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Register a new userThe various accretion modes of AM Herculis: Clues from multi-wavelength observations in high accretion states
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We report on XMM-Newton and NuSTAR X-ray observations of the prototypical polar, AM Herculis, supported by ground-based photometry and spectroscopy, all obtained in high accretion states. In 2005, AM Herculis was in its regular mode of accretion, showing a self-eclipse of the main accreting pole. X-ray emission during the self-eclipse was assigned to a second pole through its soft X-ray emission and not to scattering. In 2015, AM Herculis was in its reversed mode with strong soft blobby accretion at the far accretion region. The blobby acretion region was more luminous than the other, persistently accreting, therefore called main region. Hard X-rays from the main region did not show a self-eclipse indicating a pronounced migration of the accretion footpoint. Extended phases of soft X-ray extinction through absorption in interbinary matter were observed for the first time in AM Herculis. The spectral parameters of a large number of individual soft flares could be derived. Simultaneous NuSTAR observations in the reversed mode of accretion revealed clear evidence for Compton reflection of radiation from the main pole at the white dwarf surface. This picture is supported by the trace of the Fe resonance line at 6.4 keV through the whole orbit. Highly ionized oxygen lines observed with the Reflection Grating Spectrometer (RGS) were tentatively located at the bottom of the accretion column, although the implied densities are quite different from expectations. In the regular mode of accretion, the phase-dependent modulations in the ultraviolet (UV) are explained with projection effects of an accretion-heated spot at the prime pole. In the reversed mode projection effects cannot be recognized. The light curves reveal an extra source of UV radiation and extended UV absorbing dips. An Ha Doppler map obtained contemporaneously (abstract abridged)
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A study of AM Her low states in September 1990 and 1991 and June-July 1997 is reported from a coordinated campaign with observations obtained at the Haute-Provence observatory, at the 6-m telescope of the Special Astrophysical Observatory and at the 2.6m and 1.25m telescopes of the Crimean observatory. Spectra obtained at different dates when the source was in low states at a comparable V magnitude, show the presence of strong Zeeman absorption features and marked changes in emission lines with a day-to-day reappearance of the HeII (4686AA) emission lines in 1991. Despite this variability, the magnetic field inferred from the fitting of the absorption spectrum with Zeeman hydrogen splitting, is remarkably constant with a best value of (12.5$pm$0.5)MG. Detailed analysis of the UBVRI light curves shows the presence of repetitive moderate amplitude ($sim$ 0.3-0.5 mag) flares predominantly red in colour. These flares are attributed to small accretion events and are compared to the large ($sim$ 2 mag.) blue flare reported by Shakhovskoy et al. (1993). We suggest that the general flaring activity observed during the low states is generated by accretion events. The different characteristics of the flares (colour and polarization) are the results of different shock geometries depending on the net mass accretion flux.
High resolution X-ray spectroscopy has revealed soft X-rays from high density plasma in Classical T-Tauri stars (CTTSs), probably arising from the accretion shock region. However, the mass accretion rates derived from the X-ray observations are consistently lower than those derived from UV/optical/NIR studies. We aim to test the hypothesis that the high density soft X-ray emission is from accretion by analysing optical accretion tracers from an X-ray selected sample of CTTSs in a homogeneous manner. We analyse optical spectra of a sample of CTTSs and calculate the accretion rates based on measuring optical emission lines. These are then compared to the accretion rates derived from the X-ray spectroscopy. We find that, for each CTTS in our sample, the different optical tracers predict mass accretion rates that agree within the errors, albeit with a spread of ~1 order of magnitude. Typically, mass accretion rates derived from Halpha and HeI 5876 Ang are larger than those derived from Hbeta, Hgamma and OI. When comparisons of the optical mass accretion rates are made to the X-ray derived mass accretion rates, we find that: a) the latter are always lower (but by varying amounts); b) the latter range within a factor of ~2 around 2x10^{-10} M_odot yr^{-1}, despite the fact that the former span a range of ~3 orders of magnitude. We suggest that the systematic underestimation of the X-ray derived mass accretion rates could depend on the density distribution inside the accretion streams, where the densest part of the stream is not visible in the X-ray band because of the absorption by the stellar atmosphere. We also suggest that a non-negligible optical depth of X-ray emission lines produced by post-shock accreting plasma may explain the almost constant mass accretion rates derived in X-rays if the effect is larger in stars with larger optical mass accretion rates.
