[vsnet-outburst 15738] MASTER OT J204200.48+041839.9 (ATEL)

[Taichi Kato]

   Maybe an ER UMa-type object?

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ATEL #5240							     ATEL #5240

Title:	Unusual Cataclysmic Variable discovered by MASTER
Author:	D. Denisenko, V. Lipunov, E. Gorbovskoy, P. Balanutsa, N.
		Tiurina, V. Kornilov, A. Belinski, N. Shatskiy, V. Chazov, A. Kuznetsov,
		V. Yecheistov (Moscow State University, SAI), V. Yurkov, Y. Sergienko,
		D. Varda, E. Sinyakov, A. Gabovich (Blagoveshchensk Educational University),
		K. Ivanov, S. Yazev, N. Budnev, E. Konstantinov, O. Chuvalaev, V.
		Poleshchuk, O. Gress, A. Frolova (Irkutsk State University), V. Krushinsky,
		I. Zalozhnih, A. Popov, A. Bourdanov (Ural Federal University), A.
		Parkhomenko, A. Tlatov, D. Dormidontov, V. Senik (Kislovodsk solar
		station of the Pulkovo observatory RAS), P. Podvorotny, V. Shumkov,
		S. Shurpakov (MASTER team members), H. Levato, C. Saffe (ICATE), C.
		Mallamaci, C. Lopez and F. Podest (OAFA)
Queries:	lipunov2007 at gmail.com
Posted:	31 Jul 2013; 21:39 UT
Subjects:Optical, Cataclysmic Variable, Transient, Variables

MASTER OT J204200.48+041839.9 - new bright CV

MASTER-Amur auto-detection system discovered OT source at (RA, Dec) = 20h
42m 00.48s +04d 18m 39.9s on 2013-07-31.59277 UT. The OT unfiltered magnitude
is 
15.8m
(limit 18.7m). The OT is seen in 6 images. There is no minor planet at
this place. We have reference image without OT on 2013-04-18.81102 UT with
unfiltered magnitude limit 17.0m. 

The new object is identical to the blue star USNO-A2.0 0900-18910404 (20
42 00.42 +04 18 40.3 R=16.3 B=16.0) = USNO-B1.0 0943-0535946 (20 42 00.425
+04 18 40.02 B1=16.86 R1=16.69 B2=17.28 R2=19.21 I=15.63). It is showing
the large variability between red Palomar plates obtained on 1951-08-09
and 1991-09-12. Comparison of the 1st and 2nd epoch POSS red plates is
uploaded to http://master.sai.msu.ru/static/OT/J204200+041839-DSS-Red.jpg
(4'x4' FOV). 

The OT is present in Sloan Digital Sky Survey as SDSS J204200.44+041839.9
with r=16.63, likely observed in bright state. It has an UV counterpart
GALEX J204200.4+041840 (FUV=16.97+/-0.03, NUV=16.90+/-0.01). However, nothing
is present at this position in 1RXS catalogue. There is no radio detection.
No variable stars are found within 10' radius in AAVSO VSX and GCVS. 

This area of sky in Delphinus was observed by <a href="http://skyview.gsfc.nasa.gov/skymorph/obs.html">NEAT
project</a> with 76 images obtained on 27 nights from 1997 Aug. 02 to 2005
Sep. 02. The variable object remained at the bright level on 2001 June
03, 20, 25, 29, 30, Aug. 20 and Sep. 22. Such a long brightening is inconsistent
with the dwarf nova outburst and can be interpreted as either a VY Scl
(anti-nova) or AM Her (polar) variable in a high state. <a href="http://nesssi.cacr.caltech.edu/DataRelease/">Catalina
Real-time Transient Survey</a> data from CSS and MLS show the variability
from ~19.5 to 15.9m. CRTS light curve also does not conform to the dwarf
nova behavior. 

If the object were a polar (magnetic CV), one should have expected the
X-ray detection by ROSAT which is not the case. Thus, the most likely hypothesis
for MASTER OT J204200.48+041839.9 is that of VY Scl type variable. However,
VY-type CVs have typical orbital periods of ~4 hr or larger and late K-class
secondary components. That, in turn, contradicts to the SDSS colors for
<a href="http://skyserver.sdss3.org/dr8/en/tools/explore/obj.asp?id=1237666210874785879">SDSS
J204200.44+041839.9</a> with u=16.79, g=16.45, r=16.63, i=16.78, z=16.91.
Large negative (g-r) and (r-i) color indices are more typical for the extremely
short-period cataclysmic variables with low-mass donor of spectral class
M6 and later. The object appears to be unusual and requires a further study.

Follow up spectral and photometric observations are strongly encouraged.
The discovery and reference images are available at: http://master.sai.msu.ru/static/OT/204200.48041839.9.png

<center><a href="http://observ.pereplet.ru/MASTER_OT.html">List of Optical
Transients discovered by MASTER</a></center>

Global MASTER Robotic Net: http://observ.pereplet.ru