SWIFT J1753.5-0127 shows the lowest optical flux since 2005

ATel #7697; V. V. Neustroev (University of Oulu), S. V. Zharikov (IA UNAM), A. S. Miroshnichenko (University of North Carolina at Greensboro), D. E. Reichart, J. B. Haislip, K. M. Ivarsen, A. P. LaCluyze, J. P. Moore, M. C. Nysewander (University of North Carolina at Chapel Hill)
on 23 Jun 2015; 12:48 UT
Credential Certification: Vitaly Neustroev (vitaly@neustroev.net)

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The black hole candidate SWIFT J1753.5-0127 has remained active since the onset of its 2005 outburst. In February 2015, the transient exhibited a gradual decrease in hard X-rays (Onodera et al., ATel #7196), and since the middle of March the hard X-ray flux (15-50 keV) remains very low as seen in Swift-BAT light curves (http://swift.gsfc.nasa.gov/results/transients/SWIFTJ1753.5-0127/). Shaw et al. (ATel #7216) also reported increased activity in the softest X-ray band (0.6-2 keV), however no optical response has been detected. In particular, the reported optical magnitudes (B=17.27 and V=16.93) were very close to the values obtained by Neustroev et al. (2014, MNRAS, 445, 2424) during 2012-2013. We note that the Swift XRT/UVOT observations presented by Shaw et al. were conducted on March 11, before SWIFT J1753.5-0127 has reached the lowest hard X-ray flux level. In order to check the current photometric state of the source, we observed SWIFT J1753.5-0127 on June 17, using a PROMPT 0.41-m telescope located at CTIO in Chile. We found the V magnitude to be 17.25+/-0.04. On June 21 the observations were performed with the B, V, R, I Johnson-Cousins filters. We found the following magnitudes:

B=17.69+/-0.08, V=17.19+/-0.04, R_c=16.89+/-0.04, I_c=16.46+/-0.05.

These values are about 0.3-0.4 mag lower than those observed during recent years. Thus, the optical flux of SWIFT J1753.5-0127 is now the lowest since the transient was discovered in 2005.