New Cataclysmic Variable in Serpens
ATel #8842; D. Denisenko, A. Dambis (Sternberg Astronomical Institute, Lomonosov Moscow State University)
on 18 Mar 2016; 22:34 UT
Credential Certification: Denis Denisenko (d.v.denisenko@gmail.com)
Subjects: Optical, Cataclysmic Variable, Transient, Variables
During the observations of the field centered at NSV 8494 remotely with iTelescope.Net T31 instrument (0.51-m astrograph + 3kx3k CCD, 55'x55' FOV) at Siding Spring observatory we have discovered new optical transient, most likely a dwarf nova. The new object is located at the position 17 19 24.03 -15 05 54.4 (J2000.0). It is present on six unfiltered images (60-sec exposures) obtained from 18:42:24 to 18:48:41 UT on 2016 Mar. 18. The OT magnitude (unfiltered with red zero point) is 17.9m. It is not present on our previous images with the same telescope on 2016 Mar. 12.78 UT and on earlier exposures from 2016 Feb. 20, 21, 24 and 25. There are no asteroids at this position on 2016 03 18.78 UT according to Minor Planet Checker.
The nearest star in USNO-B1.0 catalogue is 2.5" north-west of the OT. Checking the DSS plates shows no detectable proper motion for this star. Nothing is visible at the position of the OT on the sum of DSS images and on our combined image from Feb. 20 - Mar. 12 (15x60-sec exposures, limiting magnitude ~21). There is nothing at this position in AAVSO VSX, 1RXS, GALEX, 2MASS and WISE. The object was not reported before by MASTER, ASAS-SN and other surveys via TNS.
We suppose the optical transient to be a new cataclysmic variable (dwarf nova). It was assigned the name DDE 46 following the numbering scheme used before. Photometric and spectroscopic follow-up is encouraged. Discovery and reference iTelescope T31 images are uploaded to http://scan.sai.msu.ru/~denis/DDE46-T31-2016Mar18-12.jpg with the color-combined DSS finder chart available at http://scan.sai.msu.ru/~denis/DDE46-BRIR5x5.jpg (2x zoom).
The object is located 22.5' North of the suspected variable NSV 8494 with no type of variability known since its discovery in 1936. We have identified it with the UV source GALEX J171856.6-152733 (FUV=17.58 NUV=17.66) and near-IR source 2MASS J17185668-1527336 (J=14.94 H=14.76 K=14.71). NSV 8494 is identical to the star USNO-B1.0 0745-0390349 (pmRA=10 pmDE=-8 B1=17.01 R1=15.54 B2=16.12 R2=N/A I=15.21). While the range of variability for NSV 8494 is given as 15.3 - 16.7 p in NSV and AAVSO VSX, it is showing the large variability on DSS plates, fading by at least 4.5 magnitudes from 15.5m on 1954-08-01 POSS-I red plate to ~20m on 1988-07-15 POSS-II red plate. NSV 8494 is most likely a cataclysmic variable of VY Scl type (anti-nova). Continued monitoring and photometric time series are encouraged to confirm the classification and to measure the orbital period, given the opportunity of having two poorly studied objects within 25' from each other. Animation of Red DSS plates centered at NSV 8494 (200x200 arc sec FOV) is posted at http://scan.sai.msu.ru/~denis/NSV8494-DSS-Red-anim.gif
Variable stars discovered by DDE