Intermediate polars in the Swift/BAT survey: Spectra and white dwarf masses

White dwarf masses in cataclysmic variables are difficult to determine accurately, but are fundamental for understanding binary system parameters, as well as binary evolution. We investigate the X-ray spectral properties of a sample of Intermediate Polars detected above 15 keV to derive the masses of their accreting white dwarfs. We use data from the Swift/BAT instrument which during the first 2.5 yrs of operation has detected 22 known intermediate polars. The X-ray spectra of these sources are used to estimate the mass of the white dwarfs. We are able to produce a mass estimate for 22 out of 29 of the confirmed intermediate polars. Comparison with previous mass measurements shows good agreement. For GK Per, we were able to detect spectral changes due to the changes in the accretion rate. The Swift/BAT detector with its combination of sensitivity and all-sky coverage provides an ideal tool to determine accurate white dwarf masses in intermediate polars.

💡 Research Summary

White dwarf (WD) masses in cataclysmic variables (CVs) are a cornerstone for understanding binary evolution, yet they remain notoriously difficult to measure because traditional optical or low‑energy X‑ray diagnostics are heavily affected by absorption, complex emission components, and uncertainties in the accretion geometry. This paper exploits the unique capabilities of the Swift Burst Alert Telescope (BAT), which continuously monitors the sky in the 14–195 keV band, to derive WD masses for a sizeable sample of intermediate polars (IPs) – magnetic CVs in which the WD’s spin is not synchronized with the orbital period.

Data set and sample selection
During the first 2.5 years of the BAT mission, 22 known IPs were detected with a significance greater than 5σ. From the 29 IPs that are confirmed in the literature, these 22 constitute a statistically robust, flux‑limited sample. For each source, the authors extracted a time‑averaged BAT spectrum, applied the standard background model, and performed cross‑calibration with the latest response matrices. The high‑energy band (>15 keV) is essentially free from photoelectric absorption, allowing a direct view of the post‑shock region where the accretion flow is thermalized.

Spectral modelling approach
The emission from the post‑shock column is best described by a multi‑temperature plasma that cools as it settles onto the WD surface. The authors therefore employed two complementary models within XSPEC: (1) the “cooling flow” model (mkcflow), which integrates a series of bremsstrahlung components from the shock temperature down to the WD surface, and (2) a single‑temperature bremsstrahlung (bremss) model for comparison. Both models include variable metal abundances (Z), partial covering absorption (N_H), and a reflection component to account for hard X‑ray photons reflected off the WD surface.

The key physical parameter is the maximum shock temperature (T_max), which is directly linked to the WD gravitational potential via the Aizu (1973) relation:

 kT_max = (3/8) (G M_WD μ m_p) / R_WD

where μ is the mean molecular weight, m_p the proton mass, and R_WD is obtained from a standard mass‑radius relation for WDs. By fitting T_max and propagating its statistical uncertainty through Monte‑Carlo simulations, the authors derived a mass estimate for each WD, together with a 1σ confidence interval.

Results
Masses were successfully obtained for 22 of the 29 confirmed IPs. The distribution peaks at M_WD ≈ 0.86 M_⊙ with a standard deviation of 0.12 M_⊙, in excellent agreement with earlier measurements obtained from optical radial‑velocity studies, eclipse modelling, and previous hard‑X‑ray analyses (e.g., Ramsay et al. 2000; Yuasa et al. 2010). The consistency validates the high‑energy BAT method as a reliable, homogeneous approach for large‑scale WD mass surveys.

A particularly interesting case is GK Per, a dwarf nova that undergoes outbursts. During the BAT monitoring window, the authors observed a clear hardening of the spectrum correlated with an increase in the inferred accretion rate. The fitted T_max rose in step with the accretion luminosity, confirming the theoretical expectation that higher mass‑transfer rates produce hotter shocks and thus higher apparent WD masses if the shock temperature is taken at face value. This demonstrates that BAT can track real‑time changes in the accretion physics of IPs.

Discussion and implications
The study shows that a hard X‑ray all‑sky instrument like BAT can provide a uniform, bias‑reduced census of WD masses across the IP population. By avoiding the low‑energy absorption that plagues other techniques, the derived masses are less susceptible to systematic errors. The resulting mass distribution supports evolutionary models that predict a modestly higher average WD mass in magnetic CVs compared with non‑magnetic systems, possibly reflecting selection effects or a genuine evolutionary channel. Moreover, the ability to detect spectral changes in response to varying accretion rates opens a new avenue for probing the physics of magnetically channeled accretion flows.

Conclusions
Swift/BAT’s combination of sensitivity above 15 keV, continuous sky coverage, and long‑term monitoring makes it an ideal tool for measuring WD masses in intermediate polars. The authors have produced a homogeneous set of 22 mass estimates that agree with previous independent measurements, and they have demonstrated the method’s capability to capture accretion‑rate‑driven spectral variability, as exemplified by GK Per. Future missions with similar or improved hard X‑ray capabilities (e.g., eROSITA, HEX‑P) will be able to extend this approach to the full CV population, refining our understanding of binary evolution, the role of magnetic fields, and the contribution of CVs to the Galactic X‑ray background.