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Historic light curve of Swift J0243.6+6124

ATel #10989; Roberto Nesci (INAF/IAPS)
on 21 Nov 2017; 10:51 UT
Credential Certification: Roberto Nesci (Roberto.Nesci@iaps.inaf.it)

Subjects: Optical, Binary, Neutron Star, Star, Pulsar

The star Swift J0243.6+6124 was found to be slowly variable by about 0.15 mag on a time interval of about 1000 days by Stanek et al. (ATel #10811). The star is present in the UCAC4 catalog (Zacharias et al. 2012, CDS I/322A) as 758-022980 and is well visible on a set of 65 blue plates of the Asiago Observatory taken between 1967 and 1976 with the 67/90 Schmidt telescope, originally taken for search of Mira variables. Twentytwo nearby stars taken from UCAC4 catalog were used as comparison. A fainter star at 6" to the West is fairly separated in all the Asiago plates. Aperture photometry with IRAF/APPHOT was performed to get instrumental magnitudes, then a parabolic calibration curve was made for each plate using the B UCAC4 magnitudes of the reference stars. After intercalibration for self-consistency, the final photometric internal errors of the mean values of the comparison stars range between 0.05 and 0.09 mag. Swift J0243.6+6124 showed an rms spread of the individual measures of 0.14 mag, substantially larger than those of all the comparison stars, suggesting intrinsic variability. Its light curve is reported in Fig.1 with 1 sigma error bars, together with the light curve of a comparison star of similar magnitude, shifted downward by 0.7 mag for clarity. From this data set we argue that the long-term variability of Swift J0243.6+6124 was present also in the past, with amplitude fully consistent with that shown by the recent ASASSN data (Kochanek et al. 2017, PASP 129, 4502).

Figure 1