1RXS J184542.4+483134 is a new eclipsing polar

ATel #3436; E. Pavlenko (CrAO), K. Sokolovsky (MPIfR/ASC Lebedev/SAI MSU), A. Baklanov (CrAO), K. Antonyuk (CrAO), O. Antonyuk (CrAO), D. Denisenko (IKI)
on 17 Jun 2011; 07:24 UT
Credential Certification: E. Pavlenko (pavlenko@crao.crimea.ua)

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We present time-resolved ground-based optical and space-based Swift UV and X-ray observations of the cataclysmic variable 1RXS J184542.4+483134 (USNO-B1.0 1385-0291789 18:45:42.622 +48:31:30.84, J2000; Monet et al. 2003 AJ, 125, 984) recently identified by Denisenko & Sokolovsky (2011 AstL, 37, 91) and Denisenko & Smirnov (2011 PZP, 11, 10).

Photometry with the 2.6-m Shajn and 1.25-m AZT-11 telescopes of the Crimean astrophysical observatory was conducted on 2011 April 30, May 02, 03 and April 25, 26, respectively, for the total duration of about 14.6 hrs. Observations with the Shajn telescope were obtained in BVRcIc bands with the 30 s exposure time (60 s for B). Unfiltered AZT-11 images were obtained using 180 s exposure.

The Swift satellite observed 1RXS J184542.4+483134 on 2011 May 02 (quasi-simultaneously with Shajn observations) for the total exposure of 9.7 ks. The UV-Optical Telescope (Swift/UVOT) was used with the W1 filter and obtained multiple 200 s exposures during each pointing to improve time resolution. The archival 1.7 ks Swift observation of the object obtained on 2010 May 20 was also analyzed.

The weak X-ray source is detected by Swift/XRT (0.3-10 keV) above the 3 sigma level during both epochs at the position of 18:45:42.37 +48:31:28.6 (+/-5", J2000), only 3.4" from the optical position. The source shows significant variability:

 
   Date        XRT countrate 
2010-05-20   0.0078 +/-0.0024 
2011-05-02   0.0030 +/-0.0007 

The number of collected photons is insufficient for a detailed spectral analysis, however we note that photons both below and above 1 keV were observed, indicating a hard spectrum. Archival ROSAT/PSPC (0.1-2.4 keV) data from the Bright Source Catalogue (Voges et al. 1999 A&A, 349, 389) predict (assuming the power law spectrum with the photon index of 1-3 and Galactic N_HI between zero and 5.23x10^20 cm^-2, Kalberla et al. 2005 A&A, 440, 775) a Swift/XRT countrate of about an order of magnitude higher than observed in 2011.

The optical and UV light curves show a periodic asymmetric wave with its shape and amplitude differing between bands. The highest amplitude ~2.6 mag is observed in the B band (Bmax ~17.6), it is decreasing towards shorter (~1.2 mag in the UV W1 band) and longer (~2.0 mag in I) wavelength. The most striking detail of the optical lightcurve is the deep (amplitude >2 mag) and short (duration <2.45 min) eclipse. We derive the following eclipse ephemeris: HJD(UTC) = 2455684.5149 + 0.054908 x E

Taking into account the observed X-ray, UV and optical properties, and eclipses of a very compact source, we conclude that 1RXS J184542.4+483134 is an eclipsing AM Her-type star (polar) with the orbital period close to the shortest value reported for this type of objects. The asymmetric shape of the light curve could be caused by an inclined accretion column (Andronov, 1986, Soviet Astr., 30, 166). The observed high-amplitude, long-term X-ray variability is not uncommon among polars (e.g., EF Eri, Schwope et al. 2007 A&A, 469, 1027).

There are finding chart and W1BVRI light curve http://scan.sai.msu.ru/~kirx/ATel/1RXSJ184542/

Time-resolved spectroscopy and optical circular and linear polarimetry of this new eclipsing polar are encouraged.