NICER confirms the onset of a new outburst from MAXI J1807+132
ATel #16125; Giulia Illiano (INAF OAR), Alessandro Papitto (INAF OAR), Filippo Ambrosino (INAF OAR), Arianna Miraval Zanon (ASI; INAF OAR), Andrea Sanna (Univ. of Cagliari)
on 10 Jul 2023; 19:15 UT
Credential Certification: Alessandro Papitto (alessandro.papitto@inaf.it)
Subjects: X-ray, Neutron Star, Transient
On 2023 July 2 (MJD 60127.3), the Las Cumbres Observatory Network of telescopes (LCO) reported the increase in the optical flux of the transient low-mass X-ray binary (LMXB) MAXI J1807+132 (ATel #16119). NICER promptly performed targeted follow-up observations accumulating more than 18 ks of exposure so far, from July 5 22:38 UTC to July 10 16:03 UTC.
On July 8, NICER detected a sudden increase in flux by a factor of ~100, with the source reaching ~90 counts per second in the 0.2-12 keV band, well above the background level. Since then, the source's flux has continued to increase, reaching ~220 counts per second in the latest observations. Assuming that the 0.5-10 keV spectrum can be described by an absorbed power-law with photon index Γ~2.1 absorbed by a column density of 1E21 cm^-2 (Jiménez-Ibarra et al. 2019), such a count rate translates into an unabsorbed flux of ~4.2E-10 erg/cm^2/s. Such a value is similar to the maximum ones reported during the last outburst observed from the source in 2019 (ATel #13139, 13173).
We also performed a preliminary timing analysis to search for coherent X-ray pulsations in the observations carried out after the flux increase. We found no evidence for pulsations, with a 3σ upper limit on the pulse amplitude of ~5.8%.
During MAXI J1807+132 outburst in 2017 (ATel #10208) and 2019 (ATel #13097), several brightening episodes were detected (Jiménez-Ibarra et al. 2019; ATel #13173). Moreover, in 2019 NICER detected two Type I X-ray bursts confirming the source classification as a neutron star (ATel #13239). For all these reasons, additional multi-wavelength follow-up is encouraged.
NICER monitoring of MAXI J1807+132 is still ongoing. NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.
