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XB-NEWS detects a probable new outburst from Cen X-4 after 41 years

ATel #14302; Payaswini Saikia, Maria Cristina Baglio, David M. Russell (NYU Abu Dhabi), Jeroen Homan (Eureka Scientific & SRON), Stefan Waterval, D. M. Bramich (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), Jakob van den Eijnden (Univ. of Oxford)
on 6 Jan 2021; 10:09 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

Subjects: Optical, Ultra-Violet, X-ray, Binary, Neutron Star, Transient

Referred to by ATel #: 14303, 14317, 14332, 14333, 16831

Cen X-4 is a neutron star X-ray transient discovered in 1979 during an outburst. It has since been in quiescence for almost 41 years. Recently, ATel #14254 indicated that Cen X-4 might enter an outburst in the near future, based on a gradual, significant, optical brightening detected in the long term (12 years) optical monitoring of the source with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes. The last data reported in ATel #14254 were obtained in August 2020, when the optical magnitudes of the source were V = 18.31 +/- 0.17, i' = 17.40 +/- 0.11, as compared to a fainter V = 18.43 +/- 0.10, i' = 17.46 +/- 0.09 during 2017-2018. Since the start of September, the source has been Sun-constrained.

We have been closely monitoring the source with LCO, since it re-emerged from the Sun constraint at the end of 2020 December. For the analysis of the LCO data, we are making use of the 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).

We resumed our LCO observations on MJD 59213.3 (2020 December 30) after the Sun constraint ended, and found the source to be significantly brighter than before with optical magnitudes V=17.86 +/- 0.06 and i'=17.04 +/- 0.02. The most recent observations made on MJD 59219 (2021 January 5) show a steep rise in the optical emission, with V = 17.58 +/- 0.03, i'=16.69 +/- 0.02, g' = 17.66 +/- 0.03, r' = 17.02 +/- 0.02 and Y = 21.70 +/- 0.08. From our long-term monitoring of Cen X-4 with LCO, we find that the modulation of the source is approximately 0.1 mag amplitude, which is much smaller than what is required to explain the amplitude of the variability (see ATel #14254). This suggests that Cen X-4 is entering a new outburst after ~41 years in quiescence.

After the start of the optical flux rise, the Neil Gehrels Swift Observatory (Swift) / UVOT observed with the UVW1 filter, resulting in an AB magnitude of 19.74 +/- 0.09 on MJD 59211 (2020 December 28). This is consistent with the range of quiescent magnitudes shown in Bernardini et al. 2013, that reported on large variations in the UVW1 band, spanning the range of (AB) magnitudes ~21.3-19.4. Later, observations with the UVM2 filter have been performed, obtaining an AB magnitude of 19.64 +/- 0.11 on MJD 59214 (2020 December 31) and 19.33 +/- 0.08 on MJD 59218 (2021 January 4), thus suggesting a rise also at UV wavelengths. These results also show that the system is brighter now than it was in quiescence, since Cackett et al. 2013 reported a UVM2 magnitude of 19.85 +/- 0.14 on MJD 56048 (2012 May 1).

The optical monitoring of the source with LCO is still ongoing. Further quasi-simultaneous multiwavelength observations are encouraged in order to confirm the outburst and trace the rise of the source. As Cen X-4 is very close-by (distance ~ 1.2 kpc), it is a promising candidate to study how multi-wavelength emission processes are coupled at the fainter, early stages of an low-mass X-ray binary outburst. 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