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Detection of a very red source at the position of SWIFT J1753.5-0127

ATel #9741; V. V. Neustroev (FINCA, University of Turku & University of Oulu), E. Gafton (NOT), V.-J. Haanpaa (Oulu), J. Harmanen (Turku), T. Kangas (Turku), L. Kauppinen (Oulu), G. Sjoberg (AAVSO), A. Venhola (Oulu & Kapteyn Institute, Groningen)
on 10 Nov 2016; 21:06 UT
Credential Certification: Vitaly Neustroev (vitaly@neustroev.net)

Subjects: Optical, Binary, Black Hole, Transient

Referred to by ATel #: 9758, 9765, 10075, 10081, 10325

The black hole transient SWIFT J1753.5-0127 had been in outburst since 2005, but in September 2016 it started to decline to quiescence (Russell et al., ATel #9708). We observed this source on 2016 August 24-25 and September 9 with the 0.35-m Celestron C14 robotic telescope and an SBIG ST-10XME CCD camera with Johnson-Cousins BVRI Astrodon Photometric filters. The averaged B and I magnitudes did not change between these nights (B=17.72+-0.06, I=16.42+-0.03), indicating that a fading had started after 2016 September 9, although the R magnitude dropped by 0.15 mag (R=16.83+-0.03 on August 24-25 and 16.98+-0.02 on September 9).

On 2016 November 8, we observed the field of SWIFT J1753.5-0127 with the Nordic Optical Telescope (NOT) equipped with the ALFOSC imager and spectrograph. Observations were performed with the V, R and I filters. At the position of the transient we found a very red source with the following magnitudes:

V=21.25+-0.03, R=20.66+-0.03, I=19.83+-0.02.

The observed V-I colour index (1.42 mag) is significantly redder than it was during a steady state in 2012-2013 (V-I=0.74, V=16.9, Neustroev et al. 2014, MNRAS, 445, 2424) and even during a temporary fading stage in June 2015 (V-I=0.73, V=17.19, Neustroev et al. 2015, ATel #7697). However, Russell et al. (ATel #9708) reported the reddening of light during this fading that is confirmed by our earlier observations taken on 2016 September 9 (V-I=0.87, V=17.29+-0.04).