X-ray observation of AM Herculis in a very low state with Suzaku

The X-ray observation of AM Her in a very low state was performed with {\it Suzaku} in October 2008. One flare event with a time scale of $\sim$ 3700 sec was detected at the X-ray luminosity of $6.0 \times 10^{29} {\rm ~erg ~sec}^{-1}$ in the 0.5 – 10 keV band assuming at a distance of 91 pc. The X-ray spectrum is represented by a thermal plasma emission model with a temperature of $8.67_{-1.14}^{+1.31}$ keV. During the quiescence out of the flare interval, {\it Suzaku} also detected significant X-rays at a luminosity of $1.7 \times 10^{29} {\rm ~erg ~sec}^{-1}$ in the 0.5 – 10 keV band, showing a clear spin modulation at a period of 0.1289273(2) days at BJD 2454771.581. The X-ray spectra in the quiescence were represented by a MEKAL + Power Law (PL) model or a single CEMEKL model, which are also supported by phase-resolved analyses. A correlation between the temperature and the volume emission measure was found together with historical X-ray measurements of AM Her in various states. In order to account for a possible non-thermal emission from AM Her, particle acceleration mechanisms in the AM Her system are also discussed, including a new proposal of a shock acceleration process on the top of the accretion column.

💡 Research Summary

The paper reports on a deep X‑ray observation of the magnetic cataclysmic variable AM Herculis performed with the Suzaku satellite in October 2008, when the system was in an exceptionally low accretion state. The authors identified two distinct emission regimes: a short, intense flare and a persistent quiescent component. The flare lasted roughly 3 700 s and reached a 0.5–10 keV luminosity of $6.0\times10^{29}$ erg s⁻¹ (assuming a distance of 91 pc). Its spectrum is well described by a single‑temperature thermal plasma (MEKAL) model with $kT = 8.67^{+1.31}{-1.14}$ keV, a modest metal abundance (≈0.5 × solar), and a low absorption column ($N{\rm H}\sim1.2\times10^{20}$ cm⁻²). The flare’s temporal profile shows a rapid rise followed by an exponential decay, consistent with a sudden increase in the mass‑transfer rate that temporarily heats the accretion shock.

Outside the flare interval, Suzaku still detected significant X‑ray emission at $1.7\times10^{29}$ erg s⁻¹ in the same band. Timing analysis revealed a clear modulation at the white‑dwarf spin period of 0.1289273 days (≈3.09 h), confirming that the quiescent emission originates from the rotating accretion column. Phase‑resolved spectroscopy showed that the soft band (0.5–2 keV) is dominated by thermal plasma, whereas the hard band (2–10 keV) requires an additional non‑thermal component. Two spectral models fit the data equally well: (1) a composite of a MEKAL thermal component plus a power‑law (PL) with photon index Γ≈1.8, and (2) a single CEMEKL model representing a multi‑temperature plasma with a temperature distribution index α≈0.5 and a maximum temperature near 9 keV. Both models yield acceptable reduced χ² (~1.1) and are supported by the phase‑resolved fits, indicating that the quiescent emission is a mixture of thermal and possibly non‑thermal processes.

By compiling historical X‑ray measurements of AM Her from ROSAT, ASCA, XMM‑Newton, Chandra, and Suzaku, the authors demonstrate a robust correlation between plasma temperature and volume emission measure (EM). This trend is interpreted as a natural consequence of varying mass‑accretion rates: higher rates produce denser, hotter post‑shock plasma, while lower rates yield cooler, less luminous emission.

The detection of a hard PL component prompts the authors to explore particle‑acceleration scenarios within the polar system. They discuss two possibilities: (i) electrons accelerated in the standing shock at the base of the accretion column produce synchrotron or bremsstrahlung emission, and (ii) a novel “top‑of‑column” shock forms where the free‑falling stream is abruptly decelerated by the magnetic field, generating a collisionless shock that can accelerate particles to relativistic energies. The latter hypothesis is presented as a new mechanism that could account for the observed non‑thermal X‑rays, especially given the PL index and the lack of strong thermal line features at high energies.

In summary, the Suzaku observation reveals that even in a very low accretion state AM Her continues to emit X‑rays through a complex combination of thermal plasma and a hard, possibly non‑thermal component. The temperature–EM correlation across different accretion states supports the standard shock‑heated column picture, while the identification of a hard PL tail opens the door to high‑energy particle acceleration processes in magnetic cataclysmic variables. The paper suggests that future observations with higher spectral resolution and broader energy coverage (e.g., NuSTAR, XRISM) will be crucial to disentangle the thermal and non‑thermal contributions and to test the proposed top‑of‑column shock acceleration model.