Unusual Cataclysmic Variable in Centaurus

ATel #11401; I. Larin (DNTTM, Moscow), D. Denisenko (Sternberg Astronomical Institute, Lomonosov Moscow State University), S. Pogrebisskiy (Chilescope Observatory)
on 11 Mar 2018; 22:20 UT
Credential Certification: Denis Denisenko (d.v.denisenko@gmail.com)

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The new large amplitude variable star in Centaurus at R.A. = 12 48 50.8, Decl. = -41 26 54 (J2000.0) was discovered by Ivan Larin on the images obtained remotely from iTelescope.Net T31 instrument (0.50-m f/6.8 reflector + CCD + f/4.5 focal reducer) in Siding Spring, Australia. The star was near the detection limit (magnitude about 19.2) on the unfiltered 60-sec exposure taken at 17:00 UT on 2018 February 21, but became as bright as 16.5m on March 10 images between 15:07 and 15:10 UT. The variable was named Larin 2 (the second discovery by the 13-year old school student). Animation of discovery images from T31 is posted at http://scan.sai.msu.ru/~denis/Larin2-T31-anim.gif

The new variable displays some peculiar properties that have caused the special attention. Despite the deep fading on 2018 Feb. 21 image, the eight years long Siding Spring Survey light curve from CRTS (A. Drake et al., 2009) shows no eclipses on 256 epochs. The variable in SSS data is always in the 17.0-17.8 range, except for two outliers with large errors and one spurious detection at 15.8m. Then, the star has a huge color index between the ultraviolet and infrared bands: (FUV-W1)=8.3. It has an UV counterpart GALEX J124850.7-412654 with magnitudes FUV=21.31+/-0.35, NUV=21.42+/-0.47 and IR source WISE J124850.79-412654.5 (W1=13.008+/-0.025, W2=12.515+/-0.025, W3=11.486+/-0.128, W4=8.933+/-0.324). Near-infrared color index (J-K) is also very large for cataclysmic variables: 14.23-13.25=1.0 (2MASS) and 14.40-13.14=1.26 (DENIS). The object has an X-ray counterpart in XMM-Newton and Chandra catalogs: 3XMM J124850.8-412653 (flux=5.96e-15 mW/m2) and CXO J124850.796-412654.17 which shows the flux variation by a factor of 12, from 9.8e-16 mW/m2 to 1.1e-14 mW/m2.

The follow-up observations were performed remotely at the Chilescope Observatory in Chilean Andes near La Serena. 0.50-m f/3.6 Newtonian reflector with 4Kx4K FLI Proline CCD providing the 66'x66' field of view was used. A total of sixty-three 180-sec exposures in Luminance filter were obtained from 05:02 to 08:39 UT on 2018 March 11 with the short interruption from 06:45 to 07:04 UT. The light curve is presented here. The star 1.6' to the East of the variable was used as a reference star with V=16.046 in APASS DR9 (A. Henden et al., 2016). The variable shows no deep eclipses over 3.6 hours (unless it was missed during the 20-minute gap), suggesting the orbital period is larger than 4 hours. Otherwise the light curve is characterized by the short scale variability with a period about 0.0362(2)d, or 52.1 minute which can be interpreted as a rotation period of the white dwarf. The search for the possible period in Siding Spring Survey data is giving low significance peaks near 0.25357d (6.1 hr) and 0.23748d (5.7 hr).

Based on the variability of X-ray counterpart and the complicated optical light curve with no stable pattern, Larin 2 is likely a magnetic cataclysmic variable star (polar) with a large IR contribution from the secondary and a long period. Photometric time series from multiple longitudes are needed to determine the orbital period of this variable star. Spectroscopy is also encouraged to verify the proposed classification as an intermediate polar. Color-combined DSS finder chart is uploaded to http://scan.sai.msu.ru/~denis/Larin2-BRIR.jpg (10'x10' FOV).

Chilescope Observatory