Swift and NICER X-ray monitoring of the probable new outburst of Cen X-4

ATel #14303; J. van den Eijnden (Univ. of Oxford), J. Homan (Eureka Scientific & SRON), Payaswini Saikia, Maria Cristina Baglio, 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 6 Jan 2021; 10:13 UT
Credential Certification: Jakob Van den Eijnden (a.j.vandeneijnden@uva.nl)

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Cen X-4 is a close-by (~1.2 kpc) low-mass X-ray binary, hosting a neutron star accretor. After its discovery outburst in 1979, it remained in quiescence for over four decades. Recently, based on 12 years of optical monitoring with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes, ATel #14254 suggested the possible onset of a new outburst in the near future. A sharp rise in the optical and UV has been observed in the past week, suggesting that a new outburst has now started (ATel #14302). Since Cen X-4 emerged from Sun constraint on 28 December 2020, we have monitored the X-ray binary at X-ray wavelengths with the Neil Gehrels Swift observatory (Swift) and the Neutron Star Interior Composition ExploreR (NICER). Swift and NICER light curves can be found at the link below.

Swift observed Cen X-4 on 28 and 31 December 2020, and 4 January 2021, with exposure times between ~0.5 and 1 ks. The X-ray Telescope (XRT) operated in Photon Counting mode for all observations. We used the online Swift pipeline (Evans et al. 2007, A&A, 469, 379) to extract the XRT light curve. Cen X-4 is significantly detected in all observations, and is brightening over time: we measure count rates of 0.155 +/- 0.017 ct/s, 0.266 +/- 0.037 ct/s, and 0.569 +/- 0.034 ct/s in the 1st, 2nd, and 3rd observation, respectively. The latter two count rates exceed the historical maximum count rate of Cen X-4 in quiescence observed by Swift (~0.22 ct/s; Tudor et al. 2017, MNRAS, 470, 324).

NICER has been performing daily observations of Cen X-4 since January 1 2021, with exposure times between ~0.6 and ~4.2 ks. The 0.5-10 keV light curves show strong variability (by factors of up to four) on time scales of a few tens to a few hundred seconds. To reduce the intrinsic scatter we produced a light curve from daily averaged count rates. A rising trend is seen, with a large jump in count rate between January 4 and January 5.

In order to measure the X-ray flux, we fitted the NICER spectrum taken on 2 January 2021 and the Swift/XRT spectrum from 4 January 2021. Following Chakrabarty et al. (2014, ApJ, 797, 92), who modeled XMM-Newton/NuSTAR spectrum of Cen X-4, we fit both spectra in XSPEC with a model combining interstellar absorption, a neutron star atmosphere (nsatmos, with neutron-star mass of 1.9 solar mass and a radius of 9 km) and a cutoff power law (cutoffpl). This model provided a good description of the quiescent spectra. For the NICER and Swift spectra, respectively, we fix the absorption column density N_H to 9E+20 cm^-2 and the cutoff energy to 10 keV. We measured power law indices of Gamma ~ 2.0 ( NICER) and Gamma ~ 1.5 (Swift). The effective atmosphere temperature in the Swift observation was higher than in the NICER observation, ~89 eV and ~80 eV respectively, while the 0.5-10 keV flux in the cutoff power-law component had doubled. All measured parameters are broadly consistent with those reported by Chakrabarty et al. (2014).

The unabsorbed 0.5 - 10 keV X-ray fluxes measured by NICER and Swift are, respectively, F_X ~ 7.4E-12 erg s^-1 cm^-2 and F_X ~ 1.5E-11 erg s^-1 cm^-2. Assuming a distance of 1.2 kpc, these fluxes correspond to X-ray luminosities of L_X ~ 1.2E33 erg s^-1 and L_X ~ 2.6E33 erg s^-1.

The sudden X-ray brightening (to levels not seen before with Swift) suggests that Cen X-4 is entering a new outburst, supporting the report in ATel #14302 of an outburst start based on optical and UV observations. Given the proximity of Cen X-4 and the rarity of its outbursts, we strongly encourage multi-wavelength follow up. X-ray monitoring with Swift and NICER will continue.

We are grateful to the Swift and NICER teams for rapidly approving, scheduling, and performing these observations.

Swift and NICER light curves of Cen X-4