FU Tau A is a young very low mass object in the Taurus star forming region which was previously found to have strong X-ray emission and to be anomalously bright for its spectral type. In this study we discuss these characteristics using new information from quasi-simultaneous photometric and spectroscopic monitoring. From photometric time series obtained with the 2.2m telescope on Calar Alto we measure a period of ~4d for FU Tau A, most likely the rotation period. The short-term variations over a few days are consistent with the rotational modulation of the flux by cool, magnetically induced spots. In contrast, the photometric variability on timescales of weeks and years can only be explained by the presence of hot spots, presumably caused by accretion. The hot spot properties are thus variable on timescales exceeding the rotation period, maybe due to long-term changes in the accretion rate or geometry. The new constraints from the analysis of the variability confirm that FU Tau A is affected by magnetically induced spots and excess luminosity from accretion. However, accretion is not sufficient to explain its anomalous position in the HR diagram. In addition, suppressed convection due to magnetic activity and/or an early evolutionary stage need to be invoked to fully account for the observed properties. These factors cause considerable problems in estimating the mass of FU Tau A and other objects in this mass/age regime, to the extent that it appears questionable if it is feasible to derive the Initial Mass Function for young low-mass stars and brown dwarfs.
The Kepler spacecraft observed a total of only four AM Herculis cataclysmic variable stars during its lifetime. We analyze the short-cadence K2 light curve of one of those systems, Tau 4 (RX J0502.8+1624), which underwent a serendipitous jump from a low-accretion state into a high state during the final days of the observation. Apart from one brief flare, there was no evidence of accretion during the 70 d of observations of the low state. As Tau 4 transitioned into a high state, the resumption of accretion was very gradual, taking approximately six days (~90 binary orbits). We supplement Tau 4s K2 light curve with time-resolved spectroscopy obtained in both high and low states of accretion. High-excitation lines, such as He II 468.6 nm, were extraordinarily weak, even when the system was actively accreting. This strongly suggests the absence of an accretion shock, placing Tau 4 in the bombardment regime predicted for AM Herculis systems with low accretion rates. In both the high-state and low-state spectra, Zeeman absorption features from the white dwarfs photosphere are present and reveal a surface-averaged field strength of $15pm2$ MG. Remarkably, the high-state spectra also show Zeeman-split emission lines produced in a region with a field strength of $12pm1$ MG. Zeeman emission has not been previously reported in an AM Herculis system, and we propose that the phenomenon is caused by a temperature inversion in the WDs atmosphere near the accretion region.
We model the post-shock accretion column (PSAC) for intermediate polars (IPs), with parameterizing specific accretion rate between 0.0001 and 100 g cm-2 s-1 and metal abundance between 0.1 and 2 times of solar abundance, and taking into account the gravitational potential and non-equipartition between ions, electrons and ionization degree. We assume the cylinder and dipole as geometry of the PSAC. The PSAC becomes higher against the white dwarf (WD) radius for lower specific accretion rate and more massive WD, and may be comparable to the WD radius. The consideration of the dipolar geometry significantly reduces the density and temperature over the whole PSAC comparing with the cylindrical case when the specific accretion rate is lower than a threshold which the PSAC height reachs 0.2 RWD with and is decreased by the more massive white dwarf. We calculate the spectra of the cylindrical and dipolar PSACs with the wide range of the specific accretion rate. Although the spectra soften as the specific accretion rate decreases for the both geometrical assumptions under the specific accretion rate threshold, the softening is more speedy for the dipolar PSAC. The fact means that the both geometrical assumptions lead the different WD masses for each other when their spectra are applied to the IPs hosting the low accretion or a massive WD. Although the ionization non-equilibrium are also involved for the spectral calculation, the effects are trivial because the radiation from ionization non-equilibrium plasma is a few percent of the whole at most.
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