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MAXI J1807+132: Lulin observations find substantial optical variability during the X-ray fading phase

ATel #10245; A. K. H. Kong (NTHU & Oxford), R. Jin, C.-H. Tseng, E.-T. Lin (NTHU)
on 6 Apr 2017; 08:09 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)

Subjects: Optical, X-ray, Black Hole, Neutron Star, Transient

Following the reports of decaying X-ray and optical flux of the X-ray transient MAXI J1807+132 (ATel #10223,#10224,#10227), we observed the source from 2017 March 30 to April 5 with the 1m telescope at the Lulin Observatory in Taiwan. The source is clearly detected in all observations and shows night-to-night variability. In particular, the last observation taken on April 5 suggests a rebrightening and a bluer object.

By comparing with the AAVSO Photometric All-Sky Survey (APASS) data, we estimated the following magnitudes in the g and r bands.

MJD       g-band      r-band
57842.7322             20.18+/-0.17 (with thin clouds)
57844.8121  20.94+/-0.07   20.60+/-0.07
57845.8122   21.09+/-0.05   20.93+/-0.05
57846.8487   20.93+/-0.05   20.61+/-0.05
57847.8480   21.49+/-0.07   21.00+/-0.06
57848.8334   19.27+/-0.01   19.18+/-0.02

We notice on April 5 (MJD 57848.6506; roughly 5 hours before our optical observation), there is a 950-sec Swift observation. The source is not detected with the XRT. Assuming a hydrogen column of 2.3e21 cm^-2 and a photon index of 2.5 (ATel #10224), we derived a 3-sigma 0.3-10 keV unabsorbed flux limit of 9.8e-13 erg/s/cm^2, a factor of 20 fainter than that of March 29.

We strongly encourage multi-wavelength follow-up observations to monitor the evolution of the outburst of this source.