[vsnet-alert 25409] DDE 175 = IPHAS J190356.18+090158.4 at historical brightest and rising

Thu Feb 18 02:05:00 JST 2021

In [vsnet-alert 25174] I have wished DDE 175 in Aquila to go into outburst
in 2021. On the very first observation after the winter conjunction with
the Sun (Feb. 07.52 UT) it has brightened to 16.8CR despite fading by 1.2m
in a month before the seasonal gap (from 18.3r on Oct. 1 to 19.5r on Nov.
1, 2020).

Since I have published a call for observations in ATel 14376 on Feb. 7th,
ZTF has observed this star on Feb. 10 and 11 showing the further
brightening.

DDE 175 = IPHAS J190356.18+090158.4
  20210210.5520  16.13r  ZTF (Masci et al., 2019)
  20210211.5559  16.02r  ZTF

Update! While I was writing this message, the new data have appeared at ZTF
Lasair website in nearly real time:
https://lasair.roe.ac.uk/object/ZTF18aaywchl/
  20210213.5490  16.08r  ZTF
  20210217.5503  15.78r  ZTF

ATLAS observations in cyan filter show the rise by 0.4m in 6 days from Feb.
10 to 16:

DDE 175 = IPHAS J190356.18+090158.4
  20210210.6420  17.82c  ATLAS (Tonry et al., 2018)
  20210216.6546  17.44c  ATLAS

ZTF DR4 light curve in g and r filters (Masci et al., 2019):
http://scan.sai.msu.ru/~denis/DDE175-ZTF-DR4-LC.gif
ATLAS light curve in o and c filters (Tonry et al., 2018; Smith et al.,
2020) simultaneous with ZTF DR4 data:
http://scan.sai.msu.ru/~denis/DDE175-ATLAS-2018-2020.gif

Sudden drop by 2 magnitudes in ATLAS orange filter around February-March
2020 (MJD=58900) requires explanation. ZTF has the star steady at 17.8r
from MJD=58950 to 59050.

The classification of this variable star remains unclear. It may be a
symbiotic variable, an EXOR, some kind of extreme cataclysmic variable or
pre-supernova a few years (or months) before the explosion. The problem is
that DDE 175 doesn't fit any of the three types above. It has too large
amplitude and short recurrence time for a symbiotic, too long outbursts and
rebrightenings for CVs and too blue colors for EXORs (J-K=1.07 despite g-r
from 2.5 to 3.0). Yet there's no detection by WISE.

The spectroscopy and simultaneous multi-color observations are strongly
encouraged. Of special interest are observations in g and r bands. There is
a sign in ZTF data that the star becomes redder when it brightens.

Denis Denisenko
http://scan.sai.msu.ru/~denis/VarDDE.html