TCP J10240289+4808512 (UG)
R.A. 10h24m02.89s, Decl. +48°08'51.2" (J2000.0)
2019 May 25.4972 UT, 11.8 mag (CCD, unfiltered)
Discoverer: Tadashi Kojima (Gunma-ken, Japan)
2019 05 25.497 UT
Discovered by T. Kojima, Gunma-ken, Japan, who found this on three frames (limiting mag.= 13.8) taken by Canon EOS 6D + 200-mm f/3.2 lens. Nothing is visible at this location on two frames taken on 2019 May 7.476 UT with 135-mm lens under the limiting mag.= 13.
2019 05 24.166 UT
Recent observations by the ASAS-SN Sky Patrol (Shappee et al. 2014ApJ...788...48S and Kochanek et al. 2017PASP..129j4502K) confirm this transient: 2019 May 23.247 UT, gmag. fainter than 16.74; 24.166, 11.65; no previous outbursts were recorded since 2012 Jan. 11; complete light curve at https://asas-sn.osu.edu/light_curves/09c6e373-f4d6-4b51-a5ae-6d33af960e… --- Patrick Schmeer (Saarbrücken-Bischmisheim, Germany)
2019 05 25.62 UT
The likely (blue) progenitor is USNO-A2.0 1350-07764996 (Bmag. 18.8, Rmag. 19.0) with Gaia DR2 position end figures 02.697s, 51.01" (equinox J2000.0, epoch 2015.5; Gmag. 19.44; parallax 2.9864 ± 0.3584 mas, distance 0.34 ± 0.05 kpc). Other designations are CSS_J102402.7+480850, USNO-B1.0 1381-0249998, GSC2.3 N99I009022 (Fmag. 19.39, Bjmag. 19.95, Vmag. 19.03), PSO J102402.689+480851.170 (gmag. 19.55, rmag. 19.61), GALEX J102402.6+480851, etc. The transient is probably a dwarf nova outburst with an amplitude of about 8 magnitudes (WZ Sge type?). Spectroscopy as well as multiband and time-resolved photometry are strongly recommended. --- Patrick Schmeer (Saarbrücken-Bischmisheim, Germany)
Here's a spectrum taken in bad weather conditions with a one meter telescope and a LISA spectrograph at Calern Observatory in south France. Only one exposure of 600s before clouds arrived.
There is a DASCH image from March 29, 1936, which maybe shows the star in outburst, at mag 12.31. But honestly I am not sure about the identification. Old DASCH images can be rather tricky.
Hans Bengtsson (BHS)