Hard X-ray view on intermediate polars in theGaiaera

Abstract

The hardness of the X-ray spectra of intermediate polars (IPs) is determined mainly by the white dwarf (WD) compactness (mass-radius ratio, M/R) and, thus, hard X-ray spectra can be used to constrain the WD mass. An accurate mass estimate requires the finite size of the WD magnetosphere R m to be taken into the account. We suggested to derive it either directly from the observed break frequency in power spectrum of X-ray or optical lightcurves of a polar, or assuming the corotation. Here we apply this method to all IPs observed by NuSTAR (10 objects) and Swift/BAT (35 objects). For the dwarf nova GK Per we also observe a change of the break frequency with flux, which allows to constrain the dependence of the magnetosphere radius on the mass-accretion rate. For our analysis we calculated an additional grid of twoparameter (M and R m /R) model spectra assuming a fixed, tall height of the accretion column H sh /R = 0.25, which is appropriate to determine WD masses in low mass-accretion IPs like EX Hya. Using the Gaia Data Release 2 we obtain for the first time reliable estimates of the mass-accretion rate and the magnetic field strength at the WD surface for a large fraction of objects in our sample. We find that most IPs accrete at rate of ∼ 10 −9 M yr −1 , and have magnetic fields in the range 1-10 MG. The resulting WD mass average of our sample is 0.79 ± 0.16 M , which is consistent with earlier estimates.

The Fe Line Flux Ratio as a Diagnostic of the Maximum Temperature and the White Dwarf Mass of Cataclysmic Variables

“…See e.g., Frank et al 2002), and T max = α 3 16 µm H k GM R , where α = 0.65 ± 0.07 for non-mCVs (Yu et al 2018). In previous works, the T max of several dozens of CVs have been measured via X-ray continuum fitting, and the derived M WD were in general consistent with the dynamically determined values (e.g., Suleimanov et al 2005;Shaw et al 2018;Suleimanov et al 2019).…”

Hard X-ray cataclysmic variables

“…5, with a mean value < M WD >= 0.84 ± 0.17 M . The modified PSR model accounting for the finite size of the magnetosphere was also recently applied to a set of 35 IPs observed with NuSTAR and Swift/BAT by Suleimanov et al (2019), who find more accurate WD masses with an average value of 0.79±0.16 M , but still consistent with previous results. Massive WD primaries were also found by Zorotovic et al (2011), using a set of "fiducial" CVs with reliable WD masses (left middle panel of Fig 5) with a mean value < M WD,fiducial >= 0.82 ± 0.15M .…”

V1082-Sgr: A magnetic cataclysmic variable with a lobe-filling companion star

“…What's more, the white dwarf's accretion rate (1.2 × 10 −9 M⊙yr −1 ) derived from Suzaku observations is lower than the companion's mass loss rate predicted by the stellar evolution code for most systems (∼ 10 −9 -10 −7 M⊙yr −1 ). Such phenomena seem usual in IPs with orbital period above 6 hours, as reported by Suleimanov et al (2019). The strong variability and the lower accretion rate of the system could be caused by the propeller mechanism.…”