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Orbital Period of Swift J2218.5+1925, A New Magnetic Cataclysmic Variable

ATEL # 2177; J. R. Thorstensen (Dartmouth), J. P. Halpern (Columbia)
on 28 Aug 2009; 22:53 UT
Password Certification: J. R. Thorstensen (john.thorstensen@dartmouth.edu)

Subjects: Optical, X-ray, Binaries, Cataclysmic Variables, Variables, Stars

The Swift XRT observed a highly variable X-ray source at (J2000.0) R.A. = 22h 18m 32.7s, decl.= +19d 25' 19" on 2009 August 2 and 4, for exposure times of 4885 and 4542 s, respectively. The average count rate was 0.14 s^-1 in the 0.3-10 keV band. Fits to an absorbed power law give photon index 1.05 +/- 0.10, NH < 9.e20 cm^-2, and average 0.3-10 keV flux of 9.5e-12 erg cm^-2 s^-1. This source can be identified with 1RXS J221832.8+192527 in the ROSAT All-Sky Survey Bright Source Catalogue. We noted a coincidence with a USNO-B1.0 star at (J2000.0) R.A. = 22h 18m 32.74s, decl.= +19d 25' 20.6' listed as having magnitudes B = 17.84, R = 17.22, and I = 16.37, which is also bright in the Swift ultraviolet filters. A spectrum of this star obtained with the 2.4m Hiltner telescope of the MDM Observatory on 2009 Aug. 21 showed strong emission lines of the Balmer series, He I, and He II, that are typical of cataclysmic variables. Further spectra obtained 2009 Aug. 25 through 28 UT showed a radial-velocity modulation with an amplitude of 340 km s^-1 on a period of 129.5 +/- 0.2 minutes. The line profiles show the complex orbital variations characteristic of an AM Her-type magnetic cataclysmic variable, or polar.


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