FO Aquarii Research Articles

The Intermediate Polar FO Aquarii Has Not Been the Same Since Recovering from a Series of Low States

FO Aqr is a bright intermediate polar that has long displayed large amplitude photometric variations corresponding to the 20.9 minutes spin period of its white dwarf. Between 2016 and 2020, the system suffered a series of unprecedented low-states, but recent data shows that it has now recovered to its normal optical luminosity. We compare the light curves obtained by K2/Kepler in 2014 with photometry from the TESS mission obtained in 2021. We find that the spin pulse that had been the dominant feature of the light curve in 2014 is now weak over the second half of the binary orbit and that a beat pulse is enhanced in the TESS photometry. Variations at approximately twice the spin frequency are now seen over the second half of the orbit. These photometric properties may be the new normal for FO Aqr now that its white dwarf has begun to spin down.

The Spin-period History of Intermediate Polars

Abstract We report the detailed history of spin-period changes in five intermediate polars (DQ Herculis, AO Piscium, FO Aquarii, V1223 Sagittarii, and BG Canis Minoris) during the 30–60 yr since their original discovery. Most are slowly spinning up, although there are sometimes years-long episodes of spin-down. This is supportive of the idea that the underlying magnetic white dwarfs are near spin equilibrium. In addition to the ∼40 stars sharing many properties and defined by their strong, pulsed X-ray emission, there are a few rotating much faster (P < 80 s), whose membership in the class is still in doubt—and who are overdue for closer study.

The Rise and Fall of the King: The Correlation between FO Aquarii’s Low States and the White Dwarf’s Spin-down

Abstract The intermediate polar FO Aquarii (FO Aqr) experienced its first reported low-accretion states in 2016, 2017, and 2018, and, using newly available photographic plates, we identify prediscovery low states in 1965, 1966, and 1974. The primary focus of our analysis, however, is an extensive set of time-series photometry obtained between 2002 and 2018, with particularly intensive coverage of the 2016–2018 low states. After computing an updated spin ephemeris for the white dwarf (WD), we show that its spin period began to increase in 2014 after having spent 27 yr decreasing; no other intermediate polar has experienced a sign change of its period derivative, but FO Aqr has now done so twice. Our central finding is that the recent low states all occurred shortly after the WD began to spin down, even though no low states were reported in the preceding quarter-century, when it was spinning up. Additionally, the system’s mode of accretion is extremely sensitive to the mass-transfer rate, with accretion being almost exclusively disk-fed when FO Aqr is brighter than V ∼ 14 and substantially stream-fed when it is not. Even in the low states, a grazing eclipse remains detectable, confirming the presence of a disklike structure (but not necessarily a Keplerian accretion disk). We relate these various observations to theoretical predictions that during the low state, the system’s accretion disk dissipates into a non-Keplerian ring of diamagnetic blobs. Finally, a new XMM-Newton observation from a high state in 2017 reveals an anomalously soft X-ray spectrum and diminished X-ray luminosity compared to pre-2016 observations.

Optical spectra of FO Aquarii during low and high accretion rates

ABSTRACT Between 2016 May and 2018 September, the intermediate polar (IP) FO Aquarii exhibited two distinct low states and one failed low state. We present optical spectroscopy of FO Aquarii throughout this period, making this the first detailed study of an accretion disc during a low state in any IP. Analysis of these data confirm that the low states are the result of a drop in the mass transfer rate between the secondary star and the magnetic white dwarf primary, and are characterized by a decrease in the system’s brightness coupled with a change of the system’s accretion structures from an accretion disc-fed geometry to a combination of disc-fed and ballistic stream-fed accretion, and that effects from accretion on to both magnetic poles become detectable. The failed low state only displays a decrease in brightness, with the accretion geometry remaining primarily disc-fed. We also find that the WD appears to be exclusively accretion disc-fed during the high state. There is evidence for an outflow close to the impact region between the ballistic stream and the disc which is detectable in all of the states. Finally, there is marginal evidence for narrow high-velocity features in the H α emission line during the low states which may arise due to an outflow from the WD. These features may be evidence of a collimated jet, a long predicted yet elusive feature of cataclysmic variables.

