Abstract
We analyze the far-ultraviolet light curve of the intermediate polar (IP) candidate SDSS-J093249.57+472523.0 (J0932+47), as observed by Hubble Space Telescope/Cosmic Origins Spectrograph. The power spectrum displays two periodicities at frequencies of 2.01 and 0.462 cycles minute−1, corresponding to periods of 29.8 and 130 s. We estimate the significance of the 29.8 s peak to be 3σ. The detection of the 130 s signal is less secure. If the 29.8 s signal is generated by a rotating white dwarf (WD), then J0932+47 would possess one of the fastest spinning WDs among known IPs. The N v/C iv and Si iv/C iv line ratios are larger than typically seen in cataclysmic variables (CVs), and anomalous line ratios are seen in several CVs with rapidly rotating WDs. Alternatively, the periodicities are in the range typical of dwarf nova oscillations.
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1. Introduction
Rapidly rotating white dwarfs (WD) in cataclysmic variables (CVs) are thought to be spun-up during an episode of thermal timescale mass transfer from its secondary star (Schenker et al. 2002). Two of the fastest spinning WD are in the propeller systems of AE Aqr (33.1 s period; Patterson 1979), and LAMOST J024048.51+195226.9 (J0240+19, 24.9 s; Pelisoli et al. 2022; J. Tweddale et al. 2024, in preparation). Other intermediate polars (IPs) known to harbor fast spinning WD components are V1460 Her (38.9 s; Ashley et al. 2020) and CTCV J2056-3014 (29.6 s; Lopes de Oliveira et al. 2020). The latter system was identified through its rapid X-ray pulses.
SDSS J093249.57+472523.0 (J0932+47 hereafter) is an eclipsing CV first identified by Szkody et al. (2004). XMM-Newton and ground based optical observations in 2005 yielded an X-ray to optical flux consistent with that of an IP (Homer et al. 2006). An orbital period of 95.48 minutes was determined by Gänsicke et al. (2009), and dwarf nova (DN) outbursts displaying superhumps have been studied by Shears et al. (2013). Here, we analyze far-ultraviolet (FUV) Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) data to search for periodicities in the light curve of J0932+47.
2. Data and Analysis
J0932+47 was observed with HST/COS over four consecutive HST orbits in 2013 January (GO-12870, PI: Gaensicke). The G140L grating was used in the FUV configuration in time-tag mode. We downloaded the "corrtag" files associated with these observations through the Mikulski Archive for Space Telescopes (MAST), DOI:10.17909/fjbc-6y79.
A light curve was constructed by grouping the time-tagged events into 2 s bins over each of the four HST orbits. The data from the first orbit showed a rapid brightening toward the end, so it was not included in the analysis. The median count rate for orbits 2–4 was 81 counts s−1. From these data, we constructed Lomb–Scargle periodograms (LSP; Lomb 1976; Scargle 1982) for each individual orbit as well as different combinations of orbits. The LSP constructed from the combination of the final three orbits shows two peaks, one near 0.5 cycles minute−1 and the second near 2.0 cycles minute−1.
Simulated periodic signals were constructed using the sampling window to estimate the frequencies and amplitudes of the signals. The short period peak has a frequency of 2.011 cycles minute−1, corresponding to a period of 29.8 ± 0.1 s. The longer period signal has a frequency of 0.462 cycles minute−1, corresponding to a period of 130 ± 2 s (see Figure 1). To test the significance of the 2.011 cycles minute−1 peak in the LSP, we subtracted low-frequency variations in the light curve and constructed LSPs from randomly shuffled photometric residuals. The maximum value in the LSP was recorded for each random shuffle and this distribution was compared with the LSP peak in the original photometric residuals. The randomized photometry equaled or exceeded the measured peak in 0.5% of the trials, indicating a 3σ significance of the detected signal at 29.8 s. The 130 s signal is very strong in orbit 2, but does not appear to be significant in comparison to the low frequency noise in the power spectra of the other three orbits. We consider the 130 s signal to be a marginal detection.
Figure 1. Top panel: the HST/COS FUV spectrum of J0932+47 averaged over the visit. J0932+47 shows ratios of N v/C iv and Si iv/C iv that are mildly anomalous compared with normal CVs. Bottom panel: the Lomb–Scargle periodogram created from orbits 2–4 of the HST/COS observation of J0932+47. Statistically significant signals are seen at frequencies of 0.462 and 2.011 cycles minute−1. The structure seen in the peaks results from the gaps in the HST data caused by Earth occultations.
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3. Discussion
J0932+47 was selected for further study based on its unusual emission line ratios seen in the HST/COS FUV spectrum (Pala et al. 2017). Normal CVs tend to have C iv emission lines that are stronger than the Si iv and N v strengths (Mauche et al. 1997). However, there is evidence that fast-spinning IPs show anomalously large N v/C iv and Si iv/C iv ratios. In extreme cases, such as in AE Aqr and V1460 Her, the C iv emission is nearly undetectable, causing the N v/C iv and Si iv/C iv ratios to exceed ∼10. J0932+47 has a mildly anomalous N v/C iv ratio of about 3.7 and Si iv/C iv of about 2.4. These ratios are similar to those seen in the propeller J0240+19 (J. Tweddale et al. 2024, in preparation). If the 29.8 s signal is indeed due to the spin of the WD, this would make J0932+47's WD one of the fastest spinners known among IPs.
An alternative explanation for the presence of the periodicities is that they result from dwarf nova oscillations (DNOs). DNOs with periods ranging between a few seconds to about a minute have been observed during CV outbursts and in nova-like systems. Their long-period cousins (lpDNOs) tend to have periods that are about four times longer (Warner et al. 2003) than the DNOs. At the time of observation, J0932+47 was in a state of early quiescence, having gone into outburst not long before the HST visit (Pala et al. 2017). FUV detections of DNOs are sparse, and have historically taken place during an outburst and in the days immediately following an outburst.
Further observations employing fast cadence photometry would be useful in determining whether the observed periodicities are due to a rapidly spinning WD or are DNOs.