Abstract
1RXS J180431.1-273932 is an intermediate polar: a cataclysmic variable star with a rapidly rotating, magnetized white dwarf. We analyze the system's Kepler K2 short-cadence light curve and identify a strong, highly coherent pulse with a period of 0.0057171 ± 0.0000002 days, which matches the proposed white dwarf rotational period identified in previous X-ray studies. We establish an ephemeris for the optical spin pulse.
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1. Introduction
1RXS J180431.1-273932 (hereafter, J1804) was first classified as an X-ray binary containing an accreting neutron star by Nucita et al. (2007), who identified a 494 s X-ray period in XMM-Newton observations. Although Nucita et al. (2007) attributed this period to the rotation of a neutron star, they acknowledged the possibility that the source of the pulse might instead be a magnetized white dwarf (WD), in which case J1804 would be an intermediate polar (IP; for a review, see Patterson 1994). Later, Masetti et al. (2012) found that the optical spectrum of J1804 closely resembles that of an IP, and they argued that the X-ray period was actually the rotational period of a WD. They therefore reclassified J1804 as an IP.
J1804 remains an understudied system, and even its orbital period is unknown. It appears that there have been no reported attempts to identify an optical counterpart of the X-ray period. Here, we fill that gap with an analysis of the Kepler K2 light curve of J1804, which, along with FO Aqr (Kennedy et al. 2016) and RZ Leo (Szkody et al. 2017), is one of only several IPs to have been observed by Kepler.
2. Data
J1804 was observed during Campaign 9 of the K2 mission for 41 days between 2016 May 22 and 2016 July 2 in the short-cadence mode of 58.8 s per image. We used lightkurve to select a custom aperture, but because J1804 is situated in a dense star field, its uncorrected light curve was extensively contaminated by the pointing instabilities of the spacecraft. To mitigate these systematic artifacts, we used the self-flat-fielding technique of Vanderburg & Johnson (2014).
3. Analysis
The light curve of J1804 (Figure 1) shows very little variation in its overall appearance, implying a reasonably stable rate of mass transfer during the Kepler observation. However, on timescales of minutes, there is a conspicuous periodic oscillation with a semi-amplitude of ∼2%, and it is noticeable throughout the entire duration of the light curve. The power spectrum in Figure 1 establishes that the frequency of this signal (174.91 cycles days−1, equivalent to 494 s) is identical to the proposed WD spin frequency (ω) detected by Nucita et al. (2007) and Masetti et al. (2012) in X-ray observations. The 2ω and 3ω harmonics of the spin frequency are also present. The power spectrum also shows signals at 391.5 cycles days−1, 440.4 cycles days−1, and 680.5 cycles days−1, none of which are harmonics of ω. Instead, Baran (2013) attributed these frequencies to instrumental effects, and we conclude that they are not of astrophysical origin in J1804.
Figure 1. Top: 40 day light curve of J1804 after application of the self-flat-fielding algorithm. Middle: zoomed-in view of the light curve. Periodic variability is more evident in this panel than in the first. Bottom: power spectrum of J1804. The fundamental spin frequency (ω) is 174.91 cycles days−1, and its next two harmonics are indicated. Instrumental frequencies in the power spectrum (Baran 2013) are each marked with an X. Candidate orbital (Ω) and beat (ω − Ω) frequencies are marked at 4.81 cycles days−1 and 170.10 cycles days−1, respectively.
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Our detection of the optical counterpart of the 494 s X-ray period supports the classification of J1804 as an IP rather than an accreting neutron star, as neutron stars generally do not produce detectable rotational signals in optical wavelengths. Furthermore, the 494 s period would be unusually slow for a neutron star, but perfectly reasonable for an accreting white dwarf (Patterson 1994).
Based on the maxima of the spin pulses, we compute a spin ephemeris of
where E is the integer cycle count and Tmax[BJD] is the Barycentric Julian Date (in Barycentric Dynamical Time) of the predicted time of maximum light. The numbers in parentheses are 1σ uncertainties on the final digit of each parameter.
In Figure 1, there is a signal of marginal significance near 4.81 cycles days−1. Equivalent to a period of 4.99 hr, this signal is a plausible candidate to be the binary orbital frequency (Ω). If this identification is correct, the spin–orbit beat frequency (ω − Ω) would be 170.10 cycles days−1, and we do see a weak signal at this exact frequency in the bottom panel of Figure 1. However, because of the low amplitudes of these two signals, the proposed identifications of Ω and ω − Ω should be treated cautiously until they are confirmed with subsequent observations.
4. Conclusion
Our Kepler K2 study of 1RXS J180431.1-273932 reveals that the WD's X-ray spin pulse has an optical counterpart, for which we report an ephemeris. There is also modest evidence of a 4.99 hr orbital period, but it requires spectroscopic confirmation.