XY Ari Research Articles

The mass of the white dwarf in YY Dra (=DO Dra): Dynamical measurement and comparative study with X-ray estimates

We present a dynamical study of the intermediate polar cataclysmic variable YY Dra based on time-series observations in the K band, where the donor star is known to be the major flux contributor. We covered the 3.97-h orbital cycle with 44 spectra taken between 2020 and 2022 and two epochs of photometry observed in 2021 March and May. One of the light curves was simultaneously obtained with spectroscopy to better account for the effects of irradiation of the donor star and the presence of accretion light. From the spectroscopy, we derived the radial velocity curve of the donor star metallic absorption lines, constrained its spectral type to M0.5–M3.5 with no measurable changes in the effective temperature between the irradiated and non-irradiated hemispheres of the star, and measured its projected rotational velocity vrot sin i = 103 ± 2 km s−1. Through simultaneous modelling of the radial velocity and light curves, we derived values for the radial velocity semi-amplitude of the donor star, K2 = 188−2+1 km s−1, the donor to white dwarf mass ratio, q = M2/M1 = 0.62 ± 0.02, and the orbital inclination, i = 42°−1°+2°. These binary parameters yield dynamical masses of M1 = 0.99−0.09+0.10 M⊙ and M2 = 0.62−0.06+0.07 M⊙ (68 per cent confidence level). As found for the intermediate polars GK Per and XY Ari, the white dwarf dynamical mass in YY Dra significantly differs from several estimates obtained by modelling the X-ray spectral continuum.

Dynamical mass of the white dwarf in XY Ari: a test for intermediate polar X-ray spectral models

ABSTRACT We present a dynamical study of the eclipsing intermediate polar XY Ari based on time-resolved near-infrared spectroscopy obtained with the EMIR spectrograph on the 10.4-m Gran Telescopio Canarias. Using main sequence template spectra taken with the same instrument setup as the target spectra, we measure a radial velocity amplitude of the late K- type donor star K2 = 256 ± 2 km s−1. We also obtain the rotational broadening of its photospheric lines vrot sin i = 141 ± 3 km s−1. From these and the eclipse geometry, we derive a donor-to-white dwarf mass ratio q = M2/M1 = 0.62 ± 0.02, an orbital inclination i = 80.8° ± 0.5° and dynamical masses $M_{1} = 1.21 \pm 0.04 \, \mathrm{M}_{\odot }$ and $M_2 = 0.75 \pm 0.04 \, \mathrm{M}_{\odot }$ (1σ). This result places the white dwarf in XY Ari as one of the three most massive known in a cataclysmic variable. Comparison with white dwarf mass estimates from X-ray spectral studies could indicate the necessity of an improvement of the X-ray models and/or analysis techniques, as a number of X-ray white dwarf masses are in disagreement with the dynamical mass value.

Comptonization as an Origin of the Continuum in Intermediate Polars

Abstract In this paper we test if the ∼0.3–15 keV XMM-Newton EPIC pn spectral continuum of IPs can be described by the thermal Comptonization compTT model. We used publicly observations of 12 IPs (AE Aqr, EX Hya, V1025 Cen, V2731 Oph, RX J2133.7+5107, PQ Gem, NY Lup, V2400 Oph, IGR J00234+6141, IGR J17195-4100, V1223 Sgr, and XY Ari). We find that our modeling is capable of fitting well the average spectral continuum of these sources. In this framework, UV/soft X-ray seed photons (with 〈kT s〉 of 0.096 ± 0.013 keV) coming presumably from the star surface are scattered off by electrons present in an optically thick plasma (with 〈kT e〉 of 3.05 ± 0.16 keV and optical depth 〈τ〉 of 9.5 ± 0.6 for plane geometry) located nearby (on top) to the more central seed photon emission regions. A soft blackbody (bbody) component is observed in 5 out of the 13 observations analyzed, with a mean temperature 〈kT bb 〉 of 0.095 ± 0.004 keV. We observed that the spectra of IPs show in general two photon indices Γ, which are driven by the source luminosity and optical depth. Low luminosity IPs show 〈Γ〉 of 1.83 ± 0.19, whereas high luminosity IPs show lower 〈Γ〉 of 1.34 ± 0.02. Moreover, the good spectral fits of PQ Gem and V2400 Oph indicate that the polar subclass of CVs may be successfully described by the thermal Comptonization as well.

