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MAXI J1807+132 is fading in both X-rays and optical wavelengths

ATel #10224; M. Armas Padilla (IAC-Tenerife), R. Wijnands, N. Degenaar (U. of Amsterdam), T. Munoz-Darias, F. Jimenez-Ibarra, D. Mata Sanchez, J. Casares (IAC-Tenerife), P. A. Charles (Southampton)
on 29 Mar 2017; 13:46 UT
Credential Certification: Rudy Wijnands (radwijnands@gmail.com)

Subjects: Optical, Ultra-Violet, X-ray, Request for Observations, Black Hole, Neutron Star, Transient

Referred to by ATel #: 10245

We have analyzed a new Swift observation (performed on 06:44UT on March 29, 2017) of the recently discovered X-ray transient MAXI J1807+132 [ATEL #10208, #10215, #10216, #10217, #10221, #10222,#10223].

The average spectrum obtained during this observation has been fitted with a power-law model affected by absorption. The hydrogen column has been fixed to the value of NH~2.3x1021 cm-2 obtained during the analysis of the previous Swift-XRT observation [ATel #10216]. The photon index is 2.5+/-0.2 and the resulting absorbed (unabsorbed) 0.3-10 keV flux is (9.2+/-0.1)x10-12 ((1.9+/-0.1)x10-11) ergs cm-2 s-1.

This represent a ~75% drop in X-ray flux (0.3 - 10 keV) with respect to the previous Swift-XRT observation (2 days ago; ATel #10216). Similarly, the UVOT data show also a flux drop (the below magnitudes are given in the AB system):

 
March 27         		March 29 
 
v > 17.56 		        v > 18.02 
b = 18.1 +/- 0.3 		b = 18.6 +/- 0.2 
u = 18.43 +/- 0.19 	        u = 18.76 +/- 0.16 
uvw1 = 18.56 +/- 0.13 	        uvw1 = 19.10 +/- 0.13  
uvm2 = 18.72 +/- 0.13 		uvm2 = 19.61 +/- 0.14 
uvw2 = 18.75 +/- 0.09 		uvw2 = 19.59 +/- 0.11 

The above results suggest that the source may be decaying to quiescence after a very short outburst. If confirmed this behavior is reminiscent of the X-ray transient MAXI J1957+032 (Mata Sanchez et al. 2017, arXiv:1701.03381) and that of the accreting millisecond X-ray pulsar NGC 6440 X-2 (Altamirano et al. 2010, ApJ, 712, L58; Heinke et al. 2010, ApJ, 714, 894). Both those sources have shown recurrent, very short (~week long) outbursts. However, we urge more X-ray observations of MAXI J1807+132 to investigate the further evolution of the outburst of this source.