Cen X-4 stirs again: XB-NEWS catches new activity in the optical 6 years after the misfired event

ATel #17911; M. C. Baglio, M. M. Messa (INAF-OAB), D. M. Russell, D. M. Bramich (NYU Abu Dhabi), G. Illiano (ICE-CSIC), K. Alabarta (INAF-OAB), Y. J. Yang (NYU Abu Dhabi), P. Saikia (Yale), A. Marino (ICE-CSIC), D. A. H. Buckley (SAAO/UCT/UFS), F. Lewis (Faulkes Telescope Project & The School's Observatory, LJMU)
on 20 Jul 2026; 15:26 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

Tweet

Centaurus X-4 (Cen X-4) is a transient neutron star X-ray binary discovered in 1979 during an outburst. After that, it has remained in quiescence for almost 41 years.
In December 2020, a rise into outburst was detected with the Las Cumbres Observatory (LCO) network of telescopes, followed by X-ray detections of the same event (ATel #14302 and #14303). The episode, however, lasted only about two weeks before fading back to quiescence without developing into a full outburst, and was therefore classified as a misfired outburst (Baglio et al. 2022).

After the misfired outburst, Cen X-4 remained in quiescence. We have continued to monitor it closely as part of an ongoing campaign covering ~50 low-mass X-ray binaries with LCO and the Faulkes Telescopes (Lewis et al. 2008), using the real-time pipeline "X-ray Binary New Early Warning System" (XB-NEWS; Russell et al. 2019; Saikia et al. 2026).

Multi-band LCO monitoring (V, g', r', i', z_s) has recently revealed a new long-term, slow rise in the optical flux of the source. Following the methodology of Baglio et al. (2022), we subtracted the ellipsoidal modulation of the K-type donor star from the folded light curves in each band; the residual (disc) flux shows a clear rise beginning around MJD 60203 (2023 November), following a preceding multi-year decline. This corresponds to a ~0.08–0.12 mag (~8–12%) brightening in V and i' over the ~2.8 yr since the reversal, with a consistent rise in g', r', and z_s. The rising trend continued until MJD 61230 (2026 July 9). A figure of the donor-subtracted light curves (all five bands) is available below.

Since then, the source has brightened considerably faster, peaking at V = 17.81 +/- 0.02 and i' = 17.05 +/- 0.01 mag on MJD 61239.9 (2026 July 18), ~0.36 mag (V) and ~0.29 mag (i') brighter than the quiescent baseline observed through June 2026. This rapid, day-timescale brightening is distinct from the slower multi-year trend above. Over the following day and a half the source fluctuated below this peak without a clear, steady further rise; the most recent detections give V = 18.06 +/- 0.02 mag on MJD 61240.9 (2026 July 19) and i' = 17.18 +/- 0.01 mag on MJD 61241.4 (2026 July 20).

Several scenarios could explain this behaviour. If confirmed as an outburst, this would be the first full outburst of Cen X-4 in 47 years, since its last one in 1979; alternatively, this could be another misfired event similar to that of December 2020 (Baglio et al. 2022). The peak of this new activity has not yet reached the peak brightness of the 2020/2021 misfired outburst (V = 17.43 mag, i' = 16.58 mag), remaining ~0.4 mag (V) and ~0.5 mag (i') fainter. This new activity is also brighter than typical pre-2020 flaring in this source, when only one or two points reached V<~17.8 mag and most flares peaked around V~17.95. A further possibility is thus that the disc is becoming progressively more active in build-up to a new outburst, i.e. an outburst precursor, as seen in the accreting millisecond X-ray pulsar SAX J1808.4-3658 (Goodwin et al. 2020), whose optical precursor preceded the proper outburst rise by several days. There, however, the precursor's blackbody temperature rose to ~9000 K then decreased to ~7000 K before the fast rise, well above the ~5400 K peak measured for the Cen X-4 misfired outburst (Baglio et al. 2022); if this is a precursor, Cen X-4 should be cooler than SAX J1808.4-3658 was. Continued LCO monitoring will help discriminate between these scenarios.
We strongly encourage prompt multi-wavelength (radio, IR, optical, UV, X-ray) follow-up.

Donor-subtracted optical light curves of Cen X-4 after the misfired outburst