A long serendipitous XMM–Newton observation of the intermediate polar XY Ari1

XY Ari is one of the few known eclipsing intermediate polars. We present results from a detailed analysis of an unpublished archival observation using XMM-Newton EPIC pn and MOS data in a quiescent state of XY Ari. The X-ray orbital modulation and spin pulse variations were investigated for energy dependent modulations in different energy bands. The broad orbital modulation observed with various observations was confirmed with XMM-Newton at hard X-ray (>1.6 keV). The EPIC light curves folded at the spin phases show a double peak profile as expected from two pole accretion. The pulse profile is found to be energy dependent. Hardness ratio variations and energy modulation depth during spin modulation can be explained by photoelectric absorption. The simultaneously fitted EPIC spectra with CEVMKL model yield maximum plasma temperature of $28^{+3.1}_{-2.9}$ keV with an iron abundance $\mathrm{Fe}/\mathrm{Fe}_\odot=0.37^{+0.06}_{-0.05}$. We find two intrinsic partial covering absorption columns of $6.2^{+1.0}_{-0.9} \times 10^{22}$ and $105.3^{+35.4}_{-30.4} \times 10^{22} \,\mathrm{cm^{-2}}$ with covering fractions of $0.53^{+0.05}_{-0.04}$, $0.41^{+0.14}_{-0.13}$ respectively. In addition, a Gaussian emission line at $6.43^{+0.01}_{-0.02}$ keV with an equivalent width of $51^{+12}_{-10}$ eV is required to account for fluorescent emission from neutral iron. The X-ray luminosity of the source is $4.2 \times 10^{32} \,\mathrm{erg \,s^{-1}}$ in the 0.2-10.0 keV energy band.

The Magnetospheric Boundary in Cataclysmic Variables

The magnetic cataclysmic variables (MCVs) present a wealth of observational diagnostics for studying accretion flows interacting with a magnetosphere. Spin-period pulsations from the rotation of the white dwarf are seen in optical light, in the UV and X-ray bands, and in polarimetry, and modelling these can constrain the size and location of the accretion footprints on the white-dwarf surface. Tracing these back along field lines can tell us about the transition region between the stream or disk and the magnetosphere. Further, optical emission lines give us velocity information, while analysis of eclipses gives spatial information. I discuss MCVs (particularly FO Aqr, V405 Aur, XY Ari and EX Hya, but also mentioning PQ Gem, GK Per, V2400 Oph, HT Cam, TX Col, AO Psc, AE Aqr, WZ Sge, V1223 Sgr and DQ Her), reviewing what observations tell us about the disk-magnetosphere boundary. The spin-period variations are caused by a mixture of geometric effects and absorption by the accretion flow, and appear to show that the accretion disk feeds onto field lines differently in different systems, being sometimes along field lines ahead of the magnetic pole and sometimes behind the pole. During outbursts, when the accretion flow increases by orders of magnitude, the disk pushes the magnetosphere inwards, and appears to feed field lines over a much greater range of magnetic azimuth. The non-equilibrium outburst behaviour shows an even richer phenomenology than in quiescence, adding DNOs and QPOs into the mix.

A precessing accretion disc in the intermediate polar XY Arietis?

Context. XY Ari is the only intermediate polar to show deep X-ray eclipses of its white dwarf. Previously published observations with Ginga and Chandra have also revealed a broad X-ray orbital modulation, roughly antiphased with the eclipse, and presumed to be due to absorption in an extended structure near the edge of an accretion disc. The X-ray pulse profile is generally seen to be double-peaked, although a single-peaked pulse was seen by RXTE during an outburst in 1996. Aims. We intended to investigate the cause of the broad orbital modulation in XY Ari to better understand the accretion flow in this system and other intermediate polars. Methods. We observed XY Ari with RXTE and analysed previously unpublished archival observations of the system made with ASCA and XMM-Newton. These observations comprise six separate visits and span about ten years. Results. The various X-ray observations show that the broad orbital modulation varies in phase and significance, then ultimately disappears entirely in the last few years. In addition, the X-ray pulse profile shows variations in depth and shape, and in the recent RXTE observations displays no evidence for changes in hardness ratio. Conclusions. The observed changes indicates that both the pulse profile and the orbital modulation are solely due to geometrical effects at the time of the RXTE observations, rather than phase-dependent variations in photoelectric absorption as seen previously. We suggest that this is evidence for a precessing, tilted accretion disc in this system. The precession of the disc moves structures out of our line of sight both at its outer edge (changing the orbital modulation) and at its inner edge where the accretion curtains are anchored (changing the pulse profile).

