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XSS J0056+4548 : a hard X-ray intermediate polar in the period gap

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 (bonnetbidaud@cea.fr)

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[1] 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".

[1] Acknowledgements to S.Galleti, R.Gualandi and I.Bruni for service observing at Loiano Observatory.


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