The disappearance and reformation of the accretion disc during a low state of FO Aquarii

Context. FO Aquarii, an asynchronous magnetic cataclysmic variable (intermediate polar) went into a low state in 2016, from which it slowly and steadily recovered without showing dwarf nova outbursts. This requires explanation since in a low state, the mass-transfer rate is in principle too low for the disc to be fully ionised and the disc should be subject to the standard thermal and viscous instability observed in dwarf novae. Aims. We investigate the conditions under which an accretion disc in an intermediate polar could exhibit a luminosity drop of two magnitudes in the optical band without showing outbursts. Methods. We use our numerical code for the time evolution of accretion discs, including other light sources from the system (primary, secondary, hot spot). Results. We show that although it is marginally possible for the accretion disc in the low state to stay on the hot stable branch, the required mass-transfer rate in the normal state would then have to be extremely high, of the order of 1019 g s-1 or even larger. This would make the system so intrinsically bright that its distance should be much larger than allowed by all estimates. We show that observations of FO Aqr are well accounted for by the same mechanism that we have suggested as explaining the absence of outbursts during low states of VY Scl stars: during the decay, the magnetospheric radius exceeds the circularisation radius, so that the disc disappears before it enters the instability strip for dwarf nova outbursts. Conclusions. Our results are unaffected, and even reinforced, if accretion proceeds both via the accretion disc and directly via the stream during some intermediate stages; the detailed process through which the disc disappears still requires investigation.

Kepler K2observations of the intermediate polar FO Aquarii

We present photometry of the intermediate polar FO Aquarii obtained as part of the K2 mission using the Kepler space telescope. The amplitude spectrum of the data confirms the orbital period of 4.8508(4) h, and the shape of the light curve is consistent with the outer edge of the accretion disk being eclipsed when folded on this period. The average flux of FO Aquarii changed during the observations, suggesting a change in the mass accretion rate. There is no evidence in the amplitude spectrum of a longer period that would suggest disk precession. The amplitude spectrum also shows the white dwarf spin period of 1254.3401(4) s, the beat period of 1351.329(2) s, and 31 other spin and orbital harmonics. The detected period is longer than the last reported period of 1254.284(16) s, suggesting that FO Aqr is now spinning down, and has a positive $\dot{P}$. There is no detectable variation in the spin period over the course of the K2 observations, but the phase of the spin cycle is correlated with the system brightness. We also find the amplitude of the beat signal is correlated with the system brightness.

Twisted accretion curtains in the intermediate polar FO Aquarii

We report on a ∼37-ks XMM–Newton observation of the intermediate polar FO Aquarii, presenting X-ray and ultraviolet data from the EPIC and OM cameras. We find that the system has changed from its previously reported state of disc-overflow accretion to one of purely disc-fed accretion. We detect the previously reported ‘notch’ feature in the X-ray spin pulse, and explain it as a partial occultation of the upper accretion pole. Maximum flux of the quasi-sinusoidal ultraviolet pulse coincides with the notch, in keeping with this idea. However, an absorption dip due to the outer accretion curtains occurs 0.27 later than the expected phase, which implies that the accretion curtains are twisted, trailing the magnetic poles. This result is the opposite of that reported in PQ Gem, where accreting field lines were found to lead the pole. We discuss how such twists relate to the accretion torques and thus the observed period changes of the white dwarfs, but find no simple connection.

