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ATEL # 1895; J.M. Bonnet-Bidaud (CEA Saclay), D. de Martino (INAF, Obs Napoli), M. Mouchet (Univ Paris-7, Obs Paris)
on 8 Jan 2009; 16:15 UT
Password Certification: BONNET-BIDAUD Jean-Marc (firstname.lastname@example.org)
Subjects: Optical, X-ray, Binaries, Cataclysmic Variables
We report on a preliminary analysis of the XMM observations of the suspected cataclysmic variable XSS0056 performed on Dec. 31, 2007 (Start time : 2007-12-31T04:33:32.000) for a total of 16 ksec (Observation ID: 0501230301). The source is detected in the EPIC-pn camera at an average net countrate of 2.7 cts/sec. A Scargle periodogram reveals a strong peak at about 184 cycles/day (with its first harmonic at a power ratio of 10) as well as a low frequency peak around 9.4 cycles/day. We identify the strong peak and its harmonic as the spin period and the low frequency peak as the orbital period. A composite sinusoidal fit with the fundamental, first harmonic and low frequencies gives: Pspin=469.75 +/- 0.268 sec and Porb= 2.568 +/- 0.0598 hr. The XMM spin period is slightly different from the value (465.68+/-0,07 s) recently derived from an RXTE observation (Butters et al. 2008, A&A 487, 271) and differently from that observation we do not detect sideband periodicities.
Additional optical spectroscopic observations were obtained at the 1.5m telescope of the Loiano Observatory on 2008 August 26th and September 9th with duration 4.0 hr and 3.8 hr respectively. The radial velocities of the strong H alpha emission line confirm the orbital period derived from the X-ray variability. From sinusoidal fits, the best orbital value is computed at 2.62 +/- 0.09 hr and 2.82 +/- 0.21 hr independently from the two observations with velocity amplitudes of 66 +/- 7 km/s and 75 +/- 14 km/s respectively. The combined observations yield a more accurate value of 2.6244 +/- 0.0007 hr with however possible aliases. These observations definitely established that the X-ray source XSS J0056+4548 is a magnetic Cataclysmic Variable of Intermediate polar type inside the so called (2-3) hr "period gap".
 Acknowledgements to S.Galleti, R.Gualandi and I.Bruni for service observing at Loiano Observatory.