X-ray observations of FO Aqr during the 2016 low state

Abstract

We present the first ever X-ray data taken of an intermediate polar, FO Aqr, when in a low accretion state and during the subsequent recovery. The Swift and Chandra X-ray data taken during the low accretion state in July 2016 both show a softer spectrum when compared to archival data taken when FO Aqr was in a high state. The X-ray spectrum in the low state showed a significant increase in the ratio of the soft X-ray flux to the hard X-ray flux due to a change in the partial covering fraction of the white dwarf from > 85% to 70 +5 −8 % and a change in the hydrogen column density within the disc from 19 +1.2 −0.9 × 10 22 cm −2 to 1.3 +0.6 −0.3 × 10 22 cm −2 . XMM-Newton observations of FO Aqr during the subsequent recovery suggest that the system had not yet returned to its typical high state by November 2016, with the hydrogen column density within the disc found to be 15 +3.0 −2.0 cm −2 . The partial covering fraction varied in the recovery state between 85% and 95%. The spin period of the white dwarf in 2014 and 2015 has also been refined to 1254.3342(8) s. Finally, we find an apparent phase difference between the high state X-ray pulse and recovery X-ray pulse of 0.17, which may be related to a restructuring of the X-ray emitting regions within the system.

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

“…X-ray observations (Kennedy et al 2017) show that the mass-transfer rate decreased by about one order of magnitude. Due to solar conjunction, few data are available for the descent to the low state, which has been rapid (time scale of weeks) as compared to the recovery time.…”

The true nature of Swift J0746.3-1608: a possible Intermediate Polar showing accretion state changes

“…The presence of star spots has been inferred for the first time in the donor of the IP AE Aqr (Hill et al 2016), although it has never been observed to undergo state changes. Star spots have been claimed to be responsible for the low states of Polars and of the state changes in FO Aqr (Kennedy et al 2017). We note that, interestingly, star spots at L1 should produce random (e.g.…”

“…This X-ray emission is then pulsed at the spin period of the WD (disc-accretion), at the beat (disc-less) or at both periods in the case of disc overflow (Hellier 1995). The vast majority of IPs are persistent systems, but a small subgroup including FO Aqr, AO Psc, and V1223 Sgr, showed low accretion states (Garnavich & Szkody 1988;Kennedy et al 2017;Littlefield et al 2018). Such fading is believed to be due to a temporarily reduction of the mass transfer rate from the donor star (Livio & Pringle 1994).…”

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

“…We speculated that the phase shift could be the result of a change in the accretion geometry but cautioned that it needed to be confirmed. Fortunately, Kennedy et al (2017) dug deeper into the issue and found that the purported phase shift was attributable to a minuscule inaccuracy in the WD spin period from Kennedy et al (2016), propagated across 2 yr. Although the period was constant throughout the K2 observations, there were small phase shifts of the spin pulse in response to the system's overall luminosity, and these shifts caused the Lomb-Scargle periodogram to yield a period that was inaccurate by just a few milliseconds (Kennedy et al 2017).…”

“…Fortunately, Kennedy et al (2017) dug deeper into the issue and found that the purported phase shift was attributable to a minuscule inaccuracy in the WD spin period from Kennedy et al (2016), propagated across 2 yr. Although the period was constant throughout the K2 observations, there were small phase shifts of the spin pulse in response to the system's overall luminosity, and these shifts caused the Lomb-Scargle periodogram to yield a period that was inaccurate by just a few milliseconds (Kennedy et al 2017). There are two critical takeaways from Kennedy et al (2017) about the spin ephemeris: (1) the phase shift from Paper I was not of astrophysical origin and, (2) more broadly, modeling a light curve of FO Aqr with a periodic function is not a reliable means of extracting either the time-averaged spin period or the time of one fiducial photometric maximum.…”