Long term fading and recent brightening in Cen X-4 in quiescence: precursor to an outburst?

ATel #14254; Stefan Waterval, David M. Russell, Maria Cristina Baglio, Payaswini Saikia, D. M. Bramich (NYU Abu Dhabi), Jeroen Homan (Eureka Scientific & SRON), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 9 Dec 2020; 10:23 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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Cen X-4 is a neutron star X-ray transient that has been in quiescence since its last outburst in 1979. We report on 12 years of optical monitoring of Cen X-4 with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes. We have acquired 228, 137, and 224 detections of the source in the V, R, and i' filters, respectively, from Feb. 14th 2008 (MJD 54510.7) to Aug. 31st 2020 (MJD 59092.4).

For the analysis of the LCO data, we are making use of the new real-time data analysis pipeline, the "X-ray Binary New Early Warning System" (XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020 and ATel #13451 for details). The light curves show a downward trend in brightness until MJD ~58000 followed by a re-brightening in all filters. For the first 2.4 years, the magnitude was V = 18.16 ± 0.10, i' = 17.28 ± 0.09, then by the turning point it had faded to V = 18.43 ± 0.10, i' = 17.46 ± 0.09 (in 2017-2018; MJD 57762-58391) and finally the source brightened to V = 18.31 ± 0.17, i' = 17.40 ± 0.11 by August 2020 (the errors represent the standard deviation of the variations within each time window).

In order to subtract the modulation of the companion star to isolate the accretion activity, we folded the light curves on a period of 0.6290630 days (McClintock and Remillard 1990) and fitted the lower envelope of the companion's modulation. After subtracting the modulation, we observe a linear downward trend followed by an upward one (see light curve linked below). The fitted downward slopes are -4.5e-3, -6.4e-3, and -6.3e-3 mJy/yr (converting magnitudes to flux densities) for V, R, and i', respectively (fitted until MJD 58016.4; V-band). The upward trend in the V and i' is measured at 1.6e-2 and 2.2e-2 mJy/yr, respectively and spans around 1000 days. This is around 3.5 times steeper than the downward one, that spans ~3000 days.

Bernardini et al. 2016 presented one decade of optical data of the LMXB V404 Cyg that preceded its 2015 outburst. In addition to a rise in the optical brightness shortly before the outburst, they observed a similar behavior in the light curves with a long term downward, and then upward trend. They notice a 0.1 mag decrease in brightness over ~2000 days followed by a 0.1 mag increase over ~1000 days. Given the similarity of the light curve of V404 Cyg compared to Cen X-4, this could be an indication that Cen X-4 might enter an outburst in the near future.

The last observation of Cen X-4 obtained with Swift/XRT on 26th September 2020 (MJD 59118) shows a count-rate of 0.025 ± 0.003 cps in the 0.3-10.0 keV range, which is consistent with previous quiescent values (Bernardini et al. 2013). This converts to a flux of 1.06e-12 (± 0.13e-12) ergs/cm2/s, assuming a photon index Γ = 1.7 and absorption nH = 8.0 ± 0.8 e20 cm^-2. No X-ray activity is evident in the MAXI and Swift/BAT light curves.

Since the start of September, the source has been Sun-constrained. We will continue to monitor Cen X-4 when the source becomes visible again. We encourage X-ray, and possible other wavelength observations of the source in case there is a brightening in the coming weeks/months. The LCO observations are part of an on-going monitoring campaign of ~ 50 low-mass X-ray binaries (Lewis et al. 2008) with LCO and the Faulkes Telescopes. We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

Optical LCO light curves of Cen X-4