The changing X-ray light curves of the intermediate polar FO Aquarii

Comparison of five X-ray observations of the intermediate polar FO Aqr reveals that the morphology of the X-ray light curve changes considerably with time. In particular, power spectra of the 1988 Ginga and 1993 ASCA data reveal strong sideband and orbital variations, whereas the 1990 Ginga observation does not. This suggests that the amount of stream-fed accretion varies with epoch, and the system was accreting predominantly from a disc in 1990. In contrast to other intermediate polars, the X-ray spin-pulse profiles show significant variations between observations, ranging from relatively sinusoidal to sawtooth-shaped at medium energies. During the 1988 and 1990 observations a notch is visible at spin phase zero, due to the presence of an interpulse at phase 0.85, which is absent during the other observations. At lower energies a narrow pulse of emission is seen at spin phase 0.2. We interpret the pulse profile from the 1990 Ginga observation using a model for accretion from a disc on to a dipolar magnetic field, the axis of which is offset from the white dwarf centre by ~ 0.15 white dwarf radii. In order to account for the later occurrence of the hardness-ratio maximum in 1988 and 1993, we suggest that the accretion-rate profile changes so that accretion is favoured along the field lines which trail the magnetic pole. This also accounts for the disappearance of the interpulse and notch in 1993.

Rapid Oscillations in Cataclysmic Variables. XIV. Orbital and Spin Ephemerides of FO Aquarii

ABSTRACTWe report on 17 years of photometry of FO Aquarii, a cataclysmic variable with a rapidly rotating, magnetic white dwarf. We give ephemerides for its variations at the 4.8 hr binary period and the 21 minute spin period. The orbital period is constant at 0.2020596(1) day. The spin period decreases on a timescale of 106 yr, but with additional wiggles on timescales of years to decades. The observed alternation between spin‐down and spin‐up suggests that the star is near its equilibrium spin period, an important prediction of the theory of magnetic cataclysmic variables.

On Martell and Kaitchuck's model for the intermediate polar FO Aquarii

view Abstract Citations (2) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On Martell and Kaitchuck's Model for the Intermediate Polar FO Aquarii Mukai, Koji ; Hellier, Coel Abstract Attention is given to the proposals of Martell and Kaitchuck (1991) that FO Aqr is a low-inclination system, rather than an eclipsing system, as recently claimed, and that the dominant periodicity (21 min) is the beat period rather than the white dwarf spin period, as is commonly supposed. It is shown that both proposals make it harder to understand the observations of this star, while there is no substantive evidence in their favor. It is concluded that FO Aqr is most likely a high-inclination system in which the secondary eclipses part of the accretion flow. Publication: The Astrophysical Journal Pub Date: May 1992 DOI: 10.1086/171345 Bibcode: 1992ApJ...391..295M Keywords: Cataclysmic Variables; Stellar Models; Stellar Spectra; White Dwarf Stars; Emission Spectra; Stellar Mass Accretion; Stellar Rotation; Astrophysics; STARS: BINARIES: GENERAL; STARS: INDIVIDUAL CONSTELLATION NAME: FO AQUARII full text sources ADS | data products SIMBAD (1)

Spectroscopy of FO Aquarii - Evidence for reprocessing in a hot wind

view Abstract Citations (6) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spectroscopy of FO Aquarii: Evidence for Reprocessing in a Hot Wind Martell, Phillip J. ; Kaitchuck, Ronald H. Abstract Results are presented on spectroscopic observations of FO Aqr, carried out on the nights of September 4-7, 1986 with the 1.8-m Perkins Telescope at Lowell Observatory. It is shown that the observed 21-min modulation of the He II 4686-A line is driven by two emission components which vary in flux and have large negative radial velocities. Arguments are presented to the effect that the modulated portion of the He II 4686-A emission originates from the reprocessing of X-rays in hot winds at these two sites. It is suggested that all other 21-min modulations are due to reprocessing. Publication: The Astrophysical Journal Pub Date: January 1991 DOI: 10.1086/169561 Bibcode: 1991ApJ...366..286M Keywords: Astronomical Spectroscopy; Cataclysmic Variables; Stellar Mass Accretion; Stellar Winds; White Dwarf Stars; Emission Spectra; H Lines; Light Curve; Magnetic Stars; Stellar Spectra; X Ray Spectra; Astrophysics; STARS: ACCRETION; STARS: BINARIES; STARS: INDIVIDUAL CONSTELLATION NAME: FO AQUARII; STARS: MAGNETIC; X-RAYS: BINARIES full text sources ADS | data products SIMBAD (1)