We report the identification of the ROSAT all-sky survey source 1RXS
J154814.5-452845as new intermediate polar and present the results from
follow-up optical and X-ray observations. The source shows pulsations with a
period of 693 s both in the optical and X-ray light curves and the detection of
a synodic frequency strongly suggests that this is the rotation period of the
white dwarf. Although the one day aliasing and the sparse optical data coverage
does not allow to unambiguously identify the orbital period, the most likely
values of 9.37 h and 6.72 h add 1RXS J154814.5-452845 to the intermediate
polars with the longest orbital periods known. The optical spectrum displays
features from the late type secondary and shows the presence of broad
absorption lines at \Hbet and higher order Balmer lines which may be a
signature of the white dwarf atmosphere, very similar to V 709 Cas. The average
X-ray spectra as obtained by the EPIC instruments on board XMM-Newton show hard
emission typical for this class of objects but also the presence of soft
blackbody-like emission similar to that seen from soft intermediate polars and
thought to arise from the white dwarf surface heated by the hard X-rays. The
best fit model comprises thermal emission from multi-temperature plasma in
collisional ionization equilibrium with a continuous temperature distribution
up to a maximum of ∼60 keV, an Fe fluorescence line at 6.4 keV and with
equivalent width of 260 eV and a blackbody component with kT of 86 eV. The hard
X-ray emission is absorbed by matter covering 47% of the X-ray source with an
equivalent hydrogen density of ∼\ohcm{23}. The remaining hard emission is
absorbed by a much reduced column density of 1.5\hcm{21} as is the soft
blackbody emission. (truncated)Comment: 14 pages, Latex, with 19 figures, accepted for publication in
Astronomy and Astrophysic
