Low level optical activity in MAXI J1820+070
ATel #14582; M. Cristina Baglio (NYU Abu Dhabi), Jeroen Homan (Eureka Scientific & SRON), David M. Russell, Payaswini Saikia, D. M. Bramich (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), Paul Roche (Cardiff University), Thabet Al Qaissieh, Aldrin Gabuya, Alejandro Palado (Al Sadeem Observatory, Abu Dhabi)
on 28 Apr 2021; 06:11 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)
Subjects: Optical, Ultra-Violet, X-ray, Binary, Black Hole, Transient
Referred to by ATel #: 15277
The black hole X-ray binary MAXI J1820+070 recently showed signs of new outburst activity at optical wavelengths (ATel #14492) between 2021 March 7 and 20. Similarly, an increase in the 0.5-10 keV flux by a factor of ~3-4 was observed by Swift/XRT on 2021 March 29 (ATel #14495), which indicated that outburst activity had also started in X-rays.
Since then, we have monitored the system regularly with the 1m and 2m optical telescopes of the Las Cumbres Observatory (LCO) network (see e.g. ATel #11418, #12128, #12534), as part of an ongoing monitoring program of ~50 low mass X-ray binaries (LMXBs; Lewis et al. 2008).
The optical re-brightening had a first peak on MJD 59300-59301 (Mar 27-28; V=17.72 +/- 0.01, R=17.13 +/-0.01, i'=17.12 +/- 0.09; see ATel #14492), then faded on MJD 59318 (Apr 14) to magnitudes of: g'=18.02 +/- 0.04, r'=17.50+/-0.01, i'=17.27 +/- 0.01. However, on MJD 59326 (Apr 22) the flux was brighter again in all bands: g'=17.51+/-0.01, r'=17.07+/-0.01, i'=16.87+/-0.01. This flux rise is more dominant at the highest frequencies, suggestive of an increasing disc component. Due to a gap in the monitoring between MJD 59318 and 59326, it is not possible to determine the exact time of the start of this rise with our LCO monitoring.
The system was also observed with the Meade LX850 16-inch (41-cm) telescope of the Al Sadeem Observatory (UAE), as part of monitoring that started in 2018, using the 'blue', 'green' and 'red' Baader LRGB CCD-Filters (that approximately correspond to the central wavelengths of the g', V and R filters, respectively; see ATel #11533 and Russell et al. 2019 for details).
On MJD 59320 (Apr 16) and MJD 59323 (Apr 19), the g'-band flux was very similar to the previous LCO fluxes: g'=17.92+/-0.11, V=17.48+/-0.10, R=16.92+/-0.07 on Apr 16; g'=17.89+/-0.08, V=17.49+/-0.08, R=16.89+/-0.05 on Apr 19. Combining the LCO and Al Sadeem monitoring, we are therefore able to constrain the date of the start of the new rise to between MJD 59323.9 and MJD 59326.5.
The last LCO observations, performed on MJD 59331 (Apr 27) reveal a new decrease in flux, with magnitudes: g'=18.09+/-0.02, r'=17.61+/-0.01, i'=17.35+/-0.01. However, the current magnitudes are still significantly brighter than the known quiescent magnitudes of the source according to the PanSTARRS catalogue (see ATel#14492), suggesting that some low-level activity is still ongoing.
The brief re-brightening was also seen in the UVOT, with the following magnitudes (AB system): V=17.15+/-0.18, V=17.64+/-0.15 and V= 17.5+/-0.14 (Apr 7, 9, 10 respectively); UVW1= 19.19+/-0.06, UVW1=18.37+/-0.06, UVW1=19.14+/-0.06 on Apr 1, 5, 17, respectively.
Swift/XRT observations were performed between March 29 and April 17. The per-observation-averaged count rates were variable by factors of up to 4 and no clear trend could be seen during the nearly three weeks of X-ray coverage. On average the count rates were a factor of ~3 above the count rates measured in February 2021. The highest observed flux was observed on April 15: (1.4+/-0.4)e-12 erg/cm^2/s (0.5-10 keV, unabsorbed). The Swift/XRT monitoring will resume soon.
The optical monitoring of MAXI J1820+070 with LCO and the Al Sadeem Observatory telescope is ongoing. The analysis of the LCO data are performed with a new real-time data analysis pipeline, the ''X-ray Binary New Early Warning System'' (XB-NEWS; see Russell et al. 2019 and Goodwin et al. 2020 for details). We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124. We thank the Swift team for scheduling and performing the daily observations.
Light curve of MAXI J1820+070