 1982) on the 200-inch Hale telescope at the Palomar observatory. The spectrum shows a reddened continuum and narrow H-alpha emission with a FWHM velocity of ~1500 km/s. We also detect narrow Ca II NIR triplet emission lines. We do not detect any [Ca II] lines.

The photometric and spectroscopic properties of AT 2021adlw resemble early phase intermediate luminosity red transients (ILRT) or outbursts of a dusty luminous blue variable (LBV) star. Additional observations to distinguish between the two scenarios are encouraged.

ZTF is supported by the National Science Foundation under Grant No. AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, and IN2P3, France. Operations are conducted by COO, IPAC, and UW.