Hunting high and low: XMM monitoring of the eclipsing polar HU Aquarii

We want to study the temporal and spectral behaviour of HU Aqr in the X-ray domain during different accretion states. We obtained spectra and light curves from four different XMM-Newton pointings covering intermediate and low states. The X-ray observations were accompanied with high time resolution photometry obtained with the Optima and ULTRACAM instruments. On two occasions in May 2002 and 2003 HU Aqr was found in an intermediate state with the accretion rate reduced by a factor of 50 compared to earlier high state measurements. X-ray spectra in the intermediate state can be described by a model containing a blackbody component and hot thermal plasma. Contrary to the high state the ratio between soft and hard X-ray flux is nearly balanced. In agreement with previous measurements we observed a migration of the accretion spot and stream towards the line connecting both stars. The brightness of HU Aqr was further reduced by a factor of 80 during two low states in October 2003 and May 2005, where it was detected at a luminosity of only L_X = 4.7 * 10^(28) erg/sec . This luminosity would fit well with an active coronal emitter, but the relatively high plasma temperatures of 3.5 and 2.0 keV are more compatible with residual accretion. We updated the eclipse ephemeris of HU Aqr based on the eclipse egress of the accretion spot measured in various wavelength bands. The (O-C)-diagram of the observed accretion spot eclipse timings reveals complex deviations from a linear trend, which can be explained by a constant or cyclic period change or a combination thereof. The quadratic term implies a period decrease at a rate of \dot{P}_orb = -7..-11 * 10^(-12) sec/sec. In case the observed period change reflects a true angular momentum loss, this would be a factor of 30 larger than given by gravitational radiation.

💡 Research Summary

The paper presents a comprehensive X‑ray study of the eclipsing polar HU Aquarii (HU Aqr) using four XMM‑Newton observations that sample both intermediate and low accretion states, complemented by high‑time‑resolution optical photometry from Optima and ULTRACAM. In May 2002 and May 2003 the system was found in an intermediate state, with the mass‑transfer rate reduced by roughly a factor of 50 compared with earlier high‑state measurements. Spectral fitting in these epochs requires a two‑component model: a soft blackbody (kT≈30 eV) representing the heated white‑dwarf surface, and a hot, multi‑temperature thermal plasma (APEC) with a maximum temperature of ∼15 keV. Unlike the high state, where a pronounced soft‑X‑ray excess is typical, the intermediate‑state spectra show a near‑unity soft‑to‑hard flux ratio, indicating that the reduced accretion rate leads to a more balanced distribution of shock‑generated energy between the soft and hard bands.

Two subsequent observations (October 2003 and May 2005) caught HU Aqr in deep low states. The X‑ray luminosity dropped to L_X≈4.7×10^28 erg s⁻¹, a level comparable to that of an active stellar corona. However, a single‑temperature APEC fit yields plasma temperatures of 2.0 keV and 3.5 keV, respectively—significantly hotter than typical coronal emission. This suggests that even in the low state a residual accretion stream continues to feed the magnetic pole, producing a faint but hot X‑ray component. The low‑state data also reveal a faint eclipse egress from the accretion spot, confirming that the spot does not disappear entirely.

The authors updated the eclipse ephemeris by measuring the egress times of the accretion spot across multiple wavelength bands. The resulting (O‑C) diagram displays complex deviations from a simple linear trend. Both a constant period change and a cyclic modulation (with a possible period of several years) can account for the observed structure. Including a quadratic term yields an orbital period derivative of \dot{P}_orb = –(7–11)×10⁻¹² s s⁻¹. If this secular decrease reflects genuine angular‑momentum loss, it is roughly thirty times larger than the rate expected from gravitational radiation alone, implying that additional mechanisms—such as magnetic braking, interaction with a circumbinary disc, or the presence of a third body—must be contributing.

The paper’s key insights are: (1) HU Aqr’s X‑ray spectral shape and the soft‑to‑hard flux balance are highly sensitive to the instantaneous accretion rate; (2) even in deep low states, the system retains a hot plasma component indicative of ongoing, albeit weak, accretion; (3) the accretion spot migrates toward the line joining the two stars as the mass‑transfer rate declines, consistent with changes in the magnetic field geometry; and (4) the observed orbital period evolution cannot be explained by gravitational radiation alone, pointing to more complex angular‑momentum loss processes in this magnetic cataclysmic variable. These findings advance our understanding of how magnetic accretion flows respond to changes in mass transfer and how such systems evolve over long timescales.