An optical/X-ray re-flare from the X-ray transient MAXI J1807+132
ATel #16185; Payaswini Saikia (NYU Abu Dhabi), Jeroen Homan (Eureka Scientific), Kevin Alabarta, David M. Russell, M. C. Baglio, D. M. Bramich, Sandeep Rout (NYU Abu Dhabi), Thomas Russell (INAF/IASF Palermo), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 13 Aug 2023; 02:25 UT
Credential Certification: Payaswini Saikia (ps164@nyu.edu)
Subjects: Optical, X-ray, Neutron Star, Transient
MAXI J1807+132 is a low-mass neutron star X-ray binary transient discovered by MAXI on 2017 March during an outburst (ATel #10208). The source is currently undergoing a new outburst since the start of 2023 July, as detected at optical wavelengths (ATel #16119, #16128) and confirmed by X-ray observations (ATel #16125). From previous outbursts, the source is known to have short outbursts and several re-flares (Jimenez-Ibarra et al. 2019, Albayati et al. 2021).
We have been monitoring the source at optical wavelengths with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes, as part of an observational campaign of ~50 low-mass X-ray binaries (Lewis et al. 2008) since its discovery in 2017. We found that the source had a steep rise out of quiescence, and reached the peak of its outburst around 2023 July 9 (MJD 60134.4) with g'=16.42±0.01, r'= 16.21±0.01, i'= 16.03±0.01 and z_s= 15.89±0.01 mag. Since then the source had a steady decline with the faintest magnitudes measured as g'= 20.71±0.10, r'= 20.51±0.06, i'= 20.40±0.07 and z_s= 20.48±0.09 mag on 2023 August 1 (MJD 60157.3). Soon after, the source started to re-brighten again. The latest optical magnitudes are g'=19.42±0.02, r'= 19.41±0.03, i'= 19.44±0.05, and z_s= 19.34±0.03 mag on 2023 August 10 (MJD 60166). The lowest magnitudes prior to the re-brightening are brighter than the last measured quiescent magnitudes before the outburst started (r'= 21.14±0.08, i'= 21.17±0.17 and z_s= 20.86±0.20 mag on 2023 June 25, MJD 60120.3).
A re-flare is also observed in X-rays with NICER. After reaching a peak of ~480 cts/s (0.5-10 keV; 52 detectors) on July 14, the count rates steadily declined, resulting in a non-detection on August 1 (<1 cts/s). The next day count rates had increased to ~8 cts/s and on August 2 the re-flare reached a peak of ~225 cts/s. In the most recent NICER observation, on August 4, the source had a count rate of ~200 cts/s.
We will continue monitoring the source at optical wavelengths with LCO. We thank the NICER team for the continued monitoring of the source. Further multi-wavelength observations are encouraged in the next few days/weeks to study the nature of this renewed activity. The light curve is linked below. All LCO photometric analysis and data calibration were performed using the "X-ray Binary New Early Warning System" pipeline (XB-NEWS; see Russell et al. 2019 and Goodwin et al. 2020 for details).
Optical light curve of MAXI J1807+132
