Cen X-4 returns to quiescent levels in X-ray, optical, and UV

ATel #14317; J. van den Eijnden (Univ. of Oxford), Maria Cristina Baglio (NYU Abu Dhabi), J. Homan (Eureka Scientific & SRON), Payaswini Saikia, David M. Russell, Stefan Waterval, D. M. Bramich (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), P. Roche (Cardiff University)
on 12 Jan 2021; 18:48 UT
Credential Certification: Jakob Van den Eijnden (a.j.vandeneijnden@uva.nl)

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Cen X-4 is a neutron star low-mass X-ray binary located at a close distance of ~1.2 kpc. After its first two outbursts, in 1969 and 1979, it remained in quiescence for more than 40 years. After an initial prediction based on optical monitoring with the Las Cumbres Observatory (LCO; Atel #14254), Cen X-4 was observed to become active in both the optical/UV (ATel #14302) and X-ray bands (ATel #14303). Since then, we have continued monitoring the LMXB with the Neil Gehrels Swift observatory (Swift), the Neutron Star Interior Composition ExploreR (NICER), and LCO in the X-ray, optical, and UV bands.

After the observations reported in ATel #14303, Swift observed Cen X-4 on 7 and 11 January 2021, with both observations taken in PC mode. Using the Swift online data analysis pipeline (Evans et al. 2007, A&A, 469, 379), we extract 0.3-10 keV count rates of 0.312 +/- 0.025 ct/s and 0.079 +/- 0.011 ct/s, respectively. These count rates show a decreasing trend from the Swift peak rate on 4 January; the count rate on 11 January is consistent with the range observed by Swift in quiescence (Tudor et al. 2017, MNRAS, 470, 324).

NICER monitoring confirms this decrease in X-rays. It showed a variable but increasing trend in the 0.5-10 keV count rate up to January 6, peaking at a count rate of 9.17 +/- 0.05 ct/s (all reported NICER rates are daily averages; background (less than 0.5 cts/s) was not subtracted). The count rate then decreased gradually, reaching 2.27 +/- 0.03 ct/s on January 12. The Swift and NICER X-ray light curves can be found at the link below.

Cen X-4 is being monitored in the optical with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes (see ATel #14302 for details on the monitoring and the analysis of the LCO data). Also at these frequencies, the source shows a decrease of the flux after January 6, going from V=17.43+/-0.03, g'=17.79+/-0.03, i'=16.81+/-0.02, Y=20.87+/-0.05 on January 6 to V=17.99+/-0.05, i'=17.14+/-0.03 on January 7. The most recent observations, performed on January 12, show a further flux decrease, with g'=18.73+/-0.05, r'=17.84+/-0.03, i'=17.43+/-0.04, Y=22.31+/-0.16, thus indicating that Cen X-4 is now back to its quiescent flux level at optical frequencies. Similarly, UV observations performed with Swift/UVOT show a decisive decrease in flux, with an AB magnitude in the UVW2 filter of 20.19 +/- 0.11 and 20.62 +/- 0.12 on January 7 and 11, respectively.

These decreases in X-ray, optical, and UV flux of Cen X-4 suggest a return to previous quiescent levels. It remains, however, unclear how this brief epoch of activity will continue. Possibly, it could be a precursor flare preceding a full outburst, as was observed with XB-NEWS in optical monitoring prior to the 2019 outburst of the neutron star LMXB SAX J1808.4-3658 (Goodwin et al. 2020, MNRAS, 498, 3429). Alternatively, however, Cen X-4 could now remain in quiescence.

We are grateful to the Swift and NICER teams for rapidly approving, scheduling, and performing these observations. We will continue the X-ray, optical, and UV monitoring of Cen X-4 and report on new activity if it arises.

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.

Swift and NICER X-ray light curves