An Observation of the Intermediate Polar XY Arietis withChandra

Chandra serendipitously observed the eclipsing (80° < i < 84°) intermediate polar XY Arietis (=H0253+193) in two separate but continuous observations 5 weeks apart. XY Ari was in a quiescent state during both observations. We pursue the study of phase-resolved spectra for this system by focusing on the Fe K lines. From the combined and separate data sets, we readily detect emission lines of iron near 6.4, 6.7, and 6.9 keV at better than 99% significance, in contrast to previous results. We confirm the orbit-phased sinusoidal absorption column behavior first observed with Ginga, as well as a sinusoid-like behavior as a function of spin phase. The presence of the 6.4, 6.7, and 6.9 keV lines requires different ionization states with ξ < 2 (6.4 keV) and ξ ~ 3.5–4 (6.7 and 6.9 keV) that must vary with phase. We also detect emission lines at 3.25, 4.8, and 5.4 keV that are not instrumental in origin. The 4.8 keV line may be identified as Ca XIX (4.832 keV) and the 3.25 keV line may be Ar I K, but the 5.4 keV line has no obvious identification.

X-Ray/Optical Studies of Two Outbursts of the Intermediate Polar YY (DO) Draconis

YY Draconis (likely the same variable called DO Draconis) is one of a small number of intermediate polars (IPs) that show outburst behavior. We report results from Target of Opportunity observations with the Rossi X-Ray Timing Explorer, together with ground-based optical photometry during outbursts in 1999 September and 2000 November. Similar behavior was evident in both outbursts. At outburst, the X-ray flux increased by more than a factor of 12, and the spectrum became hotter and more absorbed compared to quiescence. While the spin pulse at 529 s is clearly present in the X-ray data at 2–4 days past outburst peak and during quiescence, it was not detected in the X-ray data closest to outburst (1.5 days). This is contrary to the large increase in spin pulse amplitude that has been seen during outbursts of the IPs GK Per, XY Ari, and EX Hya. The differences in YY Dra are likely due to its unique geometry, with two relatively equal poles located near the white dwarf equator. The equal enhancement of both poles near outburst could account for the low pulse amplitude, while unequal feeding of the poles as the magnetosphere recedes during decline could explain the changes in amplitude and pulse shape. The changing height of the shocks may also have an effect on the visibility of both poles.

K-band spectroscopy of the intermediate polar XY Ari

We present the K-band infrared spectrum of the intermediate polar XY Ari. The spectrum confirms the cataclysmic binary nature of XY Ari, showing emission lines of He i (λ2.0587 μm) and the Brackett and Paschen series of H i. The broad nature of these lines suggests an origin in an accretion disc. The spectrum is strongly reddened by absorption within the molecular cloud Lynds 1457 and shows prominent absorption features from the secondary star, from which we determine a spectral type for the secondary of M0V. The secondary contributes per cent of the K-band light. We derive a visual extinction to XY Ari of and a distance of , placing XY Ari behind the molecular cloud.

An RXTE observation of the intermediate polar XY arietis

RXTE's observation of XY Ari covered more eclipses of this close binary at a higher count rate than ever before. The eclipses located the accretion regions on the white dwarf and showed that they covered < 0.002 of the white dwarf surface. Additionally we recorded the first outburst of XY Ari seen, allowing us to watch as an unstable accretion disk overwhelmed the magnetic field of the white dwarf and pushed inwards, cutting off our line-of-sight to the lower accretion pole. We also find limits on the mass of the white dwarf.

ASCA Observations of the Eclipsing IP XY Ari

AbstractWe describe the ASCA observation of the eclipsing intermediate polar (IP) XY Ari (=H0253+19). This system was first discussed by Patterson &amp; Halpern (1990, ApJ, 361, 173) as a possible IP based upon the HEAO-1 data. This system is impossible to study in the optical or ultraviolet because of the 12 mag of optical extinction associated with the molecular cloud Lynds 1457=MBM 12. The system has an orbital period of 0.2526 day and a pulse period of 208 sec. Our preliminary analysis of the ASCA data shows that the eclipse is nearly total (the precision of the background subtraction sets the limit). The eclipse ingress and egress occur very quickly, although we have not yet derived accurate numbers. There are sufficient photons that we can perform crude phase-resolved spectroscopy. The resulting spectra, each covering about 1/4 of the orbit, show that the Fe Kα line is not constant as a function of phase. A sample spectrum is reproduced below. We expect to submit a paper describing the results to the ApJ before the end of 1996.