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A pre-discovery spectrum of MASTER OT J174902.10+191331.2

ATel #4330; R. Nesci (INAF-IAPS, Roma, Italy)
on 23 Aug 2012; 16:47 UT
Credential Certification: Roberto Nesci (roberto.nesci@uniroma1.it)

Subjects: Optical, Cataclysmic Variable

After the discovery of a new Cataclismic Variable (CV) by Denisenko et al. (ATel #4324) MASTER OT J174902.10+191331.2, I looked for the object on the Objective-Prism plates of the Digitized First Byurakan Survey (Mickaelian et al. 2007 A&A 464,1177), freely available at http//byurakan.phys.uniroma1.it/. The sky area is covered by only one plate (n.1027) of very good quality, made on 1974-05-25 with IIaF emulsion, showing spectra down to Red magnitude 16.4 on the GSC_2.3 catalogue scale. At the position of the CV candidate there is a spectrum clearly visible, with a markedly blue spectral energy distribution. The spectrum is not present in the DFBS on-line database because only stars brighter than B=17 were searched by the automatic pipeline. Unfortunately, the target is partially in the light ring halo of the bright (mag 6) star HR 6642, so that it is not easy to perform a reliable sky subtraction and derive its magnitude by integration of the spectrum in the proper bandpass. I extracted the spectrum with IRAF/apall of the CV candidate and of a few nearby "clean" stars for comparison. Approximate magnitudes B=15.9 and R=15.9 were derived, substantially fainter than the peak value of 13.3 reported in ATel #4324; the outburst of 1974 therefore happened likely some weeks before May 25. Due to the low S/N ratio and spectral resolution, no reliable emission lines are detectable. On the UK Schmidt sky survey red plate (IIIaF + RG610) there are three stars encompassing the CV candidate: the brighter of these stars (R=16.7 in the GSC2.3) is 12 arcsec due South of the target, so that formally its spectrum overlaps the spectrum of the CV: however it is substantially fainter than the CV, so that its contribution to the observed spectrum is negligible. Actually, the faintest detectable spectra on the DFBS plate n.1027 are about R=16.5, so this star is below the detection threshold. It is therefore very likely that MASTER OT J174902.10+191331.2 had an outburst in April-May of the year 1974.