Fast high-amplitude optical variations in Cen X-4 during the brief flare episode seen with REM and LCO

ATel #14332; M. C. Baglio, D. M. Russell, P. Saikia, S. Waterval (NYU Abu Dhabi), P. D'Avanzo, S. Campana (INAF - OAB), D. M. Bramich (NYU Abu Dhabi), F. Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), J. Homan (Eureka Scientific & SRON), Jakob van den Eijnden (Univ. of Oxford), S. Covino (INAF-OAB), P. Goldoni (APC/UMR), N. Masetti (INAF-OAS), T. Munoz-Darias (IAC), E. Palazzi (INAF-OAS)
on 19 Jan 2021; 08:06 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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The neutron star low-mass X-ray binary Cen X-4 has been in quiescence for the past ~41 years, after its last reported outburst in 1979.

Signs of a recent (August 2020) brightening were first reported in ATel #14254 thanks to the optical monitoring performed with the Las Cumbres Observatory (LCO) network of 2-m and 1-m robotic telescopes. A possible onset of a new outburst was then reported in ATel #14302, as observed in the LCO monitoring. A flux increase was also detected at UV and X-ray wavelengths (ATel #14303) with Swift and NICER observations. After a few days of enhanced activity however, the flux started to decrease, and the source reached again its quiescent levels at all wavelengths (ATel #14317).

Here we report on optical timing observations of Cen X-4 during the above brief flare episode. The system was observed on January 5, 2021 for ~20 mins using the g' band filter as part of the LCO monitoring, for a total of 25 exposures of 30s each. Magnitudes have been extracted and calibrated with the "X-ray Binary New Early Warning System" real-time data analysis pipeline (XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020 and ATel #13451 for details). The light curve shows strong variability on short timescales, displaying a brightening of ~0.7 mag in ~8 mins between the lowest and the highest flux points. We evaluated the fractional rms following Vaughan et al. 2003 and found a value of (15.1+/-2.4)%, which is very high for a NS-LMXB.

Cen X-4 was also continuously observed for an hour on January 5, 2021 with the 60cm Rapid Eye Mount (REM) telescope (La Silla, Cile). Strictly simultaneous, 300s integration time observations have been obtained using the griz SDSS optical filters, for a total of 9 observations per filter. The light curves show strong intrinsic variability, with a ~1 mag difference between the lowest and the highest points in z-band. The fractional rms measured for the z-band is (26.8+/-8.3)%.

We built an average spectral energy distribution (SED) using the strictly simultaneous optical (REM) data, together with a Swift/UVOT UV observation performed on January 5 2021 (ATel #14302). The UV to i'-band SED looks red, with a spectral index of alpha = -1.3+/-0.2 (where F_nu propto nu^alpha). The peak of the SED lies around the i' band frequency. The position of the z-band point is uncertain, due to the high variability of the z-band curve, sometimes being brighter and sometimes fainter than the i'-band point.
A very similar result is found from the LCO data (Bessell V, SDSS g', r', i' filters) on the same date, according to which the spectral index of the optical-UV SED is -1.6+/-0.3, which is compatible with what is found with the REM+UVOT dataset. Note that the Y-band magnitudes listed in ATel#14302 and ATel#14317 were in fact uncalibrated, and should not be used; we apologise for this error in our previous ATels.

The high fractional rms, together with the red SED that is observed, are suggestive of the emission of a variable jet during the just concluded flaring episode, as and are very similar to Swift J1357.2-0933 in quiescence (Shahbaz et al. 2013; Russell et al. 2018).

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-UV SED of Cen X-4