XB-NEWS detects a new outburst from the X-ray transient MAXI J1807+132

ATel #16119; Payaswini Saikia, David M. Russell, Kevin Alabarta, M. C. Baglio, D. M. Bramich (NYU Abu Dhabi), Jeroen Homan (Eureka Scientific), Thomas Russell (INAF/IASF Palermo), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 5 Jul 2023; 13:32 UT
Credential Certification: Payaswini Saikia (ps164@nyu.edu)

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MAXI J1807+132 is a transient galactic low-mass X-ray binary (LMXB), discovered during the decay of an outburst by MAXI in 2017 March (ATel #10208). MAXI J1807+132 had another outburst in 2019 during which type-I X-ray bursts were discovered with NICER, identifying it as a neutron star LMXB (ATel #13097). It is known to have short outbursts and re-flares, with unusually high amplitude X-ray and optical variability on day-week timescales (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 a monitoring campaign of ~50 low-mass X-ray binaries (Lewis et al. 2008) since its discovery in 2017. On 2023 July 2 (MJD 60127.3), we detected an increase in the optical flux of the source, with r'=19.61±0.04, i'= 19.68±0.05 and z_s= 19.67±0.06 mag, which is ~1.0-1.5 magnitudes brighter than the last measured quiescent magnitude a week before (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). The latest magnitudes confirm the optical rise with r'=19.80±0.09, i'=19.70±0.08 and z_s=19.65±0.17 mag on 2023 July 4 (MJD 60129.9). Prior to this, the source also had an optical flare on 2023 June 11 (MJD 60106.3) with g'=19.76±0.02, r'=19.74±0.02, i'=19.834±0.05 and z_s=19.94±0.08 mag, but returned quickly back to the quiescent state. We will continue monitoring the source at optical wavelengths with LCO. Further multi-wavelength observations are encouraged in the next few days/weeks to confirm the nature of this renewed activity, and monitor the possible rise of an outburst. 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