We report the discovery of a 1mHz optical quasi-periodic oscillation (QPO) in the candidate ultracompact low-mass X-ray binary 4U 0614+091. The ultra-low frequency QPO has no X-ray counterpart in contemporaneous RXTE/PCA data and is likely a signature of structure in the accretion disk. The QPO can be reasonably fitted with a single sine wave but with a phase jump part way through the observation, indicating that it is not coherent.We also identify a 48 min modulation, approximately consistent with the suggested orbital period of O'Brien (2005) and Shahbaz et al. (2008). If this is indeed orbital, it supports an identification of 4U 0614+091 as an ultra-compact source.
Low-mass X-ray binaries (LMXBs) can be divided into black hole candidates and neutron star systems. Those with orbital periods of hours to days are expected to have hydrogen-rich mass donors. A few ultra-compact X-ray binaries (UCXBs) containing hydrogen-deficient and possibly degenerate donors can evolve to extremely small binary separations, with orbital periods as short as a few minutes (Nelson, Rappaport, & Joss 1986). Although these systems were initially assumed to be rare, many candidates have now been found (Nelemans & Jonker 2006;in't Zand, Jonker, & Markwardt 2007).
The X-ray source 4U 0614+091 was first optically identified by Murdin et al. (1974) with a faint (∼ 18 mag), blue, variable star (V1055 Ori) located in the Galactic plane (van Paradijs & van der Klis 1994). Several arguments recently concluded 4U 0614+091 is a UCXB hosting a neutron star. Juett, Psaltis, & Chakrabarty (2001) found an enhanced neon to oxygen ratio in 4U 0614+091 similar to that seen in other UCXBs. Through sensitive optical spectroscopy, Nelemans et al. (2004) and Nelemans, Jonker, & Steeghs (2006) found significant carbon and oxygen emission lines, but no hydrogen or helium. They argued 4U 0614+091 contains a carbon-oxygen accretion disk. Recently, Madej et al. (2010) reported the dis-⋆ E-mail: yzhang@phys.lsu.edu † E-mail: rih@phys.lsu.edu ‡ E-mail: elr@astro.as.utexas.edu covery of a broad emission feature at ∼ 0.7 keV, which is attribute of O VIII Lyα emission, in the spectra of 4U 0614+09. This feature has been seen so far in two systems (Madej et al. 2011) that are receiving oxygen-rich material from the CO or ONe white dwarf. It is an endorsement of an oxygen-rich mass donor star. Thermonuclear type I X-ray bursts were observed by OSO-8 (Swank et al. 1978), and assuming they were Eddington limited, Brandt et al. (1992) deduced a distance to 4U 0614+091 probably < 3 kpc. More recently Kuulkers et al. (2010) have re-examined archival bursts from several satellites and derived a distance of 3.2 kpc. These authors argue that the bursts require more helium in the accreted matter than would be consistent with the optical spectra leaving the nature of the mass donor still in question.
A straightforward resolution of this puzzle has been elusive due to the difficulty in identifying an orbital period. Machin et al. (1990) found a 10-d photometric modulation which might be caused by precession of an accretion disc. They then scaled the precession period of Her X-1 to 4U 0614+091 to infer the orbital period. If the ratio of the disc radius to the neutron star Roche lobe radius, ρ is assumed to be ∼ 1, P orb ≈ 6.3 -9.4 hr. If ρ ∼ 0.5 then P orb ≈ 2.4 -3.3 hr. Either way, this argument suggests a non-UCXB system with a hydrogen-rich donor. However, the faintness of its X-ray emission in spite of a nearby distance suggest 4U 0614+091 has a very low accretion rate more consistent with an orbital period < 1 hr (Deloye & Bildsten 2003). Recently, O’Brien (2005) reported a tentative 50 min optical period and Shahbaz et al. (2008) also claimed a strong modulation of 51.3 mins. A spectroscopic modulation on a period close to this may also be present (Nelemans, Jonker, & Steeghs 2006). These recent results support the case that 4U 0614+091 is a UCXB system.
The orbital period is not the only periodicity expected. Superhumps at close to the orbital period can occur in short period (and usually small mass ratio) LMXBs due to the beat between the orbital period and a slowly precessing accretion disk (Haswell et al. 2001). Millihertz quasiperiodic oscillations (QPOs) also occur, but are rarely observed at optical wavelengths. The exception is 4U 1626-67 which is a LMXB source containing a 7.66 s X-ray pulsar in an ultra-compact 42 min orbit. 130 mHz (∼ 7.7s) X-ray pulsations are seen, a 48 mHz (∼ 20.8s) QPO is produced by an interaction between the pulsar’s magnetosphere and the accretion disk, and a 1 mHz (∼ 16.7 min) UV QPO were found (Chakrabarty et al. 2001). Only the 1 mHz QPO was not detected in simultaneous X-ray observations. Chakrabarty et al. (2001) suggested that the 1 mHz oscillations are due to warping of the inner accretion disk.
We present here new optical photometry of 4U 0614+091 revealing a new mHz QPO and further evidence for a ∼ 50 min period. In Section 2 we describe our data. The lightcurves are analysed in Section 3 and compared with contemporaneous X-ray observations in Section 4. Finally in Section 5 we discuss the implications of a ∼ 50 min orbital period and the possible origin of the QPO.
We performed fast optical photometry of 4U 0614+091 on three nights in December 2007 with the 2.1 m (82") Otto Struve Telescope at McDonald Observatory, Texas (Table 1). Approximately 7 hours of data were obtained on each night. For convenience we will refer to the nights as Night 1, Night 2, or Night 6 for the remainder of this work. We used the Argos CCD Photometer (Mukadam & Nather 2005). This is a CCD camera designed
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