Two new Intermediate Polars with a soft X-ray component

TL;DR

This study reports the discovery and analysis of two new Intermediate Polars with soft X-ray components, characterizing their timing and spectral properties, confirming their classification, and exploring the nature of their soft X-ray emission.

Contribution

The paper provides the first X-ray observations of two candidate Intermediate Polars, identifying their soft X-ray components and detailed timing and spectral properties, expanding the class of known soft Intermediate Polars.

Findings

Both systems show energy-dependent X-ray pulsations at white dwarf spin periods.

Detection of X-ray beat variability in one system supports a longer orbital period.

Spectral analysis reveals complex emission with both hard and soft components, heavily absorbed by material.

Abstract

We analyze the first X-ray observations with XMM-Newton of RXS J070407.9+262501 and 1RXS 180340.0+401214, in order to characterize their broad-band temporal and spectral properties, also in the UV/optical domain, and to confirm them as Intermediate Polars. For both objects, we performed a timing analysis of the X-ray and UV/optical light curves to detect the white dwarf spin pulsations and study their energy dependence. For 1RXS 180340.0+401214 we also analyzed optical spectroscopic data to determine the orbital period. X-ray spectra were analyzed in the 0.2-10.0 keV range to characterize the emission properties of both sources. We find that the X-ray light curves of both systems are energy dependent and are dominated, below 3-5 keV, by strong pulsations at the white dwarf rotational periods (480 s for 1RXS J070407.9+262501 and 1520.5 s for 1RXS 180340.0+401214). In 1RXS 180340.0+401214 we also detect an X-ray beat variability at 1697 s which, together with our new optical spectroscopy, favours an orbital period of 4.4 hr that is longer than previously estimated. Both systems show complex spectra with a hard (up to 40 keV) optically thin and a soft (85-100 eV) optically thick components heavily absorbed by material partially covering the X-ray sources. Our observations confirm the two systems as Intermediate Polars and also add them as new members of the growing group of 'soft' systems which show the presence of a soft X-ray blackbody component. Differences in the temperatures of the blackbodies are qualitatively explained in terms of reprocessing over different sizes of the white dwarf spot. We suggest that systems showing cooler soft X-ray blackbody components also possess white dwarfs irradiated by cyclotron radiation.