Photometric and spectroscopic observations of the sudden brightening of AT 2019zhd (=ZTF19adakuot)
ATel #13484; U. Munari (INAF Padova), A. Siviero (Univ. Padova), S. Dallaporta, and A. Maitan (ANS Collaboration)
on 14 Feb 2020; 22:57 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)
Subjects: Optical, Nova, Star, Transient
AT 2019zhd has been discovered by the Zwicky Transient Facility (as
ZTF19adakuot) on Dec 14, 2019 at r'=20.3 mag, and followed through a slow
and monotonic brightening to r'=18.7 reached on Feb 8, 2020, with the color
remaining around g'-r'=+0.38 (B-V~ +0.59 following Jester et al. 2005, AJ
130, 873). When ZTF re-observed the field four days later, AT 2019zhd was
found to have brightened to r'=15.44 by Feb 12, 2020. Non-simultaneous
observations suggest a blueing of the colors to g'-r'=-0.05 (B-V~ +0.17).
An independent discovery on Feb 13 by K. Nishiyama and F. Kabashima was
posted to CBAT-TOCP where the event was logged as PNV J00403785+4034529 at
unfiltered 14.9 mag.
On Feb 14.75 UT we have collected BVRI photometry of AT 2019zhd with ANS
Collaboration telescopes ID 310 and 2202, and optical spectroscopy (3300-8000 Ang
range, 2.31 Ang/pix dispersion) with the Asiago 1.22m telescope.
The photometry, color-transformed to the Landolt equatorial system, provides
B=16.185, V=15.896, Rc=15.551, and I=15.352 (+/-0.011 in all bands). They
correspond to r'=15.88 and g'-r'=+0.06, indicating a fading by 0.44 mag in
r' and a g'-r' color redder by 0.1 mag compared to ZTF discovery of the
brightening on Feb 12.
Our spectrum is dominated by a strong and smooth blue continuum, with
prominent Balmer emission lines superimposed. They are characterized by an
heliocentric radial velocity of -370 km/s, a very sharp core (basically
unresolved on our spectra, thus FWHM<100 km/s) and faint wings clearly extended up
to 1000 km/s on either side. The integrated absolute flux of Halpha, Hbeta
and Hgamma is (in units of 10(-14) erg/cm2 sec) 8.54, 3.17 and 0.90,
respectively. The radial velocity close to that of M31 (-298 km/s) suggests
a partnership of AT 2019zhd to the Andromeda galaxy. The CaII H&K
doublet is observed in absorption at a velocity similar to that of the
emission lines, while NaI doublet is seen in weak absorption at a negligible
velocity, thus originating in the interstellar medium of our Galaxy. No
other absorption or emission lines are obvious in the spectrum (HeI and FeII
in particoular).
At the 770 kpc distance of M31 and correcting for E(B-V)=0.08 foreground
Galactic reddening, our V-band value transforms to an absolute magnitude
M(V)= -8.78 mag. Such a value and the spectral appearance leave open
different alternatives for the classification of AT 2019zhd, which would
clearly benefit